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AFSCN ranging

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Lt Michael Long

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Sep 17, 1996, 3:00:00 AM9/17/96
to id qqbhnz01314, tue

Anyone out there have any insight into the Air Force Satellite Control
Network? It's an old system with an ancient PRN ranging system. I'm
needing to understand how it works. It seems to have a course and a fine
ranging mechanism. The course ranging uses a PRN code long enough to
avoid any ambiguity for the distances you expect to measure. You send
the code from the earth and the satellite turns it around. When you
receive it again you shift the bits until you're in synch with the
original bit sequence. However many bits you were off tells about the
round trip distance the signal took (accuracy tolerance depends on the
bit length). What I don't understand is how the fine ranging works.


gary clare

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Sep 17, 1996, 3:00:00 AM9/17/96
to bir...@vinthill-usag.army.mil

Sure thing...as soon as you let us know who you REALLY are. You see, I
never met an American officer who said "Im needing to understand..." or
"bits." Nice try though, Ivan.

Tom Johnson

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Sep 21, 1996, 3:00:00 AM9/21/96
to

A good source of information about the AFSCN is available in a document
called
TOR-59 published by the Aerospace Corp. I don't believe that it is
publicly
available but anyone in the military or DOD contractor should be able to
get it.
In short, the difference between the long code and the short code is
the distance (in bits) before the code begins to repeat. Since the code
is a
Psuedo-Random Number (PRN) sequence, it will repeat itself after a
certain
amount of time. The longer the code length, the longer it may take to
acquire
range lock since you must search a longer code space to sync up with the
returned
signal. Thus for LEO you use the short code but for GEO you use the
long code.
If you were to use the short code for a GEO you would obtain a false
range since
the code will repeat itself in the time it takes for the signal to
propagate
to the spacecraft and back. This is one of the things you check with
your
Remote Ground Facility crew when you see that your range is bogus after
acquiring
range lock.

Tom Johnson, Development Lead and former AF satellite jockey
Analytical Graphics, Inc.
t...@stk.com
www.stk.com

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