IMPACTS OF THERMOSPHERE AND IONOSPHERE ON DATA OF LOW ORBITING GEODETIC PRECISION SPACE MISSIONS

Geodetic stellite gravity missions have increased accuracy from mission to mission over the past decade. For future gravity field missions a laser ranging instrument between two satellites, similar to GRACE's (Gravity Recovery and Climate Experiment) K-Band ranging system, is planned. This sets new requirements to the satellites Attitude and Orbit Control System (AOCS) as the satellites needs to be in a very specific attitude. Therefore, the external influence by Earth' thermosphere and ionosphere onto geodetic Low Earth Orbit (LEO) satellites needs to be well known and determined.

In our study we focus on these impacts of thermosphere and ionosphere onto GOCE (Gravity field and steady-state Ocean Circulation Explorer). GOCE uses a gradiometer consisting of 6 accelerometers, pairwise mounted on three perpendicular axes. This way not only linear accelerations can be observed, but also angular accelerations. Dominant angular accelerations can be observed in the polar region and around the magnetic equator. It can be shown that these rotations find their way into the gravity gradients (GG) determined by GOCE. We also want to describe these rotations and propose a way to reduce and/or eliminate them within the GG via a modification of the inverse calibration matrices (ICMs), which are used for an optimal calibration of the gradiometer.

Moreover, ionospheric and magnetical influences can also be found by analyzing disturbing signals, so-called 'twangs', within GRACE accelerometer data. Analysis of these signatures reveal a temporal and spatial distribution.