Final questions for a PLB purchase

The spot is mainly optimized to send and receive messages and uses a satellite system built to send and receive messages. ACR and the satellites it uses were specifically designed for search and rescue.

The ACR has a transmitter power of 5watts. The SPOT has a transmitter power of 0.4watts. They also use different radio frequencies and I suspect the frequency used by the ACT will penetrate trees easily. The ACR also sends out a beacon signal that can be tracked by search and rescue so it still should be possible for people to find you in the event the unit cannot detect GPS satellites.

However if you can get to an open area do so. If you are in a dense forest only ground rescue is possible. However if you are in an open area helicoptor rescue is possible

"2) My Garmin eTrex 30 is fantastic in dense cover thanks to its High Sensitivity receiver. How is the signal transmission of the ACR ResQLink compared to the eTrex 30?"

I assume signal Reception? I wasn't aware the eTrex 30 had transmit capability.

Correct, reception of a signal through its high-sensitivity receiver.

No thoughts about the issue other than a technical correction?

"How is the signal transmission of the ACR ResQLink compared to the eTrex 30?"

I'm sorry, but this question just doesn't make any sense at all.

You are getting all confused between something that receives only, versus something that transmits only, versus something that does both.

–B.G.–

Bob,
Clarified in the two posts preceding yours …

Bob, the purpose of that particular question was to get a sense of how the ACR is under dense cover compared to my eTrex 30. The later has no problems whatsoever under a heavy canopy of trees.

Like I said, this doesn't make any sense.

A GPS receiver only has to receive a signal from space. Space is quiet, and the GPS satellite transmitter is blasting out a very high power signal. So, some GPS receivers are quite adept at picking out that satellite signal from the noise. In fact, the received signal is below the noise floor, and it still gets it.

A beacon first has to receive the GPS signal, just like above. Then it has to transmit out a signal going up to a different satellite in a completely different orbital altitude. Transmitting upward from underneath a tree can be problematic, and tree leaves have a different effect on different frequencies. The transmit problem is much different from the receive problem.

What is agreed is that moving out into the open helps all reception and transmission. If you were incapacitated, then that might be difficult.

When I get to that bad thought, I think of the Marine who fell off a trail several years ago and broke both legs when he landed in a ravine. A few weeks later he crawled some miles out to a highway. I'll bet he wished that he had an emergency beacon.

–B.G.–

Steven makes some good points, but there is more to know:

The ACR actually uses the government-run NOAA weather (and probably military) satellites, the SAR system piggybacks onto that backbone. Includes a cooperative network of government satellites around the world. SPOT uses a private, commercial satellite system that some will say means by its very nature is less reliable.

Yes the ACR has a transmitter power of 5 watts while SPOT only a fraction of a watt. But signals at the same power travel differently at different frequencies, plus the height of each system's satellites' orbits is different — so the ACR has to send its signal farther. I don't know for sure how these differences net out, but I do believe that the net effect is the ACR signal is more likely to reach its satellite.

The ACR does have a beacon signal, but that is a "local" signal — the idea is that the satellite signal will be able to get SAR within a few hundred yards of you, then once in that range they can pick up the beacon to get within 10 meters or so of you. But don't expect SAR to pick up the beacon unless they were first alerted by the satellite and responded in the area near you.

The ACR satellites are positioned such that anywhere in the continental USA and virtually all of the civilized world there is a satellite in the sky within line-of-sight (assuming not blocked by dense forest, steep canyon walls, etc). The SPOT satellites have spread orbits that move the satellites across the sky, and sometimes there might not be a satellite in range.

If you hit the SOS button, that signal needs to go up to the satellite and then be relayed back down to a response center. If the satellite that the ACR uses cannot relay your SOS then it "holds" it until it is in range of a response center and re-transmits, but my understanding is that the SPOT's satellites do not hold the signal, so it will only work if both you and a response center are in range of the same satellite for an immediate transmission.

FYI I'm an ACR owner. It is a serious tool optimized for getting out an SOS and having SAR response, a true PLB — SPOT is not a true PLB but rather a communication device that can also send an SOS.

Nope, the ACR doesn't signal under canopy any better than SPOT. No difference. None. Nadda. Zip.

Back in the days before you could test the things, people used to claim all sorts of signaling mystical powers for the COSPAS-SARSAT PLBs. Now that folks can test them, reality has sunk in – they aren't magic, and they aren't superior.

