[IGSMAIL-8113] Upcoming convention regarding PCO and biases

[Banville, Simon]

Dear IGS community,

The IGS PPP-AR working group has recently discussed the need for a convention regarding the application of phase center offset (PCO) corrections to the computation and use of biases.

Several GNSS satellites have now been assigned different PCO values on each frequency in the IGS ANTEX file. Consequently, geometry-free biases such as differential code biases (DCBs) or widelane (aka Melbourne-Wubbena) biases will take on different values whether or not PCO corrections have been applied to the observables. Furthermore, transforming these geometry-free biases into observable-specific biases (OSBs), such as the ones defined within the IGS Bias SINEX format, will again result in inconsistencies.

This issue was encountered during the IGS repro3 clock/bias combination, where some analysis centers applied the PCO corrections and others not, resulting in inconsistent biases. When users of these products also use a different convention (e.g. applying PCO corrections when the analysis center did not), estimated parameters may be affected and issues with ambiguity resolution might occur, resulting in sub-optimal solutions.

Analysis centers of the IGS have agreed that applying PCO corrections to biases is the way forward given multi-GNSS and multi-frequency signals. Therefore, all IGS analysis centers computing such biases will be required to adopt this convention. Similarly, users of IGS products will need to adopt this convention as well to ensure consistency.

This convention will be adopted at the same time as the switch to ITRF 2020, planned around the summer of 2022. This timeframe allows for several months to make the necessary changes in your software, if necessary. A follow-up email will be sent a few weeks prior to the change as a reminder.

The IGS repro3 product combination is underway, and this convention will be applied to the combined biases, ensuring that users can process long time series using a unique convention.

If you have any question or concerns, please contact me.

Best Regards,

Simon Banville, PhD
Senior Geodetic Engineer, Surveyor General Branch
Natural Resources Canada/Government of Canada
Chair of the IGS PPP-AR working group

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