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Tue, Apr 20, 2004

PLB Problems

Some Work Poorly, Some Just Don't Work

All GPS-equipped 406 MHz Personal Locator Beacons (PLBs) and marine Emergency Position Indicating Radio Beacon (EPIRBs) are not created equal. In a finding of a recent test of PLBs and EPIRBs from three manufacturers, the GPS location function of one manufacturer's products routinely failed to acquire a GPS location when tested under other than ideal conditions. An equally important test finding was that the international standards to certify the GPS-enabled 406 MHz beacons fail to take into account the real-world conditions that often exist when beacons are activated.

PLBs became legal in the continental US last July and many pilots have purchased them as a practical and affordable alternative to installing a 406 MHz ELT, an expensive proposition. The FAA has even gone on record as recommending them to General Aviation pilots who want the advantages of 406 MHz distress alerting. They have the added advantage of being able to also be taken along on outdoor sports or boating activities, not being tied to the aircraft as is an ELT.

PLBs have been particularly popular for outdoor sports enthusiasts who venture beyond well traveled trails and campgrounds, out of bounds skiers, paddlers of wilderness rivers and the like.  GPS-enabled EPIRBS have been available for boats for a number of years and many boaters are also looking to PLBs as a more compact and potentially less expensive alternative.

The test was conducted by the Equipped to Survive Foundation and funded in part by BoatUS Foundation for Boating Safety and Clean Water and West Marine. The beacons were specifically tested for their GPS functionality, or their ability to "self-report" their location to Geostationary (GEO) satellites, which can relay the location information nearly instantaneously to rescuers. Other issues such as battery life and signal attenuation in some real-world conditions were also examined.

All of the tested emergency beacons primarily use the 406 MHz distress alerting frequency in conjunction with the COSPAS-SARSAT system of Low Earth Orbit satellites and Doppler principles to provide position information. The report did find that all of the tested beacons successfully allowed a Doppler location to be derived - even in circumstances such as the bottom of a narrow and deep gorge - a minimal acceptable level of distress alerting. However, the purpose of GPS-enhanced 406 MHz beacons is to save precious rescue time by supplying much more precise location information via their own GPS-derived location through GEO satellites - ever present in the sky - rather than waiting for an orbiting satellite to appear in view and then obtain a less accurate Doppler computed location.

Equipped to Survive Founder Doug Ritter said, "Unfortunately, in the testing, the off-the-shelf  McMurdo Ltd. 'Precision 406 MHz GPS EPIRB' (also known as G4 406 MHz GSP EPIRB)and McMurdo Ltd. 'Fastfind Plus 406 MHz Personal Location Beacon' (also known as the Fastfind Plus 406 MHz PLB) failed to reliably acquire a GPS location 'fix' under operational 'real-world conditions.' The tests revealed that purchasers of these GPS-equipped 406 MHz beacons - who paid a premium for the added GPS technology in anticipation of potentially shortening rescue response with faster location information and increased location precision - are apparently not getting what they paid for, and are operating under false expectations. This lack of GPS data could result in tragedy that might have otherwise been prevented," continued Ritter.

BoatUS Foundation President Ruth Wood said, "Emergency beacons are often called upon to send an alert during inclement weather or less-than-perfect conditions. We tested the six beacons aboard a life raft, floated them in the water tethered to an inflatable or held by a swimmer in moderate one-to-eight-foot swells, the study showed clearly that the not all these beacons operated equally," she said. Inland performance was mixed, with McMurdo's PLB failing to acquire a GPS fix where handheld GPS receivers had no difficulty acquiring a location.

The evaluation was divided into three distinct phases: Baseline, Maritime and Inland. Within each of the latter two phases, a series of tests was designed to replicate real-world conditions, with variables such as sea state, limited horizon, forest canopy, mountains and the number of GPS satellites in view.

On the water, the McMurdo beacons failed to acquire a GPS location in all planned scenarios tested.  The McMurdo EPIRB did acquire a location on the water in one instance when it was given a special opportunity with ideal conditions.  The ACR EPIRBs acquired a location in all but one tested scenarios.

Inland performance was mixed, with McMurdo's PLB failing to acquire a GPS fix in a small forest clearing where handheld GPS receivers had no difficulty acquiring a location, in a simulated rain scenario, or when initially activated with an obscured sky view and then relocated to where it could view the GPS satellites. In the baseline testing, the McMurdo PLB failed to acquire a new location when relocated. McMurdo's marine GPS-enabled "Precision" EPIRB did similarly poorly in the marine tests. The conclusion is that users of the McMurdo self-locating beacons may expect to find that GPS-derived location may not be transmitted unless environmental conditions are generally benign and the beacon is stable, and unless there is a largely uninterrupted sky view covering most of 180 degrees above and 360 degrees around the beacon location. The ACR Electronics GyPSI PLB and Techtest 500-27 PLB fared much better.

Additionally, the study identified other factors that could impact a 406 MHz beacon's GPS performance, as well as some generic 406 MHz beacon flaws worthy of note. A PLB that relies on an external GPS source is entirely dependent on the performance of that external GPS source - and the quality of GPS receivers varies significantly; submerging the base of some PLBs' antenna in water can adversely affect their ability to successfully transmit an alert under some conditions or to provide an effective 121.5 MHz homing signal.

The Equipped to Survive Foundation issued 17 specific Conclusions and 23 critical Recommendations for action based on the test results. Said Ritter, "The bottom line is that while the international COSPAS-SARSAT system does an extraordinary job of saving lives, the COSPAS-SARSAT testing standard for GPS performance needs to better reflect real-world conditions. Consumer expectations regarding performance of these emergency beacons are very high - this is one area where those expectations must be better met."



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