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THEME 3: MULTI-SENSOR INTEGRATION
Aircraft Approach & Landing
Standalone GPS and conventional code-phase differential GPS are unable to meet the stringent navigation requirements in most airborne applications, because the performance of satellite-based navigation systems are dependent on both the number and geometric distribution of satellites tracked by the receivers. Due to the limited number of GPS satellites, a sufficient number of visible satellites cannot be guaranteed everywhere, 24 hours a day. Even when some low elevation satellites can be tracked, the measurements from these satellites are contaminated by relatively high atmospheric noise. Therefore, this intrinsic shortcoming of satellite-based positioning systems results in poor accuracy in the vertical component, which is typically about two-to-three times worse than that of the horizontal components.
Previous research has shown that some means of augmentation can address these drawbacks in order to meet the specified requirements. Airport pseudolites have been suggested as a means of satisfying the performance requirements of CAT II/III approach systems. Airport pseudolites are ground-based transmitters that emit GPS-like signals, enhancing GPS navigation by providing increased accuracy, availability and integrity. Navigation accuracy improvement can occur due to better local geometry, as measured by a lower vertical dilution of precision (VDOP), which is crucial in aircraft precision approach and landing applications. Accuracy and integrity enhancement can also be achieved by using an airport pseudoliteÕs integral data link to support differential modes of operation and timely transmission of integrity warning information. Availability is increased because airport pseudolites provide additional ranging sources to augment the GPS constellation.
DSO National Laboratories, Singapore, and SNAP have jointly developed a local area augmentation system prototype able to provide high accuracy navigation for aircraft precision approach and landing. A prototype airport pseudolite has been configured, as shown in Figures 1 & 2, for this application. The pseudolite consists of a GPS signal generator with power amplifier and rubidium reference clock. The airport pseudolite signal generator is a modified Spirent Communications single-channel GPS simulator (GSS4100P) with pulsing function.
Figure 1: L1 C/A pseudolite configuration block diagram
Figure 2: L1 C/A pseudolite physical configuration
In April/May 2003, the first flight test period was carried out at the Wedderburn Airfield, Sydney, Australia, in which a total of 40 approach and landings were performed. The post-processed results of the flight tests showed that carrier-smoothed code-phase differential GPS/pseudolite can attain vertical and horizontal accuracies consistent with the Required Navigation Performance (RNP) for CAT III. The second series of flight tests were conducted in November/December 2003 to demonstrate the feasibility of a real-time integrated GPS and pseudolite for precision approach and landing.
Figure 3: Wedderburn Airfield ground configuration
Figure 4: Beech Duchess aircraft from the UNSW Aviation Department
Figure 5: The typical trajectory of the aircraft approach during the flight tests, with each circuit taking approximately 6-8min to complete
Figure 6 shows the typical DGPS/DPL solution errors over the final phase of an approach. These errors are the carrier-smoothed DGPS solution error without pseudolite substracted from the carrier-smoothed DGPS/DPL solution with the pseudolite (the absolute DGPS error with respect to the truth reference generated using the GrafNav/GrafNet GPS post-processing software package was at the submetre-level of accuracy).
On the other hand, Figure 7 shows the flight test results compared with the post-processed ÔtruthÕ reference data over two of the flight days. The horizontal axes show the distance with respect to the ground reference station.
Figure 6: Typical difference between the GPS-only DGPS position errors and GPS/PL - DGPS/DPL position errors
Figure 7: Flight test results
From these test results, it was possible to conclude that the performance of the flight test at the 100 feet decision height: along-track sensor accuracy of 0.531m (95%), cross-track sensor accuracy of 0.438m (95%) and vertical sensor accuracy of 0.730m (95%).
More details on this research issue can be found in:
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SOON, B.H.K., POH, E.K., BARNES, J., LEE, H.K., ZHANG, J., LEE, H.K., & RIZOS, C., 2003. Preliminary results of the carrier-smoothed code-phase differential GPS/pseudolite system. 6th Int. Symp. on Satellite Navigation Technology Including Mobile Positioning & Location Services, Melbourne, Australia, 22-25 July, CD-ROM proc., paper 48. (Download PDF)
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SOON, B.H.K., POH, E.K., BARNES, J., ZHANG, J., LEE, H.K., LEE, H.K., & RIZOS, C., 2003. Flight test results of precision approach and landing augmented by airport pseudolites. 16th Int. Tech. Meeting of the Satellite Division of the U.S. Institute of Navigation, Portland, Oregan, 9-12 September, 2318-2325. (Download PDF)
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SOON, B.K.H., BARNES, J., LEE, H.K., ZHANG, J., RIZOS, C., WANG, J., & LEE, H.K., 2004. Real-time flight test results of an integrated GPS/INS/pseudolite autolanding system. GNSS2004, Rotterdam, The Netherlands, 16-19 May, CD-ROM proc., paper 16. (Download PDF)
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