Refinement of observation requiremets for GOCE, Task 2


Task 2, Results and Status.


Work has started Nov. 1, 1999 on Task 2.

A gravity field simulator has been developed, and is now being tested. A fortran program harmexg.f is used for the simulation.

Coefficient error-estimates for GOCE have been received from P.Visser, TUDelft. Thanks ! (Ref.: GOCE End-to-End Performance Analysis, ESTEC Contr. No. 12735/98/NL/GD, 1998).

Problems have occurred when using error estimates of spherical harmonmic models, because the coefficient errors do not depend on the local gravity field variation. For details. Comments have been received by Gabriel Strykowski and P.Visser. Thanks.

Problems have occurred in the subroutine gpotdr, which sums series by Clenshaw summation, when used in harmexg for degrees larger than 1200. The problem especially shows up for high latitudes (abs(lat) > 60 deg.). Programs provided by H.-G. Wenzel have then been used instead (for the evaluation of GPM98A.MOD, GRM3A.MOD and GTM3A.MOD). Note 1998-11-19 by cct.

In order to account for the local gravity field variation, the ideas described in the note on simulated GOCE errors were implemented in the program harmexg. Accordingly the error-standard-deviations per degree and order (cenm, senm) are optionally multipied by a scale-factor (scf) or multiplied with a linear function, 1 at degree 0 and 2*scf at the maximal degree. The scale-factor is computed as the ratio between the gravity variance calculated from (cenm, senm) and the gravity variance calculated for the area in question. In case the program is not used to generate values for an area (but e.g. only for a point), the variance may be taken from a table of variances computed for 3 degree equal-area blocks covering the Earth.

Grid-files have been generated and stored in a directory esafig, and the names are constructed so they reflect the error-model, the area and whether it is geoid (geo) or gravity anomalies (gra).
(1) EGM96 (egm), (2) GOCE-errors (goce), (3) scaled GOCE-errors (goce+lc) and (4) linearily modified GOCE errors (goce+ll)
Areas: (1) Mediterranean Region, (med) (2) Himalay-region (him) , (3) Antarctic (arc) and (4) Indian Ocean (ind).
Selected figures:
Mediterranean region, Geoid variation from EGM96
Mediterranean region, Geoid variation from GOCE
Mediterranean region, Geoid variation from GOCE, constant scaling
Mediterranean region, Geoid variation from GOCE, linear scaling
Mediterranean region, Gravity variation from EGM96
Mediterranean region, Gravity variation from GOCE
Mediterranean region, Gravity variation from GOCE , constant scaling
Mediterranean region, Gravity variation from GOCE , linear scaling
Topography in Region
Himalaya region, Geoid variation from EGM96
Himalaya region, Geoid variation from GOCE
Himalaya region, Geoid variation from GOCE, constant scaling
Himalaya region, Geoid variation from GOCE, linear scaling
Himalaya region, Gravity variation from EGM96
Himalaya region, Gravity variation from GOCE
Himalaya region, Gravity variation from GOCE , constant scaling
Himalaya region, Gravity variation from GOCE , linear scaling
Topography in Region



The status as of 1998-11-23 was presented to Task 3 representatives, see the minutes .

The problem of a non-homogeneous gravity field was described to the GOCE MAG on 1998-12-14 at ESTEC.

Last update 1999-06-05 by CCT , e-mail: cct@gfy.ku.dk