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Author Topic:   AIS Personal Locator Safety Device
jimh posted 05-28-2012 08:44 AM ET (US)   Profile for jimh   Send Email to jimh  
There has been much prior mention and discussion of personal locator beacons or PLB devices as safety equipment for a person in distress. The PLB devices send a radio signal at UHF to orbiting satellites. An alternative approach is more down to earth: a personal automatic identification system or AIS transponder to be worn by individual crew members that will send an AIS position broadcast on VHF over a range of four miles. One such device--perhaps the only such device--is the


This device, also called a Safelink R10, is a small AIS transponder intended to be fitted to a life jacket or PFD and worn by someone in the water. When activated, the Safelink R10 deduces its position using its own built-in global positioning system (GPS) receiver, and sends a personal AIS transmission with that position information. Range is anticipated to be about four miles to a typical surface vessel.

With many recreational vessels now equipped with AIS receivers, any vessel within range should receive the Safelink R10 transmission, and, once the GPS receiver obtains a position fix, the receiving vessel will have a very accurate position for the source of the transmission. In contrast, almost no recreational vessels have receivers and direction finding equipment needed to detect and home-in on a signal from a personal locator beacon or PLB.

The use of an AIS rescue device makes sense on boats with more than one person aboard. If someone goes overboard wearing an AIS locating device, the remaining crew aboard can maneuver the vessel to their location. If you are boating alone, a PLB is a better choice.

jimh posted 05-28-2012 11:36 AM ET (US)     Profile for jimh  Send Email to jimh     
[Moved reply to a separate article. Please use this thread to comment on the AIS personal locator device, particularly in comparison to a personal locator beacon that transmits to satellites. Thanks]
jimh posted 05-28-2012 01:21 PM ET (US)     Profile for jimh  Send Email to jimh     
A GPS receiver requires a certain time to acquire a position fix. The time to first position fix can vary depending on the knowledge the GPS receiver has about the present time, date and current general location, and also if the GPS receiver has an accurate ephemeris of the satellite orbital information. The GPS receiver uses this data to predict what satellite will be overhead based on its assumed position and the date and time. It then tries to receive that satellite first. If there is no time, date, or position information, and no satellite ephemeris, the receiver has to listen for all 32 GPS satellites. Modern GPS receivers can often listen for and acquire more than one satellite at a time, so the initial acquisition time--often called cold acquisition time--can be reasonably short.

An emergency device with a GPS receiver may not be keeping track of time and date. If that is the case, when the power is activated the device will not know much to help enhance satellite acquisition time. I tried to find a specification for the GPS receiver in the Safelink R10, but I could not locate any documentation. As a comparison, the GlobalSAT BU-353 GPS receiver is a reasonably modern GPS receiver. Its specifications suggest that its time to position fix from a cold start will be 45-seconds. However, the BU-353 also notes that it is designed with an integral "super-Cap" (or very large storage capacitor) which is probably used to maintain a very low power consumption real-time clock. Since the Safelink R10 is operated only from battery power, one might wonder if its GPS receiver were provided with power for a real-time clock from the storage battery. The drain would be very small, but, on the other hand, one would not want the battery to be depleted when the device was deployed in an actual emergency. It would be interesting to see the specification for position acquisition from a cold start for the GPS in the Safelink R10.

Another factor in GPS acquisition is the influence of water on the path to the GPS satellites. Microwave signals do not propagate through water, and I would anticipate that the GPS receiver in the Safelink R10 must be designed to operation with some tolerance to water.

jimh posted 07-22-2012 11:35 AM ET (US)     Profile for jimh  Send Email to jimh     
I just received an email from a vendor touting this device. It is apparently now approved for sale in the USA. The retail price is approximately $300.
hauptjm posted 07-25-2012 03:07 PM ET (US)     Profile for hauptjm    
Here is the "skinny" from Homeland Security and the Coast Guard regarding AIS. If you haven't seen this, I thought you might be interested.
jimh posted 07-25-2012 08:22 PM ET (US)     Profile for jimh  Send Email to jimh     
The linked article (above) just seems to be a very general overview of the automatic identification system. Homeland Security is not mentioned in the article. Perhaps the link points to the wrong resource.
Chuck Tribolet posted 07-25-2012 09:03 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
My Seiko digital watch runs a couple of years on a little tiny
battery. There's no reason the clock in this AIS
transponder couldn't run for several years, particularly if
it's kept charged up.

And there's another player. It's a handheld, seriously
waterproof, DSC VHF with a built-in GPS receiver:

Given that DSC VHF radios are far more common than AIS
receivers, I think it's a better choice in the OH BLEEP, I


6992WHALER posted 07-25-2012 10:09 PM ET (US)     Profile for 6992WHALER  Send Email to 6992WHALER     
I have to agree 100% with Chuck, I think a hand held DSC VHS radio with built in GPS is a better and more useful tool than any of the other location devices including AIS. For the kind of boating most people do in Boston Whaler boats.

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