Untitled Document

Advances in the analysis of space geodetic data

Paul Tregoning 1

1 Research School of Earth Sciences, Australian National University , Canberra , ACT 0200, Australia

In 2006 the focus of geodetic research and its applications to geophysical studies has continued to be to improve the accuracy of the analysis so that smaller geophysical signals can be detected. This required revising some of the models and assumptions used to compute the theoretical range from the ground-based receivers to the transmitting GPS satellites.

Studies showed that the accuracy of the a priori surface pressure used to compute the hydrostatic atmospheric delay was critical for estimating accurate changes in height, required for studies such as glacial isostatic adjustment of Antarctica and North America caused by the melting of the Antarctic and Laurentide ice sheets. Software was modified to permit the use of observed surface pressure, values from numerical weather models or from a recent model derived from a spherical harmonic fit to surface pressure of the ECMWF global weather model (Boehm et al., 2006). The use of more accurate a priori pressure removed height errors of up to 10 mm, with an empirical error relation of ~0.2 mm height error per 1 hPa error in pressure (Tregoning and Herring, 2006).

Sub-daily periodic variations in atmospheric pressure cause periodic elastic deformations of the surface of the Earth of up to 3 mm. If unaccounted for, such errors limit the capacity of space-geodetic techniques to identify other geophysical signals of this magnitude or smaller. The spatio-temporal variations of the diurnal and semi-diurnal atmospheric tides were characterised to try to understand why applying corrections for the associated elastic crustal deformation actually degraded the analysis (Tregoning and van Dam, 2005). From an analysis of several years of global pressure data, we demonstrated that the amplitudes of the periodic pressure variations change throughout the year, yet none of the available deformation models for use in geodetic analyses account for the temporal variation in amplitude. The research is ongoing to characterise the variations, generate new models for atmospheric tidal deformation and apply them to the analysis of geodetic data. This component of the research formed part of the Masters degree of Laurent Millet, a visiting student from Ecole Normale Superieure, Cachan , France .

Figure 1. Difference between a) surface pressure derived from "standard" sea level pressure and the mean surface pressure derived from the model based on ECMWF global pressure (Boehm et al., 2006). b) station heights using the two sources of a priori pressure. c) Relation between a priori pressure differences and height differences.

References: Boehm, J. R. Heinkelmann and H. Schuh (2006) Global Pressure and Temperature (GPT): A spherical harmonic expansion of annual pressure and temperature variations for geodetic applications, submitted to Journal of Geodesy.

Tregoning, P. and T. A. Herring (2006) Impact of a priori zenith hydrostatic delay errors on GPS estimates of station heights and zenith total delays, Geophysical Research Letters, in press.

Tregoning, P. and T. van Dam (2005) Atmospheric pressure loading corrections applied to GPS data at the observation level, Geophysical Research Letters, 32, L22310, doi:10.1029/2005GL024104 .