Abstract:
In this study an evaluation of the satellite laser ranging (SLR) errors associated with pressure sensor height offsets was conducted. Site log sheets from satellite ranging stations were retrieved from the International Laser Ranging Services (ILRS) and examined. It was noted that some log sheets were updated more than a decade ago. In order to ascertain and assess the accuracy of height offsets between the pressure sensor and the telescope invariant point (IVP), an electronic survey was conducted. The feedback received was compared with the site log sheet data and some discrepancies were noted. Furthermore, in order to determine the effect of pressure variations on the range bias, simulations were performed on the same dataset but with different barometric pressure values. This was accomplished by adjusting pressure values in the source code of the analysis software, the Satellite Data Analysis Software (SDAS), before each run. The SDAS was developed by Prof. Ludwig Combrinck at the Hartebeesthoek Radio Astronomy Observatory (HartRAO), South Africa. The focus was to examine the standard deviation of the Observed minus Computed (O-C) results where it was noted that each alteration of pressure caused a variation of the O-C residuals. The differences of pressure sensor height offsets (pressure as a function of height) and SLR range biases were characterized across the SLR network and the station range bias (Rb) examined to determine if there is any correlation with the O-C residuals whenever pressure values changed. Overall, the analysis illustrated that, while the current atmospheric models are robust and capable of achieving sub-millimetre level accuracy, it is crucial to put more emphasis on the site activities that, if unattended will contribute to the ranging errors. It is vital to monitor constantlythe stability of pressure sensors. For example, it was noted in May 2019 at the Geodetic Observatory Wettzell, that their instrument had developed pressure drifting of -0.025 hPa/year. The Wettzell incident underscores the need for regular calibration of pressure instruments especially those that have been in service for more than a decade. Further, the site log sheets ought to be updated regularly and stations that reported estimated height offsets should be encouraged to measure them accurately. Additionally, the height of a meteorological instrument is currently ambiguous and ought to be explicitly stated.
