The ESOC GLONASS Analysis Centre

Table of contents:

1. INTRODUCTION

2. PROCESSING STRATEGY

3. PROCESSING DESCRIPTION

4. RESULTS

• ESOC IGLOS (the extended IGEX project) products
• IGLOS (the extended IGEX project) Orbit Comparison

1. INTRODUCTION

2. PROCESSING STRATEGY

The strategy for the processing of GLONASS data was derived from the current set up that is used for the IGS products. This way the IGLOS (the extended IGEX project) processing can take advantage of the high system automation that makes possible the processing of big amounts of data with limited human and computer resources. The introduction of GLONASS processing capabilities in our software was not straight forward. A number of changes have been necessary in the preprocessing and orbit determination stages. The main ones are the following:

• The use of the GLONASS frequencies in the preprocessing program GPSOBS.
• The use of undifferenced measurements as basic observable has been the main change. Previously the processing was based on phase double-differences and satellite clock biases were estimated using the pseudoranges in a second step. The use of undifferenced data is advantageous in three ways:
• Clocks and orbits are estimated at the same time and are consistent with each other.
• More observations can be used. The limitation of the use of only the measurements which can be combined in a double difference link disappears. This approach was developed for IGLOS (the extended IGEX project) processing and was also later adopted for our IGS processing.
• The assessment of the GLONASS receivers can be much accurately characterized with this approach.
• The lack of a priori solar radiation pressure force models for the GLONASS satellites triggered the implementation of empirical models in our orbit determination package BAHN.
• The orbit determination program BAHN was modified to allow for the estimation of time dependent parameters, typically receiver and satellite clock biases.

The final strategy was fine-tuned taking into account our computer limitation resources. GPS orbits and clocks that have already been estimated for IGS are used as input data.

3. PROCESSING DESCRIPTION

3.1 PREPARATION

RINEX (Receiver INdependent EXchange format) observation and navigation files are retrieved from CDDIS and stored in our computer. A reduced number of IGS ITRF core stations are downloaded from our internal archive. The role of the IGS stations is to fix the solution to the ITRF so that GLONASS orbits and station coordinates are represented in the current ITRF representation. Seven well distributed stations are selected. Every station has several back-ups that are used in case of unavailability. ITRF core station map as of Aug 2002.

All GLONASS observation files are re-formatted with teqc (UNAVCO RINEX formatter) to correct for phase overflow problems that have been detected in some receivers.

Click for further infomation about the Station Network

3.2 PREPROCESSING

Three days of data constitute the data arc and are available on the disk for the preprocessing program GPSOBS. IGS GPS orbits and clocks from the ESA final solution are also input to GPSOBS. The ionospheric free combination is formed. An elevation cut-off angle of 10 degrees is used. Cycle slip detection is based in a propagated orbit from the previous day.

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:

• GLONASS: 0.000, 0.000, 1.620m in satellite x, y, z.
• GPS Block I: 0.210, 0.000, 0.854m in satellite x, y, z.
• GPS Block II/IIA: 0.279, 0.000, 1.036m in satellite x, y, z.
• GPS Block IIR: 0.000, 0.000, 0.000m in satellite x, y, z.

Satellite antenna corrections are applied. The output is an observation file which contains the ionospheric free phase combination plus pseudoranges. A five minutes sampling is enough for the orbit estimation. The satellite clock biases are also estimated with the same sampling ( it would be more convenient the use of all the available data with 30 second sampling but computer memory limitations do not allow this option ).

The observation file is filtered to select the passes with at least 4 observations and with a controlled noise between phase and pseudorange for every accepted pass. Phase observations of satellite gv-75 are consistently rejected because they are found to be very noisy. The orbit of this satellite is estimated only with the pseudorange and no clock bias estimations are possible.

3.3 IGLOS (the extended IGEX project) ORBIT AND GEODETIC PARAMETER ESTIMATION

The orbit determination is the next step. BAHN is a batch least-squares estimator for dynamic orbit determination and has been modified to allow for the estimation of time dependent parameters. The following parameters are estimated:

• Solar radiation pressure coefficients
• Orbital parameters:
  • Initial position and velocity
  • Five coefficients of the empirical force model
  Orbital parameters are estimated as constant through the 72-hour (or 120-hour) orbital arc.
• Undifferenced carrier phase ambiguities estimated as real values.
• GLONASS-GPS receiver bias (only for combined GPS/GLONASS receivers): a constant delay between GLONASS and GPS pseudoranges (This parameter is receiver dependent and it can be as big as 1 microsecond).
• Station coordinates: 7 IGS stations tightly constrained to the current ITRF positions (3 mm). Remaining GPS/GLONASS station positions more freely estimated (4 m).
• Tropospheric zenith path delay estimated as linear function for every station in 2-hour intervals.
• Pass biases
• Earth orientation parameters: x and y pole position and rates and LOD estimated as linear functions for 24-hour intervals. (From 1998 to 2001)
• Station and satellite clock biases: estimated as time-dependent parameters (i.e. one value for every observation time). GPS satellite clock bias values are constrained to the ESOC precise values to produce values aligned with the GPS system time. The evolutions of the drift of receivers connected to Hydrogen masers is also constrained to stabilize the clock estimates.

