Should WAAS Get Rid of the MET Instruments?

Cheryl Griffith, Stephen Peck, Juan Ceva, Rajendra Malla, William Bertiger, Ronald Muellerschoen

Abstract:	Hughes Aircraft is currently designing the algorithms of the FAA’s Wide-Area Augmentation System (WAAS). This paper presents some of the particulars of the system design and algorithms for the troposphere delay estimation. The current WAAS specifications call for the use of meteorological (MET) instruments at the WAAS Remote Stations in order to calibrate the delay that the GPS signals suffer due to its propagation through the Troposphere. It has been long debated whether algorithms that use meteorological information can outperform algorithms that do not use the surface information. The latter algorithms implement a simple stochastic estimate of the delay. In addition, the current Minimum Operational Performance Standards (MOPS) define the troposphere compensation algorithm that the user employs. A separate issue addresses the impact of this algorithm on user range errors. This paper compares the performance of the MOPS algorithm, baseline troposphere algorithm (MET based) and the real-time troposphere estimation filter to true troposphere ,vertical delays. (Troposphere slant delay will not be evaluated in this paper.) The MOPS algorithm only accepts position inputs, and is thus limited to the full variability of the troposphere at a given location. MET data contains surface pressure, temperature and humidity information. The pressure can be used to predict dry tropospheric delays very accurately, however, the wet data varies significantly, and can only somewhat be predicted by surface MET data. A real-time filter uses the receiver measurements to estimate the vertical delay. The filter assigns a single stochastic parameter representing vertical delay at each receiver location. This algorithm is quite accurate at predicting vertical delay due to the direct measurements of slant delay inherent in the receiver measurements. Eighteen months of radiosonde observation data are used to compute true vertical delay. This data also provides surface MET data. Tropospheric delays are computed and compared to the true delay using both the MOPS and the MET algorithms. Statistics for vertical errors are analyzed for average and worst case performance. The troposphere Iestimation algorithm uses a post-processed troposphere truth reference which has been validated to the sub centimeter level. The performance results are used to prese:nt a case for removing the MET requirement at the WAAS Remote Stations.
Published in:	Proceedings of the 1997 National Technical Meeting of The Institute of Navigation January 14 - 16, 1997 Loews Santa Monica Hotel Santa Monica, CA
Pages:	911 - 920
Cite this article:	Griffith, Cheryl, Peck, Stephen, Ceva, Juan, Malla, Rajendra, Bertiger, William, Muellerschoen, Ronald, "Should WAAS Get Rid of the MET Instruments?," Proceedings of the 1997 National Technical Meeting of The Institute of Navigation, Santa Monica, CA, January 1997, pp. 911-920.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In