Schlatter Family Site

My APRS
Mobile Station

The photos below show my APRS stand-alone mobile tracker.  The system uses a Garmin eTrex Camo GPS receiver, an AEA PK-12 packet TNC, and a Yaesu FT-470 handheld 2-meter transceiver.  This setup is not pretty but it gets the job done.  Below each photo is an explantion of what you are seeing.  The system is in a plastic box that I shove under the front seat. 

This is obviously a homebrew project.  All parts, except the big plastic box, came from Radio Shack.  I have not filled in a lot of details as I figure any amateur radio operator should be able to look at this idea, modify it for himself/herself, and put together an APRS stand-alone tracker -- this is just one way to do it. 

As with anything, give it a smoke test.  Put it together and fire it up in the house.  Turn on your main station computer on which you track APRS stations and you will see the stand-alone tracker broadcasting its location -- your house.  Once it's running fine inside, disconnect the power and antenna, take it to the car/truck, plug it into vehicle power and an antenna, and let it rip. 

I have two radios in my car -- this one and an dual-band 2 meter-450 MHZ mobile rig.   I have two antennas -- one is a 1/4-wave 2-meter whip for the APRS stand-alone tracker, the other is a dual-band antenna for the mobile rig.  The APRS antenna is a through-the-glass antenna mounted on the rear window.  The antenna for the mobile rig is mounted in the center of the vehicle roof.  I have not experienced any problems with interference between the two.

What's in this photo

• Big plastic box (cover not shown) that holds everything except the GPS receiver.
• Garmin eTrex Camo GPS receiver -- on the left, with a power-data cable connected.
• Red and black main power cable, running out of the main plastic box to a cord that plugs into the cigarette lighter where it gets 12 VDC.
• Silver thing on the left end of the big plastic box is the power supply box.   More about that later.
• Round black plug at the bottom right of the big plastic box provides power to the Garmin.
• At the bottom center-right is a black plug with a silver plug connected.  This is the data connection from the GPS to the TNC.  The black plug is the serial connector on the GPS power-data cable, the silver plug is a serial connector going to the TNC.
• Black box on the right, under the handheld radio is the AEA PK-12 TNC.
• Radio is the Yaesu FT-470.  The grey plug stuck into the top of the radio is the control line to-from the TNC.  No antenna is connected.   When the system is in use, coax to the antenna runs out the same hole as the power cord (lower left corner of the box).
• The stuff wadded up between the TNC and the side of the box is just that -- plastic bubble wrap wadded up to keep the TNC from moving around.
• The red thing lying on top of the TNC is a big rubber band that I use sometimes to hold the lid on the big plastic box.
• It was carefully designed to look like a rat's nest.

How it works

The system needs:

• Power:  12 volts DC to the transceiver, 9 volts DC to the TNC, and 12 volts DC to the GPS receiver.
• A data connection from the GPS receiver to the TNC.
• Control connections to/from the TNC and the transceiver.
• Antenna to the transceiver.

And this is how I put these connections together.

Power. 

• Notice the red and black cord running out of the hole in the bottom left corner of the plastic box.  This is the main power cord.  It terminates into a 2-conductor plug so I can connect it to a normal power supply or to an automobile cigarette lighter plug.  Connected to the main power cord is a cord that plugs into the car's cigarette lighter -- that's the coiled black cord at the top center of the photo, between the plastic box and the keyboard.
• Notice inside the big plastic box there is a small box with an aluminum cover -- that's the shiny item on the left side of the big platic box -- call this the power supply box.  The red and black cord takes 12 VDC into the small black plastic box with the aluminum cover.  Inside that box is a 9-volt DC regulator mounted on a PC board.   12 VDC from the main power cord goes into this regulator and 9VDC comes out.   The power cord for the TNC connects to the output of the regulator to provide regulated 9 VDC to the TNC.
• It does not show in these photos but, mounted on the bottom of the power supply box is a barrier type terminal strip to which the main power cord is connected.  This provides a place to get 12 VDC positive and negative, directly from the main power plug. 
• Notice in the bottom left corner of the big plastic box are two round black connectors, one plugged into the other.  The left-hand side of this connection is a female cigarette lighter jack with its leads connected to 12 VDC at the barrier strip on the side of the power supply box.  The Garmin GPS receiver has a power-data cord; the power plug for the GPS receiver is plugged into the female connector, thereby picking up 12 VDC.
• In place of a battery, the Yaesu has an automobile power adapter.  The power cord from that runs to the terminal strip on the power supply box where it picks up 12 VDC for the transceiver.  Underneath the handheld is a coiled, shiny black cord -- that's the power cord for the handheld.
• Essentially, the power supply box serves as a power distribution center -- 12 VDC comes into the power supply box where 12 VDC powers a 9 VDC regulator and 12 VDC is connected to a terminal strip.  9 VDC goes to the TNC and everything else needing 12 VDC connects to the terminal strip.

Data

• Connected to the Garmin GPS is a Garmin power-data cable.  The power plug is connected to 12 VDC power as described above.  The data plug is connected to the TNC through a jumper -- the Garmin data output is a DB-9 connector, the TNC needs a DB-25 so I use a jumper with a DB-9 on one end and a DB-25 on the other.  This connection is seen at the bottom right-center of the big plastic box where the black DB-9 from the Garmin plugs into the silver DB-9 on the jumper.  You could use a standard DB-9 to DB-25 adapter. 
• Look at the bottom left corner of the TNC.  You see a round black plug.   That's the radio control input-output of the TNC.  Connected to this plug is the grey cord you see coiled up.  It carries audio and PTT signals between the radio and the TNC.  It terminates in the molded plastic plug that is plugged into the top of the radio.

Putting it into operation

Operation is simple -- set it up inside using a 12 VDC power supply in place of the vehicle cigarette lighter. First, connect the TNC to your desktop or laptop computer and, following the instructions in the owner's manual, set up the TNC to operate as a stand alone tracker.  Set up your Garmin so it puts out NMEA data.  The TNC is now set up to look for a NMEA data stream and the Garmin is now set up to send a NMEA data stream.   Set the radio to transmit and receive on 144.39 MHZ

Connect everything to power, connect Garmin data output to TNC serial input, connect the TNC to the radio, connect an antenna.

Turn on the Garmin, lay it where it gets a good view of the sky and wait a minute for it to acquire.

Turn on the radio and the TNC.  You should have another computer and receiver running APRS on 144.39 MHZ where it is monitoring APRS activity.  As the standalone tracker transmits, you will see it showing up on the screen.  It's working.

Now, do not turn off anything -- TNC, radio, Garmin, nothing -- disconnect the main power cord from the power supply.

Put the lid on the box, take it out to the car/truck.  Lay the Garmin where it has a good view of the sky.  Connect an antenna by running the antenna coax through the hole in the left corner of the box.  Plug the main power cord into the cigarette light of the vehicle, shove the box under the seat, and forget about it.  To turn off, pull the plug from the cigarette lighter -- this way, you kill the main power but everything is turned on, waiting for main power to come back on.

This link goes back to my APRS article.