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Taking the ‘search’ out of Search and Rescue

A bit late in the day I have been researching replacements for my Class B 121.5Mhz EPIRB which will be relegated to the grab bag and used as a PLB (Personal Locator Beacon) in an emergency. Cospas-Sarsat ceased satellite processing of 121.5/243 MHz beacons 1 February 2009. These beacons will only be able to be detected by ground-based receivers and aircraft for example actually engaged in a SAR operation that had been initiated using the new systems.

A bit of internet research reminded me that the old analog system was accurate to around 20km (10.8nm) and that a SAR (search and rescue) operation would only be launched after two satellite passes – which could mean a delay of about 2 to 3 hours, often it would take 6 hrs to resolve the location by using multiple passes of the weather satellites system. The other factor to note is that the old 121.5Mhz system transmitted using about 75-100 milliwatts of power as opposed to the new beacons that are using 5 watts of power – a stronger transmission is a good thing in bad weather or storm conditions or when your location may be obstructed by things like cliffs!

New C-S System OverviewSatellites receiving the old analog 121.5Mhz and the new digital 406Mhz systems still use doplar shift techniques to try and resolve the location of the beacon. But the new system is accurate to about 5km (2.6nm) as opposed to the old 20km (10.8nm). The old analog system only transmits a tone so the SAR operation cannot determine what is going on until they locate the beacon and find out that it isn’t a false alarm or some errant piece of electronics like a set top box on TV! The new system transmits actual data digitally which can then be linked automatically to your registration data bringing up who to phone and details of your vessel and so on. In the UK you must register your 406Mhz beacon with the MCA

Is added GPS a useful feature?

If the unit you buy has the added facility to transmit GPS location data as well then the accuracy is even greater – about +/-125m. It isn’t as accurate as your chartplotter GPS simply GEOSAR Dec2008 smallbecause the transmission of the GPS data is limited by the message length of the protocol being used. Even if your beacon is destroyed in the emergency it only needs to transmit for a few minutes for the satellites to pick it up and resolve the location. Pretty damn good I think:-)

The main satellite system for picking the beacon’s signal is the SARSAT system, but in addition the geostationary GEOSAR satellites that cover more than 80% of the earths surface also pick up signals and can relay the GPS location data even though they cannot compute location themselves using the doplar effect.

So how do you choose a 406Mhx beacon for your boat?

The choice is then between Category I (auto deployment) and Category II (manual deployment) 406Mhz beacons…and also units with or without GPS.

I have prepared a small table of units available in the UK market below. Click on the images for a larger view.




Clearly I think you should get one with built in GPS, and if you can get a 16 channel GPS all the better. I dont think the CatI auto deployment is worth the extra money…


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