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Re: Marked differences in published elements?



On Sun, Jul 03, 2005 at 10:24:24PM -0400, Ka8syx2003@aol.com wrote:
> Folks,
> 
> Because I've been between home qths lately, I haven't had opportunity to be 
> QRV, but was poking around on the computer this evening.  During past QSOs with 
> some of you, I've mentioned that I pull all of my predictions from WinOrbit 
> 3.6 to printed ephemeris and hand carry them outside where I set up for a given 
> pass.  Tonite I made side by side comparisons between the predictions I was 
> getting off of Satscape 2.02 pro and off the WinOrbit.  I like Satscape because 
> it provides some "sexier" views and is more "real-time" feeling.  Anyhow, I 
> was watching what FO-29 was doing this evening during an approximately 0140 to 
> 0200 pass on both programs and discovered marked differences in predicted 
> location, variances of several degrees in alt, az and lat/lon subsatellite point.  
> Comparing the elements that I routinely download from AMSAT with those the 
> Satscape took via "internet update" from Celestrak illustrated a possible cause 
> of the differences:
> 
> 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
> KEY: A-CATALOGNUM B-EPOCHTIME C-DECAY D-ELSETNUM E-INCLINATION F-RAAN
> G-ECCENTRICITY H-ARGPERIGEE I-MNANOM J-MNMOTION K-ORBITNUM Z-CHECKSUM
> 
> AMSAT.ORG (05181.AMSAT)
> FO-29
> 1 24278U 96046B   05181.59521482  .00000020  00000-0  53797-4 0  9873
> 2 24278 098.5427 229.3523 0351282 141.4784 221.1994 13.52914251438002
> 
> www.celestrak.com/NORAD/elements/amateur.txt
> FO-29                   
> 1 24278U 96046B   05184.18367414 -.00000027  00000-0  60488-5 0  9890
> 2 24278  98.5431 231.5914 0351209 134.7619 228.2674 13.52913791438356
> 
> There are some big variations between the two sets.  I realize that the AMSAT 
> numbers are for Julian 05181 and the Celestrak figures are apparently three 
> days fresher, but can that small difference (when the Satscape launchpad 
> suggests a user only needs to correct/update his keps once a month or at most 
> frequently every few weeks) cause such wide discrepancies?
> 
> Which can I believe?  

Hello Ray,

The short answer is: Both.

The main parameters are pretty constant (except some measurement uncertainties -
you can certainly ignore the fifth or sixth decimal place)

Mean motion: 13.5291...
Inclination: 98.543...
Eccentricity: 0.03512...

Earth oblateness causes a precession motion, for inclinations less than 90 degrees
RAAN decreases, for inclinations over 90 degrees RAAN increases with time.
Another effect of earth oblateness is the motion of the line of apsides,
argument of perigee changes at these inclinations even faster than RAAN.
Since the orbit is near circular, this change does not influence very much
the communication distances (as with highly elliptic orbits of AO-10, -13, -40).
Mean anomaly is dependent on the time when the satellite crosses the "electric fence",
and changes therefore opposite to the argument of perigee.

Your tracking program is taking care of all this changes and give very similar
predictions for both TLEs.

Vy 73, Viktor OE1VKW
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