the car APRS system
While Tobi was staying with us, we went on a tour of the west coast. We took along a fair amount of my amateur radio gear and used my bike trip tracking system to track our route.
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.