Status

This document has been deprecated and is for informational purposes only.

The Backtrack Orbit Search Algorithm was recommended for informational use on October 2008.

NASA Earth Science Community Recommendations for Use

The TWG bases its recommendation on an analysis of the following factors in a NASA context:

Strengths

The reference implementation of the Backtrack Orbit Search Algorithm is currently in operational use at NASA’s National Snow and Ice Data Center Distributed Active Archive Center (NSIDC DAAC) as well as at NASA's Earth Observing System (EOS) Clearing House (ECHO). Users cite the following strengths:

• Other NASA missions would definitely benefit from the adoption of this algorithm. It provides more accurate results than traditional methods for searching orbital data due to its use of the original orbit parameters of the satellite.
• The backtrack algorithm simplifies the orbital mechanics by limiting itself to circular orbits and simplifies the math involved by using a hybrid of spherical trigonometry, Cartesian solid geometry, Euclidean planar geometry, and simple algebra.
• The Backtrack Orbit Search Algorithm reference implementation generates SQL queries that can be used with any conventional relational database.

Weaknesses

The Backtrack Orbit Search is undeniably complex. In the words of the author: “While the Backtrack algorithm uses a simplified orbit model it is still not simple. As the algorithm uses concepts and techniques from Spherical Trigonometry, 3D Cartesian Geometry, ordinary Euclidean Geometry, and basic Algebra, a strong math background is highly recommended.”

The reference implementation that is freely available provides some relief from this complexity to would-be users.

In addition, in an operational setting, the Backtrack Orbit Search Algorithm does require the specification of orbital parameters that not all providers can provide when their data are ingested. These parameters need to be hard-coded into data catalogs, resulting in some degradation of flexibility.

Applicability

The high volume of today's remotely sensed Earth science data creates a strong motivation to minimize the amount of data delivered to the end user. The goal is to get users everything they need but nothing they don't need (false positive intolerance). One way to decrease the amount of unneeded data delivered is to increase spatial search accuracy. Unfortunately, the most voluminous data, orbital swath data, is also the most difficult to run spatial searches on. The Backtrack Orbit Search Algorithm provides an efficient and accurate spatial search method for this orbit data.

Limitations

The TWG acknowledges the existence of other orbit search approaches and the fact that an extensive study to compare the performance and accuracy characteristics of the Backtrack Orbit Search Algorithm against these alternatives has not been conducted. A qualitative analysis shows that the Backtrack Orbit Search algorithm is competitive with orbit propagator methods; however, quantitative tests will be needed to settle any accuracy claims. Currently, this comparative evaluation and testing are outside the scope of the SPG.