SSTV + APRS = APRN (Automatic Picture Relay Network)


The SSTV pictures shown in the animation below were transmitted at MakerFaire 2010, in San Mateo, CA. Ham Radio members of the Foothills Amateur Radio Society and the Palo Alto Amateur Radio Association helped demonstrate APRN at the Amateur Radio booth at the MakerFaire. The MakerFaire is one of the biggest makers event in the world, with tens of thousand visitors each year. Specials thanks to NE6RD, AF6DS, K6XOX, KI6VMU, KB6JPA and many others for helping out preparing and running this demonstration of APRN.

This demonstration was designed by KI6TSF to illustrate how geolocation information can be combined with pictures sent over the air. Kids as well as grown-ups had a lot of fun watching it and had a lot of interesting questions. One kid said "Oh this is not your grandfather's ham radio anymore!". APRN was invented by WB4APR Bob Bruninga, the inventor of APRS. More information about APRN can be found at http://www.usna.edu/Users/aero/bruninga/aprntxt.html


Google Earth APRN Fly-By version 1.0 by KI6TSF, 2010
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SSTV pictures can be geotagged with APRS position packets so that receiving stations can layout those pictures on a map. In this demonstration, ham radio operators ran around the MakerFaire grounds with SSTV cameras, GPS, APRS trackers and a radio transmitter. They transmitted SSTV pictures of the various booths located within the site. Each transmission was terminated by a APRS packet carrying the position of the picture. Both the pictures and the positions were transmitted on the same frequency (145.500 MHz - experimental part of the VHF band). The base station located at the booth took care of decoding the SSTV pictures and the APRS packets while generating a live feed of data that Web browsers can consume and display in real time. The received pictures are shown in the Google Earth Plugin. Hopefully your Web browser is recent enough to run the Google Earth plugin and to display the pictures.


The diagram above shows how this could be achieved in more detail. On the transmitter side, an OT+ APRS tracker set in Mic-E mode combines SSTV audio tones with APRS position packets. A packet is immediately sent after a picture transmission by intercepting the SSTV camera PTT release by the tracker. On the receiving side, MMSSTV is used to decode the pictures automatically. AGWPE, a software TNC, is in charge of decoding the APRS packets. A Java APRN program retrieves the decoded SSTV pictures from MMSSTV each time it received a APRS packet from AGWPE. This program parses the APRS packet and does some image manipulation to tag the received picture (not shown here). This program also generates a continuously updating JSON feed describing the APRN data via a Web server. On the viewer side (client side) a Web browser runs Javascript code that retrieves the continuously updating JSON feed and generates Google Earth tours in KML which are passed and interpreted by the GE Plugin.


APRN is a great way of adding useful metadata information to pictures taken from moving operators. From hot air balloons to parachute mobile to bicyclists, sending and receiving pictures annotated with geolocation information can increase the excitment by a great factor. A friendly repeater around the area will allow SSTV and APRS traffic to be transmitted on its input frequency. A typical SSTV picture in Robot 36 mode takes 36 seconds on the air, which is very reasonnable QSO time in ham speak.


APRN can be used on HF to provide receiving stations with the GPS location of the sending station. Since HF is less forgiving than VHF for data transmissions, it would be more appropriate to transmit the APRS packets at 300 baud instead of 1200 baud. A SSTV mode appropriate for HF would be Scottie DX.


APRN could be used via satellite too. Due to the limited number of ham radio operators doing APRN in any given area, this is a great way of reaching more people. Not all amateur radio satellites will allow SSTV and packet transmissions on their transponders. In the case of the ARISS station on the ISS, APRS is already allowed and supported since the station carries an APRS digipeater. With some coordinated effort, it should be possible to send APRN traffic via split frequencies (for both SSTV and APRS) and recombine them at the receiving end.

July 2010, KI6TSF