SPLB Sound Pressure Level (B-weighted)

Sound pressure is the local pressure deviation from the ambient (average, or equilibrium) pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The SI unit for sound pressure is the pascal (symbol: Pa). The instantaneous sound pressure is the deviation from the local ambient pressure p0 caused by a sound wave at a given location and given instant in time. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time (or space). In a sound wave, the complementary variable to sound pressure is the acoustic particle velocity. For small amplitudes, sound pressure and particle velocity are linearly related and their ratio is the acoustic impedance. The acoustic impedance depends on both the characteristics of the wave and the medium. The local instantaneous sound intensity is the product of the sound pressure and the acoustic particle velocity and is, therefore, a vector quantity.

The sound pressure deviation p is

p = \frac{F}{A} \,

where

F = force,
A = area.

The entire pressure ptotal is

p_\mathrm{total} = p_0 + p \,

where

p0 = local ambient pressure,
p = sound pressure deviation.

Sound pressure level

Sound pressure level (SPL) or sound level Lp is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibels (dB). Sometimes variants are used such as dB (SPL), dBSPL, or dBSPL. These variants are not permitted by SI.

L_p=10 \log_{10}\left(\frac{p^2_{\mathrm{{rms}}}}{p^2_{\mathrm{ref}}}\right) =20 \log_{10}\left(\frac{p_{\mathrm{rms}}}{p_{\mathrm{ref}}}\right)\mbox{ dB} \,

where pref is the reference sound pressure and prms is the rms sound pressure being measured.

The commonly used reference sound pressure in air is pref = 20 μPa (rms). In underwater acoustics, the reference sound pressure is pref = 1 μPa (rms).

It can be useful to express sound pressure in this way when dealing with hearing, as the perceived loudness of a sound correlates roughly logarithmically to its sound pressure. See also Weber-Fechner law.

Measuring sound pressure levels

dBSPL: A measurement of sound pressure level in decibels, where 0 dBSPL is the reference to the threshold of hearing. Often the calibration is done for 1 pascal is equal to 94 dBSPL.

When making measurements in air (and other gases), SPL is almost always expressed in decibels compared to a reference sound pressure of 20 μPa, which is usually considered the threshold of human hearing (roughly the sound of a mosquito flying 3 m away). Thus, most measurements of audio equipment will be made relative to this level. However, in other media, such as underwater, a reference level of 1 μPa is more often used. These references are defined in ANSI S1.1-1994. In general, it is necessary to know the reference level when comparing measurements of SPL. The unit dB (SPL) is often abbreviated to just "dB", which gives some the erroneous notion that a dB is an absolute unit by itself.

The human ear is a sound pressure sensitive detector. It does not have a flat spectral response, so the sound pressure is often frequency weighted such that the measured level will match the perceived level. When weighted in this way the measurement is referred to as a sound level. The International Electrotechnical Commission (IEC) has defined several weighting schemes. A-weighting attempts to match the response of the human ear to pure tones, while C-weighting is used to measure peak sound levels. If the (unweighted) SPL is desired, many instruments allow a "flat" or unweighted measurement to be made. See also Weighting filter.

When measuring the sound created by an object, it is important to measure the distance from the object as well, since the SPL decreases in distance from a point source with 1/r (and not with 1/r2, like sound intensity). It often varies in direction from the source, as well, so many measurements may be necessary, depending on the situation. An obvious example of a source that varies in level in different directions is a bullhorn.

Sound pressure p in N/m2 or Pa is

p = Zv = \frac{J}{v} = \sqrt{JZ} \,

where

Z is acoustic impedance, sound impedance, or characteristic impedance, in Pa·s/m
v is particle velocity in m/s
J is acoustic intensity or sound intensity, in W/m2

Sound pressure p is connected to particle displacement (or particle amplitude) ξ, in m, by

\xi = \frac{v}{2 \pi f} = \frac{v}{\omega} = \frac{p}{Z \omega} = \frac{p}{ 2 \pi f Z} \,.

Sound pressure p is

p = \rho c \omega \xi = Z \omega \xi = { 2 \pi f \xi Z} = \frac{a Z}{\omega} = c \sqrt{\rho E} = \sqrt{\frac{P_{ac} Z}{A}} \,,

normally in units of N/m2 = Pa.

where:
Symbol SI Unit Meaning
p pascals sound pressure
f hertz frequency
ρ kg/m3 density of air
c m/s speed of sound
v m/s particle velocity
ω = 2 · π · f radians/s angular frequency
ξ meters particle displacement
Z = c ? ρ N·s/m3 acoustic impedance
a m/s2 particle acceleration
J W/m2 sound intensity
E W·s/m3 sound energy density
Pac watts sound power or acoustic power
A m2 Area

The distance law for the sound pressure p is inverse-proportional to the distance r of a punctual sound source.

p \propto \frac{1}{r} \, (proportional)

\frac{p_1} {p_2} = \frac{r_2}{r_1} \,

p_1 = p_{2} \cdot r_{2} \cdot \frac{1}{r_1} \,

The assumption of 1/r2 with the square is here wrong. That is only correct for sound intensity.

