Re: [amsat-bb] Questions: Portable digital satellite station

[Bob Bruninga <bruninga@xxxxxxxx>]

On Thu, 3 Oct 2002, William Leijenaar wrote:

> My major goal is to make an system that can send some small digital messages
> (just ascii text). The PACSATs can do it, but I am not sure about the APRS
> satellites. So my first question is:
>
> Is it possible to send messages to one specific amateur via APRS
> satellites? Is the couverage world-wide ? (because the APRS satellites
> have no digital storage, when I'm right and then there must be some
> remote areas where the messages are not forwarded)

Here's how the APRS satellites (PCsat, ISS, UO-22) work:

0) You're right, APRS is live, there is no store-forward (but EMAIL is OK)
1) An APRS "message" is usually ONE LINE of text that stands alone.
2) APRS messages may be addressed to anyone on the air "live" (worldwide)
3) The APRS satellites relay packets live to everyone in the footprint.
4) AND - Dozens of APRS SatGates all over the world capture ALL packets
   heard on the downlink and feed them into the worldwide APRS internet
   live feed (telnet to second.aprs.net any time, day or night and  you
   will see worldwide packets there, LIVE)
5) Thousands of terrestrial IGates all over the world monitor the SatGates
   (via the internet) and if they ever see a "LIVE" packet addressed to
   anyone that this IGate has seen "locally", then this IGate will pull
   the packet off the internet and send it back to RF on the local APRS
   channel snywhere on the planet... LIVE.
6) There is no store-and-forward.  But if an APRS station is on the air
   (either on an APRS terrestrial frequency) or on the downlink live,
   any where in the world, then the packet "should" get to him... within
   seconds... or never...
7) BUT - if the APRS message is addressed to "EMAIL" instead of a call,
   then once that packet is seen on second.aprs.net by the EMAIL engine
   run by WU2Z in New Jersey, then that packet is pulled off the internet
   feed and the first "word" of the email is assumed to be an Email
   address and so it is wrapped up into a standard SMTP email and sent on
   its way via conventional means.  Thus anyone, anywhere on the planet
   can send an EMAIL from just a Kenwood HT when an APRS satelltie is in
   view.

Now, of course, we are all more amazed when it DOES work this way, than
when it doesnt, but after all, this is a hobby.  Here are the countries
known to be on the air LIVE with internet SATgate feeds.  If you are
within say  2000km of these area, you have a chance of getting a message
out...

  USA all over (Hawaii and Alaska some times)
  Europe all over
  South Africa (sometimes)
  Antarctica (sometimes)
  Japan
  Brazil (sometimes)
  Australia/NZ

WE ALWAYS NEED MORE SATGATES and IGATES.  If you have full time internet
access and a spare TNC/2m radio, and only a WHIP antenna, then you too can
contribute!

So you have to be in the middle of the Pacific or the South Atlantic to
get away from APRS...  But APRS is not like BBS's or email.  It is more
like PAGING.  Try to say what you need to say in one line and APRS is
perfect.  Get verbose, and you are using the wrong medium...

Every APRS user can send three basic packets:

POSIT:  contains your LAT/LONG/CSE/SPD and 20 chars of text comment
STATUS: 63 characters of relatively static Text saying what you're up to..
MSGS:   Variable message text addressed to ALL, BULLETINS or individuals

Each station can only have one POSIT and only one STATUS, but he can send
as many MSGS as he wants.

Even when PCsat dies, we have 2 more funded follow-on digipeaters in work,
scheduled for launch in 2003, so this APRS capability will only GROW.  We
invite all AMSATS on the drawing board to include this dumb-digipeater
concept so that users anywhere, anytime, can uplink from an HT, a message
to anyone anywhere...

And lets get those satgates on the air.

see http://www.ew.usna.edu/~bruninga/astars.html



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