Low-Cost APRS Hardware - Combining the TinyTrak3 with the Motorola i58sr


Written by Fred Chen, VE7CX

27 December 2004


The Automatic Packet Reporting System (APRS) was developed by Bob Bruninga WB4APR and combines Amateur Radio with Global Positioning System (GPS) technology.  A Terminal Node Controller (TNC) converts GPS coordinates into packets which are then broadcast on a network of digipeaters.  This information is made available on the Internet at map.findU.com or www.canaprs.net and is automatically updated at regular intervals.  APRS is particularly useful for asset tracking particularly where multiple assets (e.g. search teams, emergency responders, mobile field teams, etc.) are involved, or when the location of a specific asset (e.g. EOC, hospital, fire hall, etc.) is important.

I became interested with Automatic Packet Reporting Systems (APRS) in the spring of 2000 because I was intrigued with the concept of combining Amateur Radio with Global Positioning System (GPS) technology.   My main purpose for using APRS was specifically for mobile tracking.  There was no practical reason for having it; I just wanted to be able to do it.  Sometimes, that's the simplest answer to anyone's approach (or obsession) to their favourite hobby!  I already owned a spare handheld 2-metre transceiver and a TNC (which was suitable for APRS after I had the firmware upgraded).  I was only missing a GPS from the setup.  I eventually purchased one and had the basics of what I needed for a mobile tracker.

My First APRS Mobile Tracking Station

My requirements for a mobile tracking station was that it had to be small, unobtrusive, and portable (for quick disassembly when not in use for security reasons).  Powering the system also needed to be functional and easily adaptable to a variety of mobile scenarios (e.g. motor vehicle, bicycle, pedestrian, aircraft, etc.) using standard 12V DC power connections.  My APRS setup at the time consisted of a Garmin 12XL GPS, a Garmin GA 27C remote antenna, a Kantronics KPC-3 TNC (with Version 8.2 firmware), and an Icom IC-2AT 2-metre FM handheld transceiver.  I soon discovered that installing all that equipment in my vehicle for use as a mobile tracker was not as convenient as I expected.   The equipment was bulky because of the multiple cables, and it was a big hassle to disassemble everything when it wasn't being used.  As a consequence, my APRS equipment lived a very short life as a mobile tracker and was kept in storage most of the time.  It became useful once a year during Field Day as a demonstration mode, earning the obligatory 100 bonus points.

I always felt that dedicating a KPC-3 and a Garmin GPS for APRS was a bit overkill, especially since both pieces of equipment could be used for so many other different applications.   My KPC-3 was later required in another application so a replacement for the TNC was necessary if I ever wanted to use APRS again.

A Practical Low-Cost Alternative   

I eventually replaced my TNC with a Byonics TinyTrak3 mobile tracker.  I was first introduced to the TinyTrak a few years ago and was quite impressed by its extremely compact size and functionality.  The TinyTrak measures only 2 1/4" x 1 5/8" x 3/4".   The TinyTrak was especially designed to be a mobile tracker so it didn't need to have all the extra features of a TNC.  I purchased my TinyTrak3 as a build-it-yourself kit in April 2004.  Assembly was not difficult and I accomplished it in a single evening.

The next piece of equipment I replaced was my Garmin GPS.  I acquired a Motorola/Nextel i58sr after discovering that other radio amateurs in the USA were using the phone's built-in GPS in APRS applications.  The i58sr outputs data to NMEA via a serial data cable making it ideal for interfacing to the TinyTrak.  The i58sr does not need to be activated on a cellular phone network, but it does need a SIM card in order to access the phone's functions, including the all-important GPS.  The phone is also capable of calling the Emergency 9-1-1 number in the event of an emergency regardless of whether the phone is activated or not.  However, you should confirm this with your local emergency services provider before relying on this feature.

I didn't have to replace my Icom IC-2AT 2-metre FM handheld transceiver because it was still useful in my particular setup.  However, if you don't have a 2-metre transmitter, I recommend any basic 2-metre transceiver that can transmit on 144.390 MHz.  This transmitter need not be anything fancy.  In fact, there are many older, inexpensive 2-metre transceivers on the used or second-hand market that are ideal for this application. 

