Features October 15, 1999 Issue

GPS 406 MHz EPIRBs: ACR vs. NAT

ACR and National Airborne Technology have taken two distinct approaches to transmitting position data along with distress signals. We favor NAT’s integral GPS over ACR’s peripheral connection, but it’s twice as expensive.

The prime electronic rescue device has for many years been the EPIRB (Emergency Position Indicating Radio Beacon).

The original concept of the EPIRB was a device that would transmit a distinctive signal signifying an emergency on frequencies (121.5 MHz civilian and 243 MHz military) reserved worldwide for this purpose. This frequency would, in theory, be constantly monitored by aircraft flying overhead, which on receipt would report it to the appropriate authorities.

As years passed, several drawbacks to this system became evident. The number of EPIRBs steadily increased and so did the number of false activations. These occur both accidentally and maliciously. There was nothing in place to identify a malicious signal caused by someone’s warped idea of fun, someone who activated his EPIRB simply to attract attention. Also, when an older 121.5 MHz/ 243 MHz EPIRB is accidentally activated, its owner may have no indication that it is transmitting. Unlike the newer 406 MHz models, a flashing strobe was not required as part of their design approval. In fact, with those units it was entirely possible that your first indication of your EPIRB activating was the US Coast Guard pulling in front of your slip and telling you they had tracked a signal to your slip! This situation led to much frustration on the part of the Coast Guard and aviation authorities. Most planes soon stopped continuously monitoring the 121.5 MHz. and 243 MHz EPIRB frequencies. As a result, when the 406 MHz EPIRB system was developed, unique individual identification numbers were assigned to each transmitter. When registered, the appropriate authorities have a database of information on each EPIRB’s owner. A distress signal received from an unusual location can be verified by a phone call to the unit’s owner. For example, a signal originating from your garage in mid-January is most likely due to a mouse problem and is not a situation requiring rescue efforts.

The original 406 MHz EPIRB monitoring system consisted of polar orbiting satellites only. This left long coverage gaps between orbits in tropical regions. To overcome this, 406 MHz receivers were added to geostationary GEOS satellites over the equator. However, because GEOS satellites are not in relative motion to the earth, they have limited ability to triangulate positions. In a worst case scenario, they can only pinpoint a signal to within approximately a 12,584-square kilometer area.

The next logical step in EPIRB technology was the addition of GPS data to the transmitted distress signal, thus pinpointing the exact location of the signal to within less than .5 square kilometers. According to Coast Guard statistics, the average rescue time for a 121.5 MHz EPIRB is 6 hours and for a conventional 406 MHz EPIRB it is 2 hours. The average rescue time for a GPS-connected 406 MHz EPIRB is estimated at 1 hour 15 minutes. This could easily save lives as well as millions of taxpayer dollars. Rather than having to search vast areas of ocean, sea and air rescue craft can theoretically head straight for a precise location, which significantly aids helicopters flying at the fringes of their operational range.

Integral vs. Connected GPS
Two different approaches have emerged for combining GPS coordinates with the EPIRB signal. The first of these is the GPIRB‰ philosophy promoted by Northern Airborne Technologies, in which a stand-alone GPS receiver is installed into the actual EPIRB. On activation, the GPS turns on, acquires a location and transmits that location when it determines that it has obtained sufficiently accurate position data. The second approach, developed by ACR Electronics, connects the EPIRB to an external GPS receiver from which it periodically updates its position data. When the two are disconnected, such as when grabbing the EPIRB and abandoning ship, continuous updating of position data, of course, will cease.

The NAT Satfind-406 GPIRB is twice the size of the ACR Rapidfix 406, and twice the cost. The Rapidfix weighs 1.9 lb. and measures 7.2" high by 4.7" in diameter. The Satfind-406 weighs 3.5 lb. and measures 12.25" high by 5.25" in diameter.

Several months ago, ACR was bought by Chelton Avionics, Inc., which already owned NAT.

Evaluation Criteria
Evaluating these units proved difficult. We examined each for quality of construction. Each model was also activated in our screen room and its signal monitored and analyzed. 406 MHz EPIRBs must conform to rigid COSPAS-SARSAT guidelines in their design and construction. COSPAS-SARSAT will not assign identification numbers to unapproved units. This insures a quality level that is generally higher than most consumer-grade marine electronics products. Simply put, it would be very difficult for a 406 MHz EPIRB maker to cut corners while still retaining the required approvals.

National Airborne Technology Satfind-406 GPIRB
NAT has pioneered the concept of building a GPS receiver directly into a 406 MHz EPIRB. It’s fairly simple. When activated, the GPIRB first transmits a standard encoded distress signal and simultaneously turns on its GPS receiver. When valid position data is acquired, a distress signal is retransmitted that includes position data from the GPS. Every 20 minutes both the GPS and EPIRB reactivate and transmit updated position data to the satellite. Rescue agencies are not only supplied with the location of the original distress signal, but are periodically updated as the survivor’s position changes. This eliminates the need for elaborate drift calculations.

