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*Subject*: [amsat-bb] Trakstar and satellite latitude.*From*: "Bill Jones" <wejones@xxxxxxxxxxxx>*Date*: Tue, 17 Apr 2001 17:51:06 -0400

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 * ***************************************************************** ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**Follow-Ups**:**Re: Trakstar and satellite latitude.***From:*Phil Karn

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