Time-variable gravity fields (RL01)

The first release of CNES/GRGS gravity field models (30-day models).

Spherical harmonic coefficients of 30-day gravity field models

These are 30-day gravity field models from SH degree 2 up to SH degree and order 50, given every 10 days, using GRACE and LAGEOS data (in ITRF2000). These 30-day solutions are given every 10 days because they are based on the addition of three successive 10-day normal equations (NE) according to the following weighting scheme (first processing):

30-day solution (n : n+29) is computed with

  • 0.5 * 10-day NE(n:n+9)
  • + 1.0 * 10-day NE(n+10:n+19)
  • + 0.5 * 10-day NE(n+20:n+29)

The next 30-day solution will be computed according to the same scheme, shifted by 10 days, etc. Therefore two successive solutions contain in part common data. The aim of this process is not to produce monthly fields, but to have a 10-day resolution, smoothed on the left and the right by underweighted data (better inversion with less constraints, and less high frequency noise). In release 2, we produce purely 10-day solutions.

Important notice

Starting from September 1st 2007, there is a shift of static reference field in the version 1 series, (i.e. starting from solution « 2007-09-01_2007-09-30 » or « 2007-244_2007-273 » or « 21062_21091 »). Before this date the static reference field is EIGEN-GL04S , after this date it is EIGEN-GL04C . Users will notice a jump in the CNES/GRGS solutions if they do not change the gravity field used as a reference at this date, and instead use the same reference field for the whole « version 1 » set.

It is also reminded to the users of the GRGS products that NO SMOOTHING OR FILTERING is necessary when using them, since they have already been stabilized during their generation process.

Missing solutions

There are some gaps in this time series, coming either from GPS, from accelerometers or from KBR data gaps. Nine monthly solutions are missing:

Dates (yyyy-mm-dd) Julian days 1950 Dates yyyy-doy
2002-11-26 -> 2002-12-25 19322 -> 19351 2002-330 -> 2002-359
2002-12-06 -> 2003-01-04 19332 -> 19361 2002-340 -> 2003-004
2002-12-16 -> 2003-01-14 19342 -> 19371 2002-350 -> 2003-014
2002-12-26 -> 2003-01-24 19352 -> 19381 2002-360 -> 2003-024
2003-01-05 -> 2003-02-03 19362 -> 19391 2003-005 -> 2003-034
2003-01-15 -> 2003-02-13 19372 -> 19401 2003-015 -> 2003-044
2003-01-25 -> 2003-02-23 19382 -> 19411 2003-025 -> 2003-054
2003-05-25 -> 2003-06-23 19502 -> 19531 2003-145 -> 2003-174
2003-06-04 -> 2003-07-03 19512 -> 19541 2003-155 -> 2003-184

Grids, images and movies

Grids, images and movies of geoid and equivalent water heights differences with static field are provided. The grids are 1° x 1° global grids of geoid height or equivalent water heights differences (in meters) between time-variable solutions and static field(at reference date if the static field includes time-variable terms). The images represent geoid or equivalent water heights differences (in millimeters) between time-variable solutions and static field (at reference date if the static field includes time-variable terms). Images are compiled in geoid or EWH movies.

Dealiasing products

The files provided here contain the so-called « dealiasing products » used by CNES/GRGS, temporally averaged in a manner consistent with the way the time-variable gravity field models are produced. In the processing of the GRACE data, the gravitational influence of the Earth atmospheric pressure on the satellites’ orbits, as well as the oceanic response to it, is taken into account in the form of ECMWF 3-D atmospheric pressure fields over the whole Earth every 6 hours (with linear interpolation in-between) and MOG2D model of the response of the oceanic domain to the atmospheric forcing (pressure and wind) at the same temporal resolution. A conversion from Pascal to normalized gravitational potential units is performed.

In the first release, the files provided here are a 30-day average of those 6-hour files, computed in the following manner :

  • weight .25 for the first 10 days of the 30-day period ;
  • weight .50 for the middle 10 days of the 30-day period ;
  • weight .25 for the last 10 days of the 30-day period.

It is consistent with the way the daily GRACE data are assembled in the 30-day models produced at CNES/GRGS.

Reference

Reference paper:Lemoine, J.-M., Bruinsma, S., Loyer, S., Biancale, R., Marty, J.-C., Perosanz, F., Balmino, G., 2007. Temporal gravity field models inferred from GRACE data, Advances in Space Research 39 (2007) 1620-1629, doi:10.1016/j.asr.2007.03.062