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Adding GPS Capability to your DSC-equipped VHF Radio

Part II


by, Capt. Eric Grubb

continued from Part I (link)


Now that we've addressed some good reasons to link GPS to your radio, here are some solutions. We'll let you choose the one that fits your budget:

1) Cheapest: Get a DCS VHF Radio, then feed it from a handheld GPS.

2) Better: Get a DSC VHF Radio, and wire it to a permanent GPS antenna.

3) Available soon: Get a DSC VHF radio with an installed GPS antenna. You'll have to make sure that the radio receiver is in a place that will "see" the satellites for best results. Also new on the market are GPS/DSC handheld VHF radios, but be aware that handhelds have limited transmitting power.

4) Best, Most Expensive: Get an integrated system ($$) that includes a GPS antenna, chartplotter and radio. This could be either a factory installation or a retrofit.

I will address 1) and 2) above, since they are within reach of most boaters. As was mentioned before, NMEA 2000 is a newer standard which allows proprietary plugs to be linked together under certain installation rules. Many new integrated systems are linked that way.

On the other hand, many radios out there still connect using the older and more generic NMEA 0183 standard. NMEA 0183 is connected by pairing a "talking" device (GPS) to a "listening" device (your radio or chartplotter). You'll wind up connecting your GPS output labeled "+" and "-" to the corresponding inputs on the radio.


Before you buy, check the specifics of your planned DSC VHF radio to see which version of GPS data feed it will require. Chances are it will be something like "NMEA 0183 ver 2.0 or higher." Be sure to match that with the output of whatever GPS receiver you plan to use.

Radio Installation:

Following are a few things to consider prior to installing your radio. First, pick a location that will provide easy access for use, and allow you to comfortably see the display. Other considerations include ergonomics, sunlight readability and possible interference between components and/or your ship's compass.

Next, before you start installing anything, figure out all your wiring runs and cabling needs.

For smaller boats that use a bracket-mount, I like to have the option of making the radio removable. I like to use a waterproof connector, and in this application, a trailer connector works because I only need four wires (DC+/- and GPS +/-).


Above, the radio with the water-tight adapter installed. Also, the terminal block awaiting installation.

If you need a new VHF antenna, plan some time to find a good location and to route its coax cable. Like the radio, consult your manual for more specific instructions.

Above the hull prepped for the radio antenna installation. When you drill holes in gelcoast, be careful not to crack it. One suggestion is to run the drill in reverse for the first few millimeters until you get past the gelcoat layer.

Be sure to use some type of sealant such as 3M's 4200 to prevent water intrusion. I used masking tape around the installation for quick clean-up.


Below, the finished installation of the Shakespeare VHF antenna



Above, an area is marked for a flush-mount GPS antenna. Below, the finished installation of the flush-mount GPS receiver.


Which Wires Go Where?

The handheld Garmin eMap pictured below has its own AA power supply, so only two wires are used, GPS NMEA + and -. For a cable, I used an "eMap to serial data cable," then cut off the serial connector. You'll find that those are currently available on eBay for less than $15.


Above, two different NMEA 0183 data feeds, one from a handheld receiver and another from a pole-mounted stand-alone GPS receiver. Right, a bench test verifying correct wire connections prior to installation.

To make the connection simple or upgradeable, you can wire directly to a wiring block between the GPS and the radio. Lastly, here is some technical information that you'll need to verify for the correct eMap output: 4800 baud rate, 8 bit, no parity, 1 stop bit, no handshake. Trust me, it works. I used the above eMap set-up for four years on the river before switching to a permanently mounted antenna on the radar arch.

For my runabout, I was starting from scratch, so I found a Standard Horizon radio that I liked, and found that the GPS 17x HVS NMEA 0183 was a compatable GPS receiver.

The following applies for either type of GPS NMEA 0183 connection:

It can be a little challenging picking the proper wires, and converting the language in your radio's manual to that of your GPS receiver's manual.

I would suggest getting both manuals out together, then jot down your notes of which NMEA wires get spliced together. Most are labeled similarly enough that you can figure it out then verify it with a bench test (pictured above).

With your scratch paper, make two columns listing the various wire colors from your device. Next, label each wire on the paper, then determine the connections. While it sounds elementary, this will prove very helpful later. There may be some unused wires, such as those for a chartplotter or those that the GPS receiver uses to switch data formats.

