Published in Estate Planning magazine, October 2002; Published in the North Carolina Bar Association Estate & Planning Fiduciary Law Section Newsletter in two parts in the May and June 2002 issues; Published in the Estate Planning Section Newsletters in the State of Washington and State of California.
Five years ago, Alaska approved the creation of self-settled discretionary spendthrift trusts. This article examines how these trusts have been used in Alaska, and analyzes planning for these trusts in light of existing authority.
Alaska trustees report that approximately 870 trusts have been formed under Alaska law by nonresidents of Alaska.4 Of these, approximately 310 are SSDS Trusts, and the balance are perpetual trusts. Most of the SSDS Trusts also used a perpetual trust plan. Approximately 110 attorneys provided the legal services for the creation of these trusts.
Alaska estate planning attorneys report that approximately 125 SSDS Trusts have been formed for Alaska residents. In addition, 200 to 300 perpetual trusts have been created for Alaskans. Several lawyers report that their standard default plan for medium and large estates now is based on a perpetual trust dispositive plan. Approximately 60% of both the resident and nonresident SSDS Trusts have involved contributions of assets which were completed gifts for federal gift tax purposes.
When the Alaska Legislature enacted statutes authorizing the creation of SSDS Trusts in 1997, the initial focus was on asset protection. Gifts to such pure asset protection trusts would often be structured to be incomplete for gift tax purposes.5 This would allow substantial funding without the payment of gift tax. Advocates of foreign trusts correctly pointed out that persons seeking maximum asset protection should look offshore.6 As a result, in the five years since passage of the Alaska statutes, the primary focus of Alaska SSDS Trusts has changed from asset protection to transfer tax reduction. The use of SSDS trusts for such tax reduction is also the focus of this article.
The following example of a planning dilemma illustrates the use of SSDS Trusts for transfer tax reduction planning. The balance of this article discusses (1) how such trusts have been structured and implemented in Alaska, (2) the use of SSDS Trusts by nonresidents of Alaska, (3) how an SSDS Trust could fail, and (4) planning analysis in view of the existing authority. Tax and asset protection issues are identified, and their merit is discussed.
The planning dilemma: Early gift giving vs. future possible needs.
Consider this planning situation: your clients are a couple in their 50s. One or both is a small business owner, executive, or professional. Their net worth is in the range of $3 million to $10 million. Substantial estate taxes could be saved if your clients made annual exclusion and applicable credit gifts to irrevocable trusts for their children and/or grandchildren. These gifts will not render the clients insolvent, nor will they be transfers made with an intent to evade existing creditors. The gifts could be structured so that they qualify for valuation discounts, and the growth of the gift assets would be excluded from your clients' estates.7Based on your clients' net worth and their anticipated future earnings, it appears that these gifted amounts would not be needed by them. Nevertheless, your clients are reluctant to give away significant assets at this point in their lives. They tell you that they might need these assets in the future if they have an unexpected financial reversal.
Your clients ask if they can be added as discretionary beneficiaries of the trust. Then, the trustee can make distributions to them if needed. You respond that if they were added as discretionary beneficiaries, the IRS could successfully argue that the trust assets should be included in their gross estates at death and be taxed under the federal estate tax.
The reason is that your state has a statutory or case law policy that provides that if the settlors are discretionary beneficiaries of the trust, the settlors' creditors can reach the maximum amount that the trustee could distribute to the settlors and, in many instances, this would be all the assets in the trust.8 Therefore, the settlors could run up debts and the settlors' creditors could reach the trust assets to satisfy these obligations. Another way of looking at the situation is that the settlors, indirectly, have retained the ability to reach the trust assets through incurring debts.
As a result, the clients in our example (above) may create an Alaska SSDS Trust, make annual exclusion and applicable exclusion amount gifts to the trust,14 and be included in the class of discretionary beneficiaries to whom an independent trustee may make distributions. A strong position exists that such assets will not be included in the clients gross estates at their deaths. If the clients need funds in the future, due to an unexpected financial downturn, the trust assets are available.
How an SSDS Trust is structured and funded.
Often, settlors first form a family limited partnership (FLP) or family limited liability company (FLLC).15 These entities are funded with investment assets such as interests in closely held businesses, real estate, and marketable securities. The clients may desire that they, or family members, be the general partners or managers. Then, the clients give to the SSDS Trust the limited partnership or non-managerial LLC interests. In this way, the clients--or their family members--retain the ability to manage the assets and to decide when distributions will be made to the trust. Giving gifts of the non-managerial interests qualifies for valuation discounts.
Trust-owned life insurance. SSDS Trusts have also become a vehicle for the ownership of insurance on the settlors' lives. For example, suppose the clients wish to purchase a second-to-die life insurance policy that will develop substantial cash value and will benefit from income tax-free inside buildup. However, the clients may want the ability to reach the value of the policy if they have a financial reversal.
If the policy is owned by an SSDS Trust, the independent trustee may borrow from the insurance company, or even cash in the policy, in order to make discretionary distributions that are needed by the settlors. The fact that the settlors are discretionary beneficiaries of the trust does not appear to be enough to conclude that they have retained incidents of ownership in the policy.17 Nevertheless, careful choices of trustees and drafting are necessary to ensure that incidents of ownership are not attributable to the clients.
The dispositive plan. A typical SSDS Trust will provide that the independent trustee has absolute discretion to make distributions to a class of beneficiaries that includes the settlors and their descendants. This absolute discretion is provided to avoid an exception to the Alaska spendthrift rule for any portion of a trust's income or principal which must be distributed to the settlor.18 Further, absolute discretion avoids contentions that a beneficiary (or the beneficiary's creditors) can force a trustee to make distributions pursuant to an ascertainable standard stated in the trust instrument.19 For instance, a creditor could argue that maintenance or support includes the payment of the beneficiary's creditors. Alternatively, a creditor could argue that a trustee is required, pursuant to the ascertainable standard, to distribute assets to an insolvent beneficiary. Then, the creditor could attempt to attach the distributions.
Choice of trustees. At least one trustee of an SSDS Trust should be an independent trustee. This independent trustee has all trust distribution powers under the absolute discretion standard. In order to preserve the independence of the trustee, there must not be any agreement between the independent trustee and the settlor regarding distributions.
The existence of such an agreement would allow the settlor's creditors to reach the trust assets because the settlor would have a right to the distribution of the assets.22The result would be inclusion of the assets in the settlor's gross estate.23 Such an agreement could be written, verbal, or implied through a pattern of distributions.24 It would be more likely that a court might imply an agreement between the trustee and settlor if the independent trustee had a relationship with the settlor. Such relationships would include being a close relative, close friend, or employee. Because the transfer tax advantages depend on the premise that the settlor's creditors cannot reach the assets in the trust, it is very important to choose a trustee who will minimize the risk that an implied agreement will be found.
In addition to an independent trustee, some clients wish to appoint a family trustee who will have some or all of the administrative responsibilities for the trust. These are not tax-sensitive duties,25 and should not affect the creditor protection of the trust. However, if the trust owns insurance policies on the life of a family trustee, the managerial duties relating to such policies must be reserved for the independent trustee to avoid inclusion of the insurance proceeds in the family trustee's gross estate.
Future amendments. The newness of SSDS Trusts and the ambiguous state of the law has encouraged drafters to build flexibility into the trust instrument. The independent trustee, or an independent trust protector, is often given the authority to amend the trust instrument in order to adjust for future changes in the tax law.27A trust protector may be given the power to eliminate the settlor as a discretionary beneficiary of the trust,28 and to change the choices of trustees. The goal of such provisions is to allow future adjustments so that the trust assets will not be included in the settlor's gross estate if the tax law is interpreted or changed in a manner indicating that such inclusion is likely.
Use of SSDS Trusts by nonresidents of Alaska.
The framers of the Alaska SSDS and perpetual statutory trust provisions considered that persons located outside Alaska might well be interested in using such trusts. Consequently, they enacted statutory provisions which the framers thought would establish a sufficient Alaska nexus so that Alaska law would apply to nonresident trusts.
Subsequent Alaska legislation facilitating SSDS Trusts.
How could an SSDS Trust fail?
As discussed above, this type of transfer tax planning first depends on the asset protection foundation. Once an adequate asset protection foundation exists, the inquiry shifts to analysis of two federal estate tax provisions (Sections 2036 and 2038) and the contention regarding the Contract Clause.
Residents of states that have enacted SSDS statutes have a strong position concerning the asset protection foundation.32 Nonresidents who establish SSDS trusts have additional issues relating to whether they have an adequate asset protection foundation: the Full Faith and Credit Clause and the bankruptcy court scenarios.
If the transfer tax issue is contested, the asset protection foundation will be hypothetical. There will not be a creditor trying to reach the assets of the trust. The court will need to decide if an adequate asset protection foundation exists for the settlor in question. Such a foundation may well not need to be perfect and without any theoretical weaknesses.
Application of Sections 2036 and 2038. With the above analytical approach in mind, first consider the provisions of the estate tax. If applicable state law prevents the settlor's creditors from reaching the trust assets, Section 2038 does not apply because, as of the date of the settlor's death, the settlor does not have the power to revoke the trust by relegating creditors to the trust assets. The remaining estate tax issue is whether, pursuant to Section 2036(a)(1), the settlor has retained enjoyment of, or the right to income from, the trust assets. Initially, the plain language of Section 2036(a)(1), which requires retention does not seem to apply to a settlor-beneficiary, who may receive distributions only pursuant to the absolute discretion of an independent trustee.
If Section 2036 is amended to expressly include SSDS Trusts, and if such amendment is stated to be a change in the law, then its effect should be prospective. Existing SSDS Trusts should be grandfathered. If the amendment is stated to be only a clarification of the law, this issue of statutory interpretation will continue for existing SSDS Trusts. Nevertheless, as a practical matter, the Service may take a much less aggressive position in regard to trusts formed prior to the amendment.
