Rescue 21 goes on air

The first installation of Rescue 21, the U.S. Coast Guard’s new and long-awaited coastal VHF communication system, became operational on the morning of Dec. 20, 2005, at a ceremony in the Coast Guard Air Station hangar in Atlantic City, N.J. The new Rescue 21 system replaces the VHF National Distress and Response System (NDRS) that has been in use since the early 1970s.

The NDRS, a great advance when introduced more than 30 years ago, has never been able to provide the geographic coverage or level of communication certainty desired by both the Coast Guard and the boating community. A number of its shortcomings were made painfully clear in the Morning Dew incident on Dec. 29, 1997, when a mayday call that likely was from a sailboat in distress just outside Charleston, S.C., was not heard clearly enough to result in an immediate search-and-rescue mission.

Rescue 21 was designed with full knowledge and awareness of the shortcomings of the NDRS and will, when complete, provide coverage for 98 percent of the 95,000 miles of U.S. coastline, using a total of 44 manned stations. In addition to the shore stations, VHF/DSC radios are being installed on the 655 Coast Guard vessels operating in coastal waters.

The Rescue 21 system relies on digital selective calling (DSC) as the preferred hailing system for transmitting mayday calls and for establishing voice radio communication on the marine VHF band. DSC substitutes a very brief digitally encoded signal on channel 70 (the DSC hailing channel) for the traditional voice call on channel 16 (the International Distress and hailing channel). However, many vessels are not yet equipped with fixed-mount, DSC-capable radios and since the system must be able to work with calls made from handheld radios (there are few DSC-capable handheld transceivers), the Rescue 21 system includes an advanced radio direction-finding capability that can rapidly determine the likely position of vessels making voice calls. The Rescue 21 system specification requires it to operate with a signal from a one-watt transmitter whose antenna is only two meters above the sea at a distance of 20 nautical miles.

The advent of Rescue 21 resolves a classic chicken-and-egg problem that has retarded the use of DSC by recreational and small commercial vessels operating in U.S. coastal waters. Even though VHF/DSC capability has been mandatory on SOLAS (Safety of Life at Sea) vessels as a part of the Global Maritime Distress and Safety System (GMDSS) since Feb. 1, 1999, Coast Guard shore stations have had virtually no DSC capability, and there has been very limited DSC capability on the smaller Coast Guard vessels. While more than 25,000 recreational boaters have obtained the mobile marine service identity (MMSI) numbers needed for operation of the DSC function in their radios, relatively little use has been made of the system. The activation of each new Rescue 21 station should dramatically change this situation as mariners become aware of the benefits of DSC.

Each of the new Rescue 21 operating stations is connected to multiple antenna sites, most of which have new and taller (average height 350 feet) antenna towers fitted with multiple VHF/DSC transceivers. The receivers monitor channel 70 in addition to VHF channel 16.

Improving search and rescue

With the advent of Rescue 21 and the growing population of DSC-capable radios on both Coast Guard and other vessels, DSC hailing will be the most effective and efficient way to make a mayday call. You or, at your instruction, anyone on board, can send a distress call by lifting the safety cover over the red “Distress” or “Emergency” button and pressing it for five seconds. The call, which lasts about one third of a second, tells the receiving station who is calling, the calling vessel’s position and the nature of the distress. An audio tone will confirm transmission of the call. The mayday call will continue to be sent at brief, random intervals until a DSC response is received from the Coast Guard.

A DSC mayday call will cause all DSC-capable radios in range to sound a distinctive tone, alerting anyone nearby, display the call information on their LCD screens and record the call details in their distress-call memories. The Coast Guard can use your MMSI to interrogate the MMSI database and obtain a description of your vessel. The connection between your GPS or Loran receiver and the radio will provide your latitude and longitude (with a two decimal place resolution – about 60 to 100 feet) to every radio that receives your call. The Coast Guard’s DSC response to your call will be announced by an audio tone and indicated on the radio’s LCD. Your radio will automatically stop sending the DSC distress message and switch to channel 16, enabling you to hear the Coast Guard’s voice response and instructions. At the same time, reception of your DSC distress call by any DSC radio in range of your position may bring assistance from a nearby vessel even before the Coast Guard can arrive. Some DSC radios compute and display the range and bearing to a distress call, others provide the position of a calling station to a chart plotter.

The Coast Guard’s DSC response signal will automatically switch your radio to channel 16 for reception of the Coast Guard’s voice response to your call. There is no need to re-press the button. Once the DSC mayday call has been initiated, a voice mayday call should be made on channel 16 to alert any stations or vessels in range that may not have DSC equipment.

Routine calling

For routine communication, a number of factors make DSC hailing superior to conventional voice hailing on channel 16. Conventional channel 16 voice hailing calls may be blocked by other traffic and the call will not be heard unless someone is listening to the called radio with the audio volume at the level needed to overcome ambient noise. DSC hailing calls, however, avoid the busy channel problem since it is transmitted on channel 70, which is reserved exclusively for DSC signals.

Radios receiving a DSC hailing call will sound a tone loud enough to be heard regardless of the radio’s volume control setting. DSC hailing calls are very efficient. They provide the called station with the MMSI, they specify the working channel the calling station wishes to use, and they will usually include the calling vessel’s latitude and longitude. The called radio will automatically transmit a DSC acknowledgement of the incoming hailing call and switch to the working channel designated by the caller. The entire process is completed in a few seconds. If your MMSI is stored in the called radio’s memory, your vessel’s name will appear on the display – like caller i.d. on a land phone.

DSC hailing provides additional significant advantages when radio reception conditions are difficult. The automatic repetition of the signal at random intervals minimizes the chance that the signal will be blocked by other DSC hailing calls or radio interference for more that a few seconds. The digital coding of the signal can be accurately decoded when reception conditions make understanding a voice call very difficult or impossible.

Purchasing a DSC radio is the first step toward taking full advantage of Rescue 21 as it is deployed around the coast of the United States and enjoying the everyday benefits of using DSC hailing for all routine calls. Next, obtain your MMSI. If your boat is not required to have a radio station license you can obtain the MMSI at no cost from BoatUS at

800-563- 536, online at or from SeaTow



Once you have the MMSI, follow the instructions that came with the radio and carefully enter the number into the non-volatile memory. Connect your GPS or Loran to the DSC radio and check for the lat/long display on the radio’s LCD. Read the instructions so you will be aware of all of the various calling, response and poling features of the system. You are now ready to make your first DSC-enabled VHF call. Enter a friend’s MMSI in your radio, select a working channel and make the call.

By Ocean Navigator