Both SPOT and ACR probably will get a signal out under canopy. Both will likely NOT be able to get a GPS fix, so their SOS signals will lack location information. Doppler with the ACR will get location to within a couple miles. Electronic bread crumbs with SPOT will do the same or better. Always important to remember that nobody knows where you are with the ACR until you have an emergency. With SPOT you should've been leaving messages every so often so that SAR knows where to start.

COSPAS-SARSAT PLBs like the ACR ResQLink are an excellent option and as long as you recognize their limitations they are cheaper than SPOT. SPOT, of course, offers more features.

In the original post –

"My Garmin eTrex 30 is fantastic in dense cover thanks to its High Sensitivity receiver. How is the signal [reception] of the ACR ResQLink compared to the eTrex 30?"

The OP is looking for a comparison between eTrex 30 receivers vs the ACR ResQLink receivers.
Not a re-hash of SPOT versus PLB's.

The PLB only transmits (unless extra gadget bought like Delorme stuff). The GPS only receives. Any PLB transmitting at 5 watts will probably get a signal out in some degree of canopy. Most PLB's do transmit at 5 watts. If not, move on. The ACR is a very good PLB.

A PLB has to use that 5 watts to hit a satellite 10x farther away. So 1/2 watt to a close satellite versus 5 watts to a far away satellite.

"A PLB has to use that 5 watts to hit a satellite 10x farther away. So 1/2 watt to a close satellite versus 5 watts to a far away satellite."

I would have expected power needed to be proportional to the square of the distance, unless the transmitter is very directional. Is it not?

"Both SPOT and ACR probably will get a signal out under canopy. Both will likely NOT be able to get a GPS fix, so their SOS signals will lack location information. Doppler with the ACR will get location to within a couple miles. Electronic bread crumbs with SPOT will do the same or better. Always important to remember that nobody knows where you are with the ACR until you have an emergency. With SPOT you should've been leaving messages every so often so that SAR knows where to start."

Zorg – Are you saying that with the ACR, a SAR team will only be able to narrow down my location to within "a couple of miles"? A couple of miles in one direction and a couple in the other means the ACR will only be able to narrow down my location by 4-9 square miles?

That makes no sense to me and I've never heard that assertion made before. Others here are welcome to correct me if Zorg is correct. Who would spend money for any device that would be so pitifully inaccurate?

"That makes no sense.."

+1

But, Wiia, be prepared for unsubstantiated innuendo, as Zorg drags this thread into to ground….

Best just to walk away, start a new thread, and re-state your original question.

The Company, at its website says, "The onboard 66 channel GPS can quickly fix the sender's position to within 100 meters and then utilizes a powerful 406 MHz signal to relay the distress call to orbiting satellites.

http://www.acrartex.com/news/product-announcements/2011/12/resqlinkplus-buoyant-personal-locator-beacon/#sthash.mbYfBKuZ.dpuf

So Zorg, please source your information about "a couple of miles".

I should never had brought up the eTrex30 in my original post. My only reason for mentioning it was that my eTrex30 is fully operational under the dense cover of trees.

I was just wondering if the ACR would be equally as effective in the same conditions.

It depends on what parts of this are working and what parts are not working. By working or not, I mean it depends on whether the device has a GPS fix to send. As an example, if you were incapacitated underneath a dense tree, then it is possible that the device can't get a GPS fix, so it has no location data to transmit. It can still transmit a beacon signal, and that crude position might be good to only a few miles, so it is better than nothing. A GPS fix means that a fairly accurate location can be transmitted, and that may be good to within 100 meters. Geez, a helicopter may not want to get any closer than that, anyway.

The point is that everybody quotes accuracy and numbers to suit their own argument, and those cases can contradict everybody else's. If you sort of diagram out what is going on for receive and transmit functions, you can sort out who is claiming what and which conditions they might have meant. If you think about which element might fail, and then which elements are still left working and what the result will be, you can come to better conclusions.

–B.G.–

"fully operational under the dense cover of trees."

That is a tough one. There is no specification for what is a tree. Does that mean a dormant broadleaf tree in winter, or a green broadleaf tree in summer? What about a conifer? Different trees have different amounts of water in the leaves and needles, and that means different amounts of attenuation in the signal.