Below is the chart I created for my runabout's installation. You will notice that there may be small differences in nomenclature between manufacturers and unexpected labels. For example, the GPS receiver provides and output, so you think it would be "TX" for transmit; in this case the required wires were "RX" for receive. The same was true with my radio, where I needed the "NMEA out" connection, even though it was going IN to the radio.


The permanent-mount Garmin 17x HVS antenna pictured above (top right) was a good solution for both my iCom 504 on my cruiser and my Standard Horizon Eclipse for the runabout, as it supplied the required NMEA 0183 format. I purchased one for each vessel.

I'll use the Garmin 17x HVS as an example for how to hook up the wires, but note that a handheld or other device would work similarly. For starters, you'll need a way to connect the GPS outputs and power supplies. For both of my boats, I used a terminal block under the helm to make all the radio to GPS connections (pictured below).


Radio Connections:

Firstly, the radio needs power, so there is a DC+ Red and DC- Black. The GPS inputs are NMEA DC+ Blue and NMEA DC- Green. Those wires go directly from the radio, through the waterproof connector, to the terminal block (below).

In general, be sure to use heat-shrink, adhesive ring and butt connectors for the best protection in a marine environment full of moisture and vibration.

GPS Connections:

The GPS receiver needs a power supply, so the first two wires were 12 V DC+ Red and 12 V DC- Black. It also had an orange remote trip wire to turn it on. Since my DC power was already wired to an accessory switch on both vessels, I simply hot-wired the orange to the Black DC-. Therefore, the GPS antenna is powered and producing data whenever that accessory switch to the radio is in the ON position.

Next, the GPS outputs needed to be matched to the radio inputs. In this case, there was an NMEA DC+ Gray and NMEA DC- White/Red. These were attached to the terminal block as well (below). I hooked them directly instead of using ring connectors because of the small size of the wires. The receiver draws minimal current and the GPS DC data output stream is 0.5 volts.



With everything in place, the final step would be a check with the voltmeter to make sure everything is wired correctly. In my case, my radio had power and came to life just fine, but my initial GPS output was 0.0 V with no indication on the radio. With a little troubleshooting, I was able to determine that the red DC+ GPS connector pictured above wasn't connected tightly the first time.

With that fixed, the GPS came to life. Seeing the GPS coordinates on the radio made the whole project worth the effort!


Above, the radio is powered at the end of the project.

MMSI Registration

With the system up-and-running, the final step in the process would be to register for your MMSI number and program that number into your radio.

As you complete the online process, I would suggest that you print out all your registration information so that you'll have a full record of what they have. If you keep this in a binder with the rest of your boat records, it will be easy to cancel your account and transfer the number upon sale of your boat.

In the event of an actual distress contact, the USCG will attempt to reach you on ch. 16, and will want to verify your boat name and MMSI. For that reason, I'd suggest that you keep the number handy by having a label near the helm.


While you're in the process of accomplishing boat projects, I can't think of a better project than one that could help save your life by speeding the rescue process. Hopefully this information will help you if you decide you'd like to wire a GPS feed to your DSC-equipped VHF radio.



Boat US MMSI Registration: link


Eric Grubb
Founder, Port KY
Licensed Master

Eric grew up around boats, trading summers on board his parents' Sea Rays for many man-hours of swabbing the decks. He grew up by the little town of Sewickley, Pennsylvania, overlooking the the Dashields Locks and Dam. He has traveled the Great Lakes, Lake Huron's North Channel, Gulf of Mexico and several rivers to include the Ohio, Allegheny, Monongahela, Kanawha, Mohawk (Erie Canal), Tennessee, Tombigbee, Black Warrior and Mobile Rivers.

As a commercial pilot, Eric flies jets and is a flight instructor. He has owned recreational boats ranging from PWCs to most recently, a flybridge convertible that he keeps in a Louisville marina (MM 590). You can also find him with his family on the "Escape Pod," an 18' fishing boat. His most memorable journey was aboard the J. S. Lewis, a 155' towboat in service since 1931.

Eric is a USCG Licensed Master with a Commerical Tow Assistance rating, and is a member of the Sons and Daughters of Pioneer Riverman and the Louisville Sail and Power Squadron. After moving to Louisville, he conceived the idea for Louisville's Port KY website while searching for information to help him become a safer and more knowledgable local boater. He has worked hard over the years to educate other boaters by promoting safety classes through Port KY and by hosting captain's classes and related events.








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