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The HAZ Forums frequently feature questions about emergency transmitters and locators.

Since technology changes quickly, previous discussions become outdated. Researching older threads can be helpful, but it is also nice to find the most current information with ease. Let's keep discussion on this topic in this thread. It will be archived and a new thread started when new technology changes the game again.

Basic Technologies:
SEND: Satellite Emergency Notification Device
PLB: Personal Locator Beacon

PLB is the simplest to explain because it really only does one thing. It is used exclusively in an emergency and sends a signal on a global emergency network that can only mean one thing. You need help and need it now. Part of the data it transmits (besides your location) is identifying information so rescuers know WHO you are ahead of time. An additional feature of the PLB is that it has a local homing beacon, so when rescuers arrive to the area (100 meter accuracy) you transmitted from, the beacon on your device directs them to your exact location. This could be useful in heavy brush, etc.

PLBs cost more to purchase, but there is no subscription required. The battery generally lasts for 5 years or so, and replacing it currently costs about $150. But it's one of those pieces of equipment you hope to never use.

In the technical nitty-gritty of it all, PLBs communicate on a global emergency network run and monitored by governments. PLBs transmit at about 5 time the power of other tranmitters. For pure emergency locating, PLBs simply have no competition.

The most popular brand of PLB is the ACR ResQLink (there are several versions).

SENDs on the other hand are generally used to communicate via satellite -- In emergencies AND non-emergencies. SENDs provide extra features that some users desire, primarily the ability to send "peace of mind" messages to loved ones, or other features including mapping, tracking and waypoints. For those users, SENDs are extremely popular options. But --SENDs require fee-based subscriptions to transmit communications. Let's break them down into ONE-WAY and TWO-WAY communicators.

ONE-WAY communicators allow the user to send messages to the satellite network that are then delivered to pre-determined users. Emergency signals are forwarded to rescue personnel. Non-emergency messages can be simple messages such as "I'm OK" and are sent by text message or email to family or other pre-designated users. Some devices send only pre-programmed messages, while others will pair with a smart phone and allow custom messages to be sent. An example of a one-way SEND is the SPOT Gen3.

TWO-WAY communicators do everything that ONE-WAY communicators do, but also allow the user to receive messages over the satellite network. This can obviously be helpful both in emergencies and non-emergencies. An example of this kind of SEND is the DeLorme InReach.

In the technical nitty-gritty ... SENDs transmit their data to commercial satellite networks that do not always have global reach. This is why they require a subscription fee. DeLorme uses the highly-reliable Iridium satellite constellation and SPOT uses the similar Globalstar constellation. The messages are received by a third-party and then forwarded to the appropriate entities. Think of it as On*Star for your hike! They transmit with a power level that is only about 20% of that of a PLB. This makes them more susceptible to not acquiring your location, or not sending a message when desired.

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I should add that I don't personally own one of these devices and what is written here is just a brief summary of my research. I'll gladly correct anything that is incorrect. But feel free to discuss this topic here and include your own information, experience, and knowledge.

Previous forum discussions on this topic may be viewed here:
viewtopic.php?f=3&t=8246
viewtopic.php?f=32&t=5752
viewtopic.php?f=32&t=3072

Thank you for this. Very helpful!

@azbackpackr the only references I've seen about a SPOT and "false alarms" are those who hit the SOS because they are tired and don't feel like hiking back out, or things like that. I haven't heard of any inadvertent SOS messages, so I wouldn't worry about that.

Where the SPOT does have a problem is that it is not 100% reliable. I tracked success of sending about 200 checkin messages. My results were

94% of messages were sent with an accurate gps fix
1% sent with poor gps fix
5% failed

Of the 5% that failed, some were understandable, down in a gorge or something. But some were in ideal conditions, no trees, clear to horizon, no bad weather. 95% isn't bad odds though, so I think it is worth carrying. I've used the SOS button one time and it went through.

The Delorme Inreach is perhaps a bit more reliable, but the real advantage is that you get confirmation that your message was sent. Oh, and 2 way communication. With SPOT it's press the button and hope for the best.

Copied by admin from another thread for the useful firsthand data relevant to this thread

chumley wrote:They transmit with a power level that is only about 20% of that of a PLB. This makes them more susceptible to not acquiring your location, or not sending a message when desired.

