personal locator beacon

An OC Guide to the Personal Locator Beacon

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People enjoy outdoor adventures in remote places, preferably away from human settlements, where they can immerse themselves in nature and its natural ambiance.

Even so, going into such remote places presents a risk should adventurers find themselves lost or in danger, and they need to call for help while stuck in a place where there are no people nearby, and phone signals are either too weak or non-existent.

This means that they cannot phone their friends or local emergency agencies to send a rescue team. This raises the question:

How can adventurers avoid being cut off from emergency agencies?

The answer lies in portable, pocket-sized equipment that can broadcast distress signals (that are bundled with location details) to search and rescue teams, which allows these teams to reach the distressed person(s). This equipment is called the personal locator beacon.

The Personal Locator Beacon

locator beacon
S.J. de Waard [CC BY-SA]
This is a portable, land-based digital device that generates and transmits distress signals to a series of satellites that support search and rescue operations.

It is an off-grid beacon designed to be used in a place where there is no cellular coverage.

Even so, what is a beacon? A beacon is simply a device that can broadcast a directional signal that can be used for navigation.

If this beacon is powered by electricity and can send location data to rescue teams, then it functions as an emergency locator and transmitter device.

The PLB is simply a portable emergency locator transmitter (ELT) designed for personal use.

The Emergency Locator Transmitter

The ELT first came into mass use following the adoption of the 1972 law requiring all airplanes to have functioning radio beacons that can support search and rescue (SAR) missions.

As expected, this early ELT was simply a battery-powered radio transmitter that could broadcast distress signals.

A distress signal is an internationally recognized call for help (SOS) that can be communicated through audible sounds, broadcasted radio signals, or display of a distinct visually observable object or direct illumination.

The distress signal of an ELT is usually sent as radio-waves which are then picked up by agencies concerned with search and rescue. Most ELT operates at 121.5 megahertz (MHz) or very high frequency (VHF), which includes the military-assigned 243Mhz VHF.

ELT designs and operations were improved in 1995 following the implementation of tight regulations on ELT serviceability and performance.

Currently, the distress signals sent by ELTs are monitored by satellites, and information relayed to ground stations that plan, coordinate and execute the rescue missions.

Some ELTs are equipped with a security alert system that broadcasts coded distress signals when the user is under attack, for instance from terrorists or from pirates (in the high seas).

Legal Statutes and Universal Regulations

The regulations governing the use of ELTs are codified in Article 1.93 of Section IV in Chapter I of the Radio Regulations of the International Telecommunication Union (ITU).

According to these regulations, an ELT is required to broadcast a distress signal as a data burst that has been encoded according to the Cospas-Sarsat (C/S) T.001 and T.018 specifications for C/S 406 MHz distress beacons.

The T.001 is the first-generation specification, while the T.018 is the second-generation specification.

The requirement for data conformity to these specifications ensures that the positional data (beacon location) is bundled with the call-for-help (distress call) into a data packet that can be converted into a radio signal that is broadcast at 406Mhz.

This data burst usually occurs in cycles of one burst per every 50 seconds, and it can be detected by two international constellations of satellites called COSPAS and SARSAT (which are described below).

Therefore, the ELT transmits an emergency call-for-help and beacon location during each data burst, and for this reason, it is designated by ITU standards as an emergency position-indicating radio beacon (EPIRB).

According to ITU standards, a PLB is a portable ELT that operates at 406Mhz and is designed for personal use by anyone who is far away from ordinary emergency services such as 9-1-1.

PLBs can be used in both terrestrial and marine/aquatic environments by hikers and boating (or diving) enthusiasts respectively.

It is mandatory that each PLB is registered by relevant authorities, and assigned a unique identification number (ID) that is linked to personal data of the owner.

This PLB ID is important because it shows who has been affected, and the emergency services can call his or her family members and relatives.

A registered PLB is designated as an approved PLB. Only approved PLBs can communicate and interface with the International Cospas-Sarsat Satellite Programme.

Why are Satellites Needed for PLB-based Search and Rescue?

satellite

The PLB is a last resort ELT that is used when there is no cellular coverage in the area where the user is located.

Likewise, the PLB must be able to send the distress signal to SAR authorities. So, how do SAR authorities receive distress calls and locate the user?

The answer is that there are a number of specialist satellites orbiting the earth that serve to pick up the distress signals broadcast by PLBs and then relay them to ground stations that pass along the signals to SAR authorities.

These specialist satellites are called (search-and-rescue) SAR Satellites, and they operate under a satellite-aided SAR initiative called the International Cospas-Sarsat Satellite Programme (ICSSP) which was initiated in 1979 and ratified in 1988.

Currently, 45 countries and SAR agencies participate in ICSSP, and ICSSP operations have been harmonized to conform to regulations and statutes set by ITU, International Civil Aviation Organization (ICAO), and the International Maritime Organization (IMO).