Interestingly, if there were a statutory change in Section 2036 with regard to SSDS Trusts, there is no certainty that the change would be designed to produce inclusion of the trust assets in the settlor's gross estate. Congress' recent legislative changes in the transfer tax area have gone in the other direction.38 A reasonable argument can be made that Section 2036 should be amended to expressly allow a settlor to create an SSDS Trust in any jurisdiction in order to solve the planning dilemma described earlier. Such an accommodation might help alleviate the tension between complete repeal of the estate tax and sunset, which exists under the Economic Growth and Tax Relief Reconciliation Act of 2001 (2001 Tax Act).
The full faith and credit scenario. Now consider the full faith and credit scenario involving a nonresident settlor. Assume that a hypothetical creditor sues the settlor in the settlor's domiciliary state and obtains a judgment. Next, assume that as part of that suit, or in a separate action in the domiciliary state, the creditor proceeds against the trustee of the SSDS Trust in order to enforce the judgment against the trust's assets.
If the creditor fails to convince the bankruptcy court to so construe the code, then alternatively the creditor can argue the choice of law issue. This issue assumes that the creditor forces the settlor into involuntary bankruptcy in the settlor's state of domicile. The bankruptcy court will have personal jurisdiction over the Alaska trustee based on the court's national jurisdiction. The court will need to resolve the choice of law issue.60If (1) the bankruptcy court applies the domiciliary state's choice of law rules, (2) those rules follow the Restatement of Conflict of Laws,61 and (3) the court determines the issue is one of validity of the trust, then the bankruptcy court may determine that the Alaska SSDS statute violates a strong public policy of the domiciliary state.62 As a result, the trust assets will be included in the bankruptcy estate.
First, it is important to consider the difference between pure asset protection cases and transfer tax litigation. The highly publicized recent asset protection cases involved extreme facts and equities that would influence most courts to sympathize with the plaintiff-creditor.65 The situation is quite different when the asset protection issue is hypothetical, and needs resolution only so that the transfer tax issue may be determined.
The above analysis establishes that the asset protection foundation for a nonresident settlor using an Alaska SSDS Trust is not absolute. The interesting question is whether such a foundation needs to be perfect for transfer tax purposes. The above analysis describes theoretical approaches for a creditor to reach the SSDS Trust assets, if the facts are right and if a court follows a specific decision-tree. Are these approaches certain enough to undermine the asset protection foundation, for transfer tax purposes, of a carefully implemented Alaska SSDS Trust created by a nonresident? This is the crucial issue for nonresident settlors.
Why don't we have more authority?
Five years have elapsed since the enactment of Alaska's SSDS Trust statutes. However, authority and review remain sparse. The IRS has refused to rule further on such trusts.66 Despite the formation of numerous SSDS Trusts, practitioners in Alaska and Delaware report that as yet there is no audit experience. Consequently, there has been no administrative or judicial review of such trusts.
With respect to residents of states that have enacted SSDS statutes, the Service's estate tax statutory position appears weak. The Contract Clause contention becomes factually irrelevant as time expires. Therefore, a challenge may occur only if there is faulty implementation of the trust. Resolution of such a fact-dependent case will not be helpful for the resolution of other cases involving properly implemented trusts.
A legislative resolution of the effectiveness of transfer tax planning with SSDS Trusts is also unpredictable. At some point before 2010, Congress will likely rethink the transfer tax changes enacted by the 2001 Tax Act. Section 2036 could be amended to resolve this area. But which way?
In view of the above-described limited arguments available to the Service with respect to residents of an SSDS state, and the fact-specific character of the issues involving nonresident settlors, there may continue to be a lack of significant judicial authority in this area.68 If the tax question does arise, the Service and the estate's representative often may find a negotiable resolution.
What should planners do? Evaluate your clients' tolerance for ambiguity, and the downside.
Clients considering the use of an SSDS Trust for transfer tax reduction purposes should be fully advised of the present lack of significant authority. Planners and their clients need to be aware that such authority in this area may continue to be slow in coming. Those uncomfortable with such ambiguity should not use an SSDS Trust. For clients who are still interested, an analysis should be made of the downside risk.
If the SSDS Trust approach were to fail because of one of the issues discussed above, the following transfer tax consequences would occur. The trust assets and their appreciation will be included in the settlor's gross estate and be subject to estate tax.69Further, the settlor has lost the benefit of the annual exclusion gifting that was made to the trust. The settlor's estate retains the use of the applicable credit amount that was originally allocated to the completed gift to the trust.70The settlor has lost the cost of creating and maintaining the trust.
What if the settlor made attempted completed gifts that were larger than the settlor's annual exclusion and applicable credit amounts and, as a result, paid out-of-pocket gift tax? In addition to the above consequences, the settlor would have lost the use of the out-of-pocket tax amount during the settlor's lifetime. Moreover, if the federal estate tax is permanently repealed, the payment of the gift tax would have been unnecessary.
The main downside risk would appear to be that the settlor has lost the opportunity to do some different planning with the settlor's annual exclusion gifts and with the portion of the settlor's applicable credit amount used for the SSDS Trust. Would the settlor have done such different planning? How do the risks and rewards of such different planning compare to the SSDS approach? These are the key questions that the estate planner and interested clients need to resolve.
1Alaska's statute was passed in April 1997. Delaware immediately followed suit and enacted its version in July 1997. In 1999, both Nevada and Rhode Island passed statutes designed to implement SSDS Trusts. In this article, an SSDS Trust means an irrevocable trust which authorizes an independent trustee, in such trustee's absolute discretion, to make distributions to a class of beneficiaries which includes the sttlors.
2In this article, a perpetual trust means an irrevocable trust created in a jurisdiction that has abolished the rule against perpetuities, and therefore the trust can continue as long as it has assets. Many SSDS Trusts are also designed as perpetual trusts. Alaska perfected its abolition of the rule against perpetuities in 2000. See note 20 infra.
3The general factual information stated above and elsewhere in this article concerning Alaska SSDS and perpetual trusts is based on anecdotal, as opposed to scientific, research by the author. The author surveyed institutions and individuals that had indicated an interest in acting as a trustee for nonresident trusts. Also contacted were Alaska estate planning attorneys likely to be involved in creating SSDS Trusts for Alaska residents or in helping non-Alaska attorneys form such trusts.
4The numerical counts stated in this article refer to the number of trust instruments. Each trust instrument may create one or more separate subtrusts.
5Alaska Statute 34.40.110(b)(2) allows the settlor to retain a power to veto a distribution from the trust or a testamentary special power of appointment. Either approach makes the gift incomplete.
6Giordani and Osborne, Alaska Asset Protection Trusts: Will They Work?, included in Osborne and Schurig, Asset Protection: Domestic and International Law and Tactics, Special Pamphlet to 1997-4 Supplement (Clark Boardman Callaghan, 1995); Osborne, Asset Protection and Jurisdiction Selection: Clearing Up Your Situs Headaches, 33 U. Miami Heckerling Inst. on Est. Plan. (1999); Hogan, Once More Unto the Breach: Planning for a Conflict of Laws With Alaska and Delaware Self-settled Spendthrift Trusts, 14 Prob. & Prop. (Mar./Apr. 2000).
7For example, assume that your clients give gifts totaling $1 million and that they are both 50 years of age. Further assume that these assets grow at a net rate of 5% per year and that the second to die of your clients lives to age 84. The gift assets will grow to $5,454,648. Subtracting the $1 million initially given to the SSDS Trust leaves growth of $4,454,648. Assume this growth is taxed at a federal estate tax rate of 45%. This would produce taxes of $2,454,592. This is the tax amount that would have occurred 34 years from now if your clients had not given the $1 million to the SSDS Trust. Using the same 5% rate, avoiding this tax produces a present value savings of approximately $449,000.
8Prior to 1997, almost all states had such a policy.
9Reg. 25.2511-2(b) provides that a gift is complete if the donor has so parted with dominion and control as to leave in him no power to change its disposition, whether for his own benefit or for the benefit of another . . . . The above reasoning illustrates that this test is not satisfied by SSDS Trusts in most states.
10Section 2036 would probably apply because the settlors have retained the enjoyment of, and income from, the property by their ability to incur debt which their creditors may satisfy from trust assets. Section 2038 would apply because the above-described ability to relegate creditors to the trust assets allows the settlors to revoke the transfer of assets to the trust.
12Ltr. Rul. 9837007, which involved an Alaska resident; Rev. Rul. 77-378, 1977-2 CB 347.
13For a comprehensive discussion of these issues, including the relevant authorities, see Blattmachr and Zaritsky, North to Alaska--Estate Planning Under the New Alaska Trust Act, 32 U. Miami Heckerling Inst. on Est. Plan. (1998); Pennell, 2 Estate Planning §7.3.4.2 (Aspen, 6th ed.); Manley, Estate Planning and Asset Protection Using Self-Settled Alaska Trusts, 33 U. Miami Heckerling Inst. on Est. Plan. (1999), Spec. Sess.; and Shaftel, Newest Developments in Alaska Law Encourage Use of Alaska Trusts, 26 ETPL 51 (Feb. 1999). In the application for Ltr. Rul. 9837007, the Alaska settlor stated her position that the trust not be includable in her estate. In response, the Service declined to rule.
14Gifts to a fully discretionary trust cannot be split, under Section 2513, with a spouse who is a discretionary beneficiary. See Handler and Chen, Fresh Thinking About Gift-Splitting, 141 Tr. & Est. 36 (Jan. 2002); Benjamin, When Should the Option to Split Gifts Be Chosen?, 22 ETPL 24 (Jan./Feb. 1995).
15Alaska's limited partnership and limited liability company statutes have both been amended so as to maximize qualification for valuation discounts. See Alaska Statutes 32.11.110, et seq., and 10.050.010, et seq.