We use a 72-hour arc (a 120 hr arc is also possible) in order to obtain the precise orbits and clocks for each day, with 24 hours (or 48 hrs) before and after the central day. Orbits and clocks are combined to produce the sp3 files which are delivered to CDDIS.

The accuracy exponents are set taking into consideration the phase and pseudorange residuals of the orbit determination and the orbit comparison to the overlapping fit of the previous day.

The normal equations of the terrestial reference frame are combined to produce weekly SINEX files.

3.3.1 Measurement models

• Troposphere: Saastamoinen
• Ionosphere: first order term removed by using the so called ionospheric free combination.
• A-priori coordinates: ITRF values used when available, if not our best estimate.
• Plate motions: ITRF values used when available, if not Nuvel-NNR.
• Tidal displacements: Wahr model used for solid earth tidal displacement. Ocean loading tides with coefficients from the OSO model (when available). Pole tide and atmospheric loading are not modelled.
• Ground antenna phase centre calibrations: from the igs_01.pcv set
(when available).

3.3.2 Dynamic models

• Geopotential: GEM-T3 up to degree and order 8 with the GM (398600.4415 km3/s2), C21 and S21 from the IERS standards.
• Third-body forces: Sun, Moon and four planets regarded as point masses. Ephemeris from JPL DE200, GM of Sun 132712440000.0 km3/s2, GM of Moon 4902.7991 km3/s2.
• Solar radiation pressure, Click for a detailed description.
• Tidal forces: Wahr model for solid earth tides, Schwiderski for ocean tides.

3.3.3 Reference frames

• Inertial: Geocentric, mean equator and equinox of 2000 Jan. 1 at 12:00 (J2000.0).
• Terrestrial: ITRF reference frame realized through a set of 7 ITRF stations with tightly constrained coordinates.
• Interconnection: Precession, IAU 1976 Precession Theory; Nutation, IAU 1980 Nutation Theory; Celestial pole offsets from IERS Bulletin B; relation between UT1 and GMST, Aoki 1982; Pole and LOD estimated as linear functions for 24-hour intervals; sub-daily tidal variations in pole and UT1 with the Ray model.

3.3.4 Numerical integration

• Adams-Bashforth/Adams-Moulton predictor-corrector of order 8 started with a Runge-Kutta/Shanks of order 8. Integration step of 6 minutes.

3.4 POST-PROCESSING AND QUALITY CONTROL

The orbits obtained with BAHN are combined with the precise clocks and output every 15 minutes in a file with the sp3 format. Quality control is performed by checking:

• Post-fit carrier phase and pseudorange residuals per station and satellite.
• Orbit overlaps between consecutive days.

3.5 MULTI-ARC PARAMETER ESTIMATION

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:

1. Every final daily IGLOS (the extended IGEX project) BAHN run produces unconstrained normal equations for station coordinates and Earth orientation parameters. Other parameters are eliminated (supposed free) from the solution.
2. The normal equations from the seven days of the week are used by BATUSI to produce our weekly IGLOS (the extended IGEX project) SINEX file with our estimate of station positions and eops in a free network solution.

4. RESULTS

The routine processing of GLONASS data started in March 1999 with the analysis and submission of GPS week 0980 (Oct 1998), the official start of the IGLOS (the extended IGEX project) campaign. Currently we have covered the processing of the whole campaign with the delivery of results of GPS week 1039. We are continuing the processing beyond this date as long as our internal resources allow it and a meaningful network of GLONASS receivers continues in operation.

Our routine IGLOS (the extended IGEX project) products are the following:

• Daily orbits and clocks in the sp3 format b file: esrwwwwd.sp3. These are values at 15 minute intervals and include the accuracy codes.
• Weekly summaries: esrwwww7.sum.
• Weekly free network station coordinate solution and Earth orientation parameters in the SINEX format: esrwwww7.snx.
  * wwww: gps week , d: week day (0-6) , 7: weekly

Final products are available at CDDIS, grouped by gps week, in the directory:
ftp://cddisa.gsfc.nasa.gov/igex/products

We are also producing and archiving satellite clock bias files at 5 minute intervals. For this we are using our own internal format. They are available on request.

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