Backtrack Orbit Search Alg. Technical Note - TWG Final Report

ESDS-RFC-010 Technical Working Group Final Report

The ESDS-RFC-010 Technical Working Group (TWG) has conducted a review of ESDS-RFC-010, Backtrack Orbit Search Algorithm with the following conclusion:

That the Standards Process Group should endorse ESDS-RFC-010 as a Technical Note.

Recommendation

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 as well as at the NASA-developed Earth Observing System (EOS) Clearinghouse (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, 3-D 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 which not all providers can provide when their data is 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.

History ESDS-RFC-010 was submitted by Ross Swick from NSIDC as a NASA Standards Process Group (SPG) Technical Note. A technical note, in SPG parlance is a document that contains useful information but is not a "standard". The process of review is less rigorous from that of RFC’s which are submitted as SPG Standards, however, a proposed technical note must nonetheless, be relevant to the domain of NASA Earth Science data systems, serve a useful purpose, be technically of high quality, and be well written.

The ESDS-RFC-010 TWG conducted a Technical Review within the SPG, designed to determine the technical validity of the technical note within a NASA context. The review of the Backtrack Orbit Search Algorithm technical note was completed over a period of approximately 11 months. Review comments for additional definitions and figures to make the algorithm description clearer, as well as requests for sample test case including sample input and expected outputs were received and feedback was provided to the RFC author. The RFC document was updated several times to incorporate this feedback.

The reviews were positive and showed without a question that the algorithm description along with the reference implementation had significant benefits to NASA's missions producing remote sensed swath data.

Based on the responses received and from additional research the TWG concludes that the Backtrack Orbit Search Algorithm demonstrates sufficient usefulness to be endorsed by the SPG as a Technical Note.

RFC Overview ESDS-RFC-010 proposes the Backtrack Orbit Search Algorithm as an ESDS Technical Note. Its functionality is described in the excerpt below:

The Backtrack Orbit Search Algorithm is a spatial search method for orbital data. With the dramatic increase in satellite-borne sensor resolution, traditional methods of spatially searching for orbital data have become inadequate. As data volumes increase end-users of the data have become increasingly intolerant of false positives. As computing power rapidly increases, end-users have come to expect equally rapid search speeds. Meanwhile data archives have an interest in delivering the minimum amount of data that meets users' needs. This keeps their costs down and allows them to serve more users in a timelier manner. The Backtrack Orbit Search Algorithm has a degree of accuracy that rivals predict methods while being faster, less costly to implement, and less costly to maintain than other methods.

The idea behind Backtrack Orbit Search is that while spatial search of swath data is difficult in general, Earth Science swath data has a number of characteristics that make the task a lot easier. Remotely sensed data is valuable to Earth scientists because it is frequent, regular, and global. For the purposes of doing Earth Science, scientists have an interest in keeping the data as consistent as possible. Among other things, that means they want the sensor to have a constant field of view. An easy way to accomplish that is to put the satellite in a circular orbit. For this reason (and others), all Earth Science satellites are in a circular orbit.

The Backtrack Orbit Search Algorithm exploits this fact to greatly simplify the orbit model by just modeling an orbit as a great circle under which the Earth rotates. 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. The accuracy of the method depends on the stability of those three parameters over the life of the sensor, but there is also a scientific interest in keeping those parameters stable, so they generally stay within reason or the data aren’t useful.

As the name implies the Backtrack Orbit Search 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 equatorial crossing range, and the subsequent search is a simple, fast, Boolean search on that crossing range.

The RFC can be downloaded from:

http://www.esdswg.org/spg/rfc/esds-rfc-010

The algorithm specification, software library and additional information can also be downloaded from: http://geospatialmethods.org/bosa A reference implementation of the Orbit class is part of the NSIDC Spheres Java package. More information about the Spheres package, including downloadable source code, is available from http://geopsatialmethods.org/spheres.