The Effects of Large Ionospheric Gradients on Single Frequency Airborne Smoothing Filters for WAAS and LAAS

Todd Walter, Seebany Datta-Barua, Juan Blanch, and Per Enge

Abstract:	Both WAAS and LAAS receivers use carrier-smoothing filters to reduce the effects of multipath and thermal noise at the aircraft. For the majority of users this reduces the magnitude of the errors and leads to improved accuracy. However, the presence of a significant ionospheric gradient can introduce a bias into the output of this filter. If unmitigated, this bias can grow to be significantly larger than the noise and multipath effects the filter is employed to reduce. Such gradients are rare at midlatitudes; however, they are much more common in polarregions and sometimes a daily occurrence in equatorial regions. Therefore, this problem which may be rare in the United States or Europe will be much more significant in other parts of the world. The bias arises because the GPS code and carrier measurements are affected differently by the ionosphere. A positive delay for the code creates an equal, but opposite advance for the carrier. Therefore, as the ionospheric Total Electron Content (TEC) changes along a user’s line of sight to the satellite, the code and carrier measurements diverge from each other at twice the rate of ionospheric change. Nominally, the rate of change over a few hundred seconds is very small (less than a few millimeters per second) leading to decimeter-sized biases or less. However, if the user’s line of sight traverses a significant ionospheric gradient, the TEC can change by tens of meters in just a few minutes. This in turn can create a biased filter estimate of greater than 20 meters. This error is large compared to the overall error budget less than 0.4 m one-sigma. This paper investigates the combined effect of the TEC change and the filter bias on the WAAS/LAAS user’s differential range error and suggests a remedy to lessen the impact. This solution includes changes to the airborne algorithms to detect large differences between the code and carrier. The ability of the airborne receiver to detect this bias depends on the amount of thermal noise and multipath it experiences. This paper will review the current expected levels of noise and multipath and indicate level of performance that should be achievable.
Published in:	Proceedings of the 2004 National Technical Meeting of The Institute of Navigation January 26 - 28, 2004 The Catamaran Resort Hotel San Diego, CA
Pages:	103 - 109
Cite this article:	Walter, Todd, Datta-Barua, Seebany, Blanch, Juan, Enge, Per, "The Effects of Large Ionospheric Gradients on Single Frequency Airborne Smoothing Filters for WAAS and LAAS," Proceedings of the 2004 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2004, pp. 103-109.
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