ICSSP has enabled SAR teams to rescue more than 60,000 people since 1982. Even so, how does ICSSP work to support the rescuing of people who have sent distress signals from their PLBs.

To begin with, there are about 45 SAR satellites dedicated to ICSSP operations, and they belong to 2 systems – COSPAS and SARSAT.

The COSPAS and SARSAT systems

Cosmicheskaya Sistema Poiska Avariynyh Sudo (COSPAS) was launched in 1982 and monitored for distress signals broadcast at 121.5 MHz and 243.0MHz from vessel EPIRBs, while the Search and Rescue Satellite-Aided Tracking (SARSAT) which was launched in the same year monitored for distress signals broadcast at 406Mhz from EPIRBs.

When the PLB entered the American market in 2003, it was designed to broadcast distress signals at 406Mhz in bursts.

Currently, the duration of each burst is 440ms and the PLB uses 5 watts (W) to broadcast the signal burst, which allows the signal to reach the satellite orbitals in outer space.

It is also evident that the COSPAS-SARSAT system is a tri-band system that can detect distress signals broadcast at 121.5 MHz, 243 MHz, and 406 MHz. Likewise, it is evident that PLBs rely more on the SARSAT satellite system.

The satellites in the COSPAS-SARSAT system are organized into 3 constellations based on their orbital properties and range.

These constellations are the geostationary (or geosynchronous) earth orbit (GEO), the low earth orbit (LEO), and the medium earth orbit (MEO).

LEOSAR satellites

The 5 SAR satellites in LEO are designated as LEOSAR satellites, while the 9 satellites in the GEO constellation are designated as GEOSAR satellites.

The satellites in MEO (MEOSAR) are positioned at an altitude of 19,000-24,000 kilometers and are the next generation satellites that complement the standard GEOSAR-LEOSAR system, though they operate in a similar way to LEOSAR satellites.

The LEOSAR satellites are positioned 2000 kilometers above sea level where they orbit the earth continuously, with each satellite orbiting the earth in 100-128 minutes, which calculates to each satellite orbiting the earth 11.25 times per day.

Therefore, LEOSAR satellites have an orbital period of 100-128 minutes with 11.25 periods-per-day. Moreover, the orbits of each satellite are distinct (never cross each other’s orbits), and orbits are set such that a single orbital period allows for coverage of the entire earth.

For the person in distress, this means that his/her location is inspected once every 100-128 minutes and the detected signal is picked up.

This signal is then forwarded to the ground station when the satellite passes over it. This is because a LEOSAR satellite can pick or forward a signal only if the PLB or ground station is in the line-of-sight.

Relatedly, each LEOSAR satellite has a store-and-forward module that allows it to pick the distress signal, locate its source, and then store the information till it passes over a ground station where it forwards the information, which is then relayed to SAR teams.

The LEOSAR satellite uses Doppler processing to calculate the Doppler Shift which allows for trilateration of the PLB location.

GEOSAR satellites

The GEOSAR satellite is positioned in a stationary spot directly above the (earth) equator. Even so, it is known that the earth rotates on its axis, and this means that the satellite needs to move at the same speed as the earth’s rotation if it is to remain on the same spot above the earth.

The 9 GEOSAR satellites are fixed in their position above the earth (that is, they are geostationary), with the arrangement of these satellites around the earth tracing an orbit above the equator, hence the designation geostationary orbit.

The 9 satellites are positioned to achieve coverage of the entire earth, which means that every place on earth is visible to the GEOSAR system at any time.

Usually, the orbit is placed higher than LEOSAR and MEOSAR satellites, and it is therefore called high earth orbit (HEO).

As expected, the geostationary nature of each satellite means that it cannot use Doppler Shift to locate the PLB, and thus the GEOSAR satellite only detects the distress signal and forwards it without adding any location data.

It is for this reason that one is advised to acquire a PLB with the capability to identify its location using the global positioning system (GPS).

A GPS enabled PLB does bundle GPS coordinates with the distress signal before transmitting it to the GEOSAR satellite.

GEOSAR and LEOSAR constellations

As expected, the GEOSAR and LEOSAR constellations complement each other, and this makes the SAR satellite system effective and reliable. Also, the time latency is minimized.

To summarize, when the distress signal is detected by the SAR satellites, the location of its ELT is triangulated by LEOSAR satellites, and the information forwarded to ground stations located in 200 countries, and no one is charged (that is, the SAR services provided under ICSSP framework are free).

So, how does a PLB operate and broadcast distress signals that are detected and forwarded by these SAR satellites?

How a PLB Works

As mentioned earlier, PLB is a portable, pocket-sized ELT, and this means that its design and construction features core components of a basic ELT.

These components are the power source, antenna, transmitter, (GPS receiver), switch, and the programming module.

The power source in a PLB is a portable long-lasting, class 2 battery that can power the device at -28.9°C for at least 24 hours.

