Satellite signaling

In 1970, a plane carrying two U.S. congressmen went down in a remote region of Alaska. Despite the plane’s known flight plan, departure time and destination, the massive search effort was unsuccessful; the plane, the crew and the passengers were never found. As a result, Congress enacted legislation that required U.S. aircraft be equipped with emergency location beacons. The beacon was to activate automatically after a crash and send out a signal for search units to home in on. This led to the development of emergency locator transmitters (ELTs) for aircraft and emergency position-indicating radio beacons (EPIRBs) for marine use. The original homing-type EPIRBs were classified as class-A, -B and -C units.

Homing-type EPIRBs

Class-A beacons were designed to automatically float free and activate; class Bs were manually deployed and activated. Both, when activated, transmitted simultaneously on 121.5 MHz (civilian distress) and 243 MHz (military distress) frequencies. Their signals were detectable by both aircraft and satellites. Class-C EPIRBs operated only on VHF channels 15 and 16. They were not detectable by satellites. Their effectiveness depended on the receiving station — aircraft, coastal station or another vessel — monitoring channel 16, identifying the brief tone bursts as an EPIRB, switching to channel 15 and homing in on the signal to determine the location of the distressed craft. Recognition of class-C EPIRBs was discontinued by the U.S. Coast Guard in 1999.

This system worked, after a fashion, and was certainly better than sitting in a life raft with a handful of flares and a whistle. However, this original EPIRB setup did have some serious limitations. The frequencies used are in the VHF band. It is a line-of-sight system, which means that an aircraft, satellite or another vessel would have to be within the line of sight of the distressed craft. Satellites that received distress signals from class-A and -B EPIRBs were not able to store these signals but transmitted them immediately. If no ground station was within line of sight and able to receive them, then the signal was not received by the rescue organization.

In the case of another vessel receiving the signals, the EPIRB transmitter would be at sea level, and the potential rescue craft would have to be within a few miles, depending upon the height of its VHF antenna. Also, there are the problems of clutter, interference and false alerts — up to 99.8 percent false alerts, according to some sources — consequently, confirmation was required before search-and-rescue teams could be dispatched. This resulted in rescue response times of between four and six hours in most cases and up to 12 hours in some cases. Position accuracy is not all that good — a radius of some 12 miles, or more significantly, a search area of about 450 square miles. Finally, with this system, there is no way to determine who, or what type of vessel, is in distress. The U.S. Coast Guard no longer recommends these types of EPIRBs for purchase, and it is expected that the system of polar orbiting satellites — Cospas-SARSAT (search-and-rescue satellite-aided tracking) will cease detecting alerts on 121.5 MHz, probably by the year 2008.

406-MHz EPIRBs

It was apparent that a more exclusive and reliable satellite-based system was needed, one whose distress signal could be encoded to identify the specific vessel in distress and permit some determination of its location. The 406-MHz EPIRBs transmit an encoded distress signal on a frequency of 406 MHz and also a homing signal on 121.5 MHz. The distress signal is detectable by the Cospas-SARSAT polar orbiting satellites and also by the GOES (Geostationary Operational Environmental Satellites) system of geostationary weather satellites, providing true worldwide coverage. The category-I types automatically deploy and activate when in contact with water. They can also be manually activated while they are in their mounting brackets and can be manually deployed. Category-II types must be manually deployed but will automatically activate when not in their brackets and in contact with water.

The polar orbiting satellites receive the distress signal, store it until they are over a ground station, and then retransmit the signal to the ground station, which then notifies the rescue services. The position of the 406-MHz EPIRB is determined by calculations using the apparent change in the frequency of the signal that occurs when the source (EPIRB) and the receiver (satellite) are in motion relative to each other (Doppler shift). The frequency stability of the 406-MHz EPIRBs, which directly affects position accuracy, is some 10 times greater than that of the 121.5-MHz signal. Thus, the position accuracy of these devices is considerably greater, approximately three miles on the first satellite pass and to within one mile after three satellite passes. That amounts to a search area of from about 11 square miles, worst case, to three square miles, best case.

406-MHz EPIRB distress signals all but eliminate false alerts and also identify you, your vessel by type and size, and your emergency contact information. In terms of response time, the distress alert is essentially instantaneous. The position information may be delayed — as much as an hour or more — depending on whether or not GPS data is available.

GPS-enabled EPIRBs

Adding GPS capability to the EPIRB enables it to make better use of the geostationary satellites. These satellites are capable of detecting 406-MHz signals and retransmitting them to ground stations. However, they are not capable of locating the source of the signal, since they don’t move relative to the EPIRB (no Doppler shift). A GPS-EPIRB can include position information — latitude and longitude — in the distress signal, enabling the geostationary satellite to immediately relay your distress alert and your position to the rescue ground station.