When it was fully operational, how do you know? Many GPS receivers will temporarily keep showing you a seemingly valid latitude/longitude position, even after they have lost a usable signal. You have to look at the EPE numbers to know whether it is really going to hell or not. If you've never seen the EPE numbers, then you really don't know. On some GPS receivers, you can look at the view of the satellites and their position in the sky and their received signal quality, so you can begin to tell if the tree is really hurting you or not. On some GPS receivers, they are kind of quiet about telling you when the signal is marginally gone.

–B.G.–

“fully operational under the dense cover of trees.”

Equipped to Survive tested a different PBL years ago. I assume things are now than they were then –

“We started with a baseline test with a wide open sky, lots of satellites visible, and the Fast Find consistently sent out its GPS location in the first 406 MHz data burst, 50 seconds after deployment. We then started to artificially create much more difficult circumstances, using a “space blanket” with metalized lining to block GPS signals down to no more than 3-4 satellites visible and forcing difficult geometry. Even under the most difficult circumstances we could create using the blanket, such as at the bottom of a 5-galloon bucket surrounded by the space blanket, it still got a GPS fix consistently within the first 1-2 bursts with a single case of 3 bursts.

We then tried it under a large deciduous tree, close to the trunk, with similar results, the worst performance being the second burst.” [my emphasis]

Gee wizz, imagine that – It works under a tree – and they were doing their best to make it Not work.

wiiawiwb –

To answer your question in your original post, it's very unlikely that the GPS receiver in a PLB will match your Etrex. I've used/tested a lot of different GPS receivers and the receivers that Garmin has been manufacturing in the last few years are generally a leap ahead of the others. I have not found another receiver that will consistently hold a satellite lock indoors. The Garmin's manage to do this and it will also tell you the error circle, which can be large when indoors but would be a decent enough accuracy for a rescue. I have tested both the Delorme units (original inReach and the inReach SE) and also the Spot and neither of these devices can get a GPS signal indoors. I haven't tested any PLB's indoors but I would expect that they would fail to get a GPS lock. I mention this just to point out that Garmin receivers are exceptional, but that doesn't mean the others aren't very capable receivers. They are just not as good as the Garmin receivers. For rescue purposes, you are not going to need to receive GPS signals inside a building, so the other receivers will be varying degrees of OK.

Having said that, I'm not sure that having a PLB with a receiver as good as your Etrex should be what you are using to evaluate your needs? Since you said you don't need to communicate with anybody, then buy a PLB. The other devices have expensive plans because they are communicators, so why would you even think about buying a communicator? Save the bucks and go with the PLB, since none of them have a Garmin receiver anyway, why think about that?

This is a tad disconcerting and far more complicated than I thought. It makes me wonder why Garmin hasn't gotten into the PLB market.

I guess, in the end, you press the magic button and just hope and pray that it works as expected.

I'm sorry then to be the one to give you the truth then. Doppler accuracy would give SAR a 4 square mile box to search in. Out on the open ocean where the search is usually done with a helicopter, searching this box is pretty fast. But SAR (Search And Rescue) inland is usually coordinated by the county Sheriff, who may or may not have a helicopter available. Most likely the search will be conducted on foot and it can take a while, depending on conditions, to locate somebody.

This is something everybody should understand clearly anyway – "rescue" depends on the local SAR. If you are on the coast, it is likely that the Coast Guard will help with the search. If you are in the Sawtooths in Idaho, (like the recent plane downing), SAR is run by the county Sheriff, staffed by real people who have lives, facing nasty conditions without super high-tech equipment. I have read a couple of cases where SAR knew the exact location of the injured, but were unable to reach them, and the injured died.

Remember that you are talking about the worst-case scenario where you activate the beacon under dense canopy. The device can't get a GPS fix so the distress signal is sent without coordinates. One of the problems with worst-case analysis is that we can get statistically unrealistic "worst-case". I.e. – don't over-think this unless you really believe you must be able to signal your location from inside a cave.

The ACR ResQLink is a fine device. I think it is an excellent tool for responsible backcountry practice.

I have the ACR ResQlink.
In test mode it fails when I stand with my body to the south of the device.
I'm not sure which part fails… the GPS or the rescue signal, but I know it fails because it lights up the red led lights; not the green ones that signal a successful alert.

Billy