Availability of LAAS to Calibrate IRS for CAT IIIB

Young C. Lee, Curtis A. Shively and Thomas T. Hsiao

Abstract:	In the application of the FAA’s GPS Local Area Augmentation System (LAAS) for Category IIIB (CAT IIIB) precision approach and landings, there is concern that ground-based Radio Frequency Interference (RFI) may preclude use of GPS LAAS during the low-altitude part of such procedures. One means under consideration for overcoming such interference is the tightly-coupled integration of LAAS avionics with an Inertial Reference System (IRS) onboard the aircraft. This paper examines the availability of LAAS to calibrate the IRS to provide lateral guidance sufficient for a CAT IIIB landing if GPS is interfered with after the aircraft reaches the Final Approach Fix (FAF). Vertical guidance after the FAF is assumed to be provided by some combination of baroaltimeter and the aircraft’s radar altimeter in conjunction with a terrain data base onboard the aircraft. Unaugmented GPS is used for initial calibration of the IRS lateral errors during the en route and initial terminal area phases of flight. When the aircraft enters LAAS VHF Data Broadcast (VDB) coverage, a final calibration of the IRS is performed using the improved accuracy provided by LAAS differential corrections. Of several LAAS error components, the residual ionospheric delay is the largest, since it increases with distance from the LAAS ground facility (LGF). It is assumed that an accurate calibration for gravity deflection is already known within the VDB coverage region. The paper models the growth (drift) of IRS errors during the coasting period after calibration ends at the FAF. This model is used to determine the lateral calibration accuracy needed from LAAS so that the IRS lateral accuracy after coasting still meets the requirement for a CAT IIIB landing. Several scenarios and variations are considered corresponding to making or not making a turn after departure, different aircraft maneuvers in the terminal area and different orientations of the entire scenario relative to direction of earth rotation. To be conservative, the result from the most demanding scenario is then used to determine the availability of GPS satellite geometry to meet the final lateral calibration accuracy requirement. The availability analysis also considers two types of “equipment” in the LGF and aircraft. The first type meets ground and airborne requirements for CAT I approaches and the second type would meet the more stringent requirements for CAT III approach and landing operations. For the ionospheric delay error, a typical gradient of 3.5 mm/km was assumed. Under these assumptions, adequate availability (0.999) would be achieved with CAT I equipment and GPS/LAAS alone at all 78 locations in CONUS analyzed. Only slight improvement in availability would result from using CAT III equipment since the dominant error from the ionosphere is the same as for CAT I equipment. However, with the use of Wide Area Augmentation System (WAAS) GEOs as additional ranging sources, availability would improve to better than 0.9999 everywhere for both types of equipment. Thus, WAAS GEOs might be used to compensate for lost availability if the ionospheric gradient was larger than the 3.5 mm/km assumed in the analysis.
Published in:	Proceedings of the 2005 National Technical Meeting of The Institute of Navigation January 24 - 26, 2005 The Catamaran Resort Hotel San Diego, CA
Pages:	587 - 604
Cite this article:	Lee, Young C., Shively, Curtis A., Hsiao, Thomas T., "Availability of LAAS to Calibrate IRS for CAT IIIB," Proceedings of the 2005 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2005, pp. 587-604.
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