karn (07/16/82)
The person who asked about orbit tracking programs for the shuttle
(cobb@nbs-vms) hit one of my favorite topics. You asked for it!
I have been working off and on with various orbit tracking programs
for several years, primarily to track the amateur radio satellites
built by the Radio Amateur Satellite Corporation (AMSAT) and the
USSR radio clubs.
The best general purpose orbit tracking program available for
free public use was originally written in BASIC by Dr. Thomas A.
Clark, W3IWI, president of AMSAT. A listing of his program appears
in the AMSAT publication ORBIT magazine issue #6. You can get a
copy of this issue by sending $2 to
AMSAT
PO Box 27
Washington, DC 20044
(This address is also good for general inquiries about AMSAT and its
programs)
Tom's program has been translated into a number of dialects of
BASIC, PL/1, HP-41C, etc, and are available from
Amsat Software Exchange (ASE)
Box 338
Ashmore, IL 61912
for small donations ($5-$15) to cover costs.
One of the guys running the ASE, Bob Diersing, N5AHD, runs a CBBS on
(512) 852-8194 where you can read some info about the available
programs and get Keplerian elements for a number of satellites.
Tom's article includes a very good discussion of the theory behind
his program, which is a general one handling elliptical
orbits as well as circular ones. It uses the "Keplerian Element"
sets derived by the North American Air Defense Command (NORAD)
that are available free from NASA. These numbers describe the size and
shape of the orbit and the orientation of the orbit plane at a
specified time (the epoch), as determined from optical and radar
observations.
There is another satellite tracking program available,
more comprehensive than the Clark program, in that it has a solar
ephemeris for checking observer and satellite illumination. This is
very useful for finding passes in which the satellite is illuminated
and the observer is in darkness; under these conditions even a small
(2 foot) amateur radio satellite can be spotted with a pair of
binoculars. Something as large as Skylab or the Shuttle is very
easy to spot if you know when and where to look.
It was written and is being sold by Sat Trak International in
Colorado Springs for about $80. I have taken their software
(originally in FORTRASH), and am converting it to a set of modular
subroutines in C for use both in tracking and for orbit determination
(deriving the Keplerian elements from transponder ranging measurements,
which AMSAT will need to do for Phase III-B). Since these program cost
money and belong to someone else, I can't place my versions in the public
domain, but it might be alright to give them to persons who have already
purchased the SatTrak software. (Sort of like paying your dues to
AT&T to get a Berkeley Unix tape).
During the last shuttle mission, I tracked down a set of elements
by phone (mail would have been too slow) through a public relations person
at Johnson Space Flight Center. He had some trouble obtaining them,
mentioning that I was the only person to ask for them, but he was
quite helpful. I later posted them to net.columbia after verifying
that they were reasonably accurate. By looking up the locations of
the NASA tracking stations and running the tracking program with the
Keplerian elements, I could verify its accuracy with the 900 410-6272
number. I found it very convenient to know when the shuttle was in
range before calling the Dial-It number.
I suspect that if there is enough demand for them during future
flights, they would be more readily available.
Unfortunately, the shuttle performs many maneuvering burns during a
flight. For example, STS-4 increased its orbital altitude and
eccentricity 2 days after I got the Keplerian elements, throwing off
my predictions.
For those of you without computers (are there such people here?),
approximate predictions can be made with a Mercator map and an
acetate overlay. The overlay must contain a sine wave with an
amplitude equal to the orbital inclination (28.5 degrees for STS-4,
larger for the earlier flights). The trace period is such that equator
crossings occur at the correct spacings, remembering that the earth is
rotating under the orbit, so that each equator crossing occurs
farther to the west. On the Mercator map, draw "range circles"
around each tracking station, with the radius being given by
r * acos(r/r+h)
where r = radius of the earth
h = height of the satellite above earth surface (same units)
When the spacecraft enters a range circle, it is above that
station's horizon.
Note that the map methods for tracking satellites are workable only
when the orbits are reasonably circular. Satellites in elliptical
orbits (e.g., the new Phase-IIIB spacecraft that will be launched in
January) follow weird S-shaped ground tracks; for these, a computer is
really the only practical answer.
On the topic of listening in, NASA uses a domestic satellite to
distribute a service called "NASCOM", which is how the networks get
their feeds. I called the public relations departments at both the
Johnson and Goddard space flight centers and got two different
answers as to which satellite and transponder is used. Since I am
not rich enough to have a home satellite TV receiver (spending my money
instead on ham satellite gear), I could not verify which is correct
(I was trying to get the info to get my local CATV company to carry it).
Anyway, maybe somebody with a dish can check these out:
Satcom I, Transponder 9 (JSC answer)
Satcom II, Transponder 13 (Goddard answer)
Rumor has it that unmanned launches (e.g., Delta) are also covered
on NASCOM. Neither of these was on Satcom III-R, so my local CATV company
couldn't carry it.
Hope this info helps.
Phil Karn, KA9Q/2
Bell Labs
Murray Hill, NJ