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Summer 2003 Issue |
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SATELLITES A Keplerian Elements Primer By Tom Webb,* WA9AFM |
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Regardless of the tracking software you employ, information on the orbital characteristics of the various satellites you plan to work must be provided to the program. This information is called Keplerian elements, or Keps for short. They are the “fuel” for our tracking software. Without these numbers to crunch, you wouldn’t be able to track a single satellite. Keplerian elements were named in honor of Johannes Kepler, a 17th century physicist and mathematician who developed Kepler’s Three Laws of Planetary Motion. Keps are, in a very real sense, a snapshot of a satellite’s orbital parameters. They tell the software the exact position of the satellite at a specific time and date. From that information the software can predict future positions of the satellite. Our two examples of the most commonly used formats, AMSAT and NASA, are for AO-7 from elements distributed on 29 May 2003. Back in the “bad old days,” to load Keps automatically any nonessential information had to be removed from the Kep files, because the older DOS-based software was unable to distinguish nonessential data from required data.
The AMSAT Format (see below) was devised to
allow simplified manual entry, thus being more “human friendly.” Each
element was identified to assure that the right numbers got in the right
places. As tracking software became more sophisticated, the ability to load Keps automatically became a common feature. Now virtually all the current software will automatically update their Keps without having to edit extraneous material. The NORAD 2-Line Format contains “just the facts” data—no identifying labels, just the necessary element values to project the satellite’s position. Both NASA and NORAD use this format for tracking predictions. When NORAD began its space-tracking mission, the tracking programs were written in Fortran. Early NORAD documents refer to this as T-card format, no doubt a reference to the common use of IBM punch cards to enter:
DECODE 2-LINE ELSETS WITH THE FOLLOWING KEY: 1
AAAAAU 00 0 0 BBBBB.BBBBBBBB .CCCCCCCC 00000-0 00000-0 0 DDDZ 2 AAAAA
EEE.EEEE FFF.FFFF GGGGGGG HHH.HHHH III.IIII JJ.JJJJJJJJKKKKKZ There is a third format called One Line Element (OLE), or Charlie Format, which is used almost exclusively by the U.S. Navy. All the Keplerian-element information is contained on one line. The Charlie Keps for AO-7 would look something like this: 075300314921046700002910174119598800118624790311207812535653 Columns 1–5 are the NORAD catalog number; columns 6 and 7, the year; column 8–10, the Julian Day number; columns 11–16, fraction of the day; columns 17–22, drag; columns 23–28, inclination; columns 29–34, right ascension of ascending node; columns 35–40, eccentricity; columns 41–46, argument of perigee; columns, 47–52, mean anomaly; and columns 53–60, mean motion. As you quickly can see, this particular format may be the height of brevity, but it certainly isn’t user friendly.
Let’s take a look at each of the elements and
what each is telling us. Let me warn you, though, that the explanation of
some of the elements may be a bit over the head of the “mathematically
challenged” (such as your learned author). Do not worry, however, because
we are only going to explore the very basic concepts behind each one. The
intent is to give you grounding in what you are seeing and in what it’s
telling you. The explanations are based on the AMSAT format, so let’s
begin. |
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