03 Tracking Mobile Stations

APRS Article 3

November-December 1999

APRS Tracking Mobile Stations

By Jeff Robbins

VE3JTR (Voice Call)

VA3JTR (APRS Call)

When I discovered APRS I knew that tracking my vehicle was something I wanted to accomplish. Some people ask me, ‘Why would I want to do such a thing?’ I can think of several reasons.

One: Safety. During winter, my YL (VA3EMH) often takes my Jeep to Kingston, and using APRS I can see at a glance where she is at anytime during the trip. If I notice that the icon on the screen has stopped, I can get on the radio and find out what is going on.

Two: Theft. The APRS equipment is hidden and locked away quite securely. Should my vehicle be stolen, I will be able, perhaps with the help of my fellow APRS hams, locate and provide police with an accurate position report of my vehicle.

Three: Fun! This stuff is just way too cool to ignore.

There are several ways to accomplish this goal. If you happen to have a laptop computer, you could just take your laptop with you in the car, and update your position manually as you move around. It will work, but not really all that conveniently. All of the APRS software packages allow hookup to a Global Positioning System Receiver to automatically update the software with the current location, then the APRS software can transmit the new locations. That would work OK, but not everyone has the luxury of owning a laptop, so another option is needed. Enter the idea of a standalone tracker. No computer needed, the GPS connects directly to the TNC, and the new positions are transmitted out at regular intervals.

So what equipment will we need to put this thing together? As you may have anticipated, a basic standalone tracker will need a GPS, a TNC, and a radio to get the information out into the RF world. Lets talk for a moment about each item.

GPS Receiver. This is the item that receives radio signals from very precisely orbiting satellites, and is able to calculate its position on earth. Details of how a GPS receiver works is beyond the scope of this article, but if you are interested, a great place to start is <http://www.howstuffworks.com/gps.htm>. The key thing to look for in choosing a GPS for APRS, is a serial port. The serial port is essential, as it is the means by which we get the position information to the TNC. The protocol used by the GPS to communicate with the outside world is NMEA-0183. All serial port equipped GPS receivers, except aviation GPS, use this protocol. This National Marine Electronics Association protocol is a collection of sentences that contain various combinations of latitude, longitude, altitude, course and speed, and many other marine related variables not utilized by APRS. I bought a Lowrance GlobalNav 212.

TNC. The only tricky thing about choosing a TNC is that is has to be able to receive the NMEA sentences from the GPS and process and transmit them out as APRS packets. Most late model TNCs come equipped with this ability already, and older TNCs usually have firmware upgrades available to add APRS capability. I am a Kantronics fan, so my mobile station is using a KPC-3 (non plus) with version 8.2 firmware.

RADIO. Any radio that is capable of 2m packet operation (1200 bps) will work fine. I personally use a Radio Shack HTX-202 connected to a 5/8 wave whip.

HARDWARE CONNECTIONS

Connecting the radio and TNC is pretty straightforward for packet radio experts and rookies alike, check your TNC documentation. The GPS – TNC connection needs a little research as every GPS seems to be different, but the actual connection is simple. Your GPS will have a serial port with at the very least, a Data Out line and a ground. You need to connect the Data Out line from the GPS to the Data In line on the serial port of your TNC. The TNC serial port is a DCE port, so Pin 2 on a DB-25 port is Data In. Connect your ground to Pin 7 and you are ready to roll. If your GPS is capable of receiving data it will also have a Data In, which can be tied to Pin 3 on your TNC DB-25, but is not necessary for a standalone tracker. Most GPS units are limited to 4800 baud on their seial port, but many newer GPS units are capable of higher speeds. Just be sure you know what speed your GPS is set to, and set your TNC to match. Connect your radio as normal for packet operation.

TNC SETTINGS

Here are my settings I use for my tracker:

INT TERM

Set the interface to terminal so that all functions are available.

ABAUD 9600

My GPS is capable of communicating at 9600 baud, but you will need to find out what your GPS can use. 4800 baud, at least, seems to be standard for most GPS units.

MYCALL VA3JTR-12

Set my callsign and SSID. An SSID of 12 will bring up a Jeep on the APRS map. Most people use 9 for a car. There are also many other choices. Surf on over to <http://www.dididahdahdidit.com/text/symbols.txt> for a complete list.

GPSHEAD 1 $GPGGA

GPSHEAD 2 $GPRMC

The KPC-3 has four buffers to store GPS data. In this case I am storing two sentences in two of the buffers. $GPGGA contains latitude, longitude and altitude data. $GPRMC contains latitude, longitude, course and speed data. I do not use the other two buffers numbered 3 and 4. For a complete overview of NMEA, set your sights to <http://vancouver-webpages.com/peter/nmeafaq.txt >.

LTP 1 APRS VIA RELAY,WIDE,WIDE

LTP 2 APRS VIA RELAY,WIDE,WIDE

I am assigning a path to each of the buffers. While I use the same path for both, you can set different paths for each buffer if you desire.

BLT 1 EVERY 00:02:00 START 00:00:10

BLT 2 EVERY 00:02:00 START 00:01:10

These lines set the bulletin frequency for each buffer. I am setting each buffer for two minute intervals, but the START delay puts them one minute apart so my TNC transmits once a minute alternating between the two GPS sentences.

LEDS OFF

The KPC-3 can disable the LEDS on the front panel in order to save power.

INT GPS

I am now setting the TNC to GPS mode, so that when it is restarted, instead of talking to a terminal, it now will look for and store GPS data in the buffers.

RESET

Reset the TNC, connect your GPS to the serial port, and start tracking. Typically, the TNC will not start transmitting packets until sentences are received from the GPS. The GPS may take several seconds to lock on to the satellites, and will not send NMEA sentences until the lock, so do not be surprised if the TNC doesn’t transmit for a minute or more.

And that should get you on the air! If you have trouble, <http://www.dididahdahdidit.com/kpctracker.htm> has a great troubleshooting article to get things sorted out.

Next time, I will discuss my new Kenwood TH-D7A radio as an APRS unit. In the mean time, check out the D7A review in the May-June 1999 issue of TCA. This was an excellent overview of the entire radio, so instead of going over the same material, I will focus on the APRS aspects of the radio. Also please note that my webpage address has changed, but all of the material is at the new site: <http://www.qsl.net/va3jtr>. All of the links listed in this article are live in the web version of the article, just read this article on my website and click on the link you are interested in!