The Impact of GPS Modernization on Standalone User Performance and Integrity with ARAIM

S. Pullen, P. Enge, S. Shaw, C. Frey, J. Frye, M. Souder

Abstract:	GPS users with demanding requirements on the safety, or integrity, of their navigation information now have several options. These include the use of separate correction messages from augmentation systems such as SBAS and GBAS and autonomous integrity verification using Receiver Autonomous Integrity Monitoring, or RAIM. Both of these approaches have advanced significantly in the last decade. In particular, Advanced RAIM, or ARAIM, extends and improves upon the traditional RAIM algorithm to detect and mitigate independent, multiple, and correlated GPS signal faults, making it possible to support applications such as aviation precision approach without requiring real-time integrity messages [1]. However, RAIM and ARAIM rely on redundant ranging measurements for detectability. As a result, recent studies have suggested that multiple GNSS constellations will be required to provide availability similar to that obtainable from GPS with augmentation [2]. For applications that require the use of GPS only, such as most US military applications and many civil applications with inexpensive, GPS-only receiver chipsets, the ongoing modernization of the GPS satellite constellation and Operational Control Segment provides a basis for significant future improvements in ARAIM availability. GPS modernization enhances ARAIM performance in two ways. Improved ranging accuracy, or lower errors under nominal (no-fault) conditions, makes it easier for ARAIM to distinguish faulty measurements from normal ones. Improved integrity, or lower un-alerted failure probabilities, relaxes the missed-detection probability that ARAIM must provide to meet the overall safety requirements for precision approach (or any other application). This paper examines the effects of GPS modernization to improve the capability of ARAIM without including other GNSS constellations. Simulations have already been conducted for military aviation users by comparing today’s GPS constellation of Block IIA, IIR, IIRM, and IIF satellites with a fully-modernized constellation with the same number of GPS Block III satellites. The results show major improvements in ARAIM performance and availability for Block III, but this comparison over-simplifies the progression of GPS modernization over time. This paper will expand upon the existing results by modeling both military and civil users and by comparing ARAIM performance over the following four stages of GPS modernization: (1) Today’s GPS constellation (as of late 2012): already analyzed for military users, now expanded to include civil users of the L2 civil signal; (2) GPS constellation with dual-frequency signals: this constellation includes a mixture of Block IIF and Block III satellites, but only the presence of modernized second-frequency signals (L2 for military, L5 for civil) is modeled; (3) GPS constellation with dual-frequency civil signals: the same as (2) and may be present in about the same time frame, but it also includes modernized signals (on all frequencies) with lower nominal ranging errors; (4) GPS constellation with enhanced integrity: the same as (3) but composed entirely of GPS Block III satellites; thus the integrity enhancements of GPS III are included (already analyzed for military users). The Stanford Matlab Algorithm Availability Simulation Tool (MAAST) is used to evaluate and compare the ARAIM performance of these modernization variations. For each variation, constellations with 24, 27, and 30 satellites are simulated along with outages of satellites in particular primary orbit slots that have significant impacts on performance. The primary nominal error model (with different variations for military and civil users) is based on the use of an ARAIM "Integrity Support Message," or "ISM," which is a separate transmission that infrequently updates the health status and error-model parameters for each GPS satellite. The secondary error model is based on the User Range Accuracy (URA) parameters in the GPS navigation message, which includes quantization errors and may be more conservative than what can be supported by an ISM that is specific to a particular region of operations. The results of these simulations will illustrate the degree to which specific components of GPS modernization enhance the standalone integrity that can be obtained by users that do not combine GPS with other GNSS signals. They will also indicate which additional improvements to GPS would make the most difference to standalone GPS user performance. References: [1] J. Blanch, T. Walter, et al, “Advanced RAIM User Algorithm Description: Integrity Support Message Processing, Fault Detection, Exclusion, and Protection Level Calculation,” Proceedings of ION GNSS 2012, Nashville, TN, Sept. 17-21, 2012, pp. 2828 - 2849. [2] Phase II of the GNSS Evolutionary Architecture Study, Washington, DC, U.S. Federal Aviation Administration, February 2010.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	2637 - 2653
Cite this article:	Pullen, S., Enge, P., Shaw, S., Frey, C., Frye, J., Souder, M., "The Impact of GPS Modernization on Standalone User Performance and Integrity with ARAIM," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 2637-2653.
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