17See Ltr. Rul. 9434028. Reg. 20.2042-1(c).
19Restatement (Second) of Trusts §155, comment b (1959); Rothschild, Protecting the Estate From In-Laws and Other Predators, 35 U. Miami Heckerling Inst. on Est. Plan., pp. 17-21 through 17-23 (2001).
20In 1997, Alaska took a first step in abolishing its rule against perpetuities. This abolishment was perfected in 2000 by amendments designed to avoid the Delaware Tax Trap. Alaska Statute 34.27.051-.100. See Greer, The Delaware Tax Trap and the Abolition of the Rule Against Perpetuities, 28 ETPL 68 (Feb. 2001). Alaska is one of only several states that (1) have successfully abolished the rule against perpetuities in a manner that avoids this tax trap, and (2) also do not have a state income tax.
21These planning concepts have been thoroughly analyzed and discussed by Frederick R. Keydel and Harvey B. Wallace II in Trust Drafting for the Unforeseeable, presented at a workshop and later incorporated by Ronald D. Aucutt into Structuring Trust Arrangements for Flexibility, 35 U. Miami Heckerling Inst. on Est. Plan. (2001).
23See Reg. 20.2036-1(a) which finds retention under Section 2036 if such an agreement exists.
24Cases involving Section 2036 and an implied understanding of grantor access are discussed in Boxx, Gray's Ghost--A Conversation About the Onshore Trust, 85 Iowa L. Rev. 1195, at 1244-1251 (2000).
25Pennell, 2 Estate Planning, supra note 13, at §7.3.3, p. 320.
26Coleman, State Fiduciary Income Tax Issues, ALI-ABA Advanced Estate Planning Techniques (2002); Gutierrez, The State Income Taxation of Multi-Jurisdictional TrustsThe New Playing Field, 36 U. Miami Heckerling Inst. on Est. Plan. (2002).
27Such flexibility and suggested provisions are discussed in McBryde and Keydel, Back to the Future for the Estate Planner: Building Flexibility in Estate Planning Documents, 30 U. Miami Heckerling Inst. on Est. Plan. (1996).
28See Dodge, 50-5th T.M. (BNA), Transfers With Retained Interests and Powers, p. A-78.
31These changes are discussed in Greer and Shaftel, Alaska Enacts Additional Estate Planning Legislation, 27 ETPL 376 (Oct. 2000), and Shaftel, Newest Developments in Alaska Law Encourage Use of Alaska Trusts, supra note 13.
32This article and its hypotheticals assume that a fraudulent transfer has not been made. Both residents and nonresidents will be vulnerable to a creditor challenge if the settlers were found to have transferred assets to the SSDS Trust with an intent to evade existing creditors. Alaska Statute 34.40.110(b)(1). See Osborne, Asset Protection and Jurisdiction Selection: Clearing Up Your Situs Headaches, supra note 7, at 13-28.
33See the authorities cited in the articles listed in note 13 supra.
34Pennell, 2 Estate Planning, supra note 13, at §7.3.4.2. This commentator concludes, [t]he answer to that question has not adequately been provided by case law or rulings. Id., p. 7.345.
35Dodge, 50-5th T.M. (BNA), Transfers With Retained Interests and Powers, p. A-23.
36Section 2702(c)(2), enacted in 1990.
37Pennell, 2 Estate Planning, supra note 13, at §7.3.4.1, p. 7.334.
38The best example is the Economic Growth and Tax Relief Reconciliation Act of 2001 (2001 Tax Act or EGTRRA), which has as its ultimate goal the repeal of the estate tax.
39See note 23 supra and the text to which it relates.
40If the settlor's interest applied to all the trust's assets, the assets would all be included in the settlor's gross estate. On the other hand, if the settlor desired an interest in only part of the trust's assets, then only the proportion retained would be included in the settlor's gross estate. Reg. 20.2036-1(a). See Mahoney, 831 F.2d 641, 60 AFTR2d 87-6152 (CA-6, 1987); Estate of Tomac, 40 TC 134 (1963); Rev. Rul. 79-109, 1979-1 CB 297. This provides a hedge for the more conservative settlor and planner.
41U.S. Const. art. I, §10, cl. 1.
42Osborne, Asset Protection and Jurisdiction Selection: Clearing Up Your Situs Headaches, supra note 6, at 14-26.
44Id. See also Boxx, Gray's Ghost, supra note 24, at 1230.
45Osborne, Asset Protection and Jurisdiction Selection: Clearing Up Your Situs Headaches, supra note 6, at 14-26. Boxx, Gray's Ghost, supra note 24, at 1240.
47Boxx, Gray's Ghost, supra note 24, at 1240, n. 295.
48If the question is one of administration, the settlor's choice of law in the trust instrument controls (Restatement (Second) Conflict of Laws §273(b).) If the question is one of validity of the trust, then again, the settlor's choice will prevail, provided that this state has a substantial relation to the trust and that the application of its law does not violate a strong public policy of the state with which, as to the matter at issue, the trust has its most significant relationship under the principles stated in §6 . . . . Id., §270.
49For example, factual determinations may need to be made concerning whether Alaska has a substantial relation to the trust, and which state has the most significant relationship to the trust.
50Further analysis of this conflict of laws issue may be found in Blattmachr and Zaritsky, North to Alaska--Estate Planning Under the New Alaska Trust Act, supra note 13; Hogan, Once More Unto the Breach: Planning for a Conflict of Laws With Alaska and Delaware Self-settled Spendthrift Trusts, supra note 6; and, generally, in Moore, Choice of Law in Trusts: How Broad Is the Possible Spectrum?, 36 U. Miami Heckerling Inst. on Est. Plan. (2002).
51U.S. Const. art. IV, §1.
5218 Moore's Federal Practice §130.04 (Matthew Bender, 3d ed.).
53Id. Restatement (Second) of Conflict of Laws §92, comment e.
54Jurisdictional issues may be very fact-dependent. For example, there may be arguments that long-arm jurisdiction is appropriate due to a corporate trustee's activities in the domiciliary state, which may include advertising, attendance at conferences, articles in the national press, and website material. See Boxx, Gray's Ghost, supra note 24, at 1211-1212.
57See Osborne, Asset Protection and Jurisdiction Selection: Clearing Up Your Situs Headaches, supra note 6, at 14-24, for a full discussion of this statutory interpretation argument.
59An argument can be made that resident settlors could still rely on the Alaska SSDS Trust statute as a state law exemption independent of Bankruptcy Code section 541(c)(2). Nonresident settlors could not because section 522(b)(2) of the Bankruptcy Code limits state law exemptions to those of the debtor's domicile state.
60See the choice of law discussion above with respect to the Full Faith and Credit Clause scenario.
62This scenario has occurred involving offshore trusts. See Boxx, Gray's Ghost, supra note 24, at 1227-1230.
65 E.g., Federal Trade Comm. v. Affordable Media, LLC, 179 F.3d 1228 (CA-9, 1999); In re Portnoy, 201 B.R. 685 (S.D.N.Y. 1996); and In re Brown, 4 Alaska B.R. 279 (D. Alaska, Mar. 11, 1996).
66 See note 13, supra. Only two private letter rulings exist: Ltr. Rul. 9837007, which concluded that gifts were complete when made to an Alaska SSDS Trust designed for transfer tax reduction, and Ltr. Rul. 200148028, which found that gifts were incomplete when made to a Delaware trust designed only for asset protection, and also ruled that the Delaware trust was not a grantor trust for income tax purposes.
67 In regard to personal jurisdiction issues, Professor Boxx states, unfortunately, a decision that would expose the trust assets to the judgment in this context would be too fact-specific to have much relevance to future cases, since it would turn on personal jurisdiction of a particular state over a particular trustee. However, depending on the policy analysis done to determine personal jurisdiction, the decision could be a sufficient cautionary tale that would make the trusts less attractive or, at least, affect future litigation strategy. See Boxx, Gray's Ghost, supra note 24, at 1221, n. 149.
68In regard to the Section 2036(a)(1) issue, Professor Pennell concludes, [t]his issue will take time to resolve, and there may be fits and starts as various courts analyze the question. See Pennell, 2 Estate Planning, supra note 13, at §7.3.4.2, pp. 7.345-.346.
69 If spouses are co-settlors, conservative drafting will include a provision that states that if trust assets are included in the gross estate of the first settlor to die, then such assets will be distributed to a QTIP trust for the surviving spouse.


text:
Systems and methods for measuring velocity with a radar altimeter are provided. In at least one embodiment a method for measuring velocity magnitude of a platform in relation to a surface comprises transmitting a radar beam, wherein the radar beam is aimed toward a surface; receiving a plurality of reflected signals, wherein the plurality of reflected signals correspond to portions of the transmitted radar beam that are reflected by a plurality of portions of the surface; and applying Doppler filtering to the plurality of signals to form at least one Doppler beam. The method also comprises identifying range measurements within each Doppler beam in the at least one Doppler beam; and calculating the velocity magnitude based on the range measurements of the at least one Doppler beam.
3177348 April 1965 Daniloff et al.
2008/0246648 October 2008 Winstead et al.
European Patent Office, "Extended European Search Report from EP Application No. 16153713.9 dated Jun. 14, 2016", "from Foreign Counterpart of U.S. Appl. No. 14/613,879", dated Jun. 14, 2016, pp. 1-8, Published in: EP. cited by applicant .
Winstead et al., "Systems and Methods for Using Velocity Measurements to Adjust Doppler Filter Bandwidth", "U.S. Appl. No. 14/613,888, filed Feb. 4, 2015", pp. 1-38, Published in: US. cited by applicant .
European Patent Office, "Communication Pursuant to Article 94(3) for EP Application No. 16153713.9", "Foreign Counterpart to U.S. Appl. No. 14/613,879", filed Aug. 22, 2017, pp. 1-7, Published in: EP. cited by applicant .
