Garmin GPS 18x: Low-cost High-quality GPS Receiver for DigiTrak Autopilot of TruTrak

Garmin GPS 18x receiving unit

Four years ago, with friend Mel's help we installed the DigiTrak autopilot made by TruTrak in our Zenith 601HDS, N314LB. The autopilot requires GPS NMEA 0183 input. To save cost, we supplied that input with the already-installed Garmin GPSMAP 195.

In the DigiTrak installation manual, TruTrak advises against that choice since the 2-second update cycle of the GPSMAP 195 is not quite fast enough for the autopilot and may result in wandering flight during turns. That problem did indeed occur, as well as occasional meandering in straight-and-level flight.

Recently, the GPSMAP 195 battery needed for retention of user options failed. A replacement battery that nominally should last 10 years failed after eight weeks. Ditto for a second replacement battery. Clearly, it was time to eliminate the GPSMAP 195. Indeed, except for the autopilot, the GPSMAP 195 was no longer needed since the combination of Garmin GDL 39 and iPad installed a year ago produces much better graphics and lots of important data.

But how could we supply the autopilot with the required GPS NMEA 0183 input? A call to TruTrak provided the answer: with a Garmin GPS 18 or the more recent GPS 18x. These are tiny receiving units that can be installed on the glare shield.  There are several versions of this amazingly small and powerful GPS unit: USB, with just that connector; PC, with serial connector for the NMEA 0183 output and a cigarette lighter plug for 12/24V power input; and LVC, with just wires.

We were about to order the LVC version when our son Martin pointed out that all versions require 5V input except for the PC case, which accepts 8-30V. Since we did not want to get into the 5V supply business, the PC version was the logical choice.

 It would have been a big mistake to cut off the plug of the PC version and hook up the wires directly to 12V, since the plug converts 8-30V input into 5V and thus must be kept. This forced connection of the 12V wires to the plug. 

Here is our solution. We disassembled the plug to extract the metal tip of the "+" connection. Next we drilled a small hole into the tip, inserted the connecting wire, and soldered it in place. For the "-" connection, soldering the connecting wire to one of the spring clamps was not possible, since they are chrome-plated. Instead, we drilled a small hole into one of the clamps and attached the connecting wire with a small screw. Here is the result.
GPS 18x power plug with connecting wires
Connection to the female serial plug providing the NMEA 0183 output was simpler. We added a male serial connector and soldered the wire for the output to the #2 pin.

Installation in the plane was time-consuming but not difficult. The GPS unit has on top the name "Garmin" in white letters. The canopy reflected that name right in the middle of the field of view. We eliminated that annoying image by applying a black coating over the name with a sharpie. At the top of this post is a picture of the GPS unit on the glare shield.

 The test flight was a delight. The GPS 18x acquires satellites within at most 45 seconds. In flight, the one-second update cycle is perfect for the autopilot and results in smooth turns as well as straight-and-level flight without any meandering. And all this for under $75!

 A word of caution: Garmin also offers a GPS 18x version with a five times per second update rate. If you go for this fancier version for some application, make sure the equipment receiving the signal has sufficient baud rate to process the flow of data.

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