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RE: AO-40 data mode... APRS???



On Thu, 23 Oct 2003, Eric Christensen wrote:

> This brings up a good point...  Is anyone doing APRS on AO-40?  If not, is
> there a problem with doing that?

Not using FM.  We would use a pair of frequencies and PSK-31... Here is
what I want to do with AO-40 (The only long term viable satellite in orbit
right now).  It uses the weak-signal PSK-31 mode to be best compatible
with AO-40.

1) On each continent (or on schedule) an APRS-SATGATE station maintains a
constant uplink which is streaming any APRS-IS traffic intended for AO-40.

2) This provides a signal that APRS users can "lock onto" whenever AO-40
is in view.  Due to the high doppler of AO-40, this APRS "pilot signal"
gives them something to always find and it establishes the reference for
all other APRS users (using PSK-31).  Lets say the users tune this APRS
pilot signal to 1000 Hz on their display.

3) Now then, because the frequency is clearly established only as an
offset to the pilot, this means the APRS-SATGATE will always be able to
knnow where to listen for any APRS data transmitted up to it.  Lets say
then that the SATGATE listens for APRS encoded PSK-31 signals at 1500 Hz.
And any APRS data it hears there, it will inject into the APRS-IS just
like any other IGate.

4) All we need are this:

  a) The satgate software to keep the pilot uplink streaming any APRS-IS
     traffic intended for AO-40 on the uplink
  b) USER PSK-31 software that will "LOCK" the display to a single pilot
     signal and then vary its own uplink tones to maintain a constant
     offset by where it see's itself in the dwonlink.

The two-markers in DIGIPAN are ideal.  Hook thre red one on the pilot,
and then use the blue one to show where you want to operate.  Then QSY
your radio until you land there.  THen hit a "LOCK" button...

  c) Then we need to re-visit the exact PSK-31 protocol for sending some
APRS data.  I worked all that up back years ago, but not sure I can find
it.  Is there some other standard that has evolved into standard use.
Remember, we dont need 10' accuracy when we are looking at the whole half
the earth at a time.  So positions are just the 4 or 6 digit grid square.
But we do need a way to indicate that a message is to be an APRS message
for the APRS IS>.. so that it can be converted to packets at the
satgate...


Bob, WB4APR

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