[Kstars-devel] SatLIb verification

[Bernard GODARD]

Hi,


On 5/1/07, Jason Harris <kstars at 30doradus.org> wrote:
> Hi,
>
> So do I have an old version of SatLib or something?  Because when I
> run the demo, I am seeing some problems.

I did my test with the SatLib.h and SatLib.c code I found in the
satlib directory of kstars svn. I didn't use the demo code. I made a
custom C test program.

> First of all, It reports a
> Julian Day number that is off by about a month (and calendar dates
> reported in the output tables also reflect this month-old date).  I
> have to hack the code like this if I want the correct JD:
>
> jd=Julian_Date(cdate) + 30.0;
Just tested. same problem here!

>
> There are some ambiguities in the meanings of some parameters:
> + What time is reported by the demo?  I assume it's UT, and looking at
> the code I'm pretty sure that is correct.

That's what I did too.

> + What convention is used for geographic longitude?  Are west
> longitudes positive or negative?  From the longitude you gave for
> Toulouse, I guess west longitudes are negative.
>
I was wrong about that but the difference is not so important for
Toulouse because it's almost zero longitude. I just did the test with
west longitude positive and I have now a better match of the Heavens
Above results.

> Assuming my guesses are correct, as I said before, I can't find any
> correlation between SatLib predictions and those of
> NASA/Heavens-above/calsky.
>
> For example, for JD=2454221.5 from Tucson, AZ (lat=32.222,
> long=-110.926), SatLib gives the following pass prediction for HST:
>
> % ./demo HST
> JD=2454221.516105
> 90037B HST 2007-05-01 12:15:10   0.03 185.72  38   9  251   2727  73294 +

I have tried to re-do your test and it seems SatLib's demo finds many
more passes than Heavens Above.

But I did another test with my test program which doesn't use
SatNextPass but SatFindPosition and I made it print the epheremeris
for the 3 may 19h30 (local time = GMT-7) pass above Tucson (
32.2220°N, 110.9260°W, alt 751m) predicted by Heavens Above and I have
a very close match.
_________________________________________________________________________

test program
_________________________________________________________________________

#include "SatLib.h"
#include <stdio.h>
#include <stdlib.h>
#include <math.h>


#define NBPTS 3600



int main(int argc, char *argv[]){

char *sat1 = "HST";
char *obsname = "Tucson";
double latitude = 32.222;
double longitude = 110.926;
double altitude = 751.0;
char *TLE_file = "hst.tle";
int result = 0;
double jd_start = 2454224.60417;
double step = 1;
int i=0;
int h,m;
double s,s1,s2;
double t;
SPositionSat * pPositionSat[NBPTS];

for (i=0;i<NBPTS;i++)
  pPositionSat[i] = (SPositionSat *) malloc(sizeof(SPositionSat));

result = SatInit(obsname, latitude, longitude, altitude, TLE_file);
result = SatFindPosition(sat1, jd_start, step, NBPTS, pPositionSat);

for (i=0;i<NBPTS;i++){
  t = (jd_start-0.5)+i*step/86400.0;
  s1 = (t - floor(t))*86400;
  h = floor(s1/3600.0);
  s2 = s1 - h*3600.0;
  m = floor(s2/60.0);
  s = s2 - m * 60.0;

  printf("UT TIME : H: %d, M: %d, S : %f, ALT : %f, AZIM : %f VISI:
%c\n",h,m,s,pPositionSat[i]->sat_ele,pPositionSat[i]->sat_azi,pPositionSat[i]->visibility);
}

  return 0;
}