Frick, "Systems and Methods for Measuring Velocity and Acceleration With a Radar Altimeter", "U.S. Appl. No. 15/656,942, filed Jul. 21, 2017", Jul. 21, 2017, pp. 1-34, Published in: US. cited by applicant.
1. A method for measuring velocity magnitude of a platform in relation to a surface, the method comprising: transmitting a radar beam, wherein the radar beam is aimed toward a surface; receiving a plurality of reflected signals, wherein the plurality of reflected signals correspond to portions of the transmitted radar beam that are reflected by a plurality of portions of the surface; applying Doppler filtering to the plurality of signals to form at least one Doppler beam; identifying range measurements within each Doppler beam in the at least one Doppler beam; and calculating the velocity magnitude based on the range measurements of the at least one Doppler beam.
2. The method of claim 1, wherein calculating the velocity magnitude comprises: identifying a set of test velocities; for each velocity in the set of test velocities, calculating an error magnitude for each velocity based on the range measurements associated with the at least one Doppler beam; and identifying a velocity within a range of test velocities in the set of test velocities associated with a smallest error magnitude as the velocity magnitude.
3. The method of claim 1, wherein calculating the velocity magnitude comprises: computing the velocity based on range measurements gathered over a time period where the velocity does not change.
4. The method of claim 1, wherein calculating the velocity magnitude comprises acquiring an initial velocity estimate.
5. The method of claim 4, wherein the initial velocity estimate is based on the maximum observed Doppler shift that is above a noise floor.
6. The method of claim 1, further comprising: applying weighting factors to the range measurements associated with the at least one radar beam.
7. The method of claim 1, further comprising providing the calculated velocity magnitude to another system as navigational information.
8. The method of claim 1, further comprising dynamically adjusting at least one center frequency for at least one Doppler filter used to form the at least one Doppler beam based on an expected location on the surface.
9. The method of claim 8, wherein the expected location is determined by the calculated velocity magnitude.
10. The method of claim 8, further comprising using a map of terrain reflectivity characteristics to determine frequencies of the at least one Doppler beam.
11. The method of claim 1, further comprising: adjusting Doppler filter center frequencies over a period of time when forming the at least one Doppler beam; observing signal magnitudes over the period of time associated with at least one portion of the surface; and computing terrain reflectivity information associated with the at least one portion of the surface.
12. The method of claim 11, further comprising estimating the location of the platform relative to the surface based on correlation of computed terrain reflectivity information against a map of terrain reflectivity characteristics for the surface.
13. A system for measuring velocity of an aircraft, the system comprising: a transmitter that transmits a radar beam at a first frequency; a receiver that receives a plurality of reflected signals at a plurality of frequencies when the radar beam is reflected by a plurality of portions of a surface; and a processor that applies Doppler filtering to the plurality of reflected signals to form at least one Doppler beam and determines range measurements for the at least one Doppler beam, wherein the processor calculates a velocity estimate based on the range measurements of the at least one Doppler beam.
14. The system of claim 13, wherein the processor calculates the velocity estimate by: identifying a set of test velocities; for each velocity in the set of test velocities, calculating an error magnitude for each velocity based on the range measurements associated with the at least one Doppler beam; and estimating a velocity within a range of test velocities in the set of test velocities associated with a smallest error magnitude as the velocity estimate.
15. The system of claim 13, wherein the processor applies weighting factors to the range measurements associated with the plurality of reflected signals.
16. The system of claim 13, wherein the processor dynamically adjusts the frequency of a desired Doppler shift based on an expected location on the surface.
17. The system of claim 16, wherein the processor determines the expected location based on terrain reflectivity information collected from the at least two reflected signals, wherein the terrain reflectivity information is correlated against a map of terrain reflectivity characteristics.
18. The system of claim 17, wherein the processor determines a spectral range for the at least two reflected signals based on a spectrum of possible Doppler shifts.
19. A radar altimeter receiver, the receiver comprising: an analog to digital converter configured to convert received analog signals into digital signals; a plurality of Doppler filters configured to filter the digital signals into at least one Doppler beam, where the at least one Doppler beam is associated with a Doppler shift; a range measurer configured to calculate at least one range measurement from the at least one Doppler beam; and a velocity estimator configured to calculate the velocity of a vehicle containing the radar altimeter receiver based on the at least one range measurement.
20. The radar altimeter receiver of claim 19, further comprising at least one gate, wherein a gate in the at least one gate selects time range of samples from the digital signals for filtering by the plurality of Doppler filters.
Aircraft navigation systems frequently use velocity information calculated by different sensors onboard an aircraft to provide a navigation solution representing the location and attitude of the aircraft. For example, global positioning systems (GPS) are common sources of velocity measurements used in providing a navigation solution. Other sources of velocity measurements include measurements from an aircraft's inertial navigation system and airspeed measurements from observations of relative air pressure through a pitot tube. However, these different sources of velocity measurements are subject to different errors that increase the challenge of gathering reliable measurements of ground speed. In certain situations, a GPS may be easily jammed, causing the aircraft to rely on other less reliable sources of velocity measurement. For example, measurements from an inertial navigation system slowly drift over time and airspeed measurements via a pitot tube fail to provide accurate information about an aircraft's ground speed.
FIG. 9 is a flow diagram of a method for calculating velocity using a radar altimeter in one embodiment described in the present disclosure.
In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments.
In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures and the specification is not to be construed as limiting the order in which the individual steps may be performed. The following detailed description is, therefore, not to be taken in a limiting sense.
Embodiments described within the present disclosure provide systems and methods for measuring velocity through the use of a radar altimeter. To measure the velocity using a radar altimeter, the radar altimeter creates two or more Doppler beams at different Doppler frequencies. The radar altimeter then may apply a tracking algorithm to each of the beams to produce at least two independent altitude tracks. The range measurements produced by the different altitude tracks will be offset from one another and the magnitude of the range will be dependent on the velocity of the aircraft in relation to the measured terrain. Using trigonometric functions, the calculations based on the range differences between the different tracks can be used to determine the velocity of the aircraft in relation to the ground. The determined velocity can then be used by the aircraft as another velocity measurement to improve the accuracy of navigation solutions calculated by the aircraft, particularly when the aircraft is operating in a GPS denied environment. Also, as described in greater detail below, the velocity measurement produced by the radar altimeter may be used to improve the performance of the radar altimeter.
FIG. 1 is a block diagram of one embodiment of an aircraft 150 equipped with a Doppler beam sharpened radar altimeter 100. The term aircraft, as used herein, refers to an airplane, a helicopter, or the like. As illustrated in FIG. 1, the aircraft 150 flies in a direction having a velocity vector v. As described herein, the aircraft 150 uses a Doppler beam sharpened radar altimeter 100 to gather information about the environment through which the aircraft 150 travels. Other systems may also use a Doppler beam sharpened radar altimeter 100, such as cruise missiles, satellites, or other aerial system able to calculate altitude. In certain implementations, the system containing the Doppler beam sharpened radar altimeter 100 may also be referred to as a platform. Systems and embodiments for a Doppler beam sharpened radar altimeter 100 are described in further detail in U.S. Pat. No. 7,911,375, entitled "DOPPLER BEAM-SHARPENED RADAR ALTIMETER" filed on Jun. 3, 2009, herein incorporated in its entirety by reference and referred to herein as the '375 patent.
The radar altimeter 100 comprises a transmitter 102 and a receiver 104. The transmitter 102 transmits signals 106 (for example, pulsed radar signals) towards a target 110. The target 110 can be terrain or any other detectable object. The receiver 104 receives or detects return signals 108 reflected from the target 110. Due to the aircraft 150 flying with velocity v, the return signals 108 will be Doppler shifted. As the aircraft 150 flies towards the target 110, the return signals 108 are Doppler shifted to be higher in frequency than the signals 106. As the aircraft 150 flies away from the target 110, the return signals 108 are Doppler shifted lower in frequency. The maximum Doppler shift occurs along the line of flight of the aircraft 150. For targets perpendicular to the direction of motion of the aircraft 150, there is zero Doppler shift because there is no relative velocity between the perpendicular target and the aircraft 150.
In certain embodiments, the radar altimeter 100 may break down the Doppler spectrum of the return signal 108 into a plurality of smaller sub-spectrums (referred to herein as frequency bins or bins). A bin corresponds to a bounded frequency range within the spectrum of the returned signal. Each frequency bin corresponds to a different range of Doppler shift frequencies (in other words, each bin is centered on a different Doppler shift frequency). To be detectable, signals near the extremities of a filter's bandwidth may be stronger than signals with frequencies close to the center point. Because the frequency bins are also subject to this same limitation, the frequency ranges of the bins may be designed to overlap with the frequency ranges of adjacent frequency bins, such that signals received at the different frequencies of the spectrum are detectable. Each frequency bin can provide enough overlap with adjacent frequency bins to provide coverage of the different Doppler frequencies of signals reflected by target 110 and received by receiver 104. One embodiment overlaps the frequency bins by 50%, so that each frequency is covered by two bins (except for the ends of the Doppler spectrum). At an end of the Doppler spectrum, a frequency bin may be overlapped by another frequency bin that encompasses frequencies outside the Doppler spectrum. On any given frequency segment there could potentially be redundant information, but this does not effect how much signal can be picked up. Other amounts of overlap are contemplated, such as 0% overlap up to 80% overlap, or bins that overlap more than the adjacent bins.
