Estimation of GPS Ionospheric Delay Using L1 Code and Carrier Phase Observables

摘要

Free electrons in the earth's ionosphere cause a frequency-dependent group delay in the bands used for the spread-spectrum GPS ranging signals. This delay constitutes a potential source of error in timing measurements. The ionospheric delay can be removed by dual-frequency ranging using the L1 and L2 signals. Two-frequency time receivers are currently expensive, and are not very reliable when tracking low elevation satellites when Anti-Spoofing is enabled, unless decryption can be used. Single-frequency, L1, receivers can correct for the delay using a detailed model of the ionosphere scaled by data contained in the 'navigation message' broadcast by the satellites. However, because of unpredictable variations of the ionosphere, this so-called single-frequency correction is only expected to absorb 50% of the effect. The uncorrected ionospheric delay can cause significant errors in L1 timing systems such as disciplined oscillators. This unpredictable source of error is of increasing importance with the approach of the solar activity maximum. Estimates of the ionospheric delay, calculated from two-frequency geodesic measurements, are now available from several sources, but these are not easy to apply in real time. In the future, WAAS and other system real-time ionosphere estimates for some geographical areas will be available with suitable receivers via geostationary satellites. We report attempts to model the zenith ionosphere correction from observations of the L1 code and carrier phase GPS observables made with a multi- channel receiver module. Code and carrier phases are affected by dispersion with opposite signs, but carrier phase always contains an ambiguity. It will be shown that by measuring derivatives of the observables, the ionospheric delay can be estimated approximately by post-processing the output of a single- frequency receiver. The use of a single frequency avoids inaccuracy due to group delay differences in the receiver for LI and L2 signals.