[aprssig] Using APRS via the AO-51 digipeater

[Robert Bruninga]

AMSAT has enabled the 9600 baud APRS digipeater on the
AO-51 satellite.  Any APRS mobile (with 9600 baud such as a
D7 or D700) can easily hit the uplink.  But what we need are
lots of SATGates to monitor the downlink since it is so weak.
So any APRS IGate stations that have a 9600 baud TNC
can be very helpful...  see below....

The AO-51 Digipeater operates on 145.86 uplink and 435.150 +/-
Doppler downlink (9600 baud) but has some very unique
charasteristics that are different from normal digipeater expectations.

Since this is a 9600 baud digipeater only, the most probable users
will be owners of the TH-D7 and TM-D700 APRS radios with built
in 9600 baud APRS TNC's.  What is different about the AO-51
digipeater compared to ARISS or the PCSAT's is:

1) The downlink is weak and will need a beam to receive and you must tune doppler
2) The uplink is easy to hit even with the 5 watt HT to an omni antenna.
3) The digipeater is full duplex and the TNC in the D7/D700 is only
    half duplex.  THus the SENDER of a packet will usually NOT 
    see his own packet digipeated unless he is using two radios.

Because of these unique features there are two primary user modes
that can still be efficiently used on this space system:

a) two-way APRS message QSO's with stations using a decent
    downlink antenna

b) Station uplinks of position, status, or messages or email into
   the global APRS system from remote locations for receipt
   and viewing on the internet.

To make #b work, requires several AO-51 9600 baud packet
satellite gateways operating around the world and feeding their
live downlink into the global APRS internet system.  The rest
of this email will address how to uplink through this application.  
Here is what the sender should do:

1) Use a UNIQUE callsign-SSID for this experiment so that the data 
    is not overwritten when you then later operate terrestrially
2) Use the uplink path VIA PACB-1 at 9600 baud
3) Set your position rate to about once a minute during the pass.
4) Enter a CQ message into the radio with a TOCALL of ALL
5) Check http://map.findu.com/mycall* to see if you made it.

Anyone can be a SATGATE feeding into the global APRS
internet infrastructure simply by QSY'ing their normal APRS IGate
from the APRS channel to 435.15 +/- Dopplerand changing to 
9600 baud on their TNC for the duration of a pass.

Although a beam is needed to bring in the weak signal for
one SATgate, this system is unique in that the APRS internet
system combines the output of ALL SATgates and so only one
station at a time really needs to hear each packet.  THus, if 
we had as many as 6 SATgates across the USA in the north
and south, they would only need a SIMPLE 19" 3/4 wave
vertical receive antenna (no moving parts) and they would
not have to tune DOppler either, since they would only
receive well for the few minutes the satelltie was overhead.
Thus, these 435.15 fixed tuned and fixed OMNI SATgates
could provide continuous coeverage over the USA if there
were enough of them.

If you have a spare 9600 baud TNC and UHF receiver to dedicate
to this mission, it would really serve a great service.  The
19" whip over a ground plane does have almost 8 dBi gain
above 30 degrees and so it CAN hear the dowlink in the 
middle of a high elevation pass.

Anyway, hope that helps.

de Wb4APR, Bob