The Installation

My i58sr is semi-permanently mounted onto the dashboard of my Toyota Corolla using Velcro tape.  This gives the GPS antenna a good view of the satellites in the sky through the windshield.   Having the i58sr mounted on the dashboard also allows me to easily view the information on the display screen while driving; although, it's always a good idea to keep your eyes focused on the road ahead of you!  Because the i58sr looks like a phone (because it really is one), it tends not to attract as much attention to thieves as a real GPS might.  This is important if you leave your vehicle parked outside and you forget to the take the phone with you.

The Motorola i58sr mounted on the dashboard and situated to the left hand side of the steering column.

To switch the power to the APRS mobile tracker conveniently on and off, I installed a 12V DC 30A illuminated rocker switch (Radio Shack Part No. 275-712) and wired it directly to the vehicle battery through a fused Saratoga PowerPanel 4 DC power distribution panel.   I intended to power the GPS this way too, but the charging port on the serial data cable was not compatible with the DC charger I had.  So, the power to the i58sr is controlled on the phone itself (Note: NMEA data output requires a lot of power so having a spare battery around may be useful).   I terminated all DC connections using Anderson power poles where possible.  I mounted the 2-metre transmitter underneath the dashboard and out of sight using industrial-grade Velcro tape.  Velcro tape was also used to affix the TinyTrak3 below the steering column.  That way, I can still view the status LEDs on the TinyTrak if required.  Below are some photos of the installation in my 1990 Toyota Corolla:

Illuminated rocker switch (bottom left) Close-up of the illuminated rocker switch in the "ON" position.
TinyTrak 3 mounted underneath steering column. Close-up of the TinyTrak3 in operation.
IC-2AT set at 144.390 MHz and mounted underneath the dashboard with industrial-strength Velcro tape. Quarter-wave antenna installed using a trunk-lip mount.

Equipment List

Below is the list of equipment used in my APRS setup.  All prices are indicated in US Dollars and are based on buying mostly used or second-hand equipment.

    Primary Components

    Secondary Components

   Optional Components

Configuring the i58sr for APRS

In order to access any of the i58sr's functions, a SIM card must be installed.  So, when purchasing a used i58sr, make sure you purchase one with a SIM card.

I changed the default display settings of the i58sr so the GPS comes up as one of the power-up applications.  After selecting the GPS function, there will be two options to choose from:  Position or Interface (see photo to the right).  

GPS Interface

First, you will need to change the interface for the GPS from NMEA OUT to On because the phone will always default to Off after the power has been turned off.  This is probably one of the phone's battery-saving features since leaving it in the "On" position will drain the battery quickly.

GPS Position 

To obtain a GPS reading, select Position.  The display will then show the details of the last successful GPS reading taken (see photo to the right).  To obtain a new GPS reading, select RFRSH.  You will then be prompted by the following question:  "Satellite Data Is Outdated Continue?"  Select Yes and the GPS will begin scanning for satellites.  It will often take about 2 to 4 minutes before a GPS position is obtained.  

If it is successful in obtaining a position, the display will be updated with your new coordinates.  If it is unsuccessful, you will notified by a short audible tone and the following error message will then be displayed on the screen:  "Unable To Locate Sats.".  Select Retry, or move the i58sr to a new location so the GPS can get a better view of the sky before selecting Retry again.  Once its initial position has been determined, subsequent GPS readings will generally take much less time, often only a few seconds.

What's My Position Now?

For APRS, I use the callsign VE7CX-9 (the SSID "-9" suffix denotes a motor vehicle).  The current location of VE7CX-9 can be found on findU.com or the Canadian APRS Network. When I have my mobile tracker on, I have it configured to transmit its current location every 30 seconds.

Need Help?

The idea of putting together an affordable, low-cost APRS setup using the TinyTrak and an i58sr was presented by me as a Powerpoint presentation at the monthly general meeting of the Vancouver Emergency Community Telecommunications Organization on 05 January 2005.  For a copy of the presentation in PDF format, please click here.


This page was last updated 06 January 2005.