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 dimension 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 an 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. ¿ American Geophysical Union 1992