In at least one implementation, the range of the spectrum covered by the Doppler bins is determined by the velocity v of the aircraft 150. For example, the maximum magnitude of the Doppler shift occurs when the receiver 104 receives a signal reflected from a target 110 where the signal is reflected along a path that is parallel to the direction of the velocity vector v either in front of or behind the aircraft 150. The maximum magnitude of the Doppler shift is dependent on the magnitude of the velocity vector. For example, if the aircraft 150 is traveling at a high speed, the maximum Doppler shift will be greater than if the aircraft 150 is traveling at a low speed. In certain implementations, the frequency assigned to the highest bin or the bandwidth of a single filter that encompasses the possible range of Doppler shifts is based on the maximum designed velocity of the aircraft 150. However, when the aircraft is moving slower than the maximum designed velocity, there will be a number of bins within the Doppler spectrum that are unused, yet still consuming computing resources. In certain implementations, to improve the performance of the radar altimeter 100, the radar altimeter 100 may adjust the frequencies of the bins of the Doppler spectrum based on the velocity of the aircraft 150. For example, as the aircraft 150 travels less than the maximum designed velocity, the radar altimeter 100 adjusts the range of the Doppler spectrum, so that the edges of the maximum Doppler shift represented within the Doppler spectrum corresponds to the current velocity of the aircraft. In at least one implementation, the different velocities are provided by other velocity sensors located on the aircraft 150. For example, the ground velocity measurements may be provided by a GPS, an inertial navigation system, and the like. Also, in at least one implementation, the air speed, provided by a pitot tube, may also be used to provide a velocity measurement.
In certain embodiments, the radar altimeter 100 may produce the velocity measurement for determining the range of the Doppler spectrum and provide a velocity measurement to other systems on the aircraft 150. In certain embodiments, to produce a measurement of velocity v, the radar altimeter 100 acquires measurements from at least two separate Doppler radar tracks. Each different radar track provides a measurement of the range toward target 110 at a different angle from the aircraft 150. The radar altimeter 100 uses the computed ranges and the associated Doppler frequency offsets associated with the different computed ranges to calculate the velocity of the aircraft.
FIG. 2 is a high level block diagram of an embodiment of a receiver 200 for a Doppler beam sharpened radar altimeter system for providing velocity information. The receiver 200 may include an analog to digital converter (ADC) 202. The ADC 202 and the receiver 200, like receiver 104 in FIG. 1, receives reflected radar signals downconverts the received signals to baseband and an intermediate frequency, and converts the downconverted signals into digital signals for digital processing by a processor. The processor may be any type of processing unit that is capable of executing computer instructions that direct the processor to perform specific tasks on the received data. In at least one embodiment, the processor may be a digital signal processor. In a further embodiment, receiver 200 may also include a matched filter and gating 204. The matched filter and gating 204 may be used to measure the reflected energy within a prescribed time period. For example, the matched filter and gating 204 may gate the signals for further processing by selecting a certain time range of samples for analysis. Each gate in the matched filter and gating 204 is considered to represent a window of time in which the system monitors the reflected energy of the received signal (that is, the radar altimeter begins to accumulate energy at the beginning of each gate and stops accumulating this energy at the end of each gate). In certain implementations, the width of a gate (that is, width in time) is typically set to be equal to the transmit pulse width, but can be longer or shorter. For example, gate widths are generally dependent on the transmit pulse width in that as the transmit pulse width is increased the track and gate widths are also increased. Gating is not to be confused with windowing, which is discussed below.
In certain implementations, each separate Doppler channel comprises a different gate. However, a single gate may be used for each Doppler channel. When different gates are used for the different Doppler channels, the different channels may have different gate positions or gate widths. In at least one implementation, one of the Doppler channels may function as a level channel that is used to determine if a return signal's level is large enough to be considered a valid target.
In at least one embodiment, the Doppler filters 206 may use windowing schemes that are applied to the FFTs. Windowing is a digital signal processing (DSP) technique typically used to reduce the sensitivity of one FFT bin to signals present in other (usually undesired) FFT bins. Windowing also reduces the scalloping loss that occurs when the frequency of a signal of interest lies half-way between the center frequencies of two adjacent bins. Windowing widens the response of each of the FFT frequency bins but does not change their spacing. Different windowing schemes increase the bandwidth of the frequency bins by different amounts with varying effects on any given bin's response to signals outside that bin. Windowing schemes that can be used include but are not limited to a Hamming window, Hann window, and Blackman window. The window that is selected may affect scalloping loss. Also, zero-padding the samples before performing the FFT may increase the resolution of the FFT in the frequency domain.
Advantages of applying an FFT and a windowing scheme includes that the narrower bandwidth of each frequency bin reduces the detected noise present at the signal acquisition logic. In one alternative embodiment, a bank of band pass filters may separate the return signal into the frequency bins in place of the FFTs. Each individual band pass filter is focused on a different frequency of the Doppler spectrum. These band pass filters may overlap each other in a similar manner as the frequency bins from FFTs.
Further, receiver 200 may include range measurements 208. When performing range measurements 208, the receiver 200 may identify the different range measurements from the different Doppler beams. As one having skill in the art may appreciate, range measurements 208 identifies the range to the targets within each of the various Doppler beams by calculating the elapsed time between transmission of a signal and the reception of the received signal and multiplying the velocity of the signal by the elapsed time. Each Doppler beam is associated with a location on the reflective surface, which the aircraft is traveling towards (or away from) at a different velocity than other locations on the reflective surface. For example, when the aircraft is travelling parallel to the ground, the magnitude of the aircraft's velocity directly towards the ground beneath the aircraft is zero, resulting in a Doppler shift of zero. Also, the magnitude of the velocity of the aircraft towards other locations on the ground that are not directly below the aircraft are between zero and the magnitude of the aircraft's velocity. Due to the difference in velocity, each reflected signal that is reflected from a particular location will have a different Doppler shift than a reflected signal that is reflected from other locations. Thus, a Doppler filter effectively forms a Doppler beam in the direction in which the reflections are shifted by that Doppler Frequency. The range measurements 208 calculates the range for each of the different Doppler beams based on the frequency and time of reception of a received signal in each Doppler filter.
In at least some implementations, the receiver 200 also includes velocity estimation 210. Velocity estimation 210 calculates the velocity of the aircraft from the range measurements calculated from multiple Doppler beams over time. To calculate the velocity, velocity estimation 210 first buffers a series of range and Doppler beams verses time for the different Doppler beams. As the receiver 200 has identified the range to the terrain within each of the Doppler beams, the velocity estimation 210 is then able to calculate the magnitude of the aircraft velocity. In particular, velocity estimation 210 is able to calculate the velocity of the aircraft in relation to the ground. Velocity estimation 210 then outputs the estimated velocity to be used by other systems on the aircraft. In at least one implementation, the velocity estimation 210 provides the estimated velocity of the aircraft to the Doppler filters 206, wherein the Doppler filters 206 adjust the filters that are used to form the different Doppler beams.
In certain embodiments, the receiver 200 dynamically controls different Doppler beams to desired positions in frequency where it is expected that the reflected signals at those Doppler frequencies will support sufficiently accurate velocity estimation. For example, over flat terrain, there is little change in range associated with a region that provides zero Doppler offset. As such, the Doppler beam associated with zero Doppler offset provides ambiguous measurements unless there is observable terrain variation. Accordingly, in the case of flat terrain, the Doppler filters may be set to non-zero center frequencies. In at least one implementation, to identify a desired position, the receiver 200 may calculate reflectivity information from the reflected signals and then correlate the data against a map of terrain reflectivity characteristics. Based on the result of the correlation, the receiver 200 may change the frequencies of the different Doppler beams or may apply weighting factors to the measurements acquired through the different Doppler beams.
FIG. 3 is a block diagram of one embodiment of a Doppler beam sharpened radar altimeter system 300. The Doppler beam sharpened radar altimeter system 300 shows a system for forming two separate Doppler beams. However, more Doppler beams may be formed from the received information. The radar altimeter system 300 may be installed on a platform, such as aircraft 150 of FIG. 1. The radar system 300 includes a system timing and control controller 314, a transmitter 302 to transmit radar signals (such as signals 106) and a receiver 304 to receive return signals (such as signals 108). The radar altimeter system 300 also includes a voltage control oscillator (VCO) 306 that provides frequency control for the receiver 304 and the transmitter 302, which is under control of the system timing and control controller 314. A digitizer 320 coupled to the receiver 304 provides for analog to digital conversion. In one embodiment, the receiver 304 converts signals from their transmitted frequency down to an intermediate frequency (IF). Further illustrated in FIG. 3, the radar altimeter system 300 includes digital to analog (D/A) converters 312 and 310 to convert digital signals from a spectral analyzer 328 to the transmitter 302 and the receiver 304, respectively.
The spectral analyzer 328 comprises two or more different processing channels. As shown in this implementation, the spectral analyzer 328 comprises two processing channels. In certain embodiments, the processing channels include a track channel 330 and a PMC channel 344. In one embodiment, the Doppler channels 330 and 344 demodulate intermediate frequency signals from the receiver 304 down to baseband. In other embodiments, receiver 304 demodulates to directly output a baseband signal. Baseband is the minimum frequency that a system operates on. Here, the baseband is 0 Hz. At baseband, the only frequency component of the return signal left is the Doppler shift.
The channels 330 and 344 include gating and matched filter circuits 332 and 346, respectively. Gates are used to select a prescribed time period for a group of measurements of reflected energy. The gating and matched filter circuits 332 and 346 gate the signals from the receiver 304 by selecting a certain time range of samples that will be analyzed by the spectral analyzer 328. Each gate is considered to represent a window of time in which the system monitors the reflected energy (that is, the radar altimeter begins to coherently integrate the signal at the beginning of each gate and stops coherently integrating this signal at the end of each gate). The width of a gate (that is, width in time) is typically set to be equal to the transmit pulse width, but can be longer or shorter. Gate widths may be dependent on the transmit pulse width in that as the transmit pulse width is increased, the track and PMC gate widths are also increased. Gating is not to be confused with windowing. Windowing is discussed below in greater detail. In certain implementations, the gating and matched filter circuits 332 and 346 arrange the samples within respective buffers 338 and 352 according to the range of the returned samples within the prescribed time period. The arrangement of the returned samples is described in greater detail below.
