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Trakstar and satellite latitude.



 
   I have been using a very simple tracking program 
that I wrote myself (actually a conglomeration of 
routines borrowed from other programs found on the 
web), however I decided to "upgrade" it to do SGP4/SDP4 
models.  Since I'm not very good at pascal, I decided 
to adapt the fortran Spacetrk program that can be found 
both at NASA and at the Kelso site.  This program 
outputs X,Y,Z celestial positions rather than Lat/Lon 
and other more useful parameters, so after I got the 
program running, I added a short routine to convert the 
X,Y,Z output to Lat/Lon.   I decided that the best 
program to use to check my program would be Trakstar, 
which was available at the same sites, and is 
apparently just  a newer implimentation of the same 
routines.
   On comparing my output to that of Trakstar, my 
longitude was virtually identical, to the number of 
significant figures I used, however the latitude was 
off, by about a tenth of a degree.  This doesn't seem 
like much, but it occurred to me that latitude should 
be a fairly direct calculation from the X,Y, Z values, 
ie 
 arcsin ( z/sqrt(X^2+Y^2+Z^2))  
When I did this calculation, using the X,Y,Z values obtained from either my 
program or Trakstar, it agreed exactly with my program, 
and agreed with the output from some other programs I 
used to compare to.  
   Since the source of Trakstar is available, I tried 
to figure out what it was doing, and found that it 
first calculated the latitude, then it went through 
some iterative routine, which I didn't understand, 
(because it used some variables from some other 
subroutine which I couldn't find).
   Anyway, my question is this....  does anyone 
understand just why Trakstar uses this iterative 
routine when they could just calculate it directly 
using the above arcsin function?   Which result is 
correct?

  If anyone wants to try, I used the following keps:
ISS
1 25544U 98067A   01099.82633102  .00111422  00000-0  13228-2 0  8532
2 25544  51.5496  73.2356 0012071 262.5402 121.3376 15.60242032136402
and got X=150.757   Y=6415.907   Z=2147.297 for the celestial 
position at the Epoch time (Apr 9 2001, 19:49:55 UTC).  Using the 
arcsin function gives a latitude of 18.49977515  while Trakstar gives a latitude of 
18.6091 .  WinOrb gives 18.5 deg when using SGP4, agreeing with my result.
   I assume that Kelso's Trakstar program is more accurate, but it is beyond me to understand 
why, and even though it isn't a big difference, I'd like to know why my results are different.
Thanks   

 *****************************************************************
 *Bill Jones  N3JLQ  Sweden Maine                                *
 * wejones@megalink.net                                          *
 * Main home page   http://www.megalink.net/~wejones             *
 * WWII/B-17 page   http://www.megalink.net/~wejones/wwii.html   *
 *****************************************************************
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