An apparatus for calibrating a global navigation satellite system, GNSS, receiver system for group delay errors in GNSS transmitter signals, the apparatus comprising: a controlled reception pattern antenna, CRPA, comprising a plurality of receiving elements (12, 14) and configured to sense a plurality of signals from a plurality of GNSS transmitters using one or more transmitter frequencies; a signal conditioner configured to convert the plurality of sensed GNSS signals to a plurality of time domain digital signals; a processor configured to: convert the plurality of time domain digital signals into a plurality of filtered signals, one for each of the plurality of signals from the plurality of GNSS transmitters, using a plurality of beamforming signal weights; access group delay calibration correction data for combinations of the plurality of receiving elements, a plurality of transmitter orientations with respect to the CRPA, and the one or more transmitter frequencies; track code modulations of the plurality of filtered signals and convert the filtered signals into GNSS pseudorange measurements for the plurality of GNSS transmitters at the one or more transmitter frequencies; combine the group delay calibration correction data and the plurality of beamforming signal weights to generate weighted pseudorange error corrections for the plurality of GNSS transmitters at the one or more transmitter frequencies; and combine the weighted pseudorange error corrections and the GNSS pseudorange measurements to generate corrected pseudorange measurements for the plurality of GNSS transmitters at the one or more transmitter frequencies. a controlled reception pattern antenna, CRPA, comprising a plurality of receiving elements (12, 14) and configured to sense a plurality of signals from a plurality of GNSS transmitters using one or more transmitter frequencies; a signal conditioner configured to convert the plurality of sensed GNSS signals to a plurality of time domain digital signals; a processor configured to: convert the plurality of time domain digital signals into a plurality of filtered signals, one for each of the plurality of signals from the plurality of GNSS transmitters, using a plurality of beamforming signal weights; access group delay calibration correction data for combinations of the plurality of receiving elements, a plurality of transmitter orientations with respect to the CRPA, and the one or more transmitter frequencies; track code modulations of the plurality of filtered signals and convert the filtered signals into GNSS pseudorange measurements for the plurality of GNSS transmitters at the one or more transmitter frequencies; combine the group delay calibration correction data and the plurality of beamforming signal weights to generate weighted pseudorange error corrections for the plurality of GNSS transmitters at the one or more transmitter frequencies; and combine the weighted pseudorange error corrections and the GNSS pseudorange measurements to generate corrected pseudorange measurements for the plurality of GNSS transmitters at the one or more transmitter frequencies. convert the plurality of time domain digital signals into a plurality of filtered signals, one for each of the plurality of signals from the plurality of GNSS transmitters, using a plurality of beamforming signal weights; access group delay calibration correction data for combinations of the plurality of receiving elements, a plurality of transmitter orientations with respect to the CRPA, and the one or more transmitter frequencies; track code modulations of the plurality of filtered signals and convert the filtered signals into GNSS pseudorange measurements for the plurality of GNSS transmitters at the one or more transmitter frequencies; combine the group delay calibration correction data and the plurality of beamforming signal weights to generate weighted pseudorange error corrections for the plurality of GNSS transmitters at the one or more transmitter frequencies; and combine the weighted pseudorange error corrections and the GNSS pseudorange measurements to generate corrected pseudorange measurements for the plurality of GNSS transmitters at the one or more transmitter frequencies.