Note: The often used term "intensity of sound pressure" is not correct. Use "magnitude", "strength", "amplitude", or "level" instead. "Sound intensity" is sound power per unit area, while "pressure" is a measure of force per unit area. Intensity is not equivalent to pressure.

I \sim {p^2} \sim \dfrac{1}{r^2} \,

Hence p \sim \dfrac{1}{r} \,

Examples of sound pressure and sound pressure levels

Sound pressure in air:
Source of sound Sound pressure Sound pressure level
pascal dB re 20 μPa
Theoretical limit for undistorted sound at
1 atmosphere environmental pressure 101,325 Pa 194 dB
Krakatoa explosion at 100 miles (160 km) in air 20,000 Pa 180 dB
Simple open-ended thermoacoustic device 12,000 Pa 176 dB
M1 Garand being fired at 1 m 5,000 Pa 168 dB
Jet engine at 30 m 630 Pa 150 dB
Rifle being fired at 1 m 200 Pa 140 dB
Threshold of pain 100 Pa 130 dB
Hearing damage (due to short-term exposure) 20 Pa approx. 120 dB
Jet at 100 m 6 – 200 Pa 110 – 140 dB
Jack hammer at 1 m 2 Pa approx. 100 dB
Hearing damage (due to long-term exposure) 6×10?1 Pa approx. 85 dB
Major road at 10 m 2×10?1 – 6×10?1 Pa 80 – 90 dB
Passenger car at 10 m 2×10?2 – 2×10?1 Pa 60 – 80 dB
TV (set at home level) at 1 m 2×10?2 Pa approx. 60 dB
Normal talking at 1 m 2×10?3 – 2×10?2 Pa 40 – 60 dB
Very calm room 2×10?4 – 6×10?4 Pa 20 – 30 dB
Leaves rustling, calm breathing 6×10?5 Pa 10 dB
Auditory threshold at 2 kHz 2×10?5 Pa 0 dB

Sound pressure in water:
Source of sound Sound pressure Sound pressure level
pascal dB re 1 μPa
Auditory threshold of a diver at 1 kHz 2.2 · 10-3 Pa 67 dB

The formula for the sum of the sound pressure levels of n incoherent radiating sources is

L_\Sigma = 10\,\cdot\,{\rm log}_{10} \left(\frac{p^2_1 + p^2_2 + \cdots + p^2_n}{p^2_{\mathrm{ref}}}\right) = 10\,\cdot\,{\rm log}_{10} \left(\left({\frac{p_1}{p_{\mathrm{ref}}}}\right)^2 + \left({\frac{p_2}{p_{\mathrm{ref}}}}\right)^2 + \cdots + \left({\frac{p_n}{p_{\mathrm{ref}}}}\right)^2\right)

From the formula of the sound pressure level we find

\left({\frac{p_i}{p_{\mathrm{ref}}}}\right)^2 = 10^{\frac{L_i}{10}},\qquad i=1,2,\cdots,n

This inserted in the formula for the sound pressure level to calculate the sum level shows

L_\Sigma = 10\,\cdot\,{\rm log}_{10} \left(10^{\frac{L_1}{10}} + 10^{\frac{L_2}{10}} + \cdots + 10^{\frac{L_n}{10}} \right)\,{\rm dB}

Loudest sounds

Sound pressure levels above 194 dB at sea level produce waveforms that are distorted. Sound waves are made up of rarefaction and compression cycles but when the compression half of the wave cycle is double normal atmospheric pressure and the rarefaction half of the cycle reaches perfect vacuum (no further air molecules to remove) then the only possible increase in sound level can be achieved on the compression side of the waveform. The rarefaction half of the cycle will be clipped at any level above 194 dB. Examples of such an occurrence are large-scale manned rocket launches, sonic booms, munitions explosions, thunder, earthquakes and volcanic explosions.

SPLB Special Personal Locator Beacon

In the field of Search and Rescue (SAR), distress radio beacons, also collectively known as distress beacons, emergency beacons, or simply, beacons, are tracking transmitters which aid in the detection and location of boats, aircraft, and/or persons in distress. In the proper sense, the term refers specifically to the three types of radiobeacons (listed below) that interface with Cospas-Sarsat, the international satellite system for Search and Rescue. When activated, such beacons send out a distress signal that, when detected by non-geostationary satellites, can be located by triangulation. In the case of 406 MHz beacons which transmit digital signals, the beacons can be uniquely identified almost instantly (via GEOSAR), and furthermore, a GPS position can be encoded into the signal (thus providing both instantaneous identification and position.) Often using the initial position provided via the satellite system, the distress signals from the beacons can be homed by SAR aircraft and ground search parties who can in turn come to the aid of the concerned boat, aircraft, and/or persons.

