Photometry of global positioning system block II and IIA satellites on orbit

Observations have been taken at the U.S. Naval Observatory of the colors an d brightness of two Global Positioning System Block II and two Block HA sat ellites against calibrated standard stars. Spacecraft logarithmic brightnes s falls off with phase angle almost linearly, as with the moon or Mercury, but more steeply: 0.04-0.05 astronomical magnitudes per degree. Knowing thi s value may permit a significant refinement of the Global Positioning Syste m solar force model. The colors of all of the satellites are much redder th an they would be if the optical properties of the surfaces remained as meas ured in the factory and so show the effects of degradation in space. The hy pothesis that most of the degradation is caused by self-contamination by ef fluent from the spacecraft bodies is examined and rejected, being incompati ble with the relative brightnesses in the yellow, red, and near infrared.