The nature of the Hubble Space Telescope's (HST) low Earth orbit imposes scheduling restrictions and interruptions in the data collection periods for it's compliment of scientific instruments. During many of these times the Faint Object Spectrograph (FOS) is in a full operational configuration and is taking detector background measurements which are continually reported in HST's engineering telemetry stream. These data are primarily used to monitor the instrument for changes in behavior resulting, principally, from intermittently noisy diodes in its digicon arrays. These same data may be used to monitor temporal changes in the charged particle environment of HST's near-earth orbit. We present here the results of a study of two years of on-orbit FOS background data obtained serendipitously during periods while the FOS in an operational state, but not exposing on external, or calibration targets. These in situ data, which represent more than 100,000 discrete samples (equivalent to more than 1100 orbits) have allowed us to accurately measure variations in the background proton flux seen by the FOS. An analysis of these variations have permitted us to model the geomagnetic environment of the South Atlantic Anomaly (SAA) as a function of time as well as the change in detector background as a function of geomagnetic latitude.