PLBS vs SENDS
There's been a lot of discussion recently about options for emergency rescue beacons, and a lot of misinformation, misunderstanding, and confusion about the two different devices, so I thought I'd outline the crucial differences so people can make informed decisions. Whether you choose to take a PLB or a SEND or neither, or both is of course entirely up to you, but it's critical that you have the right information so you can make the right choice, and unfortunately the manufacturers of these products are not particularly forthcoming about the differences between the two. I won't be discussing the difference in performance between different models or anything like that - this isn't a review - I'll simply be discussing the technical differences between the two devices. The most critical thing to grasp is though they share some functions that on first glance may appear similar, the way the two devices function is completely different.
But first Satellites 101, because understanding the difference between different satellite networks is critical for understanding the difference between these devices. Satellites orbit the earth, and communicate with devices on the surface. They require line of sight, and depending on the transmission power, signal can be disrupted by interference in the form of weather, solar activity, or even clouds of migrating insects. The more powerful the transmission, the less interference there will be.
Not all orbits are equal. Orbital altitudes vary enormously from Low Earth Orbits (LEOs) of below 2,000km to very high orbits (the moon orbits the earth at an altitude of 384,000km). The lower a satellite's orbit, the shorter its orbital period will be (time taken to orbit the earth once). In addition, it's orbital speed is higher. The end result is that from the perspective of an observer on earth, the apparent time a satellite takes to cross from one horizon to another increases exponentially as the orbital height decreases. This is critical for satellite communications because this period of time represents the communication window in which a device can successfully link with a particular satellite. Satellites in LEO will cross the sky in a couple of minutes. That assumes you're in a completely flat landscape with clear views horizon to horizon. As you put yourself in steeper terrain (with a smaller and smaller view of the sky, the communication window can drop to a matter of a few seconds. In steep enough terrain, if a satellite doesn't pass directly overhead, you won't have any communication at all. Conversely, a satellite in sufficiently high altitude has the opposite problem; the moon for example is in such high orbit that the earth's rotation affects its apparent movement across our sky more than its own orbit does.
Low Earth Orbit is the easiest orbit to establish and also the easiest to maintain. As a result, the vast majority of satellites operate at these low altitudes, including every manned spacecraft, and all satellite communication satellites. The problems with coverage are mitigated by having satellite constellations consisting of multiple satellites. In fairly open terrain these ensure constant coverage. However in steep terrain there can still be gaps in coverage, particularly in areas that are not directly beneath a satellite path. Now onto the devices...
A Satellite Emergency Notification Device. Popular examples include the SPOT 3 and the DeLorme InReach. These devices are essentially the texting equivalent of a Satellite Phone, and work exactly the same way. The user subscribes to a provider, and messages are sent via a commercial comsat network such as the Iridium constellation. Critically, these are COMMUNICATION devices, not an emergency beacon. An "emergency" feature is provided, that is essentially just a "template" text message that automatically sends when the designated emergency button is pressed. This includes GPS coordinates (if available). A private commercial emergency rescue facility (in effect just a call centre) receives these messages and passes them on to the appropriate Rescue Coordination Centre (RCC) to initiate a rescue in the traditional manner.
There are not minimum standards for a SEND device as far as power output, battery life, durability, or environmental protection. However most devices how some degree of waterproofing and are low powered. Just how low-powered is sometimes a guess - you won't find the transmission power in the specs sheet for most SENDs. The Delorme InReach SE, however is 1.6w.
An issue with a SEND is battery life, as they're a "dual use" device. Because they feature tracking, "check in", communication, and sometimes altimeter, barometer and other functions, depending on how the device is used it is constantly using its battery. That means when an emergency strikes, how much battery life you have is dependent on how long it has been since you last charged it, and how much you've used it. It's similar to the battery power issue you have with using a cellphone.
The obvious advantage of a SEND, and indeed the function they're actually intended for, is two-way communication. They're primarily intended as a device for allowing periodic "check-in" so that friends and family can track your progress and be assured that all is well. However as mentioned, the device uses satellites in LEO, and as a result communication with satellites is not reliable. In steep terrain they simply may not be able to communicate with a satellite at all. Further, because of their low power output they need a clear view of the sky, and any transmission can be derailed by poor weather, tree cover, and so on.
A Personal Locator Beacon includes examples like the ACR ResQLink and McMurdo Fast Find. These are essentially small hand-held versions of the emergency beacons found on ships and aircraft and integrate with the same emergency response network used globally by world rescue agencies. Unlike a SEND, a PLB does not involve private commercial companies anywhere in its operational chain, and is not operated for profit. There is no charge for the services, and both the satellite and ground-monitoring stations are operated by government agencies. Because PLBs are a technical part of the global rescue network they must meet strict technical standards for durability, environmental protection, power output, and battery life.