As stated above, in other embodiments, the spectral analyzer 328 may comprise additional Doppler channels. For example, a third channel could function as a level channel with different gate positions or gate widths than the PMC channel 344. A level channel is used to determine if a return signal's level is large enough to be considered a valid target. However, for purposes of this description, the PMC channel 344 functions as a level channel. Also, the different Doppler channels can be used to provide information for the calculation of a velocity estimate by the radar altimeter system 300.
The radar altimeter system 300 also includes a gain circuit 308 placed before the digitizer 320 that controls the gain to provide optimal signal levels to the digitizer 320. In embodiments of the present invention, a gain may also be digital and be placed between the digitizer 320 and the spectral analyzer 328. In alternative embodiments, the channels 330 and 344 each include a gain circuit 308, which may be each assigned different gain values. The channels 330 and 344 in this embodiment include buffers 338 and 352, respectively. The buffers 338 and 352 are memories that store samples of the return signals.
A fast Fourier transform (FFT) 340 separates the return signal 108 into one or more frequency bins. An FFT 354 separates the return signals 108 into one or more frequency bins independent of the FFT 340. The FFTs 340 and 354 are shown separate from the track processor 376 and PMC 378, respectively. However, in other embodiments, the FFT 340 is implemented in software 343 and the FFT 354 is implemented in software 355.
Windowing schemes 390 and 392 are applied with the FFTs 340 and 355, respectively. Windowing is a digital signal processing (DSP) technique typically used to reduce the sensitivity of one FFT bin output compared to signals present in other (usually undesired) FFT bins. Windowing reduces the scalloping loss that occurs when the frequency of a signal of interest lies half-way between the center frequencies of two adjacent bins. Windowing widens the response of each of the FFT frequency bins but does not change their spacing. Different windowing schemes increase the bandwidth of the frequency bins by different amounts with varying effects on any given bin's response to signals outside that bin. Windowing schemes 390 and 392 that can be used include a Hamming window, Hann window, and Blackman window.
In one alternative embodiment, the spectral analyzer 328 includes a bank of band pass filters for separating the return signal into the frequency bins in place of the FFTs 340 and 354. Each individual band pass filter is focused on a different frequency of the Doppler spectrum. These band pass filters may overlap each other in a similar manner as the frequency bins from FFTs 340 and 354.
The signal processing logic of the radar system 100 includes a track processor 376 and a PMC processing unit 378 (also referred to herein as PMC 378). The track processor 376 receives an input from the FFT 340 and from the PMC 378. In certain implementations, the provided inputs comprise an array of signal magnitudes and temporal positions as provided by the respective gating and matched filter circuits 332 and 346 and FFTs 340 and 354. The track processor 376 comprises a memory 377 used for storing a tracking routine and controls the gating and matched filter circuit 332 via a feedback loop 334. Additionally, the track processor 376 provides an output to a range processor 383 and also to the gating and matched filter circuit 346. In one embodiment, the track processor 376 provides separate gate positions and gate widths for both gating and matched filter circuits 332 and 346.
The range processor 383 determines the range based on inputs from the track processor 376 and the PMC 378. In certain implementations, the range processor 383 includes an averager, an altitude adjustment, and a scaling. The averager receives inputs of range measurement or gate positions and smoothes out the gate position movements and the noise inherent in the measurements. In one embodiment of the range processor 383, the averager receives inputs from the track processor 376. The scaling converts the range measurements from units of increments of the gate position to the desired units (typically feet or meters).
The altitude adjustment serves to account for biases in the Doppler beam sharpened radar altimeter 300 that would result in incorrect altitude measurements. In one embodiment of the altitude adjustment, these measurements are compensated for to ensure a zero altitude indication when the platform is at zero feet. Values that can be adjusted to be compensated for include the length of cable between the transmitter 302 and the receiver 304 and their associated antennas, the separation between the transmitter 302 and receiver 304 antennas, the height of the antennas on the aircraft above the ground when stationary, or delays inherent to the receiver 304 or transmitter 302 (such as filter delays, pulse rise times, and the like).
In the embodiment of the radar altimeter 300 illustrated in FIG. 3, the PMC 378 receives the output from FFT 354. The PMC 378 comprises a memory 379 used for storing a power management control routine. The PMC 378 maintains the return signal received at receiver 304 at a sufficient signal strength by controlling the power level of the transmitted signal from the transmitter 302 such that velocity and altitude can be computed reliably. For example, if a return signal is detected above a threshold level, the PMC 378 will instruct the transmitter to transmit the next signal at a lower power level. Keeping the return signal strength below a threshold level decreases the probability of the platform being detected through its emissions. If a return signal is too weak, the PMC 378 instructs the transmitter to transmit at a higher power level (in order for the range gate to continue to track the nearest target) until a new target is detected with a higher power level.
The track processor 376 and the PMC processor 378 perform further signal processing on signals that are within the time period of their gates. Tracking or range measurements are done on the minimum trackable signal within each of the filters or frequency bins. The track processor 376 applies multiple range gates using the gating and matched filter circuit 332 to the return signal to create a complex-valued sample for each gate position for each transmitted pulse as described below in greater detail. The PMC 378 applies a level gate to the return signal using the gating and matched filter circuit 346.
The track channel 330 also comprises a selector 342. The selector 342 selects the frequency bin or filter in the track channel 330 to use for further signal processing. Based on the frequency bin that currently contains the return signal component of interest, the selector 342 passes the signal from the frequency bin to the track processor 376. In the embodiment shown in FIG. 3, the selector 342 is implemented in the software 343 that is executed by the track processor 376. The software 343 comprises program instructions that are stored on a suitable storage device or medium 341. Typically, a portion of the software 343 executed by the track processor 376 and one or more data structures used by the software 343 during execution are stored in a memory 377. Similarly, the PMC channel 344 also comprises a selector 356 for selecting the frequency bin or filter to use for further signal processing. The selector 356 is implemented in software 355 stored on a storage medium 358. Typically, a portion of the software 355 executed by the PMC 378 and one or more data structures used by the software 355 during execution are stored in a memory 379.
The frequency bin (or filter, for the embodiment of a bank of filters) can be selected based on any defining characteristic of a portion of the pulse having a frequency corresponding to that frequency bin. The selector 342 is in the track channel 330 and thus selects the frequency bin which contains the defining characteristic of the signal. In one embodiment, a defining characteristic is a portion of the pulse that corresponds to the nearest range after Doppler filtering. The range processor tracks the pulse corresponding to the nearest target, and the selector 342 selects the frequency bin which the characteristic of the pulse corresponding to the nearest range falls into. The track channel 330 operates to track the nearest target by enabling the selector 342 to pick the bin with the largest signal level at the current gate position and use that signal level to compare to a track threshold to determine if the gate position needs to be increased or reduced. The track threshold is a signal level that outputs a signal that can be directly computed into a range corresponding to the actual distance to the target when the range gate is in position.
Similarly, in one embodiment, the PMC selector 356 selects the frequency bin in which the pulse with the strongest signal falls into. In another embodiment, the same bin chosen by the track channel 330 is then used by the PMC channel 344 to control the transmit power to maintain the signal return level at a designed threshold or reference level. In this embodiment, the PMC channel 344 does not just pick the maximum signal level from the FFT 354 (or, in another embodiment, the bank of band pass filters) to determine which frequency bin to operate on but instead operates on the bin selected by the track channel 330 selector 342.
In at least one implementation, the range processor 383 and spectral analyzer 328 pass the range and frequency data associated with the multiple Doppler beams to a velocity processor 385. The velocity processor 385 uses the range and frequency data to calculate the velocity of the aircraft as described in greater detail within this specification. In at least one implementation, the velocity processor 385 provides a velocity estimate to the spectral analyzer 328, where the spectral analyzer 328 uses the velocity estimate to adjust the frequency range of the Doppler spectrum.
Various aspects of the Doppler beam sharpened radar altimeter 300 (for example, but not limited to, the gating and matched filter circuits 332 and 346, the gain 308, the digitizer 320, the selectors 342 and 356, and the FFTs 340 and 354) can be implemented using integrated or discrete electronic components, and combinations thereof, as known to one of ordinary skill in the art.
FIG. 4 is a block diagram illustrating the calculation of a velocity estimate performed by the track channel. As illustrated, FIG. 4 shows an A/D converter 420. In at least one implementation, the A/D converter 420 functions in a similar manner to digitizer 320 in FIG. 3. For example, the A/D converter 420 receives a downconverted and amplified analog reflected radar signal and converts the signal to a digital signal for subsequent processing. The A/D converter 420 then passes the signal to matched filter and gating 432. Matched filter and gating 432 functions similarly as described above with respect to gating and matched filter 332. For example, the matched filtering and gating 432 gate time periods of signals for storage in a buffer 438. For example, the buffer 438 stores an array 438-1 that comprises of indexed rows of signals received during a time period. Further, the matched filtering performed by matched filtering and gating 432 filters the signals received within each gate, typically, a simple coherent integration to form a complex-valued sample for each gate position. Accordingly, the row is arranged as a series of complex-valued samples, one sample for each gate position or range bin. In certain implementations, the above description applies to a pulsed systems and may also apply to frequency modulated continuous wave (FMCW) techniques or other modulation techniques that allow the formation of complex-valued samples at each range-bin.
When a series of gated radar measurements are stored in the array 438 for multiple radar pulses, data from different radar pulses for a particular range bin are scaled through window 490 and transformed by FFT 440. Both FFT 440 and window 490 function similarly to FFT 340 and window 390 described above with respect to FIG. 3. When the data in array 438-1 is windowed and transformed, the transformed data for multiple arrays 438-1 is stored in array 483-1. The array 483-1 includes multiple arrays, where each array includes rows of range bins and each row of range bins corresponds to a different frequency bin. Where the initial frequency bins correspond to negative Doppler frequency offsets and the later frequency bins correspond to positive Doppler frequency offsets. From the array 483-1, the range processor is able to calculate the range to the terrain for each frequency bin at the time the data for the particular two-dimensional array was received. The processed range data is then provided to the velocity processor 485.
