Towards Routine zero-difference GNSS Processing at Center for Orbit Determination in Europe (CODE)

The zero-difference (ZD) and double difference (DD) approach for processing GNSS observations is mathematically equivalent – meaning that in DD case the huge number of clock parameters are just pre-eliminated. The results for all remaining parameters are identical, if no numerical shortcut is done (e.g., ignoring parts of the correlations introduced by the DD approach). The advantage of processing DD observations is the reduced number of parameters, easier detection and potential correction of cycle slips, and direct access to the integer ambiguities without any phase bias parameter. On the other hand, ZD processing offers greater flexibility in network configuration and parameter handling. This is particularly advantageous when modifying the list of stations in the processing, including Low Earth Orbiting satellites (LEOs), or implementing advanced clock models, e.g., for Galileo satellites.Based on an experimentally developed ZD-based ambiguity resolution method that introduces ambiguity clusters and satellite-wise consistency corrections, the original research prototype was translated into a robust and automated routine processing chain which is suitable for operational use.The new procedure follows the structure of the established CODE DD strategy but adapts the individual processing steps, e.g. pre-processing, estimation of global parameters, handling of receiver-dependent parameters, and ambiguity resolution. Special emphasis is placed on numerical stability, the reliable handling of real-valued ambiguities, and the introduction of quality-control mechanisms designed for long-term autonomous operation. The resulting procedure enables efficient parallelization and delivers consistent orbit, clock, and ambiguity products. We have investigated the requirements on the station density for all these steps in order to optimize also the processing time. Stations, that are not needed in this context can get pre-processed based on the PPP approach and get added to the final solution only.Initial results show that the operational ZD processing chain reaches the accuracy and stability of CODE DD-based products while offering greater flexibility for future extensions. The results demonstrate that the ZD-based GNSS processing is sufficiently mature to generate stable global products on a daily basis and therefore represents a promising foundation for next-generation GNSS solutions computed at CODE.