Validation of GPS III antenna patterns

GPS III is the latest generation of modernized satellites of the Global Positioning System. Five GPS III satellites have already been launched between December 2018 and June 2021 and three further GPS III spacecraft are available for launch. Starting in 2019, satellite antenna phase center offsets (PCOs) for the L1, L2, and L5 frequency bands have been published by the manufacturer Lockheed Martin for the individual spacecraft launched so far. These PCOs are included in the igs20.atx antenna phase center model of the International GNSS Service (IGS). They are complemented by nadir-dependent phase center variations (PCVs) estimated by the Center for Orbit Determination in Europe (CODE) and the European Space Operations Center (ESOC). In Fall 2022, manufacturer-calibrated nadir- and azimuth-dependent directivity and phase patterns were, furthermore made available, which provide a more complete description of the antenna characteristics.

This contribution aims at a validation of these satellite antenna metadata. As a reference, satellite antenna PCOs and PCVs are estimated from L1/L2 and L1/L5 ionosphere-free linear combinations of a global network of GNSS tracking stations. These are compared to the ground calibrations of the GPS III transmit antennas obtained in an anechoic chamber. The manufacturer-provided directivity patterns for the main lobe of the GPS III transmit antennas can be validated with observations of a directional high-gain antenna. The 30 m dish antenna of the German Aerospace Center (DLR) located in Weilheim, Germany, was used to obtain equivalent isotropic radiated power (EIRP) measurements of all GPS III spacecraft. Repeated measurements are used to evaluate the measurement precision and elevation-dependent RMS differences allow for an assessment of the consistency of the EIRP measurements and the pre-flight gain calibrations.