Use PREDICT with "xearth" and "xplanet" to track your favorite satellites!

"earthtrack" is a socket-based application that polls PREDICT for live
tracking data, and plots positions of satellites on world maps generated
by "xearth" and "xplanet".

The concept of using "xearth" as a graphical display for PREDICT was
originally conceived by Wade Hampton who wrote an "xearth" interface
as an integral part of PREDICT's main program code.  However, it was
felt that a better approach would be to separate the "xearth" interface
code from PREDICT by treating it as a socket-based client application.
As a result, "earthtrack" was born.  The functionality of "earthtrack"
has since been expanded to work with "xplanet" as well.

"xearth" is available via the World Wide Web at:

	http://www.cs.colorado.edu/~tuna/xearth/index.html

"xplanet" is available via the World Wide Web at:

	http://xplanet.sourceforge.net/

Compile and install these applications as per their instructions.
OpenGL and Mesa libraries are not required to run "xplanet" with
"earthtrack" and PREDICT.

Build "earthtrack" by executing the "build" script in this
directory (type: ./build).  You need to be root so the executables
can be linked to /usr/local/bin.  NOTE: "earthtrack" is linked to
both "earthtrack" and "earthtrack2" in /usr/local/bin.  Executing
"earthtrack" will invoke a display using "xearth", while executing
"earthtrack2" will invoke a display using "xplanet", even though
the same binary is executed in either case.

"earthtrack" uses "xearth" to produce an orthographic image of the
earth upon which satellite names are placed over their respective
sub-satellite points.  The globe can be centered either over your
groundstation location (default), or over the location any satellite
being tracked by PREDICT.  The second method allows you to "fly with
the satellite" as it circles the globe.  The map is updated every 20
seconds.  By default, "earthtrack" connects to the PREDICT server
running on "localhost".  These defaults may be overridden through
several command-line switches.

The -h switch allows a different PREDICT server name to be specified.
The -x switch option allows additional command-line parameters to be
passed to either xearth or xplanet through earthtrack.  (Note that
the argument that follows the -x switch must be enclosed in "quotes".)
The -u switch allows a different map update interval to be specified,
and the -c switch allows the map to be centered on a specific satellite.

For example:

	earthtrack -c ISS -u 15 -h predict_server.org

allows earthtrack to invoke "xearth" as a display, centering the map
on the location of satellite "ISS", using host "predict_server.org"
as the host running PREDICT in server mode.  The satellite name
specified must be the same as PREDICT displays in Multi-Tracking
mode.  If a space appears in the name, then the entire name must
be enclosed in double quotes.

"xplanet" uses highly detailed photo-realistic maps of the world
available from a variety of sources to produce spectacular views
of the earth, and offers many features not available with "xearth".
Some of these additional features are invoked by "earthtrack2".
For example, if an uppercase -C is used rather than a lowercase
-c for the map center switch, such as in the following example:

	earthtrack2 -C ISS

The map is not only centered on the location of the ISS, but the
map is also zoomed into an area slightly larger than the footprint
of the satellite.  A range circle is also drawn on the map to
indicate the actual footprint of the spacecraft at the current
time.

If a map centered on the groundstation location is desired, then
footprint range circles for neighboring satellites are drawn on the
map.  Range circles are drawn only for satellites that are within
10 degrees of the local horizon (elevation > -10 degrees).  Such a
display may be created simply by executing "earthtrack2" without
any switches:

	earthtrack2

If the satellite being tracked is in sunlight, then the satellite
name and range circle are displayed in white.  If the satellite
is in darkness, then blue is used.  If the satellite is optically
visible to the groundstation, then yellow is used.

It is important to note that "xplanet" requires considerably more
CPU power and RAM than does "xearth".  Therefore, it may take
considerably longer than 20 seconds to update each map display
when running "earthtrack2" on slower machines.  If this is the
case, then "earthtrack2" executes no delay between "xplanet"
map updates.  Run "earthtrack2" through "nice" to reduce its
priority and associted CPU demand if desired.

"earthtrack2" may also be used to generate graphics files for use
in web server environments.  For example:

	earthtrack2 -c ISS -x "-geometry 800x600 -output graphic.gif"

will invoke "xplanet" to produce an 800x600 GIF image of the world
centered on the sub-satellite point of the ISS with a name of
"graphic.gif".  (NOTE: xplanet must be compiled with free-type
font support to allow labels to be placed in the generated
graphics files.)  This feature, along with capabilities
demonstrated in the ~/predict/clients/samples directory,
can be used to develop satellite tracking and orbital
prediction content for a web server.

"earthtrack" and "earthtrack2" exit when their connection to
the PREDICT server is broken, such as would occur if PREDICT
is terminated while "earthtrack" is still running.  Either
application may be run as background processes by placing
an ampersand (&) at the end of the command line.

For more creative uses of PREDICT and earthtrack, see John
Heaton, G1YYH's PREDICT and earthtrack modification web page
at:

	http://voyager.mcc.ac.uk/Predict/


Happy Tracking!

--
73, de John, KD2BD
December 2001

