APRS on AO-40

For linking remote travelers into the worldwide APRS system

Bob Bruninga, WB4APR

Operating a single APRS channel on AO-40: Since AO-40 is the primary bird in the AMSAT inventory that provides hemispherical coverage to all corners of the globe for many hours a day, it is reasonable that we establish a permanent APRS channel on the transponder. This can serve to link remote travelers back into the global APRS system. This permits live real-time communications between the remote operator and his intended target anywhere on the globe and only he needs the AO-40 portable station. The plan is simple. A net control station (one per continent) establishes a constant PSK signal uplink channel fed from the APRS-IS. This gives users something to tune in and lock on to as shown below. We will use PSK-31 as the modulation mode because of its excellent weak signal characteristics and ready availability to all radio amateurs. (And many other factors discussed later).

Then exactly 1000 Hz higher is where the SATGATE station is listening for remote APRS user uplinks. If the DSP software was smart enough, it couild accept multiple uplinks over a range of frequencies offset from the pilot channel. The SATGATE station will take these and then feed anything heard into the global APRS-IS!

SATGATE UPLINK: The APRS to PSK-31 conversion will convert all APRS position data (that uses the ALTNET address of "AO40xx") to a simple 6 character gridsquare as shown above. Any PHG is dropped and the course and speed is only included if the station is moving. Otherwise the position comment text comes through as written. Notice that the callsign suffix is repeated as a redundancy check and the grid square is case sensitive to allow for context checking of information validity. PSK-31 has no inherrent error detection, so these techinques allow some rudimentary sanity checking of data that can be Human read and manually entered. Thus the PSK-31 AO-40 user needs no special software to join the Global APRS system. Typically, he would prepare his basic Gridsquare format in one of his transmit buffers to trnsmit at the beginning of any QSO exchange.

The AO40 ALTNET: Traffic from the global APRS-IS intended for this SATGATE uplink will use the APRS alt-net concept. Any packet on the APRS-IS originating anywhere on the planet can be routed to the SATGATE uplink by simply using the ALT-NET address of AO40xx. This is a valid ALTNET TOCALL that will not impact normal APRS functionality but it signals the SATGATE to include it in the live uplink. There is not need to carry this through the conversion to PSK-31 because it is assumed that ALL packets on the pilot uplink meet this criteria. SImilarly, when PSK-31 user generated position data is captured from user uplinks and converted to normal APRS position or message packets, by the SATGATE for injection back into the APRS-IS, then those packets will contain not an ALTNET tocall, but a normall APRS tocall of "AP40xx" so that they are seen by any APRS station through the normal APRS filters. But the "AP40xx" gives a unique identifier that the packets came into the system via AO-40. Probably we can use the availablle two XX characters to indicate the identity of the SATGATE...

WHY PSK-31? There are several reasons why PSK-31 and this full duplex pilot uplink system is the mechanism of choice for this APRS channel on AO-40:

  • High Doppler: With over 60 KHz of Doppler during an AO-40 pass, there must be a mechanism for the user to be able to find the signal, and to track it. This includes then the mechanism for using a constant offset to identify the exact frequency for the remote user uplink.
  • Weak Signal: With the narrowband DSP signal advantage, the uplink user only needs a modest station to produce a usable signal on the transponder.
  • Low Impact: Operating with weak signals with very little margin above the transpodner noise floor permits PSK operation through AO-40 without impacting other users.
  • Ready availability: Many varieties of PSK-31 software are avaiable and since the user uplink is totally operated in QSO mode with no automatic features, it permits anyone to uplink into APRS without special software or prior considerations.

    Some AO-40 BASICS: All it takes to communicate with AO-40 is a small dish on the order of a meter across and a UHF transmitter good for 50W or so and a small UHF beam. These can easily fit in a suitcase for travel as indicated below:

    See antenna details.

    AO-40's ORBIT: AO-40 follows the same arc across the sky everyday but at different times as shown below. And this arc is just the projection of the Earth's equator onto the sky. It goes from horizon to horizon beginning in the East and ending in the West and due South it is up an angle equal to 90 minus your latitude (in the northern hemisphere). By simply pointing the axis of your dish rotator at the north star (simply North and tilted at an angle equal to your latitude) then you only have to swing the dish left and right to find AO-40. Because of AO-40's very high orbit, it is in view for 8 hours or more at a time. You can even add a simple dipole feed to your backyard satellite dish if you like:

  • Backyard dish feed photo,
  • Installed Photo,

    Easy, TV rotator (no elevation) tracking system!
    If you have a good downconverter, low noise pre-amp and circular feed and can hear AO-40 on a "prime-star" type eliptical dish, all you need to track AO-40 remotely is a simple TV rotator. Just point the axis of the rotator at the north star, mount the dish and your UHF uplink beam to the tilted mast, and you can track AO-40 horizon to horizon with just the $65 TV rotator from Radio Shack. This is beacause the vertical beamwidth of the oval Primestar dish is about 20 deg, wide enough to always see AO-40 along the geostationary arc as shown below: click here if image is broken

    APRS AO-40 SATGATE DETAILS: There are several things that the SATgate software should do to facilitate the proper operation of this system.

  • Igating: Captures all APRS-IS packets addressed to AO40xx and passes them onto the SATGATE Pilot channel.
  • Dynamics: If there is not enough APRS-IS data to saturate the uplink pilot, then the pilot channel will recirculate on a decaying algorithm older and older data to keep the channel busy and redundant. But as with any APRS system, as this data ages beyond 30 minutes it must be dropped so as to not violate the netcycle expectations of users.
  • Redundancy: Redundantly repeats all uplink "APRS" data using the original APRSdos algorithm. That is, if a new position is heard from an UPLINK station, that position is injected into the APRS-IS anew. But if not uplinked again, the packet is redundantly repeated on the APRS-IS but at an ever increasing period. Each time it is injected, the period to the next refresh is doubled. This way, new positions only have to be received once to have some redundancy in delivery, but old data will decay out of the system.
  • Messages: The same goes for messages. The SATGATE will acknowledge the receipt of a message either from the APRS-IS or from the UPLINK using the normal APRS process. Then it takes over the responsibility and the decaying algorithm to assure delivery via the AO40APRS system.

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