Hello,

This is my first post on ETS. I've tried searching the forums for my question but haven't found anything so if there is an existing thread for this please let me know. I've also read the ultimate PLB FAQ as well and did not find a suitable answer.

I'm looking to purchase a PLB. My primary outdoors activity is snowmobiling in MI Upper Peninsula and the rockies (yellowstone/wyoming). This year I'm looking to sled near Hearst ON, CA.

My question: Can I purchase a US PLB and register it for use with NOAA and also register with CA?

Also, any PLB recommendations (brand/model) would be appreciated. Cold weather use and long operating time when activated are my priorities. I'm not so concerned about size or cost and I also carry an iridium sat phone.

Thanks in advance,

Craig

ChicagoCraig:
Thanks for droping in, stick around there is a lot of discussion on PLS'S on this site. Let me give you a hand, When you put PLB in the search box add a "*" to it. the search engine uses 4 dig-gets to do a search. I received 200 hits.

No and not sure why you would want to. You register your PLB in your home country whenever possible. Once registered, it is usable anywhere you go. If the beacon is deployed, the registration information is automatically shared with the SAR agency with responsibility for whatever area of the world it is in.



ACR SARLink

Big Al,

Ahhh much better. I have some reading to do.

Thank you.

Craig

Total ETS newbie here and had some gaps in my knowledge of registration. Thank you for setting me straight.



Thanks,

Craig

Hi Doug,

I was reading the product descriptions on the ACR SARLink and the McMurdo FastFind MaxG respective mfr web sites.

Both seem to have the same operating temperatures (-20f) but the MAX G shows to have over 48 hours of operation whereas the SARLink shows over 24. GPS accuracy +/- 100m for the SARLink and +/60m for the MAX G. User replaceable battery on the MAX G and not on the SARLink. The MAX G appears to have a higher street price.

What is it about the SARLink that you recommend it over the MAX G? Asking to understand.

Thanks,

Craig

McMurdo's numbers on accuracy are bogus. All GPS PLBs have the same accuracy as it is not dependent upon the GPS, but rather on the limitations of the digital coding.

The Max G does not have true one-hand deployment, the SARLink does.

The SARLink actually has a 35 hour battery. Quite frankly, extended operating times don't matter much since you should be staying put in any land based scenario.

The SARLink has a better, latest generation 66 channel GPS chip which is more likely to get a location in difficult circumstances.

The MaxG has no dedicated GPS self test, the SARLInk has a full GPS self test.

The SARLink puts out minimum 6.3 watts ERP, the MaxG the standard 5 watts.

SARLink has a flashing LED, which could be useful.

Finally, I have greater confidence in the robustness of the ACR product than the McMurdo.

The difference for the quoted accuracy figures may be down to whether whether a 2D Circular Error Probability CEP or 3D Spherical Error Probability SEP is being quoted by the 2 different manufacturers. SEP is always worse than CEP by about 1.4 times which would account for the difference so it could well be that the accuracy figure of 60 metres quoted by McMurdo is not bogus at all.

BTW not all GPS receivers are the same, some have better accuracy due to better design in clock stability, antenna design and sensitivity and firmware differences.

Again, has absolutely nothing to do with the accuracy of the native GPS or how they compute it. The GPS itself could be accurate to within 1mm it wouldn't change a thing. The issue is the resolution that the data format can deliver. Now, if the manufacturer was to quote it's number at a specific latitude, then you could legitimately have different numbers, but this is not being done as these are world-wide devices, so worst case must be assumed, which is at the equator.

The location transmitted is offset in 4 second increments +/- from the Coarse position in 2 degree increments. At the equator, one second of latitude/longitude = 101.3 feet.

McMurdo chooses to present what it says is what an average user might expect, but IMO that is misleading unless they provide the information a consumer needs to determine what that number really means.

I assumed that the PBL, would transmit something like the much more accurate GPS NMEA 0183 GLL sentence to the satellite (minute to 2 decimal places) rather than a limited accuracy compressed binary formated positional message presumably to save data bandwidth for the satellite transmission. +- 404 feet at the equator could be problematic especially for example if under the Brazilian Jungle canopy.

So does this mean that the SPOT device is actually much more accurate in its positional fixes or is it constrained by the same limitations on transmitting its GPS fix calculation due to the constrained binary message format to the satellite? I seem to remember seeing that the SPOT device doesn't have this limitation.

It seems that the constrained transmit message could easily give SAR potentially a lot more work to do or even cost lives due to the extra search time just over the cost of transmitting an extra nibble or two to the satellite.

Is this PBL GPS format set in stone or will it be updated. It seems that the PBL GPS transmit format was instigated prior to GPS SA being turned off.

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.