CGSIC Bulletin: GPS-III Satellite Group Delay, Phase Center and Inter-Signal Bias Data

Lockheed Martin Space has released the GPS-III Satellite phase center and inter-signal bias data for SVN-74 and SVN-75.  The information has been posted on the U.S. Coast Guard Navigation Center’s website underneath the IIR/IIR-M antenna pattern data files.

The phase center and inter-signal bias data included in this new release provides additional information that supplements the antenna gain pattern data previously available.

The phase center offset data locates the electrical center of the GPS transmit antenna. This data set also includes the inter-signal corrections as measured in the factory.  The modernized satellites provide multiple civil signals in addition to the legacy L1 C/A signal, enabling robust dual-frequency or triple-frequency receiver tracking. These techniques can, for example, help eliminate ranging errors due to ionospheric path delays.  But proper operation of a multi-frequency receiver requires accommodation of inter-signal biases that arise from differences in the transmit time of each signal type.

Note that the GPS-III SVs also broadcast the Inter-Signal Corrections (ISCs) in the various LNAV/CNAV messages in accordance with all the external IS/ICDs.  The value that is being broadcast by the on-orbit constellation is not the factory measured ISCs but the ISCs estimated on-orbit by the Stanford Research Institute (SRI).