[amsat-bb] Re: ISS/STS-126 Position?

Alan Sieg WB5RMG wb5rmg at somenet.net
Tue Nov 18 11:55:06 PST 2008

Scott (& others),
Whether you subscribe to elements, or your program auto-updates, you
can usually look and see how 'fresh' your element set values are.
Most tracking programs let you see the values in use for each particular
satellite. Some have verbose descriptions, others show a compact format.

Looking at an example 2-line element set from the Amsat.org website,
you see the number 08318, in position as the "epoch" or the instant of
validity. Any element set is just a snap-shot of one moment in an orbit.
Future positions are just 'running the clock forward'. This 08318 means

   year=2008, day = 318, or the 318th day of 2008 (13 Nov)

ISS              \     /
1 25544U 98067A   08318.67148292  .00010913  00000-0  84099-4 0  6455
2 25544 051.6436 330.0973 0003809 346.3452 121.7147 15.72579683572054

So this data set for ISS was generated last week, but until something
changes the 'status-quo' it should be fairly accurate. When the shuttle
docks, there are slight changes, but generally that same data stays
fairly accurate. However the shuttle significantly slowed down to dock.
That old shuttle data set is no longer useful. At 17,500 miles/hour you
may not notice right away the effect of ISS docking from the ground.
Possibly a difference of seconds may show up eventually, but many of
us don't keep our clocks set that close. By the time the next set of
radar-derived keps for ISS show up, they will most accurately describe
the current 'docked' configuration of the orbiting complex.

For a quick tutorial on understanding Keplerian Elements by N6NKF:

P.S. Excellent viewing opportunity this evening for ISS/STS126.
I use http://www.heavens-above.com for nice tables and sky charts.
Hope your weather is clear - should be spectacular... The station complex
has never been bigger, it should be really bright !!!

   Enjoy   /;^)
  <- WB5RMG is Alan Sieg  *  http://wb5rmg.somenet.net ->

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