The Satfind-406 is supplied with a float-free mounting bracket as standard equipment. The unit itself is bright yellow with a strip of reflective tape around its top and the required lanyard cord wrapped around it circumferentially. Its flexible antenna is black and permanently attached. A slide control for manual activation and testing is located on the side of the unit. When testing, a flash of the strobe confirms that the unit is working. During our tests, the Satfind-406 GPIRB performed very well. Both signal strength, stability and spurious emissions were within acceptable values.

NAT’s GPIRB approach makes the GPS-EPIRB a stand-alone device which would not be affected by problems elsewhere, such as a power failure. It insures that a GPS signal is always available to the EPIRB. We think this is key in the type of emergency which progressively worsens, such as a ruptured through-hull that results in an electrical short. In such cases, or when it may not be necessary to abandon ship for a period of time, it is advantageous if the EPIRB has its own, self-contained GPS receiver.

Our only concern with this arrangement is the potential for long GPS acquisition times which could occur when the receiver has been shut down for long periods and is far from its last fix.

List price is $2,495, and should discount about 20%.

Bottom Line: The NAT Satfind-406 GPIRB‰ costs much more than the ACR Rapidfix 406, but its permanently attached antenna and integral GPS receiver, in our opinion, make it superior. Our only suggestion is that a less expensive, non float-free bracket be offered to the pleasure boat market.

ACR Rapidfix 406
The ACR Rapidfix 406 is smaller and lighter than the Satfind-406. It is also considerably less expensive. West Marine sells the automatic model with hydrostatic release and case for $1,199 and the manual for $949. This is not surprising considering that a GPS receiver is not built into the unit. The ACR unit relies on an externally connected GPS to obtain position data.

This model is also yellow with a translucent top. Activation and testing is accomplished by a top-mounted flip switch and is confirmed by a tone and flashing red and green LEDs which are visible through the translucent top. A series of beeps along with a sequence of flashes indicates satisfactory operation and confirms GPS data reception. Its antenna is also yellow and is banded at its top by reflective tape. The ACR’s antenna is detachable.

An external GPS is connected via an optically isolated connector that twist locks into the top of the unit. Its lanyard is rolled around a unique spinning bobbin located on the unit’s side. As expected, during testing the Rapidfix 406 performed very well. Its signal characteristics were acceptable and comparable to the model from Northern Airborne Technology.

Our concerns about this model are several. First, the concept of an externally connected GPS receiver, in our opinion, is not optimal. There is the possibility that during an emergency a boat’s GPS will fail long before the necessity to abandon ship occurs. If this happens, the EPIRB continues to store (or transmit if activated) the last known coordinates for four hours, after which it purges that data. Thereafter it functions as a standard 406 MHz EPIRB. This feature prevents it from transmitting out-of-date position data. But it still doesn’t seem as desirable as the NAT’s intgegral GPS.

Second, the Rapidfix’s GPS connector cable is thin and flimsy. Such an important connection should be made with much heavier, if not armored, cable. We asked John Jones, ACR’s product line manager, about the cable. He said it could be made more robust, but that ACR didn’t feel it necessary, because even without the GPS interface the unit still functions as an EPIRB. When asked about disconnecting it in an abandonship situation, Jones said the Category II unit’s “grab and go” bayonet connector with rubber pins will just pull out. We wonder whether the connector will detach if the unit is submerged or if it will be held under water still attached to the GPS receiver. This shouldn’t be a problem for the Category I unit which has a spring-loaded connector that separates when the hydrostatic release mechanism fires.

Third, there is no way to continuously monitor the status of the GPS-EPIRB connection other than the manual testing procedure.

Fourth, ACR does not supply a mounting bracket as a standard item with the unit. A choice of manual or auto releasing brackets must be purchased separately—about $250 for a Category I bracket, $45 for a “Quick Draw” recessed or surface mounting bracket. If no bracket is used, the integrity of the GPS connection is further compromised and the possibility of an accidental activation is greatly increased.

Finally, ACR’s antenna is easily removable. Again, we believe that foolproof reliability is compromised. Jones said that all its 406 antennas are detachable to prevent damage during shipping and that they are owner-fastened with a locking rubberized O-ring. No problems have been reported by owners, he said.

Bottom Line: The ACR Rapidfix 406 is definitely a quality piece of safety equipment, and it is the least expensive route to obtaining a GPS-connected EPIRB. In our September 1997 comparison of EPIRBs, we favored the ACR line of 121.5/243 and 406 MHz EPIRBs over other brands. But we have concerns about the GPS connector, detachable antenna, and the absence of a standard bracket.

Conclusion
Either of these two models would greatly increase your chances of rescue and reduce the time to arrival of rescue forces. Our favor, however, lies toward the concept of an EPIRB with an integral GPS receiver. There is simply too much that can go wrong with a connection to an external receiver. Ideally, additional models will be soon introduced by other manufacturers. Ultimately, an EPIRB with its own GPS receiver and verification and updating of location data from an external receiver would provide the maximum in reliability.

Contacts- ACR Electronics, 5757 Ravenswood Rd., Ft. Lauderdale, FL 33312; 954/981-3333. Northern Airborne Technology, 28 Lord St., Ste. 130, Marlborough, MA 01752; 508/303-6762.

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