the car APRS system
One of the things that struck me was how much of a pain the system was. It had always occurred to me that the pieces of technology were large, power hungry, and difficult to set up and take apart every day of biking (during the bike trips), but I assumed that moving everything into a car would make it a little more manageable. This was not the case.
The wires were continually in the way. The GPS was in a bad location. The connector for the GPS wanted to detach often. We had to remember to put the antenna for the radio in the window. It worked and was quite cool, but it was still quite a mess.
By the time Tobi left us, I had built two electronics projects that would help me make the system smaller, use less power, and be easier to transport. For the car, at least.
This is the result: the car-installed APRS system.
The circuit that you see above is a PIC microcontroller (in the big grey thing), a serial EEProm (to its left), a serial interface chip (below the EEProm), and a modem chip (lower right). The four work together to listen to the GPS, send amateur radio APRS packets onto the APRS network, store copies of GPS packets for later retrieval, and put copies of the GPS packets onto a second serial port (for a laptop or PDA to hear). The plastic box houses the 12V to 5V power supply, which is fed off of the lighter socket line in the center console.
For this system, I wanted the installation to be invisible. I didn't want to have to install and remove parts in order to use it. I want to turn on the car, have the GPS start up, have a serial cable closeby that I can plug into a laptop or PDA for maps (if I want to), and have the transmitter operate automatically. In order to do that, I had to install a GPS in the car.
The GPS is embedded in the dashboard. The installation took about three hours, but went fairly smoothly. Of course, you wouldn't know that by looking at this picture (taken in mid-installation):
The GPS is a Deluo serial GPS for Laptop/PDA. It has a magnetic base, which is currently stuck to the top of the radio inside the dashboard of the car. (There are also some wire ties holding it in for strain relief.) The GPS cable snakes down through the dashboard under the center console, which is where the APRS circuit will eventually be housed. Right now the circuit is actually on the floor of the back seat while I finish shaking out the last few bugs.
The circuit also connects to a 5 watt transmitter, which is under the passenger's seat. It's a Ramsey Electronics kit which I got for Christmas last year: the FT-146. (Apparently they've since discontinued it.) I also had a custom crystal grown for this application to put the transmitter at 144.39 MHz for APRS.
The kit went together easily, but the calibration is incorrect; the parts they shipped weren't exactly what they wrote the calibration information for, so I had to figure out how to properly tune it myself. The results are reasonable, but not stupendous: it's really about 3 watts instead of 5. No big deal for this application.
The antenna on the car is actually a coat hanger which I turned into a base loaded quarter wavelength dipole. It's not the best radiator, but it's well tuned for 144.39 MHz and easily outperforms all but one of the antennas that I own. (That one antenna is a homebrewed J-Pole, which I use on my bicycle.)
And the result? I can do bike trip drive-throughs without setting up a GPS. The circuit will store the path that I travel for later retrieval, making it no-fuss. If I want maps, I can pull out my PocketPC or laptop and hook it up. And the car can be located on the Internet.
Update, 6/2005: the GPS died a few months ago, and the replacement's RS232 lines run at a slightly lower voltage. Result: the circuit can't receive the GPS' position. I've temporarily taken the circuit out of the car until I have time to rewire the RS232 receiver for 0/+5V (RS232C) instead of -12/+12V (RS232). In the mean time, I'm using my Kenwood TH-D7A for APRS in the car, which uses a slightly different URL to locate my car.
Update, 8/2012 (!): Somewhere around 2008, I replaced most of this with a Micro-Trak 300 coupled to a Micro-Amp 3 and a professional 2m antenna. I no longer have GPS logging in the car, but it's a smaller package that I stuffed in to a metal enclosure (as RF shielding). The latest firmware for these is really quite good; transmit on turn is a very welcome addition. Sadly, the few local APRS repeaters seem to have packed up shop, so my car's location isn't generally available on the internet any longer. If you're lucky, you might find it at aprs.fi.
Also this month, I've written a program for the iPhone to drive my TH-D7 in KISS TNC mode. The TH-D7, while a good radio, is cumbersome to use with a GPS attached and has some artificial limitations on message size that I don't like. So, my new iPhone app (which will sadly never be in the App Store because of licensing problems with the serial cable manufacturer and Apple's third-party libraries issue) puts the TH-D7 into KISS mode and forms packets for it, using the iPhone's built-in GPS to boot. It's not yet perfect but it does let me send longer messages in MIC-E form. This combination broadcasts as AB3AG-8 and is somewhat more robust than the Byonics kit.
And that wasn't enough. I'm planning to photograph Burning Man this year, and my wife's not a camper. At least, not when there's no running water. So, I sez to myself, how can I send her updates that I'm okay when I'm in the middle of nowhere? Answer: APRS. There are APRS-to-email gateways. Unfortunately the APRS specification limits the size of the message somewhat artificially because of the transport it uses. So I wrote an APRS-to-Twitter gateway, which reads my MIC-E encoded packets and forwards them to my Twitter account. If you see any tweets tagged #ham that have a URL which brings you to this page, you probably saw a tweet that came via ham radio.