| Summary: | <p> One of the principal observations derived from GNSS (Global Navigation Satellite Systems) signals is ionospheric total electron content (TEC), which is a measure of the density of free electrons (i.e. ionosphere plasma density) integrated along the signal path. TEC is typically computed using the difference of dual-frequency signals from a GNSS satellite, thereby taking advantage of the frequency dispersive effects of ionosphere plasma on microwave-band propagation. However, it is difficult to distinguish between the ionosphere and other frequency-dependent effects, such as multipath and satellite antenna phase effects. Newly available triple-frequency GNSS signals allow computation of geometry-ionosphere-free combinations (GIFC) that specifically highlight the impact of residual errors from these effects. This work aims to: 1) introduce a framework for choosing linear estimator coefficients for GNSS parameters, 2) use this system to derive triple-frequency TEC estimator and GIFC coefficients, 3) introduce and summarize typical GIFC signals from real triple-frequency GPS data, 4) highlight the various frequency-dispersive effects that pervade these signals, and 5) use statistics from GIFC signals to assess the impact of error residuals on TEC estimates made using GPS signals. </p><p>
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