There are three types of distress radiobeacons compatible with the Cospas-Sarsat system:

* EPIRBs (Emergency Position-Indicating Radio Beacons) signal maritime distress,
* ELTs (Emergency Locator Transmitters) signal aircraft distress
* PLBs (Personal Locator Beacons) are for personal use and are intended to indicate a person in distress who is away from normal emergency services, e.g., 9-1-1.

The basic purpose of distress radiobeacons is to get people rescued within the so-called "golden day" (the first 24 hours following a traumatic event) when the majority of survivors can still be saved.

Since the inception of Cospas-Sarsat in 1982, distress radiobeacons have assisted in the rescue of over 20,531 persons in 5,752 distress situations. In 2005 distress radiobeacons aided in the rescue of 1,666 persons in 435 distress situations. There are roughly 556,000 121.5 MHz beacons and 429,000 406 MHz beacons. As of 2002, there were roughly 82,000 registered (406 MHz) beacons, and over 500,000 of the older unregistered kind.
General description

Most beacons are brightly colored and waterproof, EPIRBs and ELTs are larger, and would fit in a cube about 30 cm on a side, and weigh 2 to 5 kg (4 to 11 lb). PLBs vary in size from cigarette-packet to paperback book and weigh 200 g to 1 kg (? to 1? lb). They can be purchased from marine suppliers, aircraft refitters, and (in Australia and the United States) hiking supply stores. The units have a useful life of 10 years, operate across a range of conditions (?40 Celsius°C to 40 °C), and transmit for 24 to 48 hours. As of 2003 the cost varies from US$139 to US$3000, with varying performance (see below).

Classification nomenclature

The three distress radiobeacon types are further classified as follows:

Beacon modes

The most important aspect of a beacon in classification is the mode of transmission. There are two valid transmission modes:

* Digital Mode - 406 MHz beacons
o transmit a unique 15, 22, or 30 digit serial number called a Hex Code which contains encoded data such as:
+ the Country of beacon registration
+ the identification of the vessel or aircraft in distress, and
+ optionally, position data from onboard navigation equipment (GPS)
o transmits for a quarter of a second once every 50 seconds to both the GEOSAR satellites and the LEOSAR satellites
o 406 beacons will be the only beacons compatible with the MEOSAR (DASS) system.
o 406 MHz beacons must be registered (see below).
* Analog Mode - all other beacons
o A simple analogue siren tone is transmitted continuously until the battery dies.
o In the case of 121.5 MHz beacons, the frequency is monitored by most commercial airliners
o The Cospas-Sarsat system can only detect this type of beacon when a LEOSAR satellite is in view of both the beacon and a LEOLUT (satellite dish). These beacons are being phased out (see below.)

Frequency

Distress beacons transmit distress signals on the following key frequencies; the frequency used distinguishes the capabilities of the beacon. A recognized beacon can operate on one of the three (currently) Cospas-Sarsat satellite-compatible frequencies. In the past, other frequencies were also used as a part of the search and rescue system.

Cospas-Sarsat (satellite) compatible beacon frequencies

* 406 MHz UHF- carrier wave at 406.025 MHz ± 0.005 MHz

Compatible until 1 February 2009: *

* 121.5 MHz VHF ± 6 kHz (frequency band protected to ±50 kHz)
* 243.0 MHz UHF ± 12 kHz (frequency band protected to ± 100 kHz)

* NOTE: 121.5 MHz & 243 MHz beacons will become satellite-incompatible 1 February 2009.

Cospas-Sarsat incompatible beacon frequencies

* Marine VHF radio channels 15/16 - these channels are used only on the obsolete Class C EPIRBs
* The obsolete Inmarsat-E beacons transmitted to Inmarsat satellites on 1646 MHz UHF.

Types

The type of a beacon is determined by the environment for which it was designed to be used:

* EPIRBs (Emergency Position Indicating Radio Beacons) signal maritime distress,
* ELTs (Emergency Locator Transmitters) signal aircraft distress
* PLBs (Personal Locator Beacons) are for personal use and are intended to indicate a person in distress who is away from normal emergency services (i.e. 9-1-1)

Each type is sub-classified:

EPIRB sub-classification

EPIRBS are sub-classified as follows:

Recognized Categories:

* Category I - 406/121.5 MHZ. Float-free, automatically activated EPIRB. Detectable by satellite anywhere in the world. Recognized by GMDSS.
* Category II - 406/121.5 MHZ. Similar to Category I, except is manually activated. Some models are also water activated.

Unrecognized Classes:

* Class A - 121.5/243 MHZ. Float-free, automatically-activating. These devices have been phased out by the FCC and are no longer recognized.
* Class B - 121.5/243 MHZ. Manually activated version of Class A. These devices have been phased out by the FCC and are no longer recognized.
* Class S - 121.5/243 MHZ. Similar to Class B, except it floats, or is an integral part of a survival craft (lifeboat). These devices have been phased out by the FCC and are no longer recognized.

