1. Field of the Invention
The present invention is directed generally to the field of radar, and in particular to a radar system with improved interference characteristics. More particularly, the radar system belongs to the category known as "low probability of interception--low probability of exploitation" (LPI-LPE) radar. More particularly still, the preferred field of application of the present system is in the field of vehicular traffic on the ground and on roads, where fog and the like conditions obstruct normal vision. But it is, of course, also applicable to other situations; for example, for boat traffic in harbours, rivers and canals. The system is based on phased-array, monostatic limited scan volumetric radar. The intended radar range is between zero and a few hundred meters.
2. Prior Art of the Invention
Radar systems using phased-array antennae are well known, for example as in systems known as "side looking airborne radar" (SLAR), where carefully shaped radar beams permit imaging of the ground underneath an aircraft in flight. In such and similar systems, interference from other radars is not a problem. And while such systems could, in principle, be used to permit "vision-less" driving in normal vehicular traffic on roads on the ground, in practice this would only be possible where few vehicles were thus equipped each having well separated radar frequencies. As soon as indeterminate or significant numbers of vehicles are involved the problem of interference becomes intractable.
The known closest prior art to the present invention is disclosed in European Patent Application No. 92100969.2 published as No. 0 501 135 A2 on Sep. 2, 1992 entitled "Broad-band mobile radio link for high-volume transmission in an environment with multiple reflectors". U.S. patent application Ser. No. 07/823,534 filed Jan. 21, 1992, now U.S. Pat. No. 5,375,144, by Sharokh EHSANI et al corresponds to the said published European patent application. Both applications are incorporated herein by reference, where permitted.
In the above referenced European and United States patent application, which are particularly suitable for mobile radio/data links, the problems of interference are mitigated by providing two orthogonalities between different vehicle signals. The first orthogonality being that of code-division multiple access (CDMA), and the second being a small frequency separation. These techniques improve radio/data communications links under high-density usage conditions, but would not be sufficient to permit safe and reliable "radar-vision" to drivers in road traffic, or the like applications.
A further patent of interest is U.S. Pat. No. 5,031,193 granted Jul. 9, 1991 to Frederick G. ATKINSON et al, and entitled "Method and apparatus for diversity reception of time-dispersed signals". The patent teaches as follows:
A method and apparatus for diversity reception in a communication system wherein at least a dual branch receiver is provided with a stored replica of expected reference information that is correlated with the received time-dispersed signals to obtain an estimate of the transmission channel's impulse response as seen by each branch, and determine, among other things, phase error between the branch local oscillators and the time-dispersed signals. Matched filters are constructed which then coherently align the time-dispersed signals from each branch with that branch's local oscillator, also constituting the first part of the equalization. The diversity processing stage may perform bit by bit selection on the re-aligned signals, maximal ratio combining of the re-aligned signals, or equal gain combining of the re-aligned signals, following each by a sequence estimation which uses similarly selected or combined channel distortion compensation parameters to complete the equalization process on the new signal. In digital modulated carrier systems, providing expected reference information eliminates the need for carrier recovery feedback for each branch while performing part of the equalization process. PA1 variable dwell time; PA1 flexible beam shaping for transmission, reception or both; PA1 use of leakage canceller correlation loop; PA1 use of limited scan with attendant significant cost reduction (cf. J. M. Howell: Limited Scan Antennas, IEEE AP-5, Inf. Symp. 1972); and PA1 antenna can be made conformal. PA1 (a) code-division multiple access (CDMA); PA1 (b) small frequency separation (.DELTA.f); and PA1 (c) an angle-of-arrival (A-o-A) defining very narrow beam phased-array antenna.
Thus, this United States patent stores a replica of expected reference information and correlates it with actually received information to provide an estimate of the equalization necessary in the diversity receive channel to permit better reception of unexpected information.
An earlier U.S. Pat. No. 4,291,410, granted Sep. 22, 1981 to Edgar L. CAPLES et at, is entitled "Multipath diversity spread spectrum receiver". The patent discloses a multipath diversity receiver utilizing decision directed coherent integration with post detection correlation techniques.
U.S. Pat. Nos. 5,031,193 and 4,291,410 are incorporated herein by reference, where permitted.
In a recent article in the "New Scientist" (15 Oct. 1994, No. 1947) titled "CARS THAT DRIVE THEMSELVES" it is stated (page 38)"
"Controlling the car's speed is more difficult. The major challenge is building a sensor that can monitor the precise distance of the vehicle ahead and its closing speed, over a range of 100 meters to less than 1 meter. Such a sensor must be able to detect everything in the lane ahead while ignoring vehicles in other lanes. It must work accurately in all weathers and be reasonably cheap to build. `We thought the aerospace industry might have all the answers, but even military radars cannot, do all the things we need,` . . . For the moment researchers make do with hand-built radars, but they work only when the vehicle ahead is a few meters away."