There are two ways to add position information to an EPIRB’s distress signal: Build a GPS receiver into the EPIRB (integral), or physically and electronically connect the EPIRB to the vessel’s onboard GPS (interfaced). In either case, with your boat’s position known from the moment the first distress signal is received by the satellite — a matter of seconds for the interface type, and a matter of minutes for the integral type — the signal can be quickly routed to the closest rescue service anywhere in the world.

EPIRB with GPS interface

In this method, a 406-MHz EPIRB, which has been designed and built with this capability, is connected via a co-ax cable to the ship’s GPS receiver. The GPS updates the EPIRB regularly, and the EPIRB broadcasts the GPS position with the distress signal. According to National Oceanic and Atmospheric Administration estimates, this can save an average of 46 minutes in rescue time. When the two are disconnected — in an abandon-ship situation, for example — continuous updating of position data stops. This may not be as detrimental as it first appears. The fact is, given that initial position at the time of the emergency, rescue teams know your position to within one nautical mile and proceed to that position. The Cospas-SARSAT satellites keep them informed of your location via Doppler shift calculations, and when they get to within one nautical mile of your position, they use the EPIRB’s 121.5-MHz homing signal (a carryover from the original EPIRB setup) to home in on you. Also, if you have a hand-held GPS on your survival craft, your interface EPIRB should work with that.

EPIRB with integral GPS

The integral GPS type is simpler, since it requires no electrical installation and may be stowed in the survival craft ready for use after leaving the vessel. Upon activation — possibly in the survival craft — it transmits the 406-MHz distress signal, while the internal GPS begins acquiring position coordinates. GPS signals can be difficult to acquire in a “cold boot,” especially in rough sea conditions. It will keep trying to acquire a position fix for about 15 minutes, shut down for 20 minutes to conserve battery power, then begin trying again and will keep trying until it gets a fix. Once a fix is obtained, it incorporates your coordinates in the distress signal. This process of acquiring a position fix can be accomplished in anywhere from two to 12 minutes, longer under poor sea conditions. While this type of unit does provide more accurate drift fixes, as pointed out above, these are not necessarily utilized by the search-and-rescue team.

The three top suppliers of GPS-enabled EPIRBs are ACR Electronics, Northern Airborne Technology and McMurdo Pains-Wessex. ACR offers both integral (GlobalFix 406) and interfaced types (RapidFix 406) in both category-I and category-II models. However, Chelton Avionics owns both ACR Electronics and Northern Airborne Technologies, and their products employ essentially the same technologies. McMurdo Pains-Wessex offers only the Precision 406 with internal GPS in both categories I and II.

As for the size and weight of these units, EPIRBs with integral GPS typically run about 158 cubic inches and weigh about two pounds; those with interfaced GPS average about 115 cubic inches and two pounds. However, ACR has announced that as of July 1, 2003, they will be offering a 406 EPIRB with GPS interface that weighs only 17.6 ounces and occupies a volume of 46.9 cubic inches — truly a personal locator beacon.

Concerning product quality, EPIRBs must conform to rigid Cospas-SARSAT guidelines in design and construction to ensure reliability and performance standards. They will not register and assign identification numbers to unapproved devices. Nevertheless, there are significant differences in pricing, which may be accounted for, in large part, by features and functions, like provision for self-testing of the unit.

Some models offer a self-test function that does little more than check battery voltage. Others offer a full, functional self-test, which not only checks battery voltage but also creates a test message, sends the message, fires the strobe and, in the case of integral GPS models, confirms that the GPS is operational by conducting a GPS circuitry test. The results of these tests were manifested by various combinations of tones, beeps, buzzes, and flashing red and green LEDs visible through the translucent top. These are useful features but must be used with care since they place a significant drain on the batteries.

The main point to remember is that both types save time when it matters most. Minutes can be critical, particularly when hypothermia or medical emergencies are involved. Time is reduced in two ways: faster notification to the closest rescue team and telling them precisely where to go. The interface type provides earlier notification, both provide excellent position accuracy, the integral type is simpler and perhaps more convenient to hand in an emergency. Interestingly, according to NOAA, an EPIRB belonging to the fishing vessel Andrea Gail was found after the so-called “perfect storm” — tragically, it was turned off.

Ev Collier is a sailor and freelance writer based in Lynnfield, Mass.

By Ocean Navigator