Processing of the GNSS radio occultation data within the GNSS Atmosphere Sounding Project (ATMO) requires availability of the precise orbits of the GPS and Low Earth Orbiting (LEO) satellites.
The application of the radio occultation products for the operational Weather Prediction Services requires additionally generation of the satellites orbits with very low latency.
To fulfill the above requirements GFZ has developed and operates two orbit determination systems.
The first one, the so called Rapid Science Orbit (RSO) determination system produces precise orbits with 1 day delay which are used for off-line validation and gap-less generation of the radio occultation products (vertical profiles of bending angles, temperature and humidity) .
The second one, the Near-Real Time (NRT) orbit determination system generates the orbits with a frequency of one satellite revolution (approximately every 1.5 hours) and latency of 15-30 min which are prerequisite for operational generation of the radio occultation products in Near-Real Time.
Currently the NRT and the RSO processing systems deliver orbits for all GPS satellites and for the LEO satellites CHAMP, GRACE A/B, SAC-C, COSMIC 1-6, TerraSAR-X and TanDEM-X. The orbits are stored in the GFZ Information System and Data Center (ISDC) and are freely available for the scientific community world-wide.
The Rapid Science Orbit system was developed first in 2001 for the CHAMP mission to generate orbits on a daily basis and to enable processing of the radio occultation data collected by the CHAMP on-board receiver (see GPS radio occultation with CHAMP).
The RSO system generates the satellite orbits from GPS data using the GFZ's “Earth Parameter and Orbit System – Orbit Computation” (EPOS-OC) software in the so-called two-step approach.
In the first step the GPS orbits and clock biases are estimated using a globally distributed network of ground stations. In the second step, the estimated GPS orbits and clocks are used as fixed in the subsequent estimation of the orbits of the Low Earth Orbiting (LEO) satellites.
The LEO orbits generated by the RSO system are validated by independent Satellite Laser Ranging (SLR) observations and obey an accuracy of approximately 5 cm.
The RSO system was extended by the SAC-C satellite in 2003, the GRACE A/B satellites in 2004 to enable processing of the occultation data from these satellites (GRACE RO), the COSMIC 1-6 satellites (2006) (COSMIC-RO), TerraSAR-X in 2007 (TerraSAR-X RO) and TanDEM-X in 2010 (TanDEM-X RO).
The RSO system can easily be extended by other satellite missions. The continuous availability of the orbits is accured by human interaction/repair has to performed if the orbits can not be computed automatically due to data or quality problems.
To enable assimilation of the satellite radio occultation products in the Weather Prediction Systems, a Near-Real Time orbit determination system for CHAMP and GRACE was developed and became operational in June 2006.
The NRT processing system, similarly to RSO, is also using the two-step approach for the LEO orbit determination.
The frequency of the NRT orbits, in contrast to RSO, is determined by the frequency of the LEO data 'dumps' (downloads) at S-band receiving staions. F.i. by employing our station in Ny Alesund dumps arrive approximately once per satellite revolution. Once a new portion of data is available, the NRT system generates a new orbit. The latency of these orbits is very low and amounts to 15-30 minutes after the last epoch of the data used in the processing. The GPS NRT orbits, needed for the two step approach, are generated every 15 minutes with a latency of 10 minutes to be available at any time of the LEO dump.
The NRT system contains three subsystems with different sources of the GPS orbits (estimated in-house from ground data or retrieved from the International GNSS Service) what increases redundancy and reliability of the orbit products.
The system was used to generate CHAMP orbits till the very end of the mission in October 2010, for SAC-C satellite it was activated for validation purpose in a period November 2010 - August 2011.
For the GRACE-A satelite the NRT orbits are continuously generated since August 2006.
The NRT system for TerraSAR-X was activated in August 2007 (see also TerraSAR-X RO), for TanDEM-X (TanDEM-X RO) on 24 June 2010, 3 days after the launch of the satellite, what demonstrates the ability of the system to rapidly include new satellites in to the processing chain.
The accuracy of the NRT orbits depends on the NRT subsystem and is in the range of 5-10 cm obtained from the SLR validation. The system is fully automatic, needs offline and postfact human monitoring and maintenance.