This ensures that the PLB works in cold environments where a person can be trapped in. Usually, the battery should be able to keep a charge for 5 years without the need for recharging.

In most PLBs, the battery is non-removable and if its charge is depleted, then one is required to send the entire PLB back to the manufacturer for replacement. To connect the battery to the circuitry, a switch needs to be turned on.

The PLB must be activated manually, and it is for this reason that a switch is provided.

Binary Data

The battery supplies powers the circuit of the PLB which includes a programming module that handles conversion of human-readable data into binary data.

Binary data can be converted into analog signals that cause voltage or current changes.

The ability to use binary data makes the PLB a digital device that can convert data into distress signals that can be broadcast.

This conversion requires the data packet to be converted into a radio wave that is transmitted at a frequency of 406Mhz by a transmitter, and this radio wave is then received by a receiver. The transmitter and receiver are connected to antennas.

The Antenna

An antenna is a specially designed metal conductor that allows oscillating current supplied by the transmitter to create flux in electric charges as electron flows in the metal and this flux results in radiation of radio waves at a specific frequency and wavelength.

When these radio waves reach the receiver antenna, they cause charge flux which results in the generation of oscillating current that is converted by the receiver into binary data.

The receiver that receives PLB distress signals is called a transponder, and it is built into a SAR satellite.

GPS Coordinates

gps remote location

When the device is turned on, the GPS receiver captures (or acquires) the GPS coordinates of the device location from the GPS navigation satellites, and then forwards these coordinates to the programming module where they are amalgamated along with the distress call and PLB unit ID into a single data packet that can be transmitted by the PLB.

The transmitter blasts the data burst (of the distress signal) at 5 watts which ensures that the radio-waves go through tree canopies, atmosphere, clouds, and into outer space where the radio-waves are picked by transponders in the LEOSAR and GEOSAR satellites.

Audible Data Burst

The data burst is audible, which ensures that the user can hear that a distress signal has been broadcast. The SAR satellites simply rebroadcast the distress signal to an unmanned and fully automated ground station called the Local User Terminal (LUT).

The LUT performs Doppler Shift trilateration for distress signals broadcast by LEOSAR satellites, as well as perform error checking and error rectification of 406Mhz distress signal (data packet) sent by a SAR satellite.

This processing of the distress signal allows for a processed SAR message to be sent from the LUT to its mission control center (MCC).

The MCC checks the PLB unit ID (also known as PLB serial number) and retrieves its registration details from the PLB registration database.

This allows the PLB to be matched to the user identity (which are his/her names and contact details). User identification data is then added to the processed SAR message, and the resulting distress alert message is forwarded to a rescue coordination center (RCC).

The Rescue Coordination Center (RCC)

The RCC is basically the primary SAR facility for a nation, and the distress alert message it receives from an MCC allows the RCC to know who the user is and where (s)he is.

Each RCC is unilaterally operated by a SAR service, and it has its own well-defined SAR region of responsibility (SRR) as laid down by IMO and ICAO. This SRR allows for the subordination of different local SAR agencies under a single RCC.

The RCC passes the distress alert message to a local SAR agency that is nearest to the user, or the SAR agency that is well-equipped and resourced to perform SAR in the environment where the user is (for instance, in thick jungles or in marshes).

The RCC instructs the chosen SAR agency to form a SAR party that is sent to rescue the user. It also coordinates SAR with local SAR agencies if more than 2 SAR parties have been sent.

Likewise, the RCC contacts the family members, relatives, and friends of the user based on the information provided in the PLB registration data.

Doppler Shift Trilateration

As mentioned earlier, some PLBs lack a GPS receiver and this forces the SAR party to use location data acquired from Doppler Shift trilateration.

Even so, Doppler Shift trilateration has a margin of error of 2-5 kilometers (km). This means that the user can be up to 5km away from where the LEOSAR satellite locates the PLB, and this implies that the search area must have a radius of 5 kilometers.

This can be hectic for a single SAR party, and PLB manufacturers have found a way to reduce the size of the search area and even allow the SAR party to pinpoint the exact location of the user.

This is achieved through the use of a homing signal.

The Homing Signal

The homing signal is a low-powered distress signal transmitted as 121.5Mhz by the PLB. It is evident that homing signals are produced by modified transmitters that had originally debuted in vessel EPIRBs.

This homing signal can be picked up by transponders used by SAR parties, which allows them to move quickly towards the source of the signal.

Some high-quality PLBs also come with a strobe light that allows rescuers to see where the PLB user is located.

If the PLB has a GPS receiver and allows the user to send text messages, then it is called a satellite messenger, and their SAR operations are not mediated through RCCs but are instead coordinated from a Houston-based response coordination center.

Now, that the operations of the PLB and its vast supporting infrastructure have been described, how does one know which PLB to buy in the market?