Transponder landing system

A ground-based, precision aircraft landing system provides CAT I precision approach and landing guidance. The aircraft elevation position is determined by measuring differential carrier phase and time-of-arrival of the aircraft ATCRBS transponder reply. The transponder reply is received at a plurality of sensor antenna locations where it is then conveyed to a sensor, demodulated and digitized. The data is transmitted to a central processor where calibration and multipath corrections are applied. Aircraft transponder diversity antenna switching is isolated from the jitter and colored noise of transponder reply multipath by correlating differential phase jumps measured between separate sensor antennas. An estimate of the diversity antenna separation is maintained by Kalman filter processing; the estimated separation is used to correct the differential phase measurement data of aircraft elevation. The corrected phase measurement and time-of-arrival measurement is processed using another Kalman filter to achieve the desired aircraft elevation positioning accuracy. A similar differential carrier phase and time-of-arrival subsystem is applied to achieve an azimuth measurement of the aircraft position. The combined azimuth and elevation of the aircraft is then compared to the desired approach path, and the aircraft position error relative to the desired approach is communicated to the aircraft.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The Figures are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.

FIG. 1 is a perspective drawing illustrating the elements of the Transponder Landing System in accordance with this invention;

FIG. 2 is a block diagram of the AOA sensor and the antenna inputs with front-end RF assembly switching in accordance with this invention;

FIG. 3 is an illustration of error attributable to diversity aircraft antennas;

FIG. 4 is an illustration of error attributable to multipath signal transmission; and,

FIG. 5 is a flow chart illustrating processing occurring in the processors, which suitably practices the present invention.

1. A method of determining a position of an aircraft having a transponder which transmits a reply signal in response to an interrogation signal, the method comprising:

2. The method as set forth in claim 1, further comprising:

3. The method as set forth in claim 2, further comprising:

4. The method as set forth in claim 1, where the estimating a position comprises:

5. The method as set forth in claim 4, where the selected characteristics include differential phase of the reply signal on selected antennas, and the analyzing comprises:

6. The method as set forth in claim 1, wherein the reply signal propagates between the aircraft antenna and the antenna array on paths including a direct path and a reflected path, the method further comprising:

7. The method as set forth in claim 6, where the applying step comprises:

8. The method as set forth in claim 1, further comprising:

9. The method as set forth in claim 1, wherein the determining comprises:

10. A precision glide path apparatus for guiding aircraft along an approach path within operable range of an interrogator which transmits an interrogation signal, the aircraft having a transponder switchably connected between two antennas, the transponder transmitting a reply signal in response to the interrogation signal, the apparatus comprising:

11. The precision glide path apparatus as set forth in claim 10, wherein four receive channels receive input from four antennas.

12. The precision glide path apparatus as set forth in claim 10 wherein the combined position calculator calculates an angle of the reply signal relative to the plurality of antennas by interleaving among the receive channels.

13. The precision glide path apparatus as set forth in claim 10, further comprising:

14. The precision glide path apparatus as set forth in claim 10, further comprising:

15. The precision glide path apparatus as set forth in claim 10, further comprising:

16. A transponder landing system comprising:

17. The transponder landing system as set forth in claim 16, where the correction processor comprises:

18. The transponder landing system as set forth in claim 16, further comprising:

19. A position determining method comprising:

20. The position determining method as set forth in claim 19, further comprising: