NI4S Mobile APRS Tracker

Public Service Applications Andy Peterson, NI4S -- November, 2002

Introduction
My first introduction to APRS was as an adjunct to public service activities. I'd been on 2m packet back in the early 1990's, but hadn't done much with it recently.

Here, I'll describe my mobile tracker setup. I think the setup I've constructed is fairly versatile, even if it's a bit non-intuitive. I hope to shed some light on the reasons I've decided to build the setup this way.

TinyTrak II
The heart of the mobile tracker setup is the Byonics TinyTrak II. This is a small board based on the PIC microcontroller that's designed as a beacon-only APRS tracker. It has some features that REALLY stand out, in my opinion:

Only transmits positions when it's receiving good GPS data.
Has "smart beacon" mode, so transmissions become infrequent when the vehicle remains stationary.
Has the capability to retain two configs, allowing some on-the-fly flexibility.

As of November, 2002, I have not run across a similar device, designed for mobile APRS use, that's as flexible for transmit-only beaconing. The specific issues I have with some of the current products:

Alinco DR-135TP The TNC is very "dumb," not even implementing the full TNC2 command set. If the GPS gets disconnected, the TNC keeps beaconing the last known location -- no matter how old. Fixed beacon interval; must switch it on the ignition, or risk tying up the channel with frequent position updates for your stationary vehicle.
Kenwood D700a A hefty price tag =) Again, the fixed beacon interval is bad for APRS if you plan to run a tracker full-time. The integrated receive display is really cool, though.
Kantronics KPC3+ A bit expensive for a dedicated mobile tracker, but the ability to easily plug in a PC and make it a full APRS station (with advanced digipeating, no less) is attractive.

I chose to integrate my TinyTrakII into an enclosure that would provide all the power, serial, and TX/RX/PTT connections I'd ever need... at least for a little while. I'll talk more about that in a bit.

TripMate GPS
I got a refurbished TripMate GPS for $50 on DeLorme's web site. Some small modifications are needed in order to make it send NMEA data without an initialization string. There are numerous web guides on TripMate conversion, so I won't go into detail here.

Suffice to say that I tied pin 2 to 3 for auto-start, and made sure pin 9 was wired to the +5v input of the GPS, so I could provide power via the DB9 connector. Save your cash on the "GPS inline power cable" -- all they do is tee in and provide this +5v on the correct pin!

HT and amp
I picked up an old Icom IC-2AT handheld on eBay. This is a perfect APRS rig-- frequency selected by thumbwheels, not much to go wrong. Coupled with a "2w in, 30w out" brick amplifier, it makes a decent transmitting platform.

The HT I got came with a NiCd battery of unknown age. I took the cells out of the battery case, as well as most of the electronics, and added a variable voltage regulator and heatsink, so that I could provide +12v to the "charge" connector on the battery and have it supply the HT directly.

Be careful -- most of these HTs will blow up if you try to provide them 12v directly on the battery contacts! If you want to power them from 12v, make sure you know what you're doing!

"Tracker box"
I mounted the TinyTrak in a RadioShack plastic enclosure, about 5 x 7 x 2". I've heard people say that it's better to mount TinyTrak in a metal enclosure, but I've not experienced any RF problems. I wanted a somewhat large box, so I could use it as a junction point as well-- just about everything in the APRS tracker plugs into the tracker box. Here's a diagram of how it's put together for "tracker mode:"

A brief walkthrough of the subsystems:

12v/5v Power
There are two inputs, one for 12v battery and one ignition-switched. The ignition contact drives a relay coil that switches the 12v battery power on and off. I later added a switch that lets me bypass the relay to provide 12v to the unit full-time. Since TinyTrak has the "smart beacon" mode and drains little current when idle, I don't find it necessary to turn the tracker off when the vehicle is off.

After the relay, 12v power is supplied to the TinyTrak (which has an on- board regulator), some external connectors (for the HT, brick amp, etc), and a 5v @ 1A regulator. The regulator's output goes to pin 9 of the GPS serial port, so the GPS gets powered through its serial cable. This works on the TripMate GPS, but I CAN'T GUARANTEE it'll work on others.

TX/RX/PTT
TX, RX, and PTT are brought out on a 1/8" stereo jack, which maps to the Icom IC-W2A HT I used to use for the tracker box. I've made a 1/8 stereo to IC-2AT (1/8 mono + 3/32 mono) cable for the HT I now use.

Serial data
As mentioned, the GPS port is specifically wired to talk to the TripMate GPS. It acts like DTE and is also used to connect to the PC for programming, using a null-modem cable.
The "GPS Thru" port is something I added. I took the "RX" pin from the main GPS port -- received data from the GPS -- and wired it to a second DB9 acting as DCE. This can connect to a laptop to provide NMEA data for various purposes. More on that in a bit.

Some views of the tracker box:

Laptop mode
I wanted to add the capability for receiving APRS positions from other stations, as well as the ability to self-navigate. Most of my public service APRS experience has been with large bike tours covering hundreds of miles of unfamiliar roads. The ability to display my own position, updated frequently, is essential.

When adding this capability, I decided it was worthwhile to keep the TinyTrak as the beacon platform. This way, if anything goes wrong with the laptop, TNC, etc, the data still goes out. I don't have to worry about beaconing-- the TinyTrak keeps on doing its job.

For receiving positions, I use a separate TNC. To connect the RX audio to this TNC, I use a 1/8" "Y" connector, plus a custom-made "RX only" audio cable. This lets me tap into the existing TinyTrak-to-HT signal path with minimal disruption.

The TNC connects to a laptop via serial cable. The only data available on this cable is the position of other stations, plus any of my own packets that happen to be digipeated. For position data, I must rely on a separate GPS connection -- the "GPS Thru" connection on the Tracker Box.

I use the APRS+SA software package, which is a powerful APRS platform on its own. It sends APRS decodes and pertinent data to the Street Atlas USA program for plotting on street-level maps. This is a very powerful combination for the type of bike tour events I've done.

I configure APRS+ to receive positions from the TNC and plot them on the SA map. I configure SA to track from the GPS input, so that my position is drawn with SA's icons on the map created by APRS+. This has some interesting consequences:

APRS+ doesn't know where I am. This means that things like the map center need to be set as "fixed" at some sensible point.
SA can use the "automatic pan" feature to recenter the map should the chosen center point be somewhere far away.
Things like range filters, etc, have to be set up with respect to a fixed point. This isn't usually a big deal.
APRS+ isn't the one sending position beacons. This sets up an interesting "feature." When the TinyTrak sends beacons and they get digipeated, the digipeated packet gets received and plotted on the map. Together with the "current position" marker that SA generates, I can see where I am, and where everybody else THINKS I am.. similar to APRS+ "digipeat" announcement, it lets me know how far I am from where my last position report was digipeated.

The system isn't perfect, but it works pretty well. I wish it were a cleaner setup, but I think it provides a good balance and minimal reconfiguration for when I wish to add the laptop display to the system, which isn't often.

Here's a diagram of the setup with laptop included:

Conclusion
Hopefully, this has provided some idea of what a public service oriented mobile APRS setup can look like. I'd appreciate any feedback or other experiences you may have. Send email to ni4s at arrl dot net. 73!