PLBs and satellite messaging devices: a primer

In Public by Anna Debattiste

By Bruce Beckmann, Alpine Rescue Team (updated on 12-28-23)


Adventurers exploring the backcountry should always carry a device capable of sending a satellite message.  When an emergency arises and no cellular signal is available, having the ability to send an “SOS” or a message via a satellite may save a life, your own or someone else’s. 

There are now three primary types of devices that use satellites to communicate:

1)   Personal locator beacon (PLB)

2)   Satellite messaging devices, or just “satellite messenger”

3)   Satellite capable cellular phones.

In brief, a PLB sends a one-way SOS signal to a satellite that is then ultimately relayed to the local sheriff; there is no two-way communication between the hiker and the sheriff.  The satellite messenger is a two-way satellite communications device that also alerts the local authorities to your SOS call but allows for two-way messages to be sent and received between the hiker and the responding agency.   Lastly, newer cell phones capable of communicating via satellite to send an SOS message are now a third convenient way to call for help.  Each type is discussed in more detail below. 

APPLE iPhone 14 – SOS and Emergency Satellite Feature

In 2022, Apple released the iPhone 14 with SOS and emergency satellite texting capabilities. If you are out of cell coverage and have an emergency, the phone can connect via satellite to the closest emergency response center to coordinate a response. This texting capability via satellite is, however, only available during emergencies and is conducted between you and the emergency dispatch center. Texting between you and friends or family members is not supported in either emergency or non-emergency mode, at least at the time of this writing (March 2023).  Location tracking also does not appear to be an option currently.

As of this writing, the iPhone SOS feature is available on the iPhone 14 and 15 and is not available on the Apple Watch Ultra nor the iPad Pro devices – check with Apple for any changes in the SOS feature rollout within its products. Crash detection on your Apple Watch uses the iPhone 14 or 15 phone to communicate to emergency services, possibly by satellite.

This SOS feature is available in a limited number of countries.  Also, “Satellite connection might not work in places above 62° latitude, such as northern parts of Canada and Alaska.” [Use Emergency SOS via satellite on your iPhone 14 – Apple Support.]


In the very near future, several other cellular carriers (both in the U.S. and in Britain) will be entering this field.  Motorola has announced the planned release in the second quarter of 2023 of a satellite device (“defy satellite link”) that will connect to any phone via Bluetooth and provide for satellite connectivity with messaging capability – including non-emergency texts.  Service fees do apply.  It is early and hard to say for certain, but tracking may be an option with this device.  With rapid technological advancements coming, this will be an area to keep an eye on. 

The remainder of this discussion is limited to PLBs versus traditional satellite messengers.


The PLB operates on the SARSAT system (Search and Rescue Satellite Aided Tracking) on the 406 MHz frequency; the same as an aircraft’s ELT (Emergency Locator Transmitter).  [Note: SARSAT is an inter-governmental co-op of 45 nations dedicated to detecting emergency signals; the full name is COSPAS-SARSAT, which we have shortened it to SARSAT for this discussion.]   The term “PLB” has often been confused with satellite messaging devices, so remember; the PLB allows for one-way communications and the satellite messenger offers two-way communications.

When activated, the PLB sends a one-way signal to the satellites saying, “I need help,” or in other words, SOS. You cannot describe the nature of your distress call. Traditional PLB alerts are received by the SARSAT satellites, re-transmitted to the Air Force Rescue Coordination Center (AFRCC) for U.S. alerts, channeled through the state’s search and rescue (SAR) coordinator, then on to the local county or sheriff’s dispatch center, and finally to the on-call mission coordinator for the local rescue group.  Time from triggering the PLB alert in the field to reaching the appropriate state’s SAR coordinator is generally less than 10 minutes. 

PLBs enjoy better satellite communications due to having a 5-watt transmitter (e.g., able to push through dense overhead foliage) versus a 1.6-watt transmitter on a satellite messenger.  PLBs are not dependent on cellular networks and generally work around the world.  If traveling to a foreign country, contact that country or NOAA to learn about any usage restrictions. 

Before 2017, without encoded coordinates, accuracy of the target location was two to three miles, which slows down rescuer response time to locate you.  However, beginning in 2017, PLBs must transmit a GPS coordinate encoded in the signal.  This Lat/Lon coordinate narrows the search area from 2 – 3 miles to less than 100 meters, and makes finding your location much easier.  NOTE:  if you have a pre-2017 model PLB and the battery life is up, rather than replace the battery, purchase a new PLB with encoded GPS coordinates.

Once activated, PLBs also transmit a homing beacon on the 121.5 MHz band that can be picked up by ground radio detection devices, but the area of reception is limited to about ½ mile, and could be farther pending line of sight or if aircraft detection devices are used. The homing beacon is transmitted for at least 24 hours.  