PLBs communicate with specialist Rescue modules on satellites in the Cospas-Sarsat Programme - an international system of distress beacon monitoring and rescue response started in 1979 and now including 26 countries. A PLB, rather than a satellite communication device, is a distress radiobeacon. While a SEND actually sends a template text message, a PLB sends out a distress radio transmission.
A Search and Rescue payload on board seven satellites monitors and detects these distress radio transmissions, processes the signal, and then relays it to one of sixteen ground monitoring stations. From there the signal is delivered to the relevant RCC and rescue is initiated.
The seven satellites in the Cospas-Sarsat programme are in Geostationery orbit - that is they do not "cross the sky" but remain fixed over the same exact point on the earth's surface. While the Iridium satellites (for example) are at an altitude of about 780km, the Cospas-Sarsat satellites are at an altitude of about 35,000km. The result is that for a given area of the earth, any distress radiobeacon will always communicate with the same satellite, and that satellite's position in the sky will never move (across the entire length of New Zealand the angle to the nearest satellite only moves by less than 1 degree). In addition, because the satellite is at such high altitude, no matter where on earth you are the nearest satellite always appears close to directly overhead, meaning that it is always visible even in extremely steep terrain (in NZ the nearest satellite sits about 12 degrees off vertical).
Because a PLB must meet rigorous standards, and because it is a dedicated emergency device, when it is activated the remaining battery life is known and guaranteed - all PLBs must provide a minimum of 24hrs of continuous transmission at the end of the battery's 5 year shelf life. PLBs also transmit at a much higher frequency - 5W making them less susceptible to interference and able to successfully send their signal through obstructions such as a forest canopy or if beneath a pack (all attempts should still be made to give the device as clear a view of the sky as possible).
PLBs today transmit on the 406MHz frequency which is internationally dedicated to Distress Beacons. As well as being monitored by the SAR payload on satellites, all commercial aircraft are required to monitor the 406MHz frequency and can detect and report a distress radiobeacon signal. Finally, most PLBs transmit a second low powered 121.5MHz "homing" signal simultaneously. While the GPS coordinates embedded in both a SEND text message and a PLB radio distress signal will identify the approximate area where a signal originated, GPS coordinates are seldom precise enough for a SAR team to locate someone, particularly in difficult terrain. SAR teams are equipped with 121.5MHz receivers which can detect the homing signal from a PLB and pinpoint the beacon's position to within a matter of inches. This addition alone can reduce rescue time by hours or even days.
The major disadvantage of a PLB is that is does not allow for two-way communication. The main disadvantage in this case is mostly psychological - receiving confirmation that the cavalry is on its way can be highly reassuring, as opposed to simply trusting that your PLB worked. However there's a good chance a SEND message won't get through at all, and even if it does there's equal chance the response won't get through, so the value of two-way communication is perhaps over-stated.
It has also been suggested that two-way communication allows you to pass crucial information to SAR teams but personally I find this doubtful. There's very little information a party in distress can actually communicate to a rescue party that will aid the rescue, other than your location. Whether anyone is injured or not is irrelevant, for a number of reasons. To begin with, no SAR team will ever take the word of a party in distress and choose to leave equipment behind because it is not needed. Whatever you might tell them, they will follow protocol, get to you as fast as they can, and then assess you. Unless the people being rescued are medical practitioners themselves any medical assessment they provide is going to be suspect anyway. The other consideration is the nature of any injury. Medical emergencies are not, contrary to popular perception, time dependent. With the exception of a few notable examples (such as cardiac arrest and stroke) medical injuries do not get worse over time, and getting to the patient sooner does not increase their chances of survival. If you sustain a fatal injury in the wilderness, you will die. It is as simple as that. No one will ever get to you in time to save your life. And if the injury is not fatal, it's irrelevant if rescue parties bring the required treatment four hours or four days after it occurs, as long as you're protected from the dangers of exposure, dehydration, etc.
That I can think of, there is only one situation in which I can imagine 2-way communication in an emergency being useful, and that's where a large number of people require medical treatment. Such situations are likely to be rare. The only scenarios I can really think of would be a group caught in a flash flood or a bridge collapse or similar (earthquake? Rock fall?). In those scenarios the ability to notify rescuers that it is not one or two people in distress but a dozen could have significant implications.