Re: SatSpy Graphical Plots

Dave Cappellucci (
Sun, 29 Mar 1998 09:18:41 -0700

Hi All,

I have noticed several recent postings with questions pertaining to my
SatSpy program.  I am posting this in hopes of clarifying how SatSpy
displays graphical passes, especially for those with long durations.  (I
hope that I am not violating any of the conventions of this list by
posting this note.)

SatSpy is optimized for visual observation (naked eye and binoculars) of
LEO satellites.  As such, I made compromises in several areas, one of
which was the display of long passes.  When you look at a skytrace,
either single or multiple, the local sky is painted at an instant in
time.  The pass itself occurs over a period of time.  During that time,
the star background appears to move relative to the observer.  The path
of the satellite is shown correctly with respect to its azimuth and
elevation, and for LEO sats, is approximately correct for Right

For sats with long duration passes, the discrepancy between the correct
Az/El position and the approximate Ra/Dec projection can be significant. 
There are several things that you can do to mitigate this difference:

1)  Use the Geosynchronous Satellites window (Sat Views|Synch Sats...). 
This shows the position of the satellite AND the position of the sky at
the same instant in time.  You may need to adjust the filters under
Options|Sync Sats...) to view the satellite you are interested in if it
does not meet the synchronous satellite criteria set forth in
Options|Sync Sats...  

(Note: If you use this window and use the Find|Detail option to view
Ra/Dec information, there is a bug in the display of Ra/Dec.  The values
shown should have been converted to degrees from radians before display,
but were not and are therefore incorrect.)

2)  If you are using the Single Skytrace window, go to View|Ephemeris
Data from the pop-up menu to open an ephemeris data window.  The Right
Ascension/Declination values shown there are correct.

3)  You can use the undocumented feature to plot external ephemeris data
(as described in a recent posting by Jason Hatton).  To access this
feature, hit Ctrl-E from a local sky window and enter a file name.


I have added a switch in Version 3.0 which will allow you to plot
skytraces with respect to either a fixed (as in Version 2.5) or moving
background.  When the moving background is used, a satellite's trace is
shown with correct Ra/Dec values and approximate Az/El values.  (The
opposite of V2.5)

I hope that this explanation helps to clarify how passes are plotted in
this particular program.  





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