* Class C - Marine VHF ch15/16. Manually activated, these beacons operate on maritime channels only, and therefore are not detectable by satellite or normal aircraft. These devices have been phased out by the US FCC and are no longer recognized.

* Inmarsat-E - This service ended 1 December 2006; all former users have switched to Category I or II 406 MHz EPIRBS. These beacons were float-free, automatically activated EPIRBs operated on 1646 MHz. They were detectable by Inmarsat geostationary satellites, and were recognized by GMDSS. See Inmarsat-E.

ELT sub-classification

ELTs for aircraft may be classed as follows:

* A ELT, automatically ejected
* AD ELT, automatic deployable
* F ELT, Fixed
* AF ELT, automatic fixed
* AP ELT, automatic portable
* W ELT, water activated
* S ELT, survival

Within these classes, an ELT may be either a digital 406 MHz beacon, or an analog beacon (see above).

PLB sub-classification

There are two kinds of PLB:

* PLB with GPS input (internal or external)
* PLB wih no GPS input

All PLBs transmit in digital mode on 406 MHz. Additional information about PLBs can be obtained from the Ultimate PLB FAQ at Equipped to Survive.

Activation methods

There are two ways to activate a beacon:

* manually, or
* automatically

Automatic EPIRBs are water activated, while automatic ELTs are G-force (impact) activated. Some EPIRBs also deploy; this means that they physically depart from their mounting bracket on the exterior of the vessel (usually by going into the water.)

For a marine EPIRB to begin transmitting a signal (or "activate") it first needs to come out of its bracket (or "deploy"). Deployment can happen either manually—where someone has to physically take it out of its bracket—or automatically—where water pressure will cause a hydrostatic release unit to release the EPIRB from its bracket. If it does not come out of the bracket it will not activate. There is a magnet in the bracket which operates a reed safety switch in the EPIRB. This is to prevent accidental activation when the unit gets wet from rain or shipped seas.

Once deployed, EPIRBs can be activated, depending on the circumstances, either manually (crewman flicks a switch) or automatically (as soon as water comes into contact with the unit's "sea-switch".) All modern EPIRBs provide both methods of activation and deployment and thus are labelled "Manual and Automatic Deployment and Activation."

Advantages and disadvantages of the various beacons
Analog (121.5 MHz) Beacons 406 MHz (Digital) Beacons
SAR response delay SAR response delay

* SAR response to anonymous beacons can be delayed 4-6 hours, and as much as 12 hours

* Resolution or response to registered beacons is very swift. SAR response can be activated within approximately 10 minutes of beacon activation (and GEOSAR detection) if distress is evident.
* Unregistered beacons can usually be responded to after only 1 LEOSAR satellite pass; after two passes, response is immediate

False alerts False alerts

* 98.5% of 'alerts' detected are false alerts (2006 )
* Fewer than 2 in 1000 alerts and 2 in 100 composite alerts are actual distress.
* The Cospas-Sarsat system has no way of distinguishing between analog beacons and interference (from set top boxes, etc.)
* False alerts may result in a long and fruitless search by costly SAR assets, although rescue co-ordination centres typically analyse the circumstances, considering location, movement of the source and confirmatory reports before launching an operation
o Searches for interference signals and false alerts inhibit SAR assets from being available for real searches

* All alerts (100%) come from beacons (analog interference is ignored)
* Approximately 7 out of 10 of all 406 false alerts are resolved by a phone or radio call, therefore,
o SAR resources are not wasted
o SAR assets are more available for actual distress
o Persons responsible for causing false alerts can more likely avoid having to pay fines and/or paying the costs of operating SAR assets
o Follow-up to false activations allows continuous reductions in the number of false alerts
* 97.1% of all alerts are false alerts (2006 )

Information transmitted by the beacon Information transmitted by the beacon
Anonymous siren tone

* A unique 15, 22, or 30 digit serial number called a Hex Code is transmitted
* The Hex Code can contain a plethora of information, such as:
o the Country of beacon registration
o the identification of the vessel or aircraft in distress, and
o optionally, position data from onboard navigation equipment (GPS)

Beacon registration information Beacon registration information
Anonymous beacons cannot be registered

* There is no charge to register 406 beacons (see below).
o Unless otherwise advised, personal information is used exclusively for SAR distress alert resolution purposes
* Hex Code registration is mantatory in most countries of the world
* Beacons are registered with MCCs who have 24-hour access to registry data, such as:
o Name of the owner of the beacon
o Name or callsign of the ship, aircraft, or other vehicle the beacon is associated with
o Cellular, MMSI, and/or pager numbers, and/or other contact information

Transmission power Transmission power
0.1 W continuous - weak signal cannot usually penetrate debris or trees 5 W pulse mode - strong pulse reaches the satellites
Potential to be seen by a satellite Potential to be seen by a satellite

