On Reducing Temporal Aliasing with μm-High-Low SST
Murböck, Michael; Pail, Roland; Gruber, Thomas; Reußner, Elisabeth
TU München, Institute of Astronomical and Physical Geodesy, 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 SST missions based on links between low Earth orbiters (LEO) and GNSS or even geostationary satellites with ìm-accuracy. This measurement principle and its implementation were analyzed in the GETRIS project (ESA AO/1-6311/2010/F/WE) realizing a Geodesy and Time reference in space.
Closed-loop simulations of such ìm-high-low SST missions 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 for these models 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 order bands. The main advantage of high-low SST links is a large content of radial information in most of the SST observations, in contrast to low-low SST as on GRACE with mainly along-track and only very few radial information. This superior measurement geometry together with the possibility of combining several such high-low SST links with relative low technical effort shows the large potential of this gravity mission concept. This concept is analyzed in detail comparing it also with some basic future low-low SST concepts.