Tech: Testing the GlobalSat MR-350 GPS with OpenCPN

Being a catamaran owner, I naturally like to have two of everything. So ever since I started playing around with OpenCPN, I've been meaning to hook up an external GPS to provide me with a backup to my Raymarine e7D chart plotter (a.k.a. "multi-function display" or "MFD").  I was rudely reminded of this last week when my e7D's built-in GPS gave up the ghost on the eve of a 4-day cruise.

Fortunately, I had a GlobalSat MR-350 GPS receiver ready to go which I just had to hook up. (Amazon sells them for ~US$60 which is about as cheap as you can get for a waterproof, serial GPS). 

GlobalSat MR-350 GPS receiver mounted in front of helm.

The MR-350 is a serial transmission device that speaks NMEA 0183 over RS232, outputting voltage levels of +/-6V. The only gotcha is that it runs on 5V DC and, unsurprisingly, my boat's power supply is 12V DC. I therefore built a simple circuit to produce 5V from 12V using a voltage regulator with a LED power indicator, for about $2 in parts. The MR-350 cable ends in a male PS/2 connector (a.k.a. a 6-pin Mini-DIN connector).  I could have cut off the connector and soldered the wires, but instead I opted for adding a female PS/2 interface to my circuit, to make it possible to easily disconnect the GPS.

Simple GPS power/interface circuit.

Next, I connected the RS232 output to my boat computer, except, like most laptops, mine lacks a serial port thereby necessitating the use of a USB port with a serial-to-USB converter. On Windows, once the drivers are installed you'll see a new COM port under Ports in Device Manager. (The MR-350 uses the Prolific PL2303 chip and the standard PL2303 USB to serial drivers).

The final step is configuring OpenCPN to use the new COM port, which is under Settings/Connections. OpenCPN is then ready to be used as a chart plotter.  

OpenCPN's "Navigate to this" feature works like a charm, providing the familiar XTE (Cross Track Error), BRG (Bearing), VMG (Velocity Made Good), RNG (Range), TTG (Time to go), etc.

OpenCPN in navigation mode.

While OpenCPN doesn't have all the features of my e7D, it's very reassuring having a completely redundant chart plotter.

OVER.