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Session B3: Future Trends in GNSS Augmentation Systems

Alternate NAV Message Correction Table Formulation: Terse WAGE-2
Karl Kovach, The Aerospace Corporation
Date/Time: Thursday, Sep. 22, 11:48 a.m.

The GPS Legacy Navigation (LNAV) message has carried a Navigation Message Correction Table (NMCT) since 1995. The legacy NMCT is contained in page 13 of subframe 4 and is broadcast to LNAV receiving equipment every 12.5 minutes. The associated time tag for the NMCT is contained in subframe 2 of the LNAV message and is broadcast to LNAV receiving equipment every 0.5 minutes. Taken together, the NMCT and its time tag provide a ‘Wide Area GPS Enhancement’ (WAGE) capability that is analogous to the Wide Area Differential GPS (WADGPS) capability provided by a modern Satellite-Based Augmentation Systems (SBASs) such as the Wide-Area Augmentation System (WAAS) and European Geostationary Navigation Overlay Service (EGNOS). WAGE is more limited in scope than SBAS WADGPS. Due to its comparatively slow differential correction update rate (12.5 minutes), WAGE is only intended to enhance the accuracy of GPS. The SBASs have a differential correction update rate on the order of 6 seconds and are intended to enhance both the accuracy and integrity of GPS.
During the GPS modernization effort, new navigation messages were developed to accompany the new pseudoranging signals. The new Military Navigation (MNAV) messages were created as the companion for the M-code signals, the new Civil Navigation (CNAV) messages were created for the L2 CM-code and L5 I5-code signals, and the Type 2 CNAV messages (CNAV-2) were created the new L1 Cd-code signal. Each of the new message streams incorporates a new and improved version of WAGE – called “WAGE-2” – which expands the simple LNAV scalar pseudorange differential correction for each satellite into a full four-dimensional vector differential correction. WAGE-2 is thus more in-line with the SBAS differential corrections where the GPS satellite clock errors and GPS satellite radial, along-track, cross-track orbit errors are all individually addressed. Although this greatly improved the theoretical accuracy obtainable with the WAGE-2 differential corrections, the significantly larger size of the WAGE-2 messages coupled with the inherently limited data throughput rate of the MNAV, CNAV, and CNAV-2 data streams reduced the effective differential correction update rate to only about a quarter of that of the already slow legacy “WAGE-1” update rate. As the throughput demands on the MNAV, CNAV, and CNAV-2 data streams have continued to grow over time, the verbose nature of the WAGE-2 messages has stretched out the differential correction update rate to the point where the theoretical accuracy of the WAGE-2 messages is becoming moot.
To remedy the verbose WAGE-2 formulation, and to reap additional benefits from a faster time to first fix (TTFF) with WADGPS accuracy, a terse WAGE-2 formulation has been developed. Rather than taking well over a dozen paged NMCT messages for a full ‘Verbose WAGE-2’ data set, only one NMCT message is needed for the “Terse WAGE-2” data set. This paper describes the Terse WAGE-2 formulation and provides the theoretical basis for the accuracy trade-offs between precision and speed. Particular justification is provided for omitting along-track and cross-track orbit differential corrections in light of anticipated Next Generation Control Segment (OCX) performance. The fortuitous parallel between the Terse WAGE-2 User Differential Range Accuracy (UDRA) formulation and the Non-Elevation Dependent (NED) User Range Accuracy (URA) construct in the MNAV, CNAV, and CNAV-2 clock/orbit messages is developed along with suggested receiver processing algorithms to exploit those parallels.



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