* LEOSAR satellites can be spaced by up to 90-100 minutes
* Second detection is necessary due to false alerts and to resolve position—takes an additional 45-100 minutes before SAR assets can be called

* GEOSAR provides nearly-instantaneous coverage 70 degrees north and south of the equator
* Worldwide coverage via LEOSAR—6 satellites
* Future use of GNSS satellites will allow worldwide real-time coverage (MEOSAR)

Location detection Location detection

* Two "50% chance" mirror-positions are generated by Doppler triangulation after the first pass of a LEOSAR
o Due to false alerts, no reaction can occur based on first pass alert
* A second pass resolves the ambiguity and resolves the search location to a radius of 20 km
* Moving targets (usually false alerts) produce interfering anomalies; calculated positions are inaccurate

* LEOSAR uses same technique as for analog beacons, but, since beacons are uniquely identified as beacons and have improved frequency stability, response can occur based on first-pass information
* Doppler-only accuracy is within 5 km (3.1 statute miles or 2.6 nautical miles)—position is sufficiently accurate even after only one pass
* GPS Position can be encoded into the Hex Code and can be updated real-time via GEOSAR
o GPS position accuracy is within 100 m (300 ft)
* LEOSAR Doppler triangulation is less affected by beacon movement due to improved frequency stability
* In the future, near-instantaneous detection & position triangulation via MEOSAR

Age of technology Age of technology
121.5 MHz beacons were developed in the late 1960s, when car phones weighed roughly 20 lb (the first ELT TSO C91 was written in 1971. ) 406 MHz beacons use proven, modern technologies reminiscent of those found in modern cell phones.

See also Cospas-Sarsat - Advantages of 406 Beacons and Canada's National Search and Rescue Secretariat—Advantages of 406 Beacons

Although modern systems are significantly superior to older ones, even the oldest systems provide an immense improvement in safety, compared to not having any beacon whatsoever.

Phase-out of 121.5 & 243 beacons

Beginning 1 February 2009, only 406 MHz beacons will be detected by the international Cospas-Sarsat SAR satellite system. This affects all maritime beacons (EPIRBs), all aviation beacons (ELTs) and all personal beacons (PLBs). In other words, Cospas-Sarsat will cease satellite processing of 121.5/243 MHz beacons 1 February 2009.

* 121.5 & 243.0 MHz EPIRBs are already banned in the United States and in many other jurisdictions, in anticipation of the anticipated decommissioning of the 121.5/243.0 system.

* The US National Transportation Safety Board has recommended that all ELTs be switched to 406 MHz "at the earliest possible opportunity," and certainly, no later than 1 Februray, 2009. See also Avionics West's 'Boring' Article on ELTs.

According to Cospas-Sarsat (often abbreviated C-S), the international organization responsible for the Search and Rescue (SAR) satellite system, all beacon owners and users should begin taking steps to replace their 121.5/243 MHz beacons with 406 MHz beacons as soon as possible. C-S recommends that beacon owners should consider purchasing a 406 MHz beacon when the battery on an older 121.5 MHz beacon needs replacing. Typically, batteries need replacing about every five years. The sooner a beacon owner upgrades, the better the service that the Cospas-Sarsat System can provide in case the beacon is activated in a distress event.

More information about the switch to 406 is available on Cospas-Sarsat's 121.5/243 Phase-Out page.

Despite the switch to 406, pilots and ground stations are encouraged to continue to monitor for transmissions on the emergency frequencies, as many 406 beacons are also equipped with 121.5 'homers.' Furthermore, the 121.5 MHz frequency continues to be used as a voice distress frequency (especially in aviation.)

SAR response to various beacons

Emergency beacons operating on 406 MHz transmit a unique 15, 22, or 30 character serial number called a Hex Code. When the beacon is purchased the Hex Code should be registered with the relevant national (or international) authority. Registration provides Search and Rescue agencies with crucial information such as:

* phone numbers to call,
* a description of the vessel, aircraft, vehicle, or person (in the case of a PLB)
* the home port of a vessel or aircraft
* any additional information that may be useful to SAR agencies

Registration information allows SAR agencies to start a rescue more quickly. For example, if a shipboard telephone number listed in the registration is unreachable, it could be assumed that a real distress event is occurring. Conversely, the information provides a quick and easy way for the SAR agencies to check and eliminate false alarms (potentially sparing the owner of the beacon thousands of dollars in negligent false alert fines.)

An unregistered 406 beacon still carries some information, such as the manufacturer and serial number of the beacon, and in some cases, an MMSI or aircraft tail number. Despite the clear benefits of registration, an unregistered 406 beacon is very substantially better than a 121.5/243.0 beacon; this is because the Hex Code received from a 406 beacon confirms the authenticity of the signal as a real SAR alert.

