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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 -21027-4. 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 two-line element sets (TLE). TLE sets are made available to the public by the North American Aerospace Defense Command (NORAD) for non-classified objects orbiting Earth. These data sets consist of two 69-character 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 (-21027-4) 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 = CD (A/m) where CD is an object’s coefficient of drag, A is its cross-sectional area, and m is its mass. When B is modified by a reference value for atmospheric density, ρo, B* is obtained: B* = B (ρo /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 batch-least-squares 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 00000-0 -21027-4 0 9999
2 25577 86.3914 280.5847 0002046 90.5751 269.5679 14.34214463145187
For more detail on the two-line element set format used for satellite tracking, see T.S. Kelso’s FAQ on the topic.
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Re: Satellite Struggle: Newsletter Challenge (June 2017)
