ESOC is the satellite control centre of the European Space Agency (ESA). It is responsible for the operation of the ESA satellites, its ground stations, and its communications network. In order to operate the satellites that are under control of ESA, ESOC has to be able to determine their orbits, the position of the possible tracking stations, and other geodetic parameters.
A state of the art software package has been developed over a number of years at ESOC and before the IGS campaign started it was already well proven through extensive processing of data from many satellites, including satellite laser ranging (SLR) from Lageos and Starlette. Although at that time not able to handle GPS data types (pseudo-range and phase), a multi-satellite solution capability was already implemented. After submitting the proposal for ESOC participation as an IGS Analysis Centre a major effort was undertaken to develop GPS capabilities in our software. Important aspects of the use of the ESOC orbit and geodetic parameter estimation software are that this software is independent from other packages in use for GPS analysis, and the possibility of consistent processing of other geodetic satellite data with a single package (SLR, Doris and GPS).
ESOC is preparing for the use of GPS or other GNSS in operational and precise orbit determination. Some european spacecraft have been already being equipped with GPS receivers and it is foreseen that some ESA spacecraft will also use GPS. An additional application of GPS, of interest for ESOC, is the use of GPS receivers located in our ground stations to obtain ionospheric corrections for single-frequency ranging.
We have been participating as an IGS Analysis Centre from the beginning of the IGS. Our first solutions for orbital and polar motion parameters were transmitted to the CDDIS on 24 July 1992, about one month after the start of the Epoch 92 campaign. By early August the delay with respect to real time was reduced to about 10 days. Along with several other centres, ESOC continued to process IGS data after the decision of the IGS Campaign Committee in October 1992 to continue the IGS activity in the form of an "IGS Pilot Service" and then in January 1994 as the IGS Operational Service. These series have guaranteed continuity of the IGS activities after the success of the first campaign.
Switched from Double Differences to Undifferenced processing of GPS measurements. The capabilities were developed for GLONASS processing during the IGEX98 campaign.
The undifferenced phase and pseudorange measurement are used in the least squares estimation program with the ionospheric correction applied to P1/L1 measurements as follows:
Clock biases are estimated as time dependent parameters in the same process producing more accurate and consistent clock bias results.
ESOC runs GPS orbits in:
ESOC is using the observations of about 50 of the receivers in the IGS network. For our current IGS processing we use phase double differences as our basic observable, because they are especially well suited for batch estimation. With double differences the satellite and clock biases for every epoch do not need to be estimated with the same accuracy as that of the measurement, so the total number of parameters to be estimated is greatly reduced.
Precise clock biases are produced in post-processing, after the orbits have been determined.
Preprocessing is done with the program GPSOBS. GPSOBS reads RINEX (Receiver INdependent EXchange format) observation files and obtains independent ionospheric-free double difference phase combinations. An elevation cut-off angle of 20 degrees is used. Cycle slip detection is performed using two integer almost ionospheric free combinations, the 4L1 - 3L2 and the 5L1 - 4L2. Satellite centre of mass and phase wind-up corrections are performed at this step. For the satellite centre of mass correction the following values are used:
GPSOBS also estimates the station clock biases to correct the time tags of the measurements. Observations are output every five minutes.
This is performed using the program BAHN. BAHN is a batch least-squares estimator for dynamic orbit determination. We use a 48-hour arc in order to obtain the precise orbit and erp's for each day, with 12 hours before and after the central day.
Since the start of undifferenced measurement processing the clock bias values are estimated within the same least squares process as all the other quantities. No extra step is needed for their evaluation, as was the case when double differences were used.
Precise values are obtained every 5 minutes (every epoch used in BAHN) and are published in RINEX_clock files daily.
The orbits obtained with BAHN are combined with the precise clocks and output every 15 minutes in a file with the sp3 format. The erp's are output to a file with the IERS format. Quality control is performed by checking the following:
BAHN can produce a reduced set of normal equations for reference frame estimation. These equations can be accumulated for a number of arcs in order to obtain a multi-arc solution using our program BATUSI. Our weekly sequence for this is:
These solutions are available with a maximum delay of 10 hours since the last observations were collected. The strategy to obtain them is basically the same as for the final orbits but with an observation period of about 40 hours instead of 48.
These solutions are available one hour before the start of the UTC day and are obtained fitting the latest IGS rapid or ESOC orbits. THIS PRODUCT LINE HAS BEEN STOPPED SINCE MARCH 2001
Our IGS routine products are the following:
ESA Final (esawwwwd.*) products are available at CDDIS, grouped by gps week (wwww), in the directory:
ftp://cddisa.gsfc.nasa.gov/gps/products/
IGS Final(igs...), Rapid(igr...) and UltraRapid(igu...) products are available in the same directory.
ESA Rapid(esa...) and UltraRapid(esu...) products are available upon request.
We are also producing and archiving satellite/station clock bias files at 5 minute intervals. They are available on request.
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