In certain implementations, the velocity processor 485 stores the data in an array 485-1. The array 485-1 stores data that is in a lower indexed row as older data and data in a higher indexed row as more recently received data. Each column corresponds to a different Doppler frequency shift, the different frequency shifts being arranged from negative Doppler frequency shifts to positive frequency shifts. The data in the rows corresponds to range per frequency vector and is used by the velocity processor 485 to provide a velocity estimation.
In at least one embodiment, the velocity processor 485 provides an initial velocity estimate during an initial velocity acquisition. When a device using the velocity processor 485 either turns on or has yet to acquire data capable of providing a reliable velocity estimate, the device may enter a velocity acquisition mode. In the velocity acquisition mode, the device may attempt to get an initial approximate estimate of velocity from which the device can calculate more accurate estimations of velocity. In one exemplary implementation, the velocity processor 485 may identify the maximum observed Doppler shift in the data stored in array 485-1. The maximum observed Doppler shift may be either the maximum or minimum frequency bins in which a detectable signal (above the noise floor) is present. The maximum observed Doppler shift may then be used to compute a coarse estimate of velocity.
Further, the velocity processor 485 may provide continuous velocity estimate updates, where a continuous velocity estimate is an estimate provided during the normal operation of the device. To provide continuous velocity estimates, the velocity processor computes a range of frequency bins for negative and positive Doppler shift based on the current estimated velocity. In one implementation, the current estimated velocity may come from other sources of velocity, such as a GPS, an INS, and the like. Alternatively, the velocity processor may use the most recent velocity estimate as the current estimated velocity. The velocity processor then may identify a range of test velocities (denoted as set V) that encompass the estimated velocity. The velocity processor then may identify a time delta (T) over which the velocity is not expected to change significantly. The selection of duration for the time delta T may be application specific. A smaller time delta T may provide faster tracking of velocity but may also result in noisier velocity estimates.
To calculate the current velocity, the velocity processor 485 uses the range measurements at one or more frequency bins (designated as set A) from the most recent vector of range estimates. As stated above, the most recent vector of range estimates corresponds to the bottom row of array 485-1. For the set A of range measurements, the velocity processor 485 estimates the expected Doppler frequency offsets for a velocity V1 (where V1 is a velocity in the set of test velocities V) at a time T in the past. The velocity processor 485 may then interpolate the range values from the range array given T and the expected Doppler frequency offsets. The resulting interpolated set of range values may be designated as set B. When set B is calculated, the velocity processor 485 may subtract set B from A to compute the total magnitude of the error for the test velocity V1. The magnitude of error may be computed as a root mean square of the elements of set B-set A. The magnitude of error for V1 provides an estimate of how close the test velocity matches the true velocity. The velocity processor 485 uses the above procedure to calculate the magnitudes of errors for each velocity in the range of test velocities V. The velocity processor 485 computes a velocity estimate based on the resulting set of error magnitudes for the test velocities. In at least one implementation, the test velocity having the smallest estimated error magnitude is reported as the velocity. In other implementations, the test velocities in the neighborhood of the test velocity having the smallest estimated error magnitude and the associated estimated error magnitudes are interpolated to identify a velocity estimate between the test velocities in the set of test velocities. Interpolation may be performed using any form of interpolation known to one having skill in the art, for example interpolation may be performed using conic interpolation, Hermite interpolation, splines, or the like.
FIG. 5 illustrates a flow diagram for a method 500 for estimating a velocity. Method 500 proceeds at 502, where a range of frequency bins is computed for negative and positive Doppler based on current estimated velocity. Method 500 proceeds at 504, where a set of test velocities (V) is chosen that spans the range of possible velocity estimates. Method 500 then proceeds at 506, where a time delta (T) is chosen over which the velocity is not expected to change significantly. Method 500 then proceeds at 508, where a test velocity is identified in the range of test velocities.
Further, Method 500 proceeds at 510, where, for the range measurements at one or more frequency bins (designated as set A) from the most recent vector of range estimates, an expected Doppler frequency offset is estimated for the identified test velocity at the time T in the past. Method 500 then proceeds at 512, where the range values are interpolated from the range array given T and the expected Doppler frequency offsets, and the interpolated set of range values are designated as set B. Method 500 proceeds at 518, where set B is subtracted from set A and the total error magnitude for the selected test velocity is computed. Method 500 proceeds at 514, where it is determined whether there are test velocities in the set of test velocities for which an error magnitude has not yet been computed. In some implementations, the set of test velocities defines a range of test velocities. If there are test velocities remaining, method 500 proceeds to 516, where a new test velocity is selected for which an error magnitude has not been calculated. With the new selected test velocity, the method 500 returns to 510. If there are no test velocities remaining, method 500 proceeds to 520, where the test velocity having the smallest estimated error magnitude is identified. In at least one implementation, the test velocity having the smallest estimated error magnitude is reported as the velocity. In other implementations, the test velocities in the neighborhood of the test velocity having the smallest estimated error magnitude and the associated estimated error magnitudes are interpolated to identify a velocity estimate between the test velocities in the set of test velocities.
FIG. 6 is a drawing of an aircraft 600 that has a radar altimeter configured to measure a velocity V of the aircraft 600 as it travels parallel to a flat surface 602, according to one embodiment. As shown, the radar altimeter transmits a single radar beam 603 and receives three different Doppler shifted beams, while three different Doppler shifted beams are shown, the radar altimeter may receive any number of Doppler shifted beams. In particular, the radar altimeter transmits beam 603 and receives a first Doppler shifted beam 604, a second Doppler shifted beam 606, and a third Doppler shifted beam 608. By gathering the data from the three separate Doppler shifted beams, the radar altimeter on the aircraft 600 is able to calculate the velocity V of the aircraft 600.
In at least one implementation, to calculate the velocity V of the aircraft, the radar altimeter transmits beam 603 and receives the first Doppler shifted beam 604. As the aircraft 600 is moving away from portions of the surface 602, when the first beam 604 reflects off of the surface 602 and is received by the radar altimeter on the aircraft 600 that is travelling away from the surface that reflected the first beam 604, the frequency of the received first beam 604 will be less than the frequency of the transmitted beam 603. From the received first beam 604, the radar altimeter will also be able to calculate the range of the aircraft to the point on the surface 602 that reflected the first beam 604 back towards the aircraft 600.
In a similar manner, the radar altimeter transmits the beam 603 and receives the third beam 608. However, as the aircraft 600 is moving towards portions of the surface 602, when the third beam 608 reflects off of the surface 602 and is received by the radar altimeter on the aircraft 600 that is travelling towards the surface 602, the frequency of the received third beam 608 will be greater than the frequency of the transmitted beam 602. From the received third beam 608, the radar altimeter will also be able to calculate the range of the aircraft to the point on the surface 602 that reflected the third beam 608 back towards the aircraft 600.
Further, the radar altimeter transmits the beam 603 and receives the second beam 606. In contrast to the first beam 604 and the third beam 608, the second beam 606 is reflected by the ground 602 that is directly beneath the aircraft 600. As the aircraft 600 is flying parallel to the surface 602, the aircraft is not moving towards the surface 602. Thus, the second beam 606 is received at the same frequency as the transmitted beam 603. From the second beam 606, the radar altimeter is able to calculate the range from the aircraft to the surface 602.
In certain embodiments, to calculate the velocity V, the radar altimeter uses information from at least two Doppler beams. For example, the radar altimeter may calculate the velocity of the aircraft 600 using a combination of the first beam 604, the second beam 606, and the third beam 608. The radar altimeter on the aircraft 600 has already calculated the range of the ground associated with each beam. Also, the radar altimeter has also calculated the Doppler shift associated with each beam. With this information, the radar altimeter is able to calculate the velocity of the aircraft as described above in relation to FIGS. 4 and 5.
In at least one implementation, the radar altimeter is able to compare the signal power of the received beams at the different Doppler shifts with the signal power of the transmitted signal. By comparing the power levels of the different signals, the radar altimeter is able to determine the reflectivity of the surfaces that reflect the transmitted signals. Based on the sensed reflectivity, the receiver may adjust the frequencies of the Doppler beams, such that the Doppler beams are associated with surfaces having higher reflectivity, such that the received signals are more likely to provide accurate measurements. Also, the radar altimeter may correlate the reflectivity with data from a map describing the reflectivity of the different surfaces on the Earth. By correlating the reflectivity measurement with reflectivity information from a map, the radar altimeter may be able to adjust the Doppler beams to point at regions on the Earth's surface having desirable reflectivity. In at least one implementation, the radar altimeter weighs the measurements based on the reflectivity of the different radar beams when calculating the velocity. For example, if there are more than two Doppler beams, the radar altimeter may apply weights such that the radar altimeter relies on measurements from beams having higher reflectivity than beams having lower reflectivity.
FIG. 7 is a drawing of an aircraft 700 that has a radar altimeter configured to measure a velocity V of the aircraft 700 as it travels over a surface 702, where the surface varies in elevation, according to one embodiment. As shown, the radar altimeter receives three different Doppler beams, while three different Doppler beams are shown, the radar altimeter may receive other numbers of Doppler beams. In the particular example, the radar altimeter transmits beam 703 and receives a first beam 704, a second beam 706, and a third beam 708. By gathering the data from the three separately received beams, the radar altimeter on the aircraft 700 is able to calculate the velocity V of the aircraft. However, in contrast to the example provided above with respect to FIG. 4, the surface 702 varies in elevation such that the range measurements provided by the different Doppler beams are associated with sections of the surface 702 that have different elevations. To determine the magnitude of the velocity vector V, the radar altimeter may make several measurements over a period of time, such that the different Doppler beams are reflected from the same portions of surface 702.
