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Adding GPS Capability to your DSC-equipped VHF Radio

Part I


by, Capt. Eric Grubb

April, 2012

Are You GPS and DSC-Equipped?

And why is this important? First, a few definitions:


from Wikipedia: "Digital Selective Calling is a standard for sending pre-defined digital messages via the medium frequency (MF), high frequency (HF) and very high frequency (VHF) maritime radio systems. It is a core part of the Global Maritime Distress Safety System (GMDSS)...When sending a distress signal, the DSC device will at minimum include the ship's MMSI number. It will also include the coordinates if available and, if necessary, the channel for the following radiotelephony or radiotelex messages."


from Boat US : "Maritime Mobile Security Identification (MMSI) number. In cooperation with the FCC and USCG, Boat US has been given the authority to issue free MMSI numbers."

This registration links your radio to the digital database, so that if you were to have an emergency on the water, the USCG would know not just where you are, but who you are, the size and name of your boat, as well as emergency contact information and where you keep your vessel.

For more information and instructions for registering, visit the link provided at the end of Part II.

So, why do I need it?

From the USCG Navigation Center website:

"Mariner's Safety Endangered When VHF Radio Distress Alerts by Digital Selective Calling (DSC) Lack Location and Identification Information."

"...approximately 90% of VHF DSC distress alerts received by the Coast Guard do not contain position information, and approximately 60% do not contain a registered identity. The Coast Guard cannot effectively respond to a DSC distress alert sent from such a radio."

In other words, the big red DISTRESS button will not do much good unless you've set up a GPS feed and MMSI account. That's fine if you can stay with the radio to transmit your mile marker on Channel 16, but it won't help you a bit if you're on fire and need to jump out. When they call you back, you do know your location and mile marker, right?


Above, a DSC-equipped VHF radio displaying GPS inputs

By now, you've most likely heard of the USCG's Rescue 21 system. Through the use of digital technology, they have established coverage areas on both coastlines, as well as parts of the Great Lakes and Columbia River. These include monitoring of VHF, MF and HF radio frequencies over large areas of open water.

In Louisville

Closer to home, the USCG, Sector Ohio Valley, Louisville monitors VHF ch. 16 for distress calls generally between Madison and Evansville. While there are repeater antennas in the area of coverage, you would be well-served to have a 25 watt fixed-mount VHF with a good antenna versus only a 1-5 watt handheld unit. As a rule of thumb, VHF radios work via "line of sight," and yield about 1 mile per watt of power. If all you have is a handheld, you'd be better off trying to make your call from your bridge for better reception.

What about digital coverage and DSC? In this area, funding has been improved and installation is in progress for new digital functionality. Slightly different than Rescue 21, this system will offer enhanced AIS traffic services. That is, they will be able to monitor AIS-equipped boats as they transition our area. Additionally, DSC-equipped radios that have a GPS feed and MMSI registration will be identifiable by the system. This capability should be fully functional by 2013.

National Marine Electronics Association (NMEA)

The concepts behind MMSI and DSC were first drafted in 1979, and have been available to mariners for 15 years now. Since we could write chapters on the topic, I'll keep it simple: to correlate your information and a MMSI number to your radio, all you have to do is register online then program the number into your radio. To attach a GPS position feed to your radio, you'll need a GPS output from an antenna or chartplotter.

If you have a chartplotter connected to your radio, most likely you're already set. Some new radios, such as the Standard Horizon Explorer GPS GX 1700, have a built-in GPS receiver. For years, I ran with a "temporary" system that anybody could set up with a 10 year-old Garmin eMap and a $15 cable. I recently upgraded to a newer GPS receiver for each boat, giving GPS position capability to my VHF/DSC radios. In any case, having the position feed to the radio is critical to enabling for a better response to a distress situation.

There are other benefits to having DSC capability. You can set up a buddy list of MMSI numbers, and call your friends on the radio like you would on a cell phone. Their plotters have the ability to fix your position because of the GPS data feed. AIS technology adds another layer of technology, and with it you can see towboats displayed by name miles away.


Above, a Garmin eMap providing an NMEA 0183 GPS data feed to the above iCom 504 via a patch cable. This "poor man's" $50 solution worked for over 4 years, but used a lot of AA batteries.

A Few Solutions

For starters, let's assume you have a DSC-equipped VHF radio, but no GPS. You'll have to do your homework and find out what kind of data feed it will accept, such as NMEA 0183 or the newer NMEA 2000.

NMEA 0183, for example, 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. Part II of this article (below) details an installation in one of my vessels.

The newer NMEA 2000 standard is a whole other animal, designed to take the guesswork out of connecting multiple devices. Connectors are simply strung together, while adhering to certain rules concerning terminations and branches off the main run.

Speaking of a DISTRESS Call

Let's say you're rafted-up with the radio playing all day. You go to start, and now the battery is shot. Your radio still works, so you call the USCG. Will they respond? The USCG will respond to a distress call that involves the possibility of loss of life, injury or damage to property. In the above example, it is entirely possible that they may not provide assistance.

Conversely, you're at anchor and your grandmother is feeling light-headed and collapsed because she forgot to bring her medication. Since there is a risk of a loss of life, a response would most likely be appropriate.

Each situation is evaluated differently, and the response is based upon several factors. If loss of life, injury or damage to property is not involved, you should seek out the assistance of other boaters or those who provide tow services on the water.


The bottom line is we'll have fully-functional digital VHF capability in this area by 2013, if not sooner. Hopefully, this served as a primer to get you thinking about taking your boat, if not already equipped, to the next level for better rescue potential. Keep reading for installation tips...

Continued in Part II Installation: (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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