Absolute real-time ionospheric measurements from GPS satellites in the presence of anti-spoofing

Determining user position and velocity using signals from the constellation of GPS satellites depends upon receiving signals from satellites in widely disparate directions in the sky to minimize the effects of geometric dilution of precision, (GDOP). The range error due to the electron content of the earth's ionosphere is normally corrected by using signals of both the GPS L2 and L1 frequencies to automatically correct for the first order ionospheric range error effect. When a GPS satellite is just rising above the horizon it is desirable to use it in the navigation solution, as soon as it reaches an elevation above, say 5 degrees, so that the newly acquired satellite aids in producing a position solution with a minimum GDOP. However, the lower signal to noise ratio when a GPS satellite is viewed near the horizon, especially evident when anti-spoofing, (a/s), is turned on, can limit the accuracy of making measurements of absolute ionospheric range delay to values higher than approximately one meter until the satellite reaches an elevation angle greater than approximately 30 degrees. The comparative effects of a/s and high multi-path on the GPS differential group delay signal have been examined and the results clearly demonstrate that, even in a high multi-path environment, a/s is the dominant effect which limits early determination of absolute ionospheric range error. A novel, non-recursive, robust residual-adaptive filter was found to give the best results in obtaining absolute ionospheric range error corrections at low elevation angles in the presence of a/s. The times required from satellite rise until an absolute ionospheric range error of less than 0.16 meters r.m.s. could be obtained corresponded to elevation angles of approximately 30 degrees in the presence of a/s, and considerably lower with a/s turned off. Multi-path was a significant limitation in determining absolute ionospheric range error only with a/s turned off, and then only in a relatively bad multi-path environment.