[Kstars-devel] SatLIb verification

[Jason Harris]

Hi,

So do I have an old version of SatLib or something?  Because when I
run the demo, I am seeing some problems.  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;

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.

+ 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.

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 +
90037B HST 2007-05-01 12:16:32   2.48 174.36  42  11  247   2469  73294 +
90037B HST 2007-05-01 12:17:54   4.12 160.88  46  14  243   2313  73294
90037B HST 2007-05-01 12:19:16   4.49 146.25  49  16  238   2281  73294
90037B HST 2007-05-01 12:20:37   3.46 132.05  53  18  234   2379  73294
90037B HST 2007-05-01 12:21:59   1.36 119.61  57  20  229   2591  73294
90037B HST 2007-05-01 12:22:43  -0.03 113.88  59  21  226   2743  73294

(columns are satellite ID, satellite name, UT date(?), UT time(?),
altitude (deg), azimuth (deg), other stuff...)

Now, if those times are UT, then heavens-above should predict a pass
by HST at around 07:15 local time on 2007-04-30.  However, the nearest
H-A prediction is at 04:10 on 2007-05-01:

       Event                  Time             Altitude
Azimuth        Distance (km)
  Leaves shadow            04:11:07             13°              176°
(S )         1,687
  Maximum altitude         04:11:07             13°              176°
(S )         1,687
  Drops below 10° alt      04:11:55             10°              166°
(SSE)        1,851
  Sets                     04:14:57             -0°              144°
(SE )        2,742

What am I missing?

thanks for any ideas,
Jason



On 4/29/07, Bernard GODARD <bernard.godard at gmail.com> wrote:
> Hi,
>
> I did a comparison of SatLib with heavens above and PREDICT on the
> 2007 april 29 19h40 UTC pass of the ENVISAT satellite above Toulouse
> (France) :
> - the differences from SatLib and Predict on 8 points along the pass
> are less than 1 degrees
>  in both altitude and azimuth
> - the differences between heavens-above and SatLib are more pronounced :
> _________________________________________________
> HEAVENS ABOVE
> _________________________________________________
> Event	                            Time	   Altitude 	Azimuth
> Reaches 10° altitude 	        21:41:52 	10° 	87° (E ) 	
> Leaves shadow 	                21:41:02 	8° 	95° (E ) 	
> Maximum altitude 	        21:44:23 	14° 	58° (ENE) 	
> Drops below 10° altitude      21:46:54 	      10° 	28° (NNE) 	
> Sets 	                              21:50:21 	      0° 	4° (N ) 	
> _________________________________________________
> SATLIB
> _________________________________________________
> Event	                             Time	   Altitude 	Azimuth 	
> Reaches 10° altitude 	        21:43:22 	10° 	71° (E ) 	
> Leaves shadow 	                21:40:57 	5° 	93° (E ) 	
> Maximum altitude 	        21:44:43 	11° 	55° (ENE) 	
> Drops below 10° altitude      21:46:02 	      10° 	40° (NNE) 	
> Sets 	                              21:50:17   	0° 	6° (N )
> __________________________________________________
>
> The ENVISAT TLE used with Predict, SatLib and Heavens-Above was the same:
> ENVISAT
> 1 27386U 02009A   07118.76468073  .00000023  00000-0  24704-4 0  6392
> 2 27386 098.5492 186.3684 0001106 104.6779 255.4513 14.32247138269785
>
> Toulouse latitude, longitude, altitude used with all programs :
>  43.6deg
>  1.433deg
>  136m
>
> I will try to find the time to do a few other tests.
>
> Clear skies
>
>       Bernard
>
>
>
> On 4/25/07, JB BUTET <ashashiwa at wanadoo.fr> wrote:
> > Hi
> >
> > Ok, with good TLEs it seems there's a pb.
> > I have another program (in dos, really accurate) for satellites to verify.
> >
> > > Anyway, my original question was: have you done any verification of the
> SatLIb
> > > predictions against other published satellite predictions?  If so, can
> you
> > > tell me about the results and which sources you used for the comparison?
> >
> > I've ported satlib in order to make some predictions abou ISS. So I've
> > done ISS comparison with heavens-above and ...m don't remember this
> > great site... mmm... ah... no more neurons left here... calsky !!
> >
> > And errors was really minimal. (predictions were good).
> >
> > But I haven't try for others.
> >
> > May be Patrick or Bernard has some other ideas ?
> >
> > Clear skies,
> >
> > JB
> >
> >
>