Redwood Sail and Power Squadron

a unit of United States Power Squadrons®

Boating is fun..We'll show you how

VHF/GPS DSC System

The Coast Guard’s new marine radio safety network, "Rescue 21", is operational on the west coast, rescue centers now have the ability to receive instant distress alerts from DSC-capable VHF marine radios; however, 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 MMSI identity. If you would like to take the "Search" out of Search & Rescue and insure the Coast Guard can quickly reach you in an emergency, complete all the steps necessary to obtain an MMSI identifier for your vessel and connect your GPS to your VHF radio so that location information is available to the radio. Equipment manufacturers supply the information required to make those connections in the User Guide/Installation Guide for your GPS and VHF radio.

How to Install "Rescue 21" Capability on your Vessel.

Step 1: VHF Radio

Purchase a Marine VHF Radio which has Digital Selective Calling Capability (DSC).

Radios with DSC capability have a "Red" Distress button on the front.

Step 2: MMSI Number

Obtain a "Maritime Mobile Service Identity" (MMSI) number.: To utilize the Digital Selective Calling (DSC) capability of your VHF for routine operations and for automated distress hailing you need an MMSI number. MMSI is a fancy name for what is equivalent to a "phone number" for your radio. It uniquely identifies your vessel. When you request the MMSI number you supply the Coast Guard with a description of your boat. This benefits you as when you press the "Distress" button on your radio and the Coast Guard receives the distress call, your MMSI number is also transmitted and they can retrieve the description of your boat to use in their Search and Rescue efforts. The United States Power Squadrons is authorized to issue MMSIs to US recreational boats. However if you plan to cruise to Mexico or Canada, this is considered an international voyage and your must obtain your MMSI directly from the Federal Communications Commission.

Step 3

Enter the MMSI into your VHF Radio.: Now that you have obtained your MMSI number, you must entered it into your radio. The instructions for accomplishing this are contained in the User Guide or Owner's Manual for your VHF radio. If you do not have the Owner's Manual, go online to the manufacturer's website and download a copy of the manual.

Global Positioning System (GPS) Receiver

To insure the Coast Guard can find you quickly, attach a GPS receiver to your VHF radio.
The GPS must have NMEA 0183 outputs to share location information with your DSC equipped VHF Radio.
Once you follow the manufacturers instructions on how to attach the GPS to the VHF, you will be able to automatically transmit location information to the Coast Guard when you press the "Distress" button on the VHF.
In non-emergency situations, you can send your location information to a friend's DSC capable VHF radio. This might be helpful if you wanted to communicate location information to each other.

GPS receivers come in many shapes and sizes. They can be simple as the one shown above or they can be a full feature Chart Plotter. Both simple and full featured ones can be interfaced to your VHF if they have NMEA 0183 outputs. The instructions will be in your Owner's Manual. The details will be in the portion of the manual which describes installation procedures.

Step 4: Connections

Now it is time to make the connections between the VHF Radio and the GPS Receiver. Make sure you have the Owner's Manual/Installation Guide for each piece of equipment.

GPS Receiver/Chart Plotter: In the manual for this piece of equipment, turn to the section that describes the wiring aspects of the installation.
There will be a diagram which indicated the color of the wires in the wiring harness.
Two of the wires go you your battery or electrical distribution panel. Those will be "Red" for positive voltage and "Black" for the negative voltage wire.
You need to identify the wire colors for: (1) the "NMEA Out +" wire, and (2) the "NMEA Out -" wire.
In our example, the "NMEA Out +" will be "Yellow" and "NMEA Out -" will be "Brown".
Using the Installation Guide for the VHF radio, identify the wire colors for (1) "NMEA In+" and "NMEA In -".
In our example, the "NMEA In +" will be "Blue" and "NMEA In -" will be "White".
In the units are close enough together that the wires can be connected without adding an additional length of wire, they may be connected as shown below. In the illustration, a Blue Seas System 2402 Terminal Strip was used.
A wiring guide is available from the Coast Guard Navigation Center at: http://www.navcen.uscg.gov/pdf/marcomms/dsc/DSC_VHF_GPS_Install_Final.pdf describing how popular GPS receivers and plotters can be properly interconnected to a VHF marine radio. This tells exactly which colored wires need to be connected to each other.
A table which can be used as an aid for identifying which wires to connect can be found at:
VHF-GPS Wiring Connections. Note that the table contains an example as well as a "blank" worksheet, page 3, which you can complete with your information.

As alternatives to a terminal strip, you could a 4-wire Flat Trailer Light Plug if you would like to be able to remove the either the VHF or GPS or both pieces of equipment from your boat. This is useful if you have an open boat and are worried about the security of your electronics.

Step 5: Turn on the Equipment and Verify Proper Operation

If you have properly connected the wires which allow the GPS to send position data to your radio, you will see an image on the VHF display similar to the one shown below. Note that the display contains latitude and longitude information. It may also contains the time of day and an image of a satellite which indicates a connection to the GPS.

Note the satellite image in the top right corner and the lattitude and longitude location information in the lower left corner. This radio has been properly connected to the companion GPS receiver. This boater is now ready to share his location information with other boaters. He has also achieved the ability to have his radio send his location information to the Coast Guard if an emergency occurs for which it is appropriate to press the "Distress Button".

Caution: Do read your VHF radio manual so you are familiar with the proper use of the "Distress" Calling feature of your radio.

Step 6: Proper VHF Usage

Remeber, the VHF radio is your primary piece of safety gear when boating. Following proper radio procedures will insure you can summon help when needed.

- The United States Power Squadrons, in your area, offer a 2 hour seminar on the proper use of VHF/DSC radios. Contact your local Squadron for Seminar Information.
- Here is a link to online information on proper VHF Radio usage: VHF Radio Procedures