In certain embodiments, the radar altimeter receives the third beam 708. At a particular moment in time, the third beam 708 reflects off of surface feature 710. Surface feature 710 is any identifiable feature on the surface 702. For example, the feature 710 may include a building, a mountain, a hill, a valley, and the like. Further, as the aircraft 700 is moving towards the waves reflected as part of the third beam 708, when the third beam 708 reflects off of the ground and is received by the radar altimeter on the aircraft 700 that is travelling towards the third beam 708, the frequency of the received third beam 708 will be greater than the frequency of the transmitted beam 703. From the received third beam 708, the radar altimeter will also be able to calculate the range of the aircraft to the point on the ground that reflected the third beam 708 back towards the aircraft 700.
As the aircraft 700 continues to travel, the surface feature 710 may be positioned directly below the area scanned by the second beam 706. As the surface feature 710 is directly below, the radar altimeter receives measurements associated with the surface feature 710 that can be compared against the measurements of the surface feature 710 acquired by the third beam 708. Further, at a slightly later moment in time, the first beam 704 may acquire measurements of the surface feature 710, which measurements acquired by the first beam 704 of the surface feature 710 may also be compared to the measurements of the surface feature 710 acquired by the third beam 708 and the second beam 706.
In certain embodiments, to determine that a surface feature measured by the third beam 708 at one time is the same surface feature measured by the second beam 706 and the first beam 704 at different times, the radar altimeter monitors sequences of ranges measured by the different Doppler beams to identify the surface feature 710. For example, the radar altimeter on the aircraft 700 may correlate different sequences of range measurements associated with the different Doppler ranges over a period of time. Alternatively, the radar altimeter may perform curve fitting to identify the different sequences of range measurements by different Doppler beams that are associated with the same surface. By correlating the sequences of range measurements from the different Doppler beams or by performing curve fitting, the radar altimeter is able to identify range measurements of a single surface feature 710 by the different Doppler beams.
In certain embodiments, having acquired different measurements of the surface feature 710 by the multiple Doppler beams, the radar altimeter calculates the magnitude of the velocity vector V. To calculate the velocity magnitude, the radar altimeter uses corresponding measurements of the surface feature 710 from at least two Doppler beams. For example, the radar altimeter may calculate the velocity of the aircraft 700 using any combination of measurements of the surface feature 710 from the first beam 704, the second beam 706, and the third beam 708.
FIG. 8 is a diagram of an aircraft 800 having a radar altimeter that emits Doppler beams configured to measure the full range of Doppler shifts as determined by the velocity of the aircraft 800. As shown, the radar altimeter on the aircraft 800 transmits and receives three Doppler beams, a first Doppler beam 804, a second Doppler beam 806, and a third Doppler beam 808. The first Doppler beam 804 is received from a direction that is opposite to the direction of the velocity vector V. Thus, the measured frequency of the received first beam 804 will be at the minimum expected frequency for a Doppler shifted signal as the aircraft 800 is moving directly away from the surface 802 that reflected the first Doppler beam 804. However, the third Doppler beam 808 is received from the same direction as the velocity vector V of the aircraft 800 towards surface 805. As the aircraft 800 is moving directly to the surface 805 that reflects the third Doppler beam 808, the measured frequency of the received third beam 808 will be at the maximum expected frequency for a Doppler shifted signal. In contrast to the first Doppler beam 804 and the third Doppler beam 808, the radar altimeter receives the second Doppler beam 806 from a direction perpendicular to the velocity vector V of the aircraft 800. Thus, the received second Doppler beam 806 will have the same measured frequency as the transmitted beam 803.
In certain embodiments, the velocity vector V determines the maximum and minimum expected Doppler shifts for reflected signals. In certain implementations, the radar altimeter implements Doppler filters to acquire the needed information from the different Doppler Beams. As the maximum and minimum expected Doppler shifts are tied to the velocity of the aircraft 800, the radar altimeter may adjust the characteristics such as center frequency and bandwidth of the different Doppler filters based on the calculated velocity provided by the measurements of the different Doppler beams. Alternatively, the radar altimeter may adjust the characteristics of the different Doppler filters based on a velocity measurement provided by an external source of velocity information. For example, the radar altimeter may receive velocity information from a global positioning system receiver, an inertial navigation system, a pitot tube, and the like.
FIG. 9 is a flow diagram of a method 900 for calculating velocity based on measurements from a radar altimeter. Method 900 begins at block 902, where a radar beam is transmitted. For example, a radar emits a radar beam towards a surface over which an aircraft is traveling. The radar beam propagates towards the surface and reflects off of multiple locations on the surface. Method 900 then proceeds at block 904, where a plurality of reflected signals are received. For example, each reflection from a different location will exhibit different Doppler shift due to the relative velocities between the aircraft and the location.
In a further implementation, method 900 proceeds at 906, where a Doppler filtering is applied to the plurality of signals to form at least one Doppler beam. For example, the radar altimeter uses Doppler filtering to identify at least two different reflected radar beams having different Doppler shifts. Method 900 proceeds at 908, where range measurements are identified within each Doppler beam in the at least one Doppler beam. Method 900 then proceeds at 910, where the velocity magnitude based on the range measurements of the at least one Doppler beam is calculated.
Example 1 includes a method for measuring velocity magnitude of a platform in relation to a surface, the method comprising: transmitting a radar beam, wherein the radar beam is aimed toward a surface; receiving a plurality of reflected signals, wherein the plurality of reflected signals correspond to portions of the transmitted radar beam that are reflected by a plurality of portions of the surface; applying Doppler filtering to the plurality of signals to form at least one Doppler beam; identifying range measurements within each Doppler beam in the at least one Doppler beam; and calculating the velocity magnitude based on the range measurements of the at least one Doppler beam.
Example 2 includes the method of Example 1, wherein calculating the velocity magnitude comprises: identifying a set of test velocities; for each velocity in the set of test velocities, calculating a magnitude of errors based on the range measurements associated with the at least one Doppler beam; and identifying a velocity within a range of test velocities associated with the smallest magnitude of errors as the velocity magnitude.
Example 3 includes the method of any of Examples 1-2, wherein calculating the velocity magnitude comprises: computing the velocity based on range measurements gathered over a time period where the velocity does not change significantly.
Example 4 includes the method of any of Examples 1-3, wherein calculating the velocity magnitude comprises acquiring an initial velocity estimate.
Example 5 includes the method of Example 4, wherein the initial velocity estimate is based on the maximum observed Doppler shift that is above a noise floor.
Example 6 includes the method of any of Examples 1-5, further comprising: applying weighting factors associated with the at least one radar beam.
Example 7 includes the method of any of Examples 1-6, further comprising providing the calculated velocity magnitude to another system as navigational information.
Example 8 includes the method of any of Examples 1-7, further comprising dynamically adjusting at least one center frequency for at least one Doppler filter used to form the at least one Doppler beam based on an expected location on the surface that will produce an improved velocity measurement.
Example 9 includes the method of Example 8, wherein the expected location for the improved velocity measurement is determined by the calculated velocity magnitude.
Example 10 includes the method of any of Examples 8-9, further comprising using a map of terrain reflectivity characteristics to determine Doppler that will produce the improved velocity measurement.
Example 11 includes the method of any of Examples 1-10, further comprising: adjusting Doppler filter center frequencies over a period of time when forming the at least one Doppler beam; observing signal magnitudes over the period of time associated with at least one portion of the surface; and computing terrain reflectivity information associated with the at least one portion of the surface.
Example 12 includes the method of Example 11, further comprising estimating the location of the platform relative to the surface based on correlation of computed terrain reflectivity information against a map of terrain reflectivity characteristics for the surface.
Example 13 includes a system for measuring velocity of an aircraft, the system comprising: a transmitter that transmits a radar beam at a first frequency; a receiver that receives a plurality of reflected signals at a plurality of frequencies when the radar beam is reflected by a plurality of portions of a surface; and a processor that applies Doppler filtering to the plurality of reflected signals to form at least one Doppler beam and determines range measurements for the at least one Doppler beam, wherein the processor calculates a velocity estimate based on the range measurements of the at least one Doppler beam.
Example 14 includes the system of Example 13, wherein the processor calculates the velocity estimate by: identifying a set of test velocities; for each velocity in the set of test velocities, calculating a magnitude of errors based on the range measurements associated with the at least one Doppler beam; and estimating a velocity within a range of test velocities associated with the smallest magnitude of errors as the velocity estimate.
Example 15 includes the system of any of Examples 13-14, wherein the processor applies weighting factors associated with the plurality of reflected signals.
Example 16 includes the system of any of Examples 13-15, wherein the processor dynamically adjusts the frequency of a desired Doppler shift based on an expected location on the surface that will produce an improved velocity measurement.
Example 17 includes the system of Example 16, wherein the processor determines the expected location based on terrain reflectivity information collected from the at least two reflected signals, wherein the terrain reflectivity information is correlated against a map of terrain reflectivity characteristics.
Example 18 includes the system of Example 17, wherein the processor determines a spectral range for the at least two reflected signals based on a spectrum of possible Doppler shifts.
Example 19 includes a radar altimeter receiver, the receiver comprising: an analog to digital converter configured to convert received analog signals into digital signals; a plurality of Doppler filters configured to filter the digital signals into at least one Doppler beam, where the at least one Doppler beam is associated with a Doppler shift; a range measurer configured to calculate at least one range measurement from the at least one Doppler beam; and a velocity estimator configured to calculate the velocity of a vehicle containing the radar altimeter receiver based on the at least one range measurement.
Example 20 includes the radar altimeter receiver of Example 19, further comprising at least one gate, wherein a gate in the at least one gate selects time range of samples from the digital signals for filtering by the plurality of Doppler filters.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

matches:
Application No. 16153713
Application No. 16153713
v.
V.
V.
V.