20. Track-to-Track Ambiguity Resolution for Zero-Differences—Integer Phase Clocks

In this section we introduce a novel approach for GNSS ambiguity resolution at the zero-difference level, what we call Track-to-Track (T2T) ambiguity resolution. The T2T approach is based on the resolution of wide-lane and narrow-lane ambiguities between consecutive satellite tracking passes, what we call track-to-track or pass-to-pass ambiguities. To fix T2T ambiguities to their integer values, GNSS measurements from only a single GNSS receiver are used without forming any double-differences or similar combinations between different GNSS receivers. Thus, the T2T approach is especially appropriate for LEO applications, to connect very short tracking passes (typically 15–20 min) that introduce a very large number of zero(double)-difference ambiguities, and for ground networks, where the ambiguities of a single GNSS satellite can be connected over a longer period of time (e.g., one week). This opens up a new application for T2T ambiguities to monitor stability and to define code biases and GNSS clock parameters over a long period of time. In this section, we demonstrate the T2T ambiguity resolution approach using LEO and ground GPS measurements. We show that LEO T2T ambiguity resolution leads to an optimal combination of LEO and ground GPS measurements and thus opens doors to form a network of LEO satellites in space for the determination of combined GNSS/LEO terrestrial reference frame parameters. This is possible thanks to the connected LEO ambiguities over all tracking passes (about 16 ambiguities per day per GPS satellite). Hence double-differences between a LEO satellite and ground stations are connected, reducing the number of zero-difference or double-difference ambiguities with the ground IGS network by nearly 95%.

The same Track-to-Track (T2T) ambiguity resolution approach based on carrier-phase measurements could be applied to double-differences. Biases in the double-differences that are common and repeat from one GPS tracking pass to another tracking pass (e.g., multipath effects, orbit errors, etc.) will be removed when forming differences of double-difference ambiguities between consecutive tracking passes. This is particularly true for the narrow-lane ambiguities where the reduction of common systematic effects between tracking passes will significantly improve ambiguity resolution. In this way reducing the effects like near-field multipath and orbit errors, that repeat in a similar way from the track to the track.