I'm not a tech guy but does it matter that 7 of the 12 satellites that the PLBs communicate with are 22,000 miles above the Earth's surface vs. Spot and InReach's 400 miles? Is this the reason for the higher output signal? Also compare the PLB's 12 sats (7 are owned by other countries: 2 of those from India, and another from Russia) with no spares vs. 44 for SPOT and 66 plus 6 in orbit spares for inReach. Its my understanding that PLB sats are 1st weather sats and the emerg comm part is an add-on. In the event that the sat has a power problem the add-ons like sun spot and o-zone monitoring and maybe emerg comm would be shut down (I'm just guessing).

@Dave1
I'm not a technical guy either, but I did read from several different sources that the PLBs are more reliable at getting a signal out. I didn't realize that the satellites were at a different orbit level, nor do I know how much the power output is related to the distance.

Most reviews I've read show that PLBs have an excellent connection rate, while the SENDs were very good ... just not quite PLB level. The post from @w h a above seems to corroborate that. I think 95% is a very good success rate for communicating with a satellite with a handheld device!

Here's a highly technical discussion about the PLB satellite communications:
http://en.wikipedia.org/wiki/Internatio ... _Programme
You are correct that the satellites are meteorological satellites. But I personally wouldn't fear them being shut down or compromised in any way. These are the primary emergency channels for distressed mariners, aviators, etc. and 37 countries are involved in the system. When a commercial aircraft crashes, it's emergency locator pings on the same frequency (406MHz) as your PLB. In my opinion, that's pretty good assurance that the system will be maintained and operational.

I bought a ResQlink back in December and put a lot of research into it. My findings led me to believe that the PLB was my best shot at being rescued. The signal is 5 times stronger than the Delorme and 10 times stronger than the SPOT. It will broadcast the signal for up to 36 hrs. PLB's are older technology and have been used in the field successfully for a long time. The Delorme Inreach has some nice frills but is weaker. The SPOT-- well, I really couldn't find any reason to purchase this product over the others. The nice thing about the ResQLink is no subscription. I only had to pull the trigger once and I have it for the 6 year battery life. It seems like there's more than a few SPOT's and Delorme's sitting out there with un-renewed subscriptions.

@Tough_Boots
I like the reliability factor you touched on. Good points.

Spot 0.4 watts
DeLorme InReach SE 1.6 watts, max 7.5 ( found in 1 forum, couldn't verify it )
ACR ResQlink 406 5.0 watts

22000 / 400 = 55
55 times further but only 3.125 times more watts. Probably not that simple, heck E.T. phoned home with a pile of junk. Just infield test and opinions. I couldn't find technical data on antenna design and how they compared to each other. Guess ACR looks like the wise choice based on what is available. InReach looks like the fun choice but you would kinna think they'd publish technical comparison data if they had it to prove. I didn't invest much time looking, just curious.

I'm not a scientist, and I didn't even stay in a Holiday Inn Express last night. But I would make an edumakated guess that radio waves can travel quite far in space with very little power. So the higher up the satellites are, the farther the radio waves will travel with no additional power? It's a difference between the Mesosphere where the near-earth orbit satellites are and the Thermosphere where the PLB satellites are. I could be totally wrong about this. It just seems logical to me.

Apparently, in the Thermosphere, the density of air is so small that a single molecule of oxygen travels an average of one kilometer before colliding with another molecule of oxygen (thanks Wiki! ). What does that mean for the wattage of your transmitter? I have no idea!

For the record, the Space Shuttle and ISS orbits are approximately 200 miles. Less than half as high as the Iridium and Globalstar satellites.

Engineer Mode ON

So Dave's question is about transmitter power and the distance to the satellite. The distance does matter, but the design of the tranmitter is actually only half of the information needed to perform a comparison.

Briefly: Let's have an antenna radiating power: Lets say this is 5 watts of power. The radiation pattern of the antenna determines how that power is "spread out". While most PLBs probably have a dipole design, let's assume the antenna on the PLB is isotropic. An isotropic antenna is one which spreads an equal amount of power in every direction. Think of it like a sphere expanding away from the antenna.

What is the surface area of a sphere? 4*pi*r^2

Therefore at 400 miles, that 5W of power is spread over 5.20749×10^12 square meters.

Let's assume the receiving antenna is 1 meter in radius, and therefore the surface area is: 3.14 square meters

Which means the receiver gets 3.14/(5.21*10^12) amount of the power, which is .00000000000060268 * 5 W, or
.000000000003 Watts

At 22000 miles, that 5W of power at the 1 meter radius antenna is
.000000000000001 Watts

which is a lot less!

