Types of satellite-based emergency beacons
Personal Locator Beacons
Personal Locator Beacons (or PLBs) have one and only one purpose: to send your location to emergency responders, indicating that you need some kind of urgent help. They use a satellite network operated by government agencies worldwide, and send your distress call directly to a government-run rescue coordination center.
When you initiate a distress call, the following sequence of events happens (from REI's article on PLBs and locator beacons):
PLBs transmit powerful signals at 406 MHz (MegaHertz), an internationally recognized distress frequency monitored in the U.S. by NOAA (National Oceanic and Atmospheric Administration) and the AFRCC (Air Force Rescue Coordination Center).
A PLB communicates with a network of Russian, Canadian, American and French military satellites known as COSPAS-SARSAT (SARSAT is an acronym for "Search and Rescue Satellite Aided Tracking").
After receiving your transmission, these satellites "fix" on your location using a Doppler Shift method and relay your information to the AFRCC where search and rescue procedures begin. If you use a GPS-compatible PLB, you can deliver your GPS coordinates very quickly without having to wait for the satellites to determine your position.
Assuming you are in a location where the signal can be picked up by a satellite – which they almost always can be – the AFRCC will receive your call almost immediately. Most PLBs these days are GPS-integrated.
If you use a PLB with a GPS interface, the 406 MHz signal will guide rescuers to an area less than 100 meters from your position. At the same time, they will employ a tracking device to home in on the 121.5 MHz frequency put out by the PLB. When using a GPS-compatible PLB in the continental U.S, it takes only 5 minutes to alert search-and- rescue personnel of your position
So, it takes about 5 minutes with a PLB for the AFRCC to determine your location.
Commercial Satellite Based Systems
There are various commercial emergency beacons, like your SPOT Messenger, which use privately owned satellites to relay emergency messages or other messages. They usually require an annual subscription, whereas PLBs only have the up-front purchase cost. SPOT and many other companies use the GEOS International Emergency Rescue Coordination Center (IERCC) which is based in Houston, Tx. According to SPOT's info pages:
Once activated, SPOT will acquire its exact coordinates from the GPS network, and send that location along with a distress message to the GEOS International Emergency Response Center every five minutes until cancelled or until the batteries are depleted. The Emergency Response Center notifies the appropriate emergency responders based on your GPS location and personal information – which may include local police, highway patrol, the Coast Guard, our country’s embassy or consulate, or other emergency search and rescue teams – as well as notifying your emergency contacts about the receipt of a distress signal.
The IERCC is manned 24/7 with dedicated and highly trained operatives who have access to emergency responders worldwide to ensure that in an emergency your situation will be dealt with efficiently and you will receive the resources you need. In fact, the average response time from the receipt of your 911/SOS message until referring an emergency responder is only 11 minutes, wherever you are in the world.
Once a SOS/911 message has been received, the IERCC calls your emergency contact(s) to make sure that it wasn't a false alarm. They locate and notify an emergency responder, and then maintain an open line of communication, including providing updates of your location if needed. The IERCC will also keep your emergency contact(s) informed.
The response speed of the GEOS IERCC is going to depend on how well-connected they are with emergency responders in your area, but it looks like it takes about 11 minutes for SPOT's service to determine your location and contact the authorities.
Once "The Authorities" have been notified
Once they have been alerted, the time for rescue crews to actually show up is going to be dependent on many factors, including:
How easy it is to contact the appropriate SAR agency in your area, and how many levels of communication it takes to get there. For example, in a national park it's pretty easy for the AFRCC or IERCC to contact the park dispatch and initiate a SAR response. In more remote areas it may take time to contact an agency like the local Sheriff office, the National Guard, or some other agency.
How long it takes a SAR team to initiate a response after being notified. Most Coast Guard air stations and some Air Force helicopter SAR units maintain a 24 hour alert posture, with a crew able to depart within 15-30 minutes of receiving a call. Most National Guard units provide 24-hour response but may have to call crews from home during off-hours, and can have anywhere from a 15 minute to a 1 hour time from receiving a call to a helicopter lifting off.
Ground-based SAR teams which are not on 24-hour standby often take a long time to assemble, especially volunteer teams. While a first responder, such as a Sheriff SAR deputy or an NPS rescue ranger may be able to respond quickly, a fully-assembled team capable of performing technical rescue or a litter evacuation commonly takes several hours to assemble personnel and equipment.
How long it takes for a SAR team to reach the area. This depends on where the nearest SAR resources are and their availability. In the Sierras and Rockies you might see a helicopter within a matter of hours, if weather allows and resources are available. In a remote part of Alaska it may take a day for a plane to do a fly-by and assess the accessibility of the location.
How long it takes a SAR team to actually find you. These devices transmit a homing signal as mentioned above, and with GPS they'll already have a pretty good idea. If there's dense tree cover or poor visibility though, it may take time for a crew to actually locate your exact position.
Once they reach you, the time for a rescue to take place. Helicopters can't land everywhere, and ground teams can take a long time to arrive.
With either a PLB or a commercial device, the response time is going to mostly be a factor of your location, not the signal getting out. The signal will be located quickly, but rescue crews may take minutes to hours to arrive depending on the situation.
Making yourself easy to find
If you have a GPS-based device, rescue crews should be able to locate you to within ±100m, so you don't generally need a long-distance signal like a smoke column. Making a fire (if conditions are appropriate and safe to do so) can help especially at night. A signaling mirror is very helpful in daylight for catching a pilot's attention. Place yourself in a wide open area if possible. At night you can use a flashlight to get the attention of helicopters once you hear them nearby. Once you've been noticed, don't shine the flashlight directly at the cockpit as pilots will be using night-vision goggles and could be blinded. Large areas of unnatural colors, such as a bright tarp or tent, can also help you to stand out.