On Reducing Temporal Aliasing with Multi-Satellite Formations
Murböck, Michael¹; Pail, Roland¹; Reubelt, Tilo²; Sneeuw, Nico²; Gruber, Thomas¹; Daras, Ilias^1
1TU München, Institute of Astronomical and Physical Geodesy, GERMANY; ²Universität Stuttgart, Geodetic Institute, GERMANY

The monitoring of the temporal variations of the Earth's gravity field is an important task. This monitoring allows for estimating hydrological and glaciological temporal mass variations. With Satellite-to-Satellite tracking (SST) missions this can be done on a global scale. One of the largest restrictions of such a mission is temporal aliasing from both tidal and non-tidal mass variations. The goal of this work is to show the possibility of reducing temporal aliasing by double low-low SST formations.

Closed-loop simulations of double and multi SST pairs on polar and inclined orbits are performed based on full normal equations and including realistic error models. Fully normalized spherical harmonic (SH) coefficients are estimated together with their formal errors. The observations are computed on Kepler orbits in terms of gravitational acceleration differences along the line of sight between the two satellites of each pair. The background models contain static gravity, ocean tides and mass variations from the atmosphere, non-tidal ocean, continental hydrology, and ice. Corresponding error information is applied as well. The main results then are the differences of the estimated mean fields to the mean reference representing the temporal aliasing effects and all other error sources depending on the SH degree. Furthermore, observation noise is applied as stochastic time series and adequate ARMA filter models are used to de-correlate it before inversion.

Temporal aliasing from under-sampling of tidal and non-tidal mass variations leads to large errors around specific resonance SH orders. These are bands of SH coefficients with the order around the number of revolutions per nodal days and integer multiples of it (for geodetic satellites between 15 and 16). The magnitude on each SH order band of these resonances is mainly depending on the semi-major axis and therefore on the revolution time. It is analyzed how the combination of SST pairs on different altitudes can reduce the resonance effects of temporal aliasing.