But does it matter when you are talking about such tiny amounts of power?

Not really, because we don't know anything about the receiver. The receiver design may require more or less power to be received to be able to actually obtain the signal being transmitted to it.

Generally speaking, a transmitter and receiver are designed together.

For example, the low transmit power of a battery-efficient cellphone might transmit a little power, and thus need a sensitive receiver capable of receiving small signals. A small receiver in a cellphone/radio might require a big powerful transmitter in a tower to ensure the signal makes it through.

So ultimately, the transmit power doesn't tell you anything on its own. The transmitter is designed to work with the receiver. Perhaps SPOT wanted the best battery life, so they transmit the least power, and have the most sensitive receiver. PLBs, being the oldest, and designed to be put into boats and whatnot, allocated more power to the transmitter, and hence the receiver was probably designed to that transmitter power level at that distance.

Transmit power can matter in terms of overcoming interference and getting through trees and other things that try to absorb that transmitted power. For example, to a satellite 22000 miles away, the satellite is getting your signal at the same time as it is getting all energy radiated across the country in its direction. The satellite will filter out your frequency, and then you compete against everything else in the world transmitting at that same frequency. There are other techniques that don't require you to "win" this power battle on your frequency. For example, the satellite wants to be able to receive 2 different PLBs at the same time transmitting at the same power level. This will be the topic of PLBs class 102 .

chumley wrote:I'm not a scientist, and I didn't even stay in a Holiday Inn Express last night. But I would make an edumakated guess that radio waves can travel quite far in space with very little power.

Correct, let's take a satellite dish. A satellite dish is designed to move all power in one direction without allowing it to spread out very much.

A perfect (theoretical) satellite dish could manage to send ALL the transmitted energy in one direction, without allowing it to spread out at all. If the satellite dish on the satellite orbiting pluto is the same size and both antennas are pointed perfectly, that pluto satellite would receive all the transmitted power.

Now no dish is perfect, so the power does spread out, but a dish is much better than a whip antenna and can ensure that a much greater percentage of the transmit power reaches the receiver which is very, very, very far away.

Pure space (e.g. the absence of matter) absorbs no energy in generally accepted physics. So if the transmitter and receiver are both in space, you don't have to worry about anything in the atmosphere, like pesky clouds, absorbing any of the energy either.

nonot wrote:you don't have to worry about anything in the atmosphere, like pesky clouds,

And chemtrails. Don't forget chemtrails...

nonot wrote:3.14/5.21*10^12

doh, I forgot the exponentiation

I think that's what it's called. Google please don't fail me!

... :worthy:

I didn't check my math, guess I will look foolish if I messed it up

The biggest problem I see with the PLB is that you have to be conscious to activate it. If you fall and knock yourself unconscious, you will not be able to activate it.

I've been told that a similar technology used by firefighters has a "dead man's switch" that activates periodically. If the firefighter doesn't turn it off the team will go into full rescue mode. It is assumed that the firefighter has a problem or has possibly been overcome by smoke.

“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.” ― Douglas Adams, Mostly Harmless

Al_HikesAZ wrote:used by firefighters has a "dead man's switch" that activates periodically. If the firefighter doesn't turn it off the team will go into full rescue mode.

You heard that during 9-11. I believe lack of movement activates it.

Al_HikesAZ wrote:The biggest problem I see with the PLB is that you have to be conscious to activate it. If you fall and knock yourself unconscious, you will not be able to activate it.

true... my thought is that what is the likelihood of being anything more than temporarily knocked out while hiking? Those who climb, canyoneer, or mountaineer might be better off with something else. I can't think of many (if any) situations I've read about where hikers died in the wilderness after being rendered completely unconscious. I guess age/health related issues might be the exception-- stroke, heart attack, etc.

Thus for some situations, the best thing you can do is "never hike alone". I know that's not realistic/normal for me or many on this site though.

Al_HikesAZ wrote:The biggest problem I see with the PLB is that you have to be conscious to activate it. If you fall and knock yourself unconscious, you will not be able to activate it.

SPOT and InReach both have a feature that transmit your position on some preset schedule. I'm talking about a feature I've never used and I am neither an engineer or Chumley, so take this for what it is worth. I think that is the most expensive option as far as data plans. Still need someone on the other end that cares enough to see if all the little dots on the map are in one place. Power/battery use would be an issue too I'd think. But then if I am unconscious at the bottom of a ravine am I thinking?

AZ Wandering Bear wrote:I am neither an engineer or Chumley