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The Backtrack Orbit Search Algorithm

Orbit searching is by far the most accurate way to search for level 0-2 orbital swath data. Unfortunately orbital mechanics is a quite difficult field, and the most well known orbit model, the NORAD propagator, is quite complex. The NORAD propagator is designed to work with a wide range of possible orbits, from circular to extremely elliptical, and consequently requires quite a bit of information about the orbit to model it well.

In order to facilitate earth science the orbits of satellites gathering earth science data are quite restricted compared to the variety of orbits the NORAD propagator is designed to work with. Generally the earth science community would like global coverage, with a constant field of view, at the same time every day. For this reason most earth science satellites are in a sun-syncronous near-polar orbit. And even missions that are not interested in global coverage, e.g. the Tropical Rainfall Measuring Mission (TRMM), are still interested in having a constant field of view so the coverage of the sensor is at a constant resolution. For this reason all earth science satellites are in circular orbits.

The Backtrack Orbit Search Algorithm exploits this fact to greatly simplify the orbit model by just modelling an orbit as a great circle under which the Earth rotates. This reduces the number of orbital elements required for the model from 22 to 3. Moreover the NORAD propagator is designed to predict future orbits based on current status, and consequently must be reinitialized periodically to correct for cummulative error as the model spins forward. As the name implies Backtrack spins the orbit backwards, and in practice spins backwards at most one orbit, so there is no cumulative error.

There are currently two implementations of backtrack, in the spheres package and another that is incorporated in the NASA Earth Observing System (EOS) Clearinghouse (ECHO). In 2008, a NASA Technical Note was created, reviewed and revised to satisfy endorsement requirements of the NASA Earth Science Data Systems Standards Process Group. After review and revisions to the Technical Note, the Technical Working Group in January 2009 recommended that the "algorithm description along with the reference implementation had significant benefits to NASA's missions producing remote sensed swath data." The full technical note, documentation and reviewer's comments are available from NASA as the ESDS-RFC-10 Backtrack Orbit Algorithm Technical Note.

Listed below are some descriptions of the algorithm. They are listed in order of simplicity so it is suggested you start with the first link and work your way down.

A paper we did comparing Backtrack to some other popular orbit search methods (in MSWord)

A presentation based on the paper above that contains a lot more pictures (in powerpoint).

A general example and one special case.

A detailed description of the algorithm (including equations).

A detailed worked example (the example above with actual numbers).

NASA ESDS-RFC-10 Backtrack Orbit Algorithm Technical Note.