The Effects of Local Ionospheric Decorrelation on LAAS: Theory and Experimental Results

Solar cycle 23 is expected to reach a maximum in the year 2000. This has produced a recent flurry of research about the possible adverse effects of the ionosphere on the millions of GPS users. The introduction of the Global Positioning System (GPS) ushered in a new era of affordable precise navigation. Differential GPS (DGPS) corrections have reduced positioning errors from about 100 meters to roughly 1 meter, which has led to their proposed use in aircraft precision approach. However, the accuracy of differential corrections is somewhat limited by spatial and temporal decorrelation of the atmosphere. Much of the recent increase in GPS usage, as well as the development of precision landing requirements were developed near solar minimum. Although consideration was made for increased levels of sunspot and ionospheric activity, it is prudent to better characterize ionospheric decorrelation. This paper will present both theoretical and experimental results in this regard. Currently assumed value of spatial decorrelation for the ionosphere is about 2 mm/km, which is equivalent to an error of only 3 centimeters error due to the ionosphere at 15 kilometers from the DGPS reference station. However, this value is based on relatively smooth models of the ionosphere. This value seems overly optimistic when compared to a documented ionospheric gradient of 55 mm/km, which is certainly too conservative. Much of the development of the prototype landing systems has occurred in the last three or four years, close to the solar minimum. Current requirements for precision landing, which limit integrity risk to one undetected navigation hazard in a billion trials, suggest that we must fully characterize ionospheric decorrelation. This paper seeks to obtain statistical bounds for the ionospheric decorrelation effects on a LAAS architecture. Archived data from the National Satellite Test Bed (NSTB = WAAS Prototype) was analyzed to characterize the spatial gradients. The impact of filter time constants on pseudorange error was calculated. This delay artifact of the CSC architecture was examined to ensure integrity. This paper seeks to obtain statistical bounds for the ionospheric decorrelation effects on a LAAS architecture. Introduction Measurement decorrelation is an issue for precision DGPS applications such as LAAS. The rate of decorrelation determines the inter-station spacing for WAAS. Decorrelation is roughly linear with range from the reference station, increasing as you move farther from the reference station. This increase in error does not necessarily lead to a reduction in safety, since the aircraft is typically higher above local terrain when it is further from the airport. Previous GPS research in Antarctica measured a decorrelation of 50 cm over a 9 km baseline, all of which was attributed to the ionosphere [Goad]. This gradient of 55 mm/km, is 20 times larger than the typical quoted values. While polar regions typically experience larger values of TEC, this single observation is alarming since it is certainly not the worst possible value, just the worst value observed during the short campaign. Ionospheric gradients (decorrelation) can be visualized in Figure 1. As the distance between the reference station and the mobile user increases, the lines of sight will travel through different parts of the ionosphere. For a single frequency receiver, there are two observables, the pseudorange (ρ) and the carrier phase (φ). Both of Figure 1 LAAS Overview G P S G P S G P S P δ, |δ| < 100 m G P S