PLBs should be registered with NOAA to be effective for SAR teams.  This can easily be done online via Registration info includes owner’s name, contact info, emergency contacts, use of the PLB (motorized, hiking, skiing, etc.) and may also include vehicle info for trailhead checks.  It is important to do this to give rescuers some information about you in the event of an SOS activation.  Periodic updates to your registration information can be done via this link as well.

In 2023, a popular PLB is made by ACR Electronics and sells for about $400.  There are several other manufacturers on the market with varying prices and features.  We do not recommend one model over another. 

PLBs do not require a subscription fee.  When new, battery life is guaranteed for five years, even after allowing for up to a set number of test communications during the five-year time frame.  At the end of the five years, the battery should be replaced by an authorized repair center and re-certified for use.  Monthly tests of satellite and GPS connectivity are recommended. 


As of 2023, the two-way satellite communication units most commonly used are the Spot X and Garmin inReach.  There are other two-way satellite messaging devices available, and one model or manufacturer is not recommended over another, so do your homework. 

The key difference between a satellite messenger and a PLB is the ability to send and receive text messages.  The satellite messengers also utilize 1.6 watts of power versus 5 watts for PLBs.

Satellite messengers utilize a network of commercial satellites versus governmental satellites.  These units communicate directly to the commercial satellite and are not dependent on cellular networks.  Satellite messengers generally work around the world, except in the far north or south latitudes of >70°, but if traveling to a foreign country, contact that country or NOAA to learn about any existing usage restrictions. Messages can be sent at any time, with or without a pending emergency.

There are many commercial satellite constellations for backcountry emergency communication and the most popular are the Iridium and Globalstar satellite constellations, as opposed to the governmentally operated SARSAT satellite system. 

  • The Iridium satellite constellation is owned by Iridium Communications, Inc. and consists of 66 active satellites.   The Iridium system operates in the frequency range of 1616 to 1626.5 MHz. Garmin inReach uses this system.
  • The Globalstar satellite system is owned by Globalstar Inc. and consists of 24 active satellites.  The Globalstar system operates in the frequency range of 1610.73 to 1621.35 MHz.  Globalstar is used by the OnStar system in some vehicles.  SPOT and Apple iPhone 14 with the SOS feature use this system and use one frequency for U.S. units and a different frequency for the rest of the world.

Both satellite systems operate in a low earth orbit of 485 miles versus the SARSAT’s various satellites whose orbits range from 527 to 22,320 miles above earth.

In 2022, GEOS International Emergency Response Coordination Center (GEOS-IERCC), located in Houston TX, monitors the Iridium satellites for emergency traffic.  Focus Point International in San Diego, CA monitors the Globalstar system.  If an emergency signal is detected by either agency, they determine the location and alert the appropriate county’s sheriff directly, who then activates local SAR teams for that area. 

In 2023, the purchase price of a satellite messenger varies; most are in the $350 to $450 range depending on available features, and prices vary based on the vendor as well.  There are also a variety of available monthly/annual service plans which must be purchased for operation of the unit.  In comparison, a PLB does NOT require the purchase of any service plan. 

Because satellite messengers operate on commercial networks, registration with the monitoring company is required for activation.  Each company may request various types of information and some is optional.  Registration includes general information that might be needed in a rescue, e.g., name, age, address, medical info if desired, secondary emergency contact info, etc.  It is from such registration that this important emergency information is provided to the monitoring agency for use in an actual emergency and passed on to the agency responsible for the rescue response.  It is important to update this information as changes occur in your personal life.

Some folks may carry both a PLB and a two-way satellite messenger.  Each has its own unique advantages.  Considerations include:

  • PLBs, with five watts output power, may push an SOS signal through dense foliage better than a satellite messenger.  Getting the signal out under adverse conditions may lead to a faster reporting time to local SAR teams than the two-way satellite communication units. 
  • Satellite messaging devices, with1.6 watts of output power, may take more time to reach the satellites in similar conditions; but when connected, they do allow for two-way conversations. 
  • Both PLB and satellite messaging devices need clear access to the sky above you to connect to the satellites.  Terrain features, such as tight or narrow gullies, heavy dense overhead foliage and extreme weather can inhibit transmission on either unit.


