Hardware Hacking Projects for Geeks

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Part II: Advanced Hacks, Tools, and Techniques

Project Overview

This hack uses the APRS technology developed in the amateur radio com-

munity. Depending on the PDA or laptop and operating system you use for

this project, you should find a large number of software packages.

You will be putting together a position transmitter package and a data

receiver package. The transmitter gathers GPS data and sends it out using

an integrated two-way radio. The receiver package listens for the data

packets sent by the transmitter using a second integrated two-way radio

and then displays the decoded position of the transmitter on its screen.

More than one transmitter can be picked up on the receiver. Each position

transmitter has its own identification information, so you can easily track

hundreds of objects. You can see an overview of how the transmitter and

tracker work in the diagram in Figure 12-2.

You will construct the transmitter electronics and receiver electronics, and

connect them to the FRS radios and one GPS unit.

Hardware Assembly Instructions

The following instructions walk you through the construction of the trans-

mitter and the receiver for your remote object tracker.

1. Build a transmitter

You will first assemble the transmitter out of a TinyTrack electronics kit,

a low-cost GPS receiver, and a two-way radio. The kit includes all the

required electronic components and a circuit board, which means the total

assembly time should be only a few hours. The GPS receiver connects to this

circuit board via a serial port. Finally, you’ll connect the two-way radio to

the circuit board through its built-in headset and microphone ports.

APRS

In November of 1992, Bob Bruniga

(WB4APR) showed a DOS version of

an APRS system at the ARRL Digital

Communications Conference (DCC) in

New Jersey. Bob created the system

to report position, weather, and mes-

sages between people.

Figure 12-2: Tracking system overview

FRS radio

TinyTrack 3

GPS

Position transmitter

PDA

TNC

FRS radio

Position receiver and mapping software

Project Overview

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235

Chapter 12, How to Build a Remote Object Tracker

Locate a GPS receiver

There are many inexpensive new and used GPS receivers available on the

market today. You should choose one that supports NMEA (National

Marine Electronics Association) output from an RS-232 interface. Some

appropriate models are:

• Any modern Garmin, including the eTrex, GPS12, or GPS V

• Any modern Magellan, including the Meridian and Sportrak series

• DeLorme Tripmate (with a slight modification)

• DeLorme Earthmate (with a data converter)

The modern units from Garmin and Magellan have an RS-232 compatible

serial output. A cable from the manufacturer is all that is needed for the

external connections.

The DeLorme Tripmate can be used if it is slightly modified. To modify the

Tripmate for use with this hack, simply wire up two DB9 male connectors as

shown in Exhibit C. The Tripmate operates off of four AA batteries that you

install inside the case. If you want to run it from an external power source,

add a connector to the DB9 connector on the right side labeled J1. J1 will

accept a voltage from 5V to 6V, so you can connect an external battery pack

of four AA or larger batteries.

The DeLorme Earthmate cannot be modified in the same way. It sends out

data in a format incompatible with NMEA. However, its data output can

be converted using the GST-1 NMEA “sentence converter” available from

http://www.byonics.com. This sentence converter changes the non-NMEA

data that the Earthmate sends out into the standard NMEA data required

by this project.

I used a Garmin eTrex Vista with PC interface cable, which I bought

on eBay, for my system. This eTrex unit requires a special serial port

cable, which I also bought on eBay.

Obtain and build a TinyTrack

The TinyTrack is a small GPS position encoder that will transmit GPS

data over a radio. A complete kit that includes all the necessary parts

as well as a programmed control is available from http://www.byonics.

com.

Build the kit according to the instructions. It should take an hour or

two to solder all of the parts into place. After you put the kit together

it should look like Figure 12-3.

Figure 12-3: Assembled TinyTrack

Hardware Assembly Instructions

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