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High-precision applications of GNSS require accurate calibrations to correct for phase variations of the transmitting antennas. Calibrations distributed by the International GNSS Service (IGS), based upon observations from the global network of ground stations, are often used as the source of the transmitter calibrations as they are inherently linked to a specific definition of the International Terrestrial Reference Frame. The IGS provides antenna phase variations as a function of boresight angle for each basic block of navigation satellite. To support scientific missions operating in low Earth orbit, the antenna calibrations must be extended beyond the 14-degrees limit that can be observed by GNSS receivers on the Earth. Extended antenna calibrations to accommodate low Earth orbiting satellites have already been derived for the GPS Block II satellites. This paper derives an extension solution for the new GPS IIIA antenna calibration, based on a year of observations from the Sentinel-6 Michael Freilich altimetry mission and validated using Jason-3. These solutions are inherently consistent with the IGS-provided models of the Block II antenna calibrations. We use the new model for the GPS IIIA transmitter antenna calibration extensions to evaluate the impact on precise orbit solutions of both Sentinel-6 MF and Jason-3.

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