Showing papers by "Christoph Dahle published in 2015"

A Geocenter Time Series from a Combination of LAGEOS and GRACE Observations

Abstract: The geocenter motion can be inferred by evaluating Satellite Laser Ranging (SLR) observations to the LAGEOS satellites. Within the dynamic orbit determination process the degree 1 coefficients of a spherical harmonic expansion of the Earth’s gravity field are estimated. In a combined approach covering the years 2006–2011, GRACE mission GPS, K-band inter-satellite range-rate, and SLR observations are added to the LAGEOS solution via normal equations to examine possible improvements of the geocenter estimates. The particular effects on the estimates by each of the GRACE observation types are analyzed and the combined solutions are assessed and discussed. It turns out that adding GRACE data degrades the LAGEOS geocenter time series while at the same time consuming considerable computational resources.

The Effect of Pseudo-Stochastic Orbit Parameters on GRACE Monthly Gravity Fields: Insights from Lumped Coefficients

Abstract: The official GFZ RL05 monthly GRACE gravity models were processed in a two-step approach. In the first step the orbits were determined. In the second step corrections to the gravity field parameters were estimated, while the orbits were kept fixed. This led to a significant de-noising of the resulting monthly models, but accidentally also to a regularization, i.e., the estimated gravity field coefficients were biased towards the a priori model. We compare the GFZ RL05 models to a revised version RL05a that was determined in a common estimation of orbit and force model parameters. A large number of gravity field coefficients is significantly affected. We relate this effect to the one-hourly stochastic accelerations estimated for orbit determination, and to ignoring the correlations.

Gravity Field Mapping from GRACE: Different Approaches—Same Results?

Abstract: GFZ as part of the GRACE Science Data System (SDS) is routinely processing time-variable global gravity field models on monthly and weekly basis throughout the whole GRACE mission period. These operational products consist of spherical harmonic coefficients which are calculated based on the so-called dynamic method, i.e. integration of variational equations. As a matter of fact, these coefficients are imperfect due to different error sources such as inaccurate background models, instrument noise and inhomogeneous sampling and thus have to be filtered during post-processing in an appropriate way. Nevertheless, the current release named GFZ RL05 shows significant improvements compared to its precursors with an average error level of only about a factor of 6 above the pre-launch estimated baseline accuracy.

Impact of Ionosphere on GPS-based Precise Orbit Determination of Low Earth Orbiters

Abstract: Deficiencies in gravity fields derived from the orbital trajectories of Low Earth Orbiting (LEO) satellites determined by GPS-based Precise Orbit Determination (POD) were identified in recent years. The precise orbits of the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) mission are, e.g., severely affected by an increased position noise level over the geomagnetic poles and spurious signatures along the Earth’s geomagnetic equator. This is illustrated in Figure 1, showing the carrier phase residuals of a reduced-dynamic orbit determination for GOCE in m, binned to the ionospheric piercing points at 450 km altitude (Jäggi et al., 2015a). The degradation of the orbits directly maps into the gravity fields recovered from these orbits.