COSPAS – SARSAT Emergency Communications
A few months ago, I was walking around the Science Museum in London
A few months ago, I was walking around the Science Museum in London, and I spotted the ‘Apollo Survival Radio’ by the Lunar Module exhibit. That led to an interesting discussion on Twitter about how the radios were planned to be used, and I included some details in my Scanning Scene column at the time.
That, in turn, led to a chat with our editor along the lines of “Tim, you could write something about Emergency Communications, couldn’t you?” So, here we are.
In this first part of our short series on Emergency Communications, I want to look at the Emergency Position Indicating Radio Beacon (EPIRB for short), Emergency Locator Transmitter (ELT) and Personal Locator Beacons (PLB). These all transmit on 406MHz, and they are triggered, often automatically, in a distress situation. The beacons’ signals are detected by satellites, monitored by an international consortium of rescue services, known as ‘COSPAS-SARSAT’.
(Космическая Система Поиска Аарийныч Судов).
This, I am reliably informed translates loosely to The Space System for the Search of Vessels in Distress.
SARSAT – perhaps more predictably – stands for ‘Search and Rescue Satellite-Aided Tracking’.
COSPAS-SARSAT is an international organisation, defined by treaty and organised as a non-profit, intergovernmental and humanitarian body of 44 nations and agencies (Fig. 1).
It was established in 1988, following a prior agreement of 1984 and, prior to this, after the first rescue using the technology occurred in September 1982.
The system uses a network of satellites, which covers the whole of the Earth’s surface. Any alerts detected by the satellites are forwarded to over 200 countries, without any cost to the owners of the beacons or to the receiving government agencies. The system has several distinct components:
First, the distress beacons (EPIRBs, ELTs and PLBs) themselves, which are to be activated in a life-threatening situation.
Second, the Search and Rescue Repeaters (SARR) and the Search and Rescue Signal Processors (SARP), which are carried on board satellites.
Third, the Satellite downlink receivers at ground stations. These process the received signals and they are known as Local User Terminals (LUTs).
Forth, the Mission Control Centres, which distribute the received information to Rescue Co-ordination Centres – most importantly the position data received from beacons.
At the end of 2018, there were 34 Mission Control Centres in operation, with three more under development. The UKMCC is at Fareham in Hampshire.
And Finally, there are Rescue Co-ordination Centres (MCC), which call on the required agencies, for example, the Coastguard, to provide an appropriate rescue response to the emergency in question.
The EPIRB (Fig. 2), ELT and PLB beacons are all digital radio transmitters on 406MHz (406.025 or 406.028MHz). Beacons are manufactured by a variety of vendors worldwide. ELT systems are meant to be used in aircraft. EPIRBs (Fig. 2) are designed to be used aboard a marine vessel and finally, the PLB is designed to be carried by an individual. Sometimes, PLBs are carried aboard aircraft or vessels. However, depending on local regulations, this may or may not satisfy regulatory requirements.
Any of the beacon types does not transmit until activated in an emergency. There are, however, facilities which allow the beacons to be tested. Some beacons have to be activated manually by the user, whereas an ELT might be automatically activated by the shock of an aircraft crash. An EPIRB may also be automatically activated by contact with water.
EPIRBs are divided into two categories: Category 1 EPIRBs can be activated either manually or automatically. Automatic activation takes place when the EPIRB is released from its bracket, which will be equipped with a hydrostatic release. The mechanism releases the EPIRB in a water depth of between 1 and 3m. The EPIRB is buoyant, floats to the surface and starts transmitting its message. It is important that the EPIRB is mounted somewhere that it can float free of a sinking vessel.
Category 2 EPIRBs (Figs. 3 to 5) have to be manually activated, so they do need to be accessible, in the event of an emergency.
COSPAS-SARSAT do not levy subscriptions for use of the service, although some countries charge licence fees for beacon ownership – and some may charge for the cost of rescue operations.
The Space Segment
The part of the system in Space consists of SARR and SARP components on board the host satellite. There are five satellites in (polar) low-altitude Earth orbits; these are known as LEOSARs. Six more satellites are to be found in geostationary Earth orbits; they are called GEOSARs. And more than 30 satellites are located in a medium-altitude Earth orbit; these are known as MEOSARs.
The SARR or SARP are always secondary payloads on the satellite – for example, the geostationary satellites’ primary mission is likely to be meteorological. This is also true for LEOSAR satellites
The primary mission of the MEOSAR satellites is navigation. The SARR, when it receives a distress signal from a beacon, will retransmit it to a ground station in real-time. The SARP will record the data from the distress signal for later re-transmission, in the event that the satellite is not in range of a ground station when it receives the distress signal.
