RTNET® (Real Time NETwork processing Engine)
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 is designed for processing of GPS networks with the highest possible accuracy. Besides the careful modeling of all effects that may influence the GPS measurements an important feature of the RTnet model is the correct handling of the measurement dependencies (RTnet works with the full variance-covariance matrices and not only with their baseline-related parts).
- RTnet is primarily designed for real-time applications (however, post-processing is possible, too). In order to achieve high numerical stability of the estimation process a square-root filter is used instead of a simple Kalman filter.
- RTnet can estimate a large number of parameter types. In addition to the ambiguities and station coordinates the list of parameter types includes the receiver and satellite clock corrections, parameters that characterize the ionospheric and tropospheric biases etc.
- Rtnet can process data in network mode or in PPP mode in real-time or post processing.
- Real-time estimation of satellite clocks for regional or global networks is possible without the requirement of a stable reference clock (the average of all satellite clocks in view from the network is used as reference).
- Real-time results from one master Rtnet run (i.e. satellite clocks or tropospheric parameters) can be read directly on input by other slave rtnet runs.
- Processing was tested up to 10 Hz data rate.
- In addition to its original main purpose (precise network deformation monitoring) RTnet is capable to estimate DGPS corrections and generate virtual reference station data and FKP corrections for the network region.
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.