Beacons operating on 121.5 and/or 243.0 MHz simply transmit an anonymous siren tone, and thus carry no information to SAR agencies. Such beacons implicitly rely on the doppler location detection system. SAR authorities have no way of knowing whether a 121.5/243.0 MHz signal is actually a SAR signal until they physically deploy to the location and home in on the source (and sound) of the transmission. Since SAR resources are scarce (and expensive), most countries do not deploy the most useful SAR homing assets (aircraft) until ambiguity has been resolved (see doppler).

Responsible agencies

In the U.S., offshore beacons are investigated and victims rescued by the Coast Guard. On-shore beacons are investigated by local search and rescue services in Alaska. The Air Force Rescue Coordination Center is charged with land-based emergency signals, usually dispatching volunteer members from The United States Air Force Auxiliary Civil Air Patrol. In the U.S. there are no published notification systems for other locations.

Statutory requirements

In the U.S. (as in most jurisdictions) no special license is required to operate an EPIRB. The following paragraphs define other requirements relating to EPIRBs, ELTs, and PLBs.

Registration
There is no charge to register 406 MHz beacons. IT MAY SAVE YOUR LIFE.

All distress alerting beacons operating on 406 MHz should be registered; all vessels and aircraft operating under SOLAS and ICAO regulations must register their beacons. Some national administrations (including the United States, Canada, Australia, and the UK also require registration of 406 MHz beacons.

* Unless the national registry authority advises otherwise, personal information contained in a beacon is used exclusively for SAR distress alert resolution purposes

The Cospas-Sarsat Handbook of Beacon Regulations provides the status of 406 MHz beacon regulations in specific countries and extracts of some international regulations pertaining to 406 MHz beacons.

The following list shows the agencies accepting 406 beacon registrations by country:

* United States - NOAA
* Canada - NSS for civil beacons, CMCC for military beacons
* Australia - AMSA
* the United Kingdom - United Kingdom Mission Control Centre (UKMCC)
* International - Cospas-Sarsat International 406 MHz Beacon Registration Database (IBRD)

Environment-specific requirements

Aviation (ELTs)

Most general aviation aircraft in the U.S. are required to carry an ELT, depending upon the type or location of operation, while scheduled flights by scheduled air carriers are not. However, in commercial aircraft, a cockpit voice recorder or flight data recorder must contain an underwater detection beacon.

As per 14 CFR 91.207.a.1, ELTs built according to TSO-C91 (of the type described below as "Traditional ELT, unregistered") have not been permitted for new installations since June 21, 1995; the replacing standard was TSO-C91a. Furthermore, TSO-C91/91a ELTs are scheduled to phased out February 1, 2009 to be replaced by the 406 MHz ELT, a far superior unit.

Though monitoring of 121.5 and 243 MHz (Class B) distress signals by satellite is scheduled to cease in 2009, there is currently no upgrade of older ELT units mandated by the FAA for aircraft in the United States.

Marine (EPIRBs)

EPIRBs are a component of the Global Maritime Distress Safety System (GMDSS). Most commercial off-shore working vessels with passengers are required to carry a self-deploying EPIRB, while most in-shore and fresh-water craft are not.

As part of the United States efforts to prepare beacon users for the end of 121.5 MHz frequency processing by satellites, the FCC has prohibited the use of 121.5 MHz EPIRBs as of January 1, 2007 (47 CFR 80.1051). See the United States Coast Guard (USCG) brief on the 121.5/243 Phase-out.

The most current and comprehensive information about EPIRBs is provided by the Equipped To Survive Foundation. See also the 406 beacon performance review.

Personal locator beacons (PLBs)

Personal locator beacons operating on 406 MHz must be registered. PLBs should not be used in cases where normal emergency response exists (i.e. 911.)

The most current and comprehensive information about PLBs is provided by the Equipped To Survive Foundation. See also the 406 beacon performance review.

Detailed type descriptions

Current types

EPIRBs (marine)

Current marine EPIRBs are generally divided into three classes; Category I, Category II, and Class B (or Category B).

* Category I EPIRBs are considered the best but are also the most costly. The Category I - type is recommended by the IMO because a float-free bracket will deploy automatically once the vessel sinks and the EPIRB will then be activated automatically by immersion in water in the event of a disaster at sea. These EPIRBs are generally housed in a specially designed bracket on deck and the buoyant beacon is designed to rise to the surface and emit two signals, an emergency homing signal on 121.5 MHz and a digital identification Hex Code on 406 MHz that can be used to almost-immediately alert SAR authorities of the distress of the stricken vessel. Category I EPIRBs used in American waters must be registered with NOAA.

* Category II EPIRBs are similar to Category I EPIRBs but are generally manual deployment only. Also like Category I EPIRBs, Category II units must be registered. Category II EPIRBs are also generally less costly averaging less than US$1,000.

