An Ionospheric Correction Algorithm for WAAS and Initial Test Results

Alex Draganov, Tim Cashin, Joe Murray

Abstract:	The variability in electron density encountered by RF signals propagating through the ionosphere is a major source of navigation error for single frequency GPS users, especially during periods of intense geomagnetic activity. The FAA’s Wide Area Augmentation System (WAAS) is expected to improve GPS user navigation accuracy by broadcasting correction data which will include parameters to correct for the ionospheric error affecting GPS ranging measurements. Single-frequency users will receive WAAS estimates for the vertical ionospheric delay at defined geographical points with a requirement that the resulting correction errors not exceed 2 meters in 99.9% of occurrences. This paper presents a new Kalman filter-based concept for determining the WAAS ionospheric corrections, which has been developed by Stanford Telecom (STel) and is being tested during flight trials at the FAA’s National Satellite Testbed (NSTB). Vertical ionospheric delay is modeled as the Klobuchar model prediction scaled by a specialized fit function. The latter can be expressed analytically using a double expansion of Legendre polynomials as a function of latitude and longitude. Coefficients of the expansion form a part of the state vector in the Kalman filter. Hardware calibration biases (Ll/L2 differential delays) in the GPS satellites (‘5 gd ) and in WAASNSTB monitor receivers are also included as additional Kalman filter states. Processing of simulated GPS measurement data has validated the capability of the Kalman filter to estimate both the ionospheric delay and the Ll/L2 hardware biases. Variances of the ionospheric delay were estimated as a by-product of the Kalman filter processing, and served as a basis for determining the Grid Ionospheric Vertical Error (GIVE) parameter in the WAAYNSTB broadcast message. This paper compares ionospheric delay estimation results using the STel method to measured (truth) data, under both quiet and magnetic storm conditions, Sequences of maps of the vertical ionospheric delay over the Contiguous United States (CONUS) were generated from the estimated data, showing the spatial and temporal structure of the ionosphere during the quiet magnetospheric conditions and during a severe magnetic storm (3 hour Kp = 70 ) on October 4, 1995. The ionospheric delay maps are consistent with published results for the ionoshperic behavior during quiet and storm periods.
Published in:	Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996) September 17 - 20, 1996 Kansas City, MO
Pages:	789 - 797
Cite this article:	Draganov, Alex, Cashin, Tim, Murray, Joe, "An Ionospheric Correction Algorithm for WAAS and Initial Test Results," Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996), Kansas City, MO, September 1996, pp. 789-797.
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