This is extracted from a post to the GPS-TalkUsers list on Topica and it may
be one of the most fascinating things I've seen in a long time.  If you're
interested in GPS technology for the blind, this is the place to hang out
because there are some genuine gurus on this list (like this woman,
obviously) who can talk about, envision and even design stuff that'll knock
your socks off.

 --
 Walt Smith - Clearwater, FL
 [log in to unmask]

There is not a lot to tell about APRS. In areas where many hams can
"digipeat" your signal, you transmit your GPS position over packet radio and
everyone can receive it. That's the case here in the Bay area.

Bill, our city emergency coordinator for example is very in to APRS. In his
car, he has a ham tranceiver with a built-in TNC (terminal node controller)
that is constantly on, transmitting his GPS position on frequencies that by
convention have been designated for this part of the ham hobby.  At home,
with a TNC attached to my scanner, and my computer running software that can
interpret the information, I can pull up a map that shows an icon of Bill,
driving around in his car. At any given time, the map has icons for many,
many other hams as well who have their systems broadcasting their position.
If there were an emergency, and I had a sighted person to read the map, I
could locate Bill, a good thing since I'm his assistant coordinator for our
city!

Another friend has a "tiny tracker" system in the trunk of his car. I can't
describe it as I've never seen it, but it is the newest thing in APRS.  His
GPS antenna is the only thing visible. His system is always on. One day his
car was stolen. He simply went to the police with his laptop, running the
APRS maps, and fascinated, the police were able to easily track and recover
his car within the day.

To put a system like this together you can buy all new stuff which is
expensive, surface mount and just plain cool. Or you can cobble together
cheap stuff from a flea market. You can buy a cast-off GPS, TNC, ham
tranceiver and laptop.

To further complicate things, a TNC can now be emulated in software (think
of it as a radio modem) and so you can just use a laptop, a tranceiver and a
GPS. Or, some tranceivers have a TNC built-in, so you can use a
laptopwithout a sound card, a GPS and that tranceiver.

For the sending system, you can use even a very old laptop, because it
doesn't need much power. It just has to receive the incoming GPS data and
send it on to the TNC. I do not know if there exists a GPS with TNC
built-in. The receiving system, on the other hand needs to be Windows or
Linux-based and the interface is all graphical. Various receiver programs
exist which do the job of interpreting the data; some work in conjunction
with Delorme and others work with the public domain Tiger maps which aren't
that good.

Many hams are completely absorbed with APRS and others think it is truly
dumb. I think it could be a useful way for my friends to know where I am. I
have used a TNC together with an old laptop to retrieve the raw data off the
air (in this case the laptop was just a dumb terminal) and the raw data
looks a lot like NMEA sentences with call signs preceeding each sentence.
This was trivial since I'm already a ham. I just bought an old TNC from the
flea market for $10, got my fiance to solder up a cable for it using
pin-outs I got off the web, then I connected a 8088 laptop running Como to
the TNC's serial port. I tuned my ham radio to the right frequency, cabled
the whole thing together and could read all the raw APRS data I cared to
read.

There is lots of technical information about how it all works, and much of
the software and all the specs are of course open source.

The software only TNC programs are typically shareware, as are the APRS
mapping programs. Local hams are typically happy to solder for blind hams,
so getting cables isn't a problem if you're resourceful.  You can also surf
Ebay for APRS gear and find gadgets in every price range.

I haven't looked at the tech info though; it just isn't interesting enough
to me untill I have a tiny system that can implement this. I will probably
run the software-only TNC on a laptop, but I might decide to get a radio
with a built-in TNC instead.