APRS and ARISS
Details of the Automatic Packet (Position) Reporting System are available from many sources. An excellent example is The ARRL Handbook for Radio Communications 2011 or later edition.
My APRS experience has been entirely on the Kenwood TH-D72 hand-held dual band transceiver, which has GPS (Global Positioning System) and APRS built in. I don't know to what extent the following discussion applies to other hardware.
GPS, when enabled, tells me my latitude, longitude and altitude. APRS assembles call sign, latitude, longitude, altitude, some comments, and possibly the frequency to which I am listening on my alternate band, into a “Beacon” packet for transmission to other nearby APRS devices. If my packet reaches an APRS node connected to the Internet, my call sign, location and other information will be posted on a Google Map available to anyone who browses the aprs.fi website and enters my call sign. However, even if no Internet connection is reached, APRS could, for example, allow a group of people out of sight of each other to keep in touch. And if you happened to be stranded out of sight of the highway, you might attract the attention of a passing ham.
An interesting use of APRS is attempting to reach the amateur APRS digipeater on the International Space Station. As I explain later, this is not much different from reaching any other digipeater (except that the traditional “line of sight” limitation of VHF range takes on a new meaning).
Here is a photo of my Kenwood TH-D72 transceiver and Arrow Hand Held antenna (supported by a photograhy tripod) waiting to send an APRS packet.
Although the photo shows only horizontal elements of a length suitable for UHF, the complete Arrow antenna would have additional vertical elements of VHF length. This is intended for communication with a satellite, where the uplink is VHF and the downlink is UHF (or vice versa). The two director elements are cabled thru a diplexer to a single BNC cable. I don't need the dual-band capabilty because APRS uses the same frequency, UHF in this case, for up and down. I bypassed the diplexer and removed the longer elements for a better fit in the window of my condo.
I have not yet managed to reach any satellite other than the International Space Station, and probably won't with my 5 watt hand held. However, my experiences may be of some interest to other hams.
I find the heavens-above.com site a good way to find out when the “big bird”, abbreviated “ISS”, is coming my way, though there are alternatives such as amsat.org. Be sure to get all passes, and not limit yourself to the visible ones. (The Space Station is visible only when the sun shines on it but not on you.) South West or South South West are best from my location, when the elevation is low enough that my antenna doesn't touch the aluminum window frame.
I use the menu to set the “Packet Path” on the Kenwood to ARISS, the Amateur Radio International Space Station. (At times when the Space Station is not available, another node will answer, probably SSI on Salt Spring Island). I press the “TNC” (terminal node controller) key once or twice until “APRS12” shows on the screen. “12” means 1200 baud. The “A/B” key switches to the “A” band, the default band for APRS. Remember I am listening for voice on the “B” band). Then I tune to the appropriate frequency (the Space Station has temporarily changed its APRS frequency from 145.825 MHz to 437.550 MHz ). Set the Beacon “Method” to “Auto” with “Interval” about 1 minute. Too short an interval could fill the sky with your packets and be a nuisance to other hams.
Heavens-above shows three things for a Space Station pass at your location (gridsquare or latitude and longitude), the second of which, the most interesting, tells us the time and compass direction of the highest elevation above the horizon. It also shows the “rising and setting” times and the compass directions for the earliest and latest times the bird will be higher than 10 degrees above the horizon. (amsat.org refers to AOS for “acquisition of signal”, LOS for “loss of signal”, and Azimuth instead of compass points. Note that amsat.org uses UTC time).
When the Space Station is approaching its maximum elevation I make sure that “BCON” is visible on screen to indicate beacons are enabled, otherwise press the “BCON” key once to make it so.
Each APRS beacon packet contains call sign, latitude and longitude if known, and a short message of your choice such as “welcome to beautiful White Rock BC”, etc. The procedure for contacting the Space Station is the same as for any other use of APRS except for the frequency (terrestrial APRS in North America usually uses 144.390) and the “Packet Path” referred to above.
The reward for all this preparation is you may get a reply from the Space Station acknowledging that its digipeater recognized you. And perhaps it is a step closer to familiarity with APRS.
For some reason I have found the Space Station more interesting a few weeks back when its digipeater frequency was 145.825. Back then I several times received packets from as far away as southern California. And once when I had forgotten to switch off, the mag-mount antenna in a corner of my living room picked up a ham driving 100 klicks on a highway in Nevada. However this is nothing like a QSO. I know that I have reached the digipeater but have no idea who, if anyone, might have “heard” me.
I have no idea why the change of digipeater frequency would affect the results.
I have found the Kenwood TH-D72 menus fairly easy to use, but if you prefer, all these settings can be set and backed up from Windows XP or Windows 7 through a computer USB connection. All the software you need can be downloaded for free.
In summary, APRS has some more exciting uses than just telling your boss where you stopped for coffee !