There is only 20bits for the address in the PLB message format - so only 4" resolution.
If you allowed more accuracy you would have to have different formats for different beacons and there would be further confusion when the resulting position string was passed onto the local SAR (is this deg mm ss or deg mm.mm, is this WGS84, NAD83, NAD27CONUS etc. at +/-100m these don't matter)

The standard was developed for marine rescues and 100m accuracy is probably good enough to find you from a helicopter. They don't need to know which side of the life raft you sent the distress call from.

SPOT don't have to conform to any message format other than their own standards so probably send the full accuracy of the GPS chip, 5-10m in ideal condition. How much this helps in a real rescue is debatable - I can't imagine rescuers walking up and down a rescue scene looking only at their own GPS displays looking for an exact spot.

Thanks. So SPOT is approximately 10 times more accurate than a PLB. I didn't know that PLBs were subject to this constraint in their accuracy due to cost considerations.

Has nothing at all to do with cost, per se. Simply has to do with the existing limitations of a protocol designed before GPS was even widely available. There is currently efforts undergoing to revise the protocols to enhance the resolution, but it will take a few years.

As for SPOT vs PLB, there's a good deal more to consider than the raw accuracy of the transmitted position. First of all, PLBs have a homing beacon on 121.5 MHz, something SPOT lacks. Second, in the U.S, and over time more and more SAR forces all over the world, are getting 406 MHz DF capability, which means that they can home from distances in excess of 100 miles out.

Third, if SPOT does not get a GPS location, that's it. There is no back-up like the Doppler location you get with a PLB's 406 MHz transmission and LEO satellites. SPOT does have the potential to at least get SAR into the area if Tracking is engaged. That’s a somewhat big if, in my opinion, but better than nothing. Assuming someone intends to use SPOT as their distress alerting device, they would do well to ensure tracking is always turned on (which must be done manually every 24 hours, something that has occasionally been forgotten by a user, with resulting issues for SAR). Even then, without homing and depending upon the mode of travel, it can be difficult.

SPOT transmits at 400 mW while PLBs transmit at nominal 5 Watts, with the latest version form ACR transmitting at 6.3 watts. While that is partly a requirement to ensure it reaches the GEO satellites (which BTW ensure sub-5 minute message delivery compared to up to 20 minutes for SPOT, it also ensure there is enough power to punch through heavy canopy or to be picked up if bounced off the narrow walls of a canyon, for example.

It also matters where you travel. SPOT has limited coverage compared to the 406 MHz COSPAS-SARSAT system that is global. Not an issue for some, a big issue for others. Finally, as I ahve nmoited in articles, there is the issue of the SPOT interface with SAR, which is of concern, IMO, compared to the more established interface through COSPAS-SARSAT system, an issue we will be taking up in D.C. next week at the NSARC Working Group meeting.

So, again, the issue of the GPS resolution/accuracy provided in the distress alert is not so cut and dried as it may seem at first glance.

Doug,

I am further edified in the PLB domain. The two line items above solidify my choice to procure the ACR SARLink.

Thank you,

Craig

I've wanted a PLB for a couple of years, and now my wife has agreed that it would be desirable to have one for our next trip.

I've been looking at the ACR SarLink (as Doug recommends above) and the ACR AquaLink, which is almost identical except that the case is a tiny bit larger, apparently to provide the ability for it to float.

Curiously, the AquaLink appears to sell for about $7 less than the SarLink at most online retailers. I suspect that is because it doesn't include a nice a belt clip. The price difference isn't much, but it's enough to pay for shipping.

Can anyone point out a downside to choosing the AquaLink over the SarLink for use in land based environments?


Dennis

I'm interested in the differences as well. My brother in law has an AquaLink which I'm considering packing for a hard-core snowmobile trip in ON, CA instead of purchasing a SARLink.