Did GPS emergency beacons get a fix?

In a moment of stress and danger, the survival of a voyager can rest on the reliability of his or her emergency gear. So when a recent test of GPS-equipped emergency position-indicating radio beacons (EPIRBs) and personal locator beacons (PLBs) suggested one manufacturer’s units didn’t transmit the GPS position properly, the firm in question, McMurdo Ltd., scrambled to address test results that could have a profound effect on the reputation of its emergency signaling gear.

The issue of unreliable GPS EPIRBs arose as a result of tests performed in December 2003 and January 2004 by Equipped To Survive, a nonprofit foundation dedicated to survival safety that evaluates survival gear on its website (www.equipped.org). After a National Oceanographic & Atmospheric Administration/U.S. Coast Guard test of GPS-equipped EPIRBs and PLBs in March 2003 indicated problems but did not release names of the products tested, ETS decided to conduct its own independent test of GPS-enabled 406-MHz emergency beacons. These examinations detailed the performance of specific beacon models. ETS tested models from ACR Electronics of Ft. Lauderdale, Fla.; McMurdo Ltd. of Portsmouth, England; and Techtest Ltd. of Leominster, England. Two primary financial sponsors of the ETS tests were BoatU.S. Foundation and West Marine.

Doug Ritter, executive director of ETS, was not only successful in getting EPIRB manufacturers like ACR, McMurdo and Techtest to contribute EPIRBs and PLBs to the tests, he also convinced NOAA of the usefulness of the tests. “They were skeptical at first,” Ritter said. “But they came to see the value of the testing.” NOAA required all the EPIRBs and PLBs in the test to have their signals modified with special codes so the test alerts would be easily identifiable.

The central issue of the tests was the self-locating effectiveness of GPS-enabled emergency beacons — units that either have an internal GPS receiver board or can accept a GPS position from an external source. In other words, how well did these units perform in getting a GPS fix and then in sending it to Cospas/

SARSAT satellites?

What was not at issue in the test was the effectiveness of the beacons in transmitting the standard 406-MHz distress signal. The basic 5-watt signal, which contains a digital owner identification but no position information, is used by polar-orbiting Cospas/SARSAT satellites to determine a beacon’s position via the Doppler shift (the motion of the speeding satellite induces a Doppler frequency shift onto the received distress signal and from that shift it’s possible to determine the position of the beacon on the surface of the earth). While the Doppler-shift technique is much more accurate in the second-generation 406-MHz beacons than the first-generation 121.5-MHz units, the Doppler method is far surpassed in accuracy by a GPS-capable beacon, which can supply a lat/long fix to search-and-rescue authorities. With this fix, SAR forces have a much smaller area to search to find a distressed party.

It should be stressed that all the beacons in these tests work well in the basic 406-MHz Doppler mode. “The 406 system works, it works well, and it works under less than ideal conditions,” Ritter said.

For voyagers who have paid a higher price for a GPS-capable beacon, though, the effectiveness of that GPS function is clearly important. According to the ETS tests, in the marine environment the GPS-capable beacons from McMurdo (the Precision 406 EPIRB and the Fastfind Plus 406 PLB) failed in most cases to transmit a GPS position in the 30-minute time limit specified by Cospas/SARSAT. The McMurdo products were faulted for their lengthy time to first fix (TTFF). When the test data was released, the McMurdo Precision 406 EPIRB and Fastfind Plus 406 PLB clearly were not shown in a positive light.

In an effort to redress the shortcomings indicated by the tests, McMurdo went back to their Precision 406 EPIRB and Fastfind Plus 406 PLB units and made several modifications. First, McMurdo engineers worked with the company that provides the GPS engine used in the Precision and Fastfind Plus units. They added settings to the firmware (typically an EPROM chip that stores software operating code) that provide faster GPS location acquisition under less than perfect conditions.

Next McMurdo engineers focused on the firmware of the beacon by revising the GPS timing strategy. While the original emphasis of the McMurdo beacon operating software was on long-term averaging of the GPS fix results in order to get the best fix position in subpar conditions &mdash heavy rain, GPS satellite signal masking, unfavorable ionospheric conditions, etc. &mdash the new instructions place greater emphasis on acquiring a GPS fix position quickly. With GPS selective availability turned off, this first fix is now more accurate and reliable than in the past.

Finally McMurdo made a physical change to the Precision 406 EPIRB case. It added more buoyancy to the beacon package, making it less likely to roll and giving the GPS antenna a more stable view of the sky.

According to McMurdo, these changes correct the GPS problems noted by the ETS tests. McMurdo undertook land- and sea-based tests of its updated products. These were conducted on land in Santa Cruz, Calif., and at sea in U.K. waters.

According to McMurdo, the Precision 406 EPIRB achieved an average initial GPS acquisition time of 1 minute 48 seconds. And it reportedly transmitted its GPS position signal in 3 minutes 21 seconds. The Fastfind Plus 406 PLB was reported to have demonstrated similar response times.

Voyagers with GPS-equipped McMurdo EPIRBs and PLBs can contact McMurdo at 800-576-2605 to set up an upgrade of their unit. According to McMurdo all products purchased new at marine retailers will already incorporate the upgrades.

Ritter is aware of the changes McMurdo has undertaken, and he said ETS is planning a new round of tests in January 2005 that will evaluate the modified McMurdo EPIRB and PLB, as well as new PLBs from ACR Electronics and possibly EPIRBs and PLBs from other manufacturers.

Has McMurdo successfully upgraded their GPS EPIRB and PLB? “I’d love to report that McMurdo units are okay, but it’s impossible to say without testing,” Ritter said. “We’re very much looking forward to testing off-the-shelf units from consumer stock to hopefully confirm they have fixed the problems.”

The detailed summary of the ETS evaluation and McMurdo’s report on their own tests can be found at www.equipped.org/406_beacon_


By Ocean Navigator