Fail safe system with common mode avoidance
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to &#34;fail safe&#34;, or &#34;fail passive&#34; and &#34;fail operational&#34; systems such as those used to provide position and attitude information to the flight control system of an aircraft and more particularly to such systems which are also safe from &#34;common mode&#34; failures. 
     2. Description of the Prior Art 
     Systems for providing information indicative of position (for example, deviations from an ILS beam or altitude, latitude and longitude) and indicative of attitude (for example, the RF signals from an ILS or the signals from gyros and accelerometers indicative of pitch, roll and yaw) for use in vehicles such as aircraft, are well known in the art. Traditional systems employ gyroscopes and accelerometers while more recent systems use transmitted information from several satellites. Since high performance aircraft need a high degree of integrity including accuracy and dependability, and since in some situations, for example, &#34;precision approaches&#34;, two or three unique data sources are required to meet certification, &#34;fail safe&#34; and &#34;fail operational&#34; systems have been devised. &#34;Fail safe&#34; systems employ two redundant sensing systems whose outputs are checked against each other and if they are not the same, a failure is known and both outputs are discarded. &#34;Fail operational&#34; systems employ three or more redundant sensing systems and an associated algorithm to compare the outputs of the sensing systems and detect when one of them differs from the others. When this occurs, the erroneous output is discarded but the outputs of the other sensing systems may be used to control the aircraft. Prior art fail operational systems are generally classified as either: 
     1) Triplex system with three or more sensing systems each producing an output indicative of the same condition. The outputs of the sensing systems are checked against each other and if one differs from the others it is discarded and the other outputs are used. An example of such a system may be found on the Boeing 747. 
     2) Dual-dual systems with two or more pairs of sensing systems each sensing system producing an output indicative of the same condition. The outputs of the sensing systems of each pair are checked against each other and if they are not the same, then the output of that pair is discarded and the output from the other pair or pairs is used. As an example, such a system may be found on the Douglas MD-11. 
     Furthermore, in Category IIIb decision approaches, (unpiloted landing), only fail operational systems can be used thus requiring at least three data sources. 
     Many fail safe and fail operational systems are vulnerable to a &#34;common mode&#34; failure i.e. a problem that effects all of the sensors in the same manner so that their outputs are all in error by the same amount. When this occurs, the comparison of the outputs may indicate the signals are valid when in reality none of the signals can be relied on. 
     BRIEF DESCRIPTION OF THE PRESENT INVENTION 
     The present invention overcomes the &#34;common mode&#34; problem by utilizing two dissimilar sensing systems, for example, inertial reference systems (IRS) which utilize inertial equipment such as gyros and accelerometers and together are often referred to as an inertial reference unit (IRU) and satellite navigation equipment such as a global position system (GPS). These systems are responsive to entirely different measurement and computation techniques and thus can never have a common erroneous factor which has the same effect on both of the sensing systems. When the output of the GPS receiver is checked against the output of the IRU, the system is referred to as &#34;fully monitored&#34;. In the implementation of the present invention, two parallel fully monitored attitude and navigation systems are employed with each performing a check between the IRU and the GPS receiver outputs to determine if they are valid. If one system fails the validity test, the other may be used so that fail operational performance is provided. Furthermore, since they are set up in a dual-dual arrangement, they may be substituted directly into aircraft using the present dual-dual operation such as the Douglas MD-11. For systems using triplex arrangements such as the Boeing 747, the dual-dual arrangement of the present invention can be substituted for two of the three systems with the third being used for a further cross check if desired. It is also possible to implement the present invention with three fully monitored components to produce the outputs and thus employ them directly in the triplex system. 
     As mentioned, both the inertial reference unit (IRU) and the global position system (GPS) receiver produce outputs indicative of aircraft position and attitude. The IRU uses gyros and accelerometers which are very accurate in determining attitude but somewhat less accurate in determining position while the GPS receiver obtains signals from a plurality of satellites and is very accurate in determining position and is somewhat less accurate in determining attitude. Though there may be small differences, the position information from the IRU is sufficiently accurate to check the position information from the GPS receiver and the attitude information from the GPS is sufficiently accurate to check the attitude information from the IRU. 
     After the integrity check is performed, the information concerning position from the GPS receiver and the information concerning attitude from the IRU is provided to the flight control system of the aircraft. Accordingly, the fail operational operation is provided and since the two systems do not use any information common to both, common mode failures are avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For simplicity, the present invention will be described in connection with producing a fail operational or &#34;fully monitored&#34; inertial attitude reference system although it will be understood that a fully monitored position reference system or indeed any other aircraft or other system which needs fail operational performance without common mode errors can be made using the same concepts. 
     In FIG. 1, two fully monitored inertial reference systems, IRS, or alternately, AHARS 12 and 14 are shown in a dual-dual arrangement for use in providing pitch (Θ)), pitch rate (Θ), roll (Φ), roll rate (Φ), yaw (Ψ) and yaw rate (Ψ) signals to a flight control system 16 of an aircraft so as to provide a fail operational attitude sensing system. Since they are identical, only IRS 12 will be described. 
     Within IRS 12 a Global Position System GPS receiver 20 is shown having three antennas 22, 24, and 26 for receiving signals from the satellites either directly or through a ground based transceiver. GPS 20 may be like that commonly sold by Honeywell and an output indicative of pitch, roll and yaw as sensed by GPS 20 is produced on a line 28 which is presented to a comparator 30. 
     Also within IRS 12 an inertial reference unit IRU 40 is shown. IRU 40 contains a three axis gyro and an accelerometer unit such as is commonly sold by Honeywell Inc. and an output indicative of pitch, roll and yaw as sensed by IRU 40 is produced on a line 42 to the FCS 16 and, via a line 44, to the comparator 30. 
     Comparator 30 contains an algorithm which compares or determines the difference between the pitch, roll and yaw and the aircraft body rates (Θ, Φ, and Ψ) outputs as produced by the GPS 20 and the IRU 40 and determines whether they are the same within a certain predetermined amount of variance. If they are within the circle of confidence, then a &#34;valid&#34; signal is released from comparator 30 on a line 44 to the FCS 16 so that FCS 16 knows that the pitch roll and yaw signals from IRU on line 42 are valid. 
     In a similar fashion, pitch roll and yaw signals from IRS 14 are presented to FCS 16 over a line 46 and &#34;valid&#34; signals are presented from IRS 14 to FCS 16 on a line 48. 
     It should be noted that there is no &#34;common mode&#34; errors possible since IRU 40 responds to different factors than GPS 20. Thus a &#34;valid&#34; signal will always mean that the yaw pitch and roll signals from the IRS&#39;s 12 and 14 are truly valid. It is also seen that each IRS is, in fact, a &#34;fail safe&#34; system since any error whether in the GPS or the IRU will result in no &#34;valid&#34; signal. 
     Thus, FCS 16 receives two sets of pitch, roll and yaw signals and two &#34;valid&#34; signals from IRS 12 and IRS 14 when all systems are working properly. In the event that an error occurred in either IRS 12 or IRS 14, no &#34;valid&#34; signal would be presented by the faulty IRS and FCS 16 would use the signals from the other IRS. If both IRS&#39;s 12 and 14 were subjected to an error, then no &#34;valid&#34; signal would be presented to FCS at all and it would be known that none of the signals could be relied on. 
     It is seen that I have provided a fail operational system without common mode errors for the attitude and position control of an aircraft that can be directly incorporated in the existing fail operational systems on aircraft. It is also seen that the invention has uses other than for aircraft control and that those skilled in the art will find many changes and modifications to the disclosures used in connection with the preferred embodiment without departing from the scope of the invention.