How does the Reverb tool select which granules contain data in a specific geographic region?

The Backtrack Orbit Search Algorithm (BOSA) is used so that Reverb can select which granules contain data in a specific geographic region. The backtrack algorithm exploits the fact that Earth science satellites are in nearly circular orbits. By modeling an orbit as a great circle under which the Earth rotates greatly simplifies the orbit model .  The simplicity of the model allows backtrack to be more efficient than orbit propagator methods, which are designed to work with any satellite.  The simplified orbit model relies on only three parameters: inclination, period, and swath width. 

As the name implies, Backtrack works by tracing the orbit backwards.  Backtrack starts with the area of interest and answers the question, “In order for the sensor to have seen this area, where must the satellite have crossed the equator?”  There is no time dependence, so the speed of the algorithm is independent of the time range searched.  There is no cumulative error because Backtrack backs up at most one orbit.  There is no performance hit from using a lookup table because Backtrack calculates the actual equatorial crossing range.  The subsequent search is a simple, fast, boolean search on that crossing range (Spatial Search of Orbital Swath Data, Swick and Knowles, 2005).