* Class B EPIRBs, also called Category B or "Mini B", operate a 121.5 MHz homing signal only and are usually manual deployment only units. They are the cheapest units but also the least capable. Since the signal has no identification component, Class B EPIRBs are not registered. Due to their limitations, Class B EPIRBs are slowly being phased out. As the International Cospas-Sarsat program will no longer monitor Category B EPIRB signals as of February 1, 2009, this type of beacon will become fully obsolete (see above.) Although the U.S. Coast Guard no longer recommends them, they remain in wide use.

ELTs (aircraft)

ELTs used in aircraft are of the following types:

* The new 406 MHz TSO-126 ELT will be the only type of ELT detected by Cospas-Sarsat after February 1, 2009 (see above.)

Types Being Phased Out:

* TSO-C91 - 121.5 / 243 MHz unregistered - have not been permitted for new installations since June 21, 1995;
* TSO-C91a - 121.5 / 243 MHz unregistered - was the replacing standard; most current aviation ELTs are of this type.

PLBs

All PLBs must have a Hex Code on the body. Persons must register this Hex Code with their national SAR agency. See below for types of PLBs no longer used.

Obsolete types

Obsolete EPIRBs

There are also several older types of EPIRB devices which are no longer recommended for use.

* Class A - A 121.5 MHz automatic activation unit. Due to limited signal coverage and possible lengthy delays in signal recognition, the U.S. Coast Guard no longer recommends use of this type.

* Class C - Operates on VHF channel 15/16. Designed for small crafts operating close to shore, this type was only recognized in the United States. Use of these units was phased out in 1999.

* Class S - A 121.5 MHz unit similar to Class B but is often included as an integral part of a lifeboat or survival suit. Their use is no longer recommended by the U.S. Coast Guard.

* Inmarsat E - entered service in 1997. The unit is an automatic activation unit operating on 1646 MHz and detectable by the Inmarsat geostationary satellite system. This class of EPIRB was approved by the Global Maritime Distress Safety System (GMDSS), but not by the United States. In September 2004, Inmarsat announced that it was terminating its Inmarsat E EPIRB service as of December 2006 due to a lack of interest in the maritime community.

Furthermore, the U.S. Coast Guard recommend that no EPIRB of any type manufactured before 1989 be used.

Obsolete ELTs

* Any ELT that is not a 406 MHz ELT with a Hex Code will be obsolete February 1, 2009.

Obsolete PLBs

* Military forces at one time used 121.5/243.0 MHz beacons such as the "PRQ-501," which had a built-in VHF radio. These are being replaced by modern 406 PLBs.

How they work

All the systems work something like this: A beacon is activated by a crash, a sinking, or manually by survivors. The beacon's transmission is picked up by one or more satellites. The satellite transmits the beacon's signal to its ground control station. The satellite's ground station processes the signals and forwards the data, including approximate location, to a national authority. The national authority forwards the data to a rescuing authority. The rescuing authority uses its own receiving equipment to locate the beacon and makes the rescue or recovery. Once the satellite data is in, it takes less than a minute to forward the data to any signatory nation.
Overview diagram of EPIRB/COSPAS-SARSAT communication system
Overview diagram of EPIRB/COSPAS-SARSAT communication system

There are several systems in use, with beacons of varying expense, different types of satellites and varying performance. Note that even the oldest systems provide an immense improvement in safety, compared to not having a beacon.

GPS-based, registered

The most modern 406 MHz beacons with GPS (US$ 1200-$3000 in 2002) locate a beacon with a precision of 100 meters, anywhere in the world, and send a serial number so the government authority can look up phone numbers to notify next-of-kin in four minutes, with rescue commencing shortly afterward. The GPS system permits stationary, wide-view geosynchronous communications satellites to enhance the doppler position received by low Earth orbit satellites. EPIRB beacons with built-in GPS are usually called GPIRBs, for GPS Position-Indicating Radio Beacon or Global Position-Indicating Radio Beacon.

High-precision registered

An intermediate technology 406 MHz beacon (US$ 500-900) has world-wide coverage, locates within 2 km (12.5 km2 search area), notifies kin and rescuers in 2 hours maximum (46 min average), and has a serial number to look up phone numbers, etc. This can take up to two hours because it has to use moving weather satellites to locate the beacon. To help locate the beacon, the beacon's frequency is controlled to 2 parts per billion, and its power is a hefty five watts.

Both of the above types of beacons usually include an auxiliary 25 milliwatt beacon at 121.5 MHz to guide rescue aircraft.

Traditional ELT, unregistered

The oldest, cheapest (US$ 139) beacons send an anonymous warble at 121.5 MHz. They can be detected by satellite over only 60% of the earth, require up to 6 hours for notification, locate within 20 km (search area of 1200 km2) and are anonymous. Coverage is partial because the satellite has to be in view of both the beacon and a ground station at the same time—the satellites do not store and forward the beacon's position. Coverage in polar and south-hemisphere areas is poor. The frequency is the standard aviation emergency frequency, and there is interference from other electronic and electrical systems, so false alarms are common. To reduce false alarms, a beacon is confirmed by a second satellite pass, which can easily slow confirmation of a 'case' of distress to up to about 4 hours (although in rare circumstances the satellites could be position such that immediate detection becomes possible.) Also, the beacons can't be located as well because their frequency is only accurate to 50 parts per million, and they send only 75-100 milliwatts of power.

