To generate the highly precise ephemerides of Global Positioning System (GPS) satellites necessary for modern geodetic applications, one must have an accurate force model that includes the pressure of solar radiation and spacecraft thermal emission. We present the dimensions and optical parameters of Block I and Block II GPS satellites, show how they are used to form the models of the solar force, and compare predictions of these models with values estimated from tracking data. Simple approximating functions are given for the solar/thermal radiation pressure, and the problem of estimating a smaller, unmodeled force called Y bias is discussed. A simple model is given for the effect of earthshine on GPS spacecraft. Two kinds of discrepancy are noted between model predictions and actual performance. For Block II satellites, beginning soon after launch, a transient force decaying exponentially with time, perhaps due to outgassing, aliases into estimates of the solar force. Also, at the beginning of eclipse season and for at least some satellites, Y bias estimates vary rapidly for reasons not presently understood. Therefore the geodetic worker should not only use the best available force models but also should avoid using a satellite in those critical time periods when unexplained model errors are present.

Fundamental to NAVSTAR Global Positioning System (GPS)

From 20 October to 17 November 1986, three Contracting and three Government institutions took part in a cooperative evaluation of the accuracy of the Global Positioning System (GPS). In effect, acting as independent GPS users employing CIA-code single-frequency or P-code dual-frequency receivers, each organization computed satellite-to-user range prediction errors.Each institutional user computed statistical summaries of these User Range Errors (URE). The root-mean-square (rms) of the URE's across all satellites, and over all 30-days of the test, as reported by each user ranged from a high of 4.8 meters to a low of 3.1 meters. The C/A-code users experienced the largest rms URE. The GPS is required to provide 6 meter rms URE.For the same test period, the GPS Operational Control System (OCS) computed URE performance measures of 2.0 meters (rms) and 2.5 meters (rms). This suggests that these GPS performance measures might be improved.Throughout the 30 day test, users reported that the daily rms URE were approximately steady-state. However, just prior to the test, the rms URE did appear correlated over several days with rms values near 6 meters. This URE degradation appeared to improve as NAVSTARS 3,6, and 10 were less and less eclipsed by the Earth shadow.

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