This month's Challenge Question: Specs & Techs from GlobalSpec:
Column 54 through 61 of a TLE set for the IRIDIUM 20 satellite has a value of 210274. What is this number’s meaning, and what are the physical implications of its negative value?
And the answer is:
Simplified perturbation models (SGP4 in particular) are used to determine a satellite’s orbital path. They use a standard data source known as twoline element sets (TLE). TLE sets are made available to the public by the North American Aerospace Defense Command (NORAD) for nonclassified objects orbiting Earth. These data sets consist of two 69character lines that contain all of the parameters necessary to calculate a satellite’s orbital position and velocity for a period of time around the TLE “epoch” (the moment in time corresponding to the TLE data points).
The number in the challenge question (210274) represents the term of the TLE known as B* (Bstar), the ballistic drag coefficient that indicates aerodynamic drag on a satellite in the SGP4 orbit model. In aerodynamics, the ballistic drag coefficient B = C_{D} (A/m) where C_{D} is an object’s coefficient of drag, A is its crosssectional area, and m is its mass. When B is modified by a reference value for atmospheric density, ρ_{o}, B* is obtained: B* = B (ρ_{o} /2). B* represents an object’s susceptibility to drag.
Negative B* values occasionally show up in TLE sets, indicating erroneously that energy is somehow being added to the system. This is not the physical reality of drag acting on a satellite, of course. It is instead a consequence of the way SGP4 models forces with respect to the actual dynamic environment. From Revisiting Spacetrack Report #3 [PDF], we know that “SGP4 uses power density functions that require a term that encapsulates the ballistic coefficient, B*. Simplified force modeling and the batchleastsquares processing of observational data often yield a B* that has “soaked up” force model errors.”
The full TLE set for this challenge question was:
IRIDIUM 20 [+]
1 25577U 98074A 17107.88627323 .00000039 000000 210274 0 9999
2 25577 86.3914 280.5847 0002046 90.5751 269.5679 14.34214463145187
For more detail on the twoline element set format used for satellite tracking, see T.S. Kelso’s FAQ on the topic.
