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Availability Enhancements for CAT IIIB LAAS
2004 Award Winner
Curtis A. Shively, The MITRE Corporation
Thomas T. Hsiao, The MITRE Corporation
ABSTRACT
Standards are currently being developed for using LAAS for Category IIIb (CAT IIIb) operations. Since the performance requirements are more stringent than for CAT I, LAAS CAT IIIb faces the challenge of achieving adequate availability of good geometry from ranging sources used in the position solution. This paper analyzes the relative availability improvement predicted for various alternatives to enhance LAAS CAT IIIb availability. The analysis considers increasing the number of ranging sources by employing Wide Area Augmentation System (WAAS) geostationary satellites (GEOs) or airport pseudolites (APLs). Alternative error models are considered for airborne noise, airborne multipath and residual ionospheric delay. The potential of taking credit for reduction in airborne error contributions due to comparison among redundant navigation sensors such as via mid-value selection (MVS) is also explored. The analysis also includes use of a second frequency (e.g., L2) to remove ionospheric delay from the carrier smoothing process and thus permit better smoothing of multipath errors both on the ground and in the aircraft. A final alternative considered is use of highly accurate differential carrier phase measurements in a position solution to reduce vulnerability to loss of a critical satellite during the phase of the approach and landing below 500 feet above ground level. Availability under each scenario is computed for the vertical alert limit (VAL) ranging from 5.3 m (current requirement) to 10.0 m. Results show that if VAL is not increased beyond about 5.3 m, adequate (0.999) availability cannot be achieved by merely extending the CAT I architecture or by any one of the above methods alone, except using dual frequency in both the ground facility and the aircraft. However, since a second GPS civil frequency for use in the aircraft won’t be provided until circa 2014, several of the above techniques would need to be combined (e.g., reduced airborne error, reduced ionospheric error and dual frequency in the ground facility) to give adequate availability for VAL = 5.3 m. On the other hand, if VAL could be increased to approximately 7.5 m, some other alternatives alone and most pair-wise combinations of these alternatives would provide adequate availability. The final choice of a LAAS architecture for CAT IIIb will, of course, consider additional factors beyond availability, such as ease of extension from CAT I, cost and technical risk.
Publication
Copyright ©2004 IEEE. Reprinted from NAVIGATION: Journal of the Institute of Navigation, Vol. 51, No. 1, pp. 45–57.
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