These ELTs will not be monitored by Cospas-Sarsat after February 1, 2009.

Location by Doppler (without GPS)

When the beacon has no GPS receiver, the system locates the beacon from its Doppler shift as received by the quickly-moving satellites. Basically, the frequency received varies depending on the speed of the beacon relative to the satellite. The amount of shift is proportional to the range and bearing to the satellite. The instant the beacon's Doppler shift changes from high to low indicates the time when the bearing from the beacon to the satellite's ground track is 90 degrees. The side of the satellite track is determined because the rate of change of the Doppler shift is faster when the Earth is turning towards the satellite track.

In order to handle multiple simultaneous beacons, modern 406 MHz beacons transmit in bursts, and remain silent for a few seconds. This also conserves transmitter power.

Russia developed the original system, and its success drove the desire to develop the improved 406 MHz system. The original system is a brilliant adaptation to the low quality beacons, originally designed to aid air searches. It uses just a simple, lightweight transponder on the satellite, with no digital recorders or other complexities. Ground stations listen to each satellite as long as it is above the horizon. Doppler shift is used to locate the beacon(s). Multiple beacons are separated when a computer program performs a Fourier transform on the signal. Also, two satellite passes per beacon are used. This eliminates false alarms by using two measurements to verify the beacon's location from two different bearings. This prevents false alarms from VHF channels that affect a single satellite. Regrettably, the second satellite pass almost doubles the average time before notification of the rescuing authority. However, the notification time is much less than a day.

Operational testing

According to the FAA, ground testing of type A, B and S ELTs is to be done within the first 5 minutes of each hour. Testing is restricted to 3 audio sweeps. Type I and II devices (those transmitting at 406 MHz) have a self test function and must not be activated except in an actual emergency.

The Coast Guard web page for EPIRBs states: "You may be fined for false activation of an unregistered EPIRB. The U.S. Coast Guard routinely refers cases involving the non-distress activation of an EPIRB (e.g., as a hoax, through gross negligence, carelessness or improper storage and handling) to the Federal Communications Commission. The FCC will prosecute cases based upon evidence provided by the Coast Guard, and will issue warning letters or notices of apparent liability for fines up to $10,000."

Satellites used

Receivers are auxiliary systems mounted on several types of satellites. This substantially reduces the program's cost.

The weather satellites that carry the SARSAT receivers are in "ball of yarn" orbits, inclined at 99 degrees. The longest period that all satellites can be out of line-of-sight of a beacon is about two hours.

The first satellite constellation was launched in the early 1970s by the Soviet Union, Canada, France and the USA.

Some geosynchronous satellites have beacon receivers. Since end of 2003 there are four such geostationary satellites (GEOSAR) that cover more than 80% of the surface of the earth. As with all geosynchronous satellites, they are located above the equator. The GEOSAR satellites do not cover the polar caps.

Since they see the Earth as a whole, they see the beacon immediately, but have no motion, and thus no doppler frequency shift to locate it. However, if the beacon transmits GPS data, the geosynchronous satellites give nearly instantaneous response.

History

The original impetus for the program in the U.S. was the loss of Congressmen Hale Boggs (D-LA) and Nick Begich (D-AK) in the Alaskan wilderness on October 16, 1972. A massive search effort failed to locate them. The result was a U.S. law mandating that all aircraft carry an emergency locator transmitter. Technical and organizational improvements followed.

Cospas-Sarsat is an international organization that has been a model of international cooperation, even during the Cold War. SARSAT means Search And Rescue SATellite. COSPAS is a Russian acronym with the same meaning. A consortium of Russia, the U.S., Canada and France formed the organization in 1982. Since then 29 others have joined.

Cospas-Sarsat defines standards for beacons, auxiliary equipment to be mounted on conforming weather and communication satellites, ground stations, and communications methods. The satellites communicate the beacon data to their ground stations, which forward it to main control centers of each nation that can initiate a rescue effort.

The U.S. Coast Guard once promoted an emergency beacon on maritime VHF emergency channels. It now promotes the superior Cospas-Sarsat system, and no longer services emergency beacons on maritime VHF frequencies.

Current events

In a Safety Recommendation released 4 September 2007, the NTSB once again recommended that the FAA require all aircraft have 406 MHz ELTs. They first recommended this back in 2000 and after vigorous opposition by AOPA, the FAA declined to do so. This recommendation is apparently a reaction to the looming cessation of 121.5 MHz satellite processing. Citing two recent accidents, one with a 121.5 MHz ELT and one with a 406 MHz ELT, the NTSB concludes that switching all ELTs to 406 MHz by 1 February 2009 is a necessary goal to work towards.

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