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.
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)
and a poster (52x36) version of the same thing (in powerpoint).
A presentation based on the paper above that contains a lot more pictures (in powerpoint).
A graphic
introduction from the PSQ progress report (HTML-ified powerpoint, starts at slide 6)
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).
