The software that we developed has two main components: (1) real-time -epoch-server (RTES) which reads an epoch of GPS data from GPS receivers, translates, compresses, transmits via the internet to the processing computer, uncompresses and makes data available to the real-time network processing software (RTnet). (2) RTnet takes the observation, and processes the data.
Two main features of RTES are the high compression rate that can be achieved (110 bytes per 9-satellite epoch) and high reliability due to use of what we call UDP with virtual Handshake. The UDP stream goes into a hybrid queue that is capable to serve the data as both a time critical stream and a complete data set even with network interruptions.
The main characteristics of the RTnet program may be summarized as follows:
RTnet processes zero-differenced observations and the satellite and receiver clock corrections are estimated at every epoch independently. Due to one-to-one correlation with the clock biases the phase ambiguities may be resolved on the double-difference level. The double differencing of GPS measurements is optimized every epoch and the information on the resolved ambiguities is stored.
The software can use precise GPS orbits, broadcast orbits, or both combined. For real-time applications this implies that some other process can provide predicted ultra-rapid orbits from the IGS (so called IGU orbits). The software will always use the latest predicted file. If broadcast orbits are used then these will be read from the RTES server. If both orbits shall be combined then the logic works as follows: RTnet will use the IGU orbits for all available satellites, and for those satellites for which the IGU orbits are not available RTnet will use the broadcast orbits.