The Garmin inReach and SPOT X are examples of modern global two-way communication devices.  Today, there is a wide selection of vendors.  Features found in these units vary, but now most allow the user to:

  1. Send an SOS, along with the GPS coordinates and elevation of the individual requesting help.  This message is without detail – just “I Need Help”.
  2. Send pre-written text messages stored on the device to a selected recipient or recipients, and most importantly,
  3. Send and receive customized SMS text messages (like on a smart phone) from/to any selected recipient(s).  This feature allows the user to communicate with others on a non-emergency basis, or in the event of an emergency to explain the nature of the emergency and provide details. Rescuers can send you a text to advise you of what to do while they are enroute, their ETA, etc.  After an SOS activation, the monitoring agency will immediately send a reply message letting you know your message was received and will ask for more details regarding your situation. The satellite messenger sends the SOS and communicates on a different frequency and antenna to eliminate interference from normal message traffic; in other words, the SOS is a top priority signal as are the related SOS messages.   Depending on the carrier’s operating procedures, the flow of the emergency information may vary from company to company, but ultimately gets to the sheriff of the county in which the emergency exists.

Regular messages can often be directed to any cell phone number and/or to an email address, or directly between other satellite SMS enabled units.  Units may also allow direct posting to social media accounts like Facebook and Twitter.

A smart cell phone may be linked via Bluetooth to make texting and general use easier. This feature may be optional and varies based on the unit. Some units on the market, such as ZELO, Defy Satellite Link and Bivystick, require the Bluetooth linking of a cell phone for full operation. Prices for these “fobs” start as low as $99 for the unit, plus a reasonable monthly subscription fee. Always carry a backup battery pack for your phone; without the phone, these devices are no good. These fobs may require charging after 48 hours, so plan accordingly.

Satellite messengers function independent of a cell phone network; thus, no signal, no problem.  Basic mapping (scales vary) and other features found on a standard handheld GPS unit may also be provided. Again, features vary based on model, service plan, etc.

Furthermore, the Garmin inReach user may allow for his/her location to be shared.  Sharing allows a person or persons to track that individual in real time in the field on a proprietary mapping system.  These programs vary from proprietary mapping programs to the use of Google maps, and can be used on an individual’s computer, smart phone, or similar device; cellular or internet connection is required to view such activity online.  Periodic track points show current time and coordinates of each point, a visual tracking of the route a person is taking, the direction headed, elevation, and speed of travel.  Interval of transmission is generally a user selected option and there is a wide range of time intervals available; the most frequently selected interval is every 10 minutes. Read the unit’s service plan limitations and charges for frequency of intervals selected and select an interval best suited for your needs.

There are just too many companies, with varying features and subscription levels, who now provide these valuable services to discuss here.  If looking to purchase such a device, you should do your own research and determine which model and service plan meets your unique needs and budget.

Whichever device you choose, please read the manual carefully.   Enjoy and be safe out there.

Update, December, 2023 – SpaceX Touts – new features for 2024: According to ARS Technica “ARS Technica Link”: Starlink’s website update is revealing a bit more about its plans for a satellite-delivered cell phone service. The new page for Starlink Direct to Cell promises “ubiquitous coverage” from cellphone towers in space that will work over bog-standard LTE. The current timeline claims there will be
text service starting in 2024, voice and data in 2025, and “IoT” service in 2025.

“Today satellite phone connectivity still requires giant, purpose-built hardware, like the
old-school  Iridium network phones.  If you’re only looking for emergency texting, you
can also make do with Apple’s introduction of the barely there connectivity paradigm,
requiring being inside a connectivity window, holding up a phone, and following a
signal-targeting app. Starlink wants to bring full-blown space connectivity to normal
smartphone hardware.”

Below is a sample guide to the various features found, noting the differences between a PLB and satellite messaging devices.  This may not apply to all devices and is subject to change.  Do your own research carefully before making a personal selection.


Approximate cost$380 varies$450 varies
Monthly subscription feeNoneStart around $15/mo and up and vary widely
Signal Strength Wattage5 watt1.6 watt
BatteryFive-year life – replaceable through approved centers for a feeRechargeable – mini USB connection– can last 80 hours on a single charge
Register with NOAAYESNO
Register with ManufacturerNOYES
Monitored byGovernments worldwideCommercial companies worldwide
Worldwide operationYes, depending on countryYes, depending on country
Ability to send two-way messages – emergency and non-emergencyNOYES- pending model
Homing beacon (121.5 MHZ)YESNO
Offers navigation reaturesNOYES- pending model
Bluetooth pairingNOYES- pending model
Offer other navigational featuresNOYES- pending model
Weather reportsNOYES- pending model
Topo map navigationNOYES- pending model
Computer tracking real time at homeNOYES- pending model
Send SOS & your coordinatesYes – models sold after 2016YES
Ability to cancel SOSYESYES- pending model
Ability to set waypointsNOYES- pending model
AltimeterNOYES- pending model
Paring with certain fitness watchesNOYES- pending model
Social media postingNOYES- pending model