Circular Letter SC7: Satellite Gravity Field Missions SSG 2.193: Gravity Field Missions: Calibration and Validation

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1 Circular Letter SC7: Satellite Gravity Field Missions SSG 2.193: Gravity Field Missions: Calibration and Validation Many groups around the world are working hard to develop software for analyzing satellite born gravity field observations, as satellite gravity gradiometry observables, satellite-to-satellite tracking data, either in the high-low or in the low-low mode. Those groups that are responsible for these missions are already well-prepared for data processing, of course. But many others are still busy with working on analysis procedures and software developments. Indeed, there will be only a couple of years and we are confronted with a huge number of data. There are various approaches for global and regional gravity recovery procedures, space-wise, time-wise, etc. some of them are using spherical harmonics, wavelets, covariance functions or any other space-localizing gravity field representations etc.. Another problem closely related to the recovery procedure are the topics "calibration" and "validation", but also data combination with terrestrial data or any already existing data sets. To provide a simple platform for any scientist or for groups of scientists to check his/their own developments or to compare the effectiveness of their procedures to the procedures of others it seems to be useful to provide a unique data set to the international community, which can be used to compare the various analysis procedures. This is the reason that we decided to provide simulation scenarios for future satellite born gravity field missions. We computed for a time period of 30 days the orbits of CHAMP, GRACE1 and 2, GOCE and of 24 GPS satellites including the velocities, accelerations and for GOCE the tensor components for specified gravity fields and reference frame specifications. To start with, the models are very simplified, e.g. there is no noise on the data. It is intended to provide more specific error models later on when we receive response to our ideas. The simulation scenarios are specified in the data sheet given below. It is planned to compare global and/or regional recovery techniques, spherical harmonics (each parameter and degree variances) and gravity functionals in (geographic) blocks (center point and mean block values), gravity functionals in (geographic) blocks (center point and mean block) values in the region specified in the data sheet. We can specifiy and modify the test procedures after we receive your response and your comments. It is important that many groups and individuals express their interest and their willingness to paticipate in the comparisons. If there is sufficient response and interest we will prepare more sophisticated simulation material especially more advanced error models. Any comment is very welcome. It would be nice to have first results of these activities occasional the coming IAG assembly in Budapest but latest at the IUGG General Assembly. The simulation material is available in packed form on two CD-roms. You may receive it after demand. Pieter Visser Jürgen Kusche Karl Heinz Ilk

2 Data sheet for the simulations scenarios: General characteristics: PSEUDO-REAL-GRAVITY FIELD FOR GOCE, GRACE, CHAMP: EGM96 (COMPLETE UP TO DEGREE 300) PSEUDO-REAL-GRAVITY FIELD FOR GPS-SATELLITES: EGM96 (COMPLETE UP TO DEGREE 36) REFERENCE GRAVITY FIELD: OSU91 (COMPLETE UP TO DEGREE 36) MISSION PERIOD: 30 DAYS DATA SAMPLING RATE 0.2Hz, CORRESPONDING TO AN INTEGRATION STEP SIZE OF 5 SEC RELATION BETWEEN INERTIAL SYSTEM AND EARTH FIXED SYSTEM: siderial time[rad]= T[days]* D0* D-5[rad/sec] D0[rad] T[0,30days] T[days]:=TJU[Julian date] JULIAN DATE OF , 0h: GRAVITY GRADIENT GIVEN IN AN EARTH POINTING SYSTEM (local orbital system: tangential, normal to the osculating plane, and earth pointing building an orthogonal triad) RECOVERY AREA: GLOBAL COMPUTATIONS: SPHERICAL HARMONICS UP TO DEGREE 300 REGIONAL COMPUTATIONS: POINT VALUES (ANOMALIES, GEOID HEIGHTS) WITHIN 1 x1 -COMPARTMENTS, RECOVERY AREA: PHI[-24,+43 ],LAMBDA[57,132 ] Initial values for the orbits: DATE(DAY,MONTH,YEAR) TIME(HOURS,MINUTES,SECONDS) SATELLITE SEMIMAJOR AXIS (KM) ECCENTRICITY INCLINATION (rad) LONGITUDE OF THE ASCENDING NODE (rad) ARGUMENT OF PERIASTRON (rad) MEAN ANOMALY (rad) CHAMP E E E GRACE_ E E E GRACE_ E E E+00

3 GOCE E E E+00 GPS_ E E E E E E GPS_ E E E E E E+01 GPS_ E E E E E E GPS_ E E E E E E GPS_ E E E E E E GPS_ E E E E E E GPS_ E E E+00

4 E E E+01 GPS_ E E E E E E GPS_ E E E E E E GPS_ E E E E E E+01 GPS_ E E E E E E GPS_ E E E E E E+01 GPS_ E E E E E E GPS_ E E E E E E GPS_ E+05

5 E E E E E+01 GPS_ E E E E E E+00 GPS_ E E E E E E+00 GPS_ E E E E E E GPS_ E E E E E E+00 GPS_ E E E E E E GPS_ E E E E E E GPS_ E E E E E E+01

6 GPS_ E E E E E E+01 GPS_ E E E E E E+00 GPS_ E E E E E E+01