Between-satellite ambiguity resolution based on preliminary GNSS orbit and clock information using a globally applied ambiguity clustering strategy.

The use of undifferenced (UD) processing schemes of GNSS measurements is becoming more and more popular for the generation of global network solutions (GNSS orbits and clock products) within the GNSS community. As opposed to classical processing schemes, which are based on a two-step approach where the orbits (generally, the contributions to the observation geometry) are estimated in a double-difference (DD) scheme while leaving the estimation of the corresponding clock information (and other linear terms) to a second, independent UD procedure where the orbits are introduced as known, the newer designs combine both parts into a single, compact processing scheme. Although this offers a higher flexibility, some challenges arise from the handling of the many parameters, as well as from the implementation of robust ambiguity resolution (AR) strategies. The latter could lead to a prohibitive computational time for a growing size of the network due to the large amount of ambiguity parameters. To overcome that issue, we propose a new UD-AR strategy that adapts the DD-AR approach. This is accomplished by carefully inspecting the real-valued ambiguities in a stand-alone step, where the DD-AR information is explicitly considered through the use of ambiguity clusters. As a result, the preliminary satellite orbits and clock corrections are modified to become consistent with the integer-cycle property of the carrier phase ambiguities, allowing to resolve them as integer numbers in a computationally inexpensive station-wise parallelization. This strategy is introduced and explained in detail. Moreover, it is shown that the GPS and Galileo solutions generated by this procedure are at a competitive level compared to classical DD-based solutions

On the relation of GNSS phase center offsets and the terrestrial reference frame scale: a semi-analytical analysis

Phase center offsets (PCOs) of Global Navigation Satellites Systems (GNSS) transmit antennas along the boresight axis introduce line-of-sight dependent range changes in the modeling of GNSS observations that are strongly correlated with the estimated station heights. As a consequence, changes in the adopted PCOs impact the scale of GNSS-based realizations of the terrestrial reference frame (TRF). Vice versa, changes in the adopted TRF scale require corrections to the GNSS transmit antenna PCOs for consistent observation modeling. Early studies have determined an approximate value of alpha=-0.050 for the ratio of station height changes and satellite PCO changes in GPS orbit determination and phase center adjustment. However, this is mainly an empirical value and limited information is available on the actual PCO-scale relation and how it is influenced by other factors. In view of the recurring need to adjust the IGS antenna models to new ITRF scales, a semi-analytical model is developed to determine values of \alpha for the four current GNSSs from first principles without a need for actual network data processing. Given the close coupling of satellite boresight angle and station zenith angle, satellite PCO changes are essentially compensated by a combination of station height, zenith troposphere delay, and receiver clock offset. As such, the value of alpha depends not only on the orbital altitude of the considered GNSS but also on the elevation-dependent distribution of GNSS observations and their weighting, as well as the elevation mask angle and the tropospheric mapping function. Based on the model, representative values of alpha_GPS=-0.051, alpha_GLO=-0.055, alpha_GAL=-0.041, and alpha_BDS-3=-0.046 are derived for GPS, GLONASS, Galileo, and BeiDou-3 at a 10° elevation cut-off angle. These values may vary by dalpha~0.003 depending on the specific model assumptions and data processing parameters in a precise orbit determination or precise point positioning. Likewise changes of about +/-0.003 can be observed when varying the cut-off angle between 5° and 15°