This is a Clojure wrapper of a Ruby implementation of the Backtrack Orbit Search Algorithm (BOSA) originally written in Java and maintained by NSIDC. References to "the Java code" or "the NSIDC spheres code" in the refer to the original library which is available here. Along with the original code, geospatialmethods.org also has a [description of the algorithm] (http://geospatialmethods.org/bosa/).
Orbiting satellites have sensors that trace a circular path around the Earth. It is difficult to express this ground track as a 2D polygon with lat/lon coordinates. Most of the orbits we deal with have a long, narrow, sinusoidal shape that would require hundreds of points to describe. Further, their minimum bounding rectangle covers nearly the whole Earth and some orbits have ground tracks that cross themselves within a single granule of data.
Instead of describing the orbit as a complex 2D polygon, we want to answer a different question: Where would the orbit need to cross the equator in order to see a given point on the Earth?
Instead of using a complicated 2D polygon query, we could instead perform a simple range query: "Find all data granules whose orbit crosses the equator between longitudes a and b."
See the orbit.rb Ruby code for more information.