When the COSPAS-SARSAT system was first launched, it comprised solely of Low-Earth-Orbit satellites. Although the orbits of the series of satellites were arranged to cover the entire surface of the globe (because each satellite has a relatively low orbit), there will be periods of time when a LEOSAR satellite may not be over a particular geographic location. Therefore, a distress beacon may be undetected until the next satellite comes into range; there will then be a delay in relaying the distress signal back to the ground.
Because this may not happen in real-time, LEOSAR spacecraft have ‘store-and-forward’ functionality. This ensures that a signal detected when the spacecraft is out of range of a ground station can be successfully retransmitted, once the ground station comes into the footprint of the LEOSAR. The five satellites of this type orbit around every 100 minutes.
Even if the distress beacon is not transmitting a GPS position, it is possible for the ground stations receiving the signal from the LEOSAR to determine the bearing and range of the beacon, with respect to the satellite. This is done by means of calculations of the Doppler shift of the signal (at a known frequency), as the satellite passes over the beacon. Range and bearing are measured from the rate of change of the received frequency. This will vary, according to where the satellite is along its orbit, and due to the rotation of the Earth.
If the frequency of the beacon is varying rapidly, this suggests that the satellite is nearly overhead the beacon’s position. If you’ve ever tried to track a satellite’s signals, you’ll know that, on overhead passes, as the satellite climbs high in the sky above you, the Doppler shift of the signal is very rapid indeed. You could try with one of the amateur satellites or an NOAA weather satellite on 137MHz if you fancy a practical experiment. In fact, the NOAA weather satellites form part of the COSPAS-SARSAT system, so it will be an authentic test!
Because these satellites are in geostationary orbit, this means, of course, that they are turning at the same speed as the earth – so there is no relative motion between the distress beacon and the satellite, and thus no doppler shift. The GEOSAR satellites can only relay the message received from the distress beacon, rather than augment the message with any positional information. However, if the beacon’s data payload includes a GPS position then, of course, this can be relayed directly to the SAR agencies.
The existing constellation (series) of GEOSARs covers all area of the Earth. Therefore, as soon as a distress beacon is activated, it should be detected by one of these satellites (with the exception of the polar regions, which are out of range of the GEOSARs).
These satellites are the latest addition to the COSPAS-SARSAT system. They combine the advantages of the LEOSAR (ability to detect the beacon position from Doppler shift) and GEOSAR (coverage). It is planned that there will be over 70 of the MEOSAR satellites – which will form the majority of the space segment of the COSPAS-SARSAT system.
MEOSAR satellites will consist of SARR transponders as secondary payloads on a variety of navigation satellite constellations, including Galileo, GLONASS, BEIDOU and, of course, GPS. The location of a distress beacon can be calculated by the ground station, by analysing the Doppler signals with respect to the satellite’s path. This can also be achieved by measuring the time difference of arrival of a beacon’s transmission, due to the distance between the beacon and each MEOSAR satellite in view.
Data from the MEOSAR satellites started to be distributed on December 13th, 2016. However, data from the MEOSAR constellation was used prior to the official implementation of the system. This was done, in order to establish the location of EgyptAir Flight 804, which crashed in the Mediterranean.
A particularly interesting feature of the MEOSAR system is that – when using the Galileo navigation satellites – the MEOSAR system will be able to download data back to the distress beacon, using the ‘Return Link Service’ of the Galileo system. It’s anticipated that this will be used to confirm that the distress beacon has been received by the SAR authorities.
Ground stations monitor the satellites, using either satellite dishes or phased arrays. These Local User Terminals (LUT) are established by individual national administrations or agencies. Signals received are transferred to the appropriate Mission Control Centre (MCC).
At the end of December 2016, the LEOSAR satellites were tracked and monitored by 53 LEOLUTs (Low Earth Orbit Local User Terminals), the GEOSAR satellites were checked by 21 GEOLUTs and the MEOSAR satellites were watched from 17 MEOLUTs.
Evolution of the COSPAS-SARSAT System
When this system was first introduced in 1982, the satellites monitored the two original types of distress beacons: EPIRBs and ELTs. By the early 2000s, the PLBs became available for individuals engaged in recreational activities beyond the reach of normal telephone systems – or perhaps small aircraft pilots or mariners, where regulations permitted.
Before COSPAS-SARSAT came into operation, the civilian aviation community had been using the 121.5MHz frequency for distress, and the military had been using 43MHz (with 121.5MHz as a backup).
Initially, ELTs for commercial aircraft were built to transmit on 121.5MHz, a frequency well monitored worldwide. Military beacons were built to transmit on 243MHz.
When COSPAS-SARSAT first came into being, satellites monitored the beacon alerts on 406MHz, 121.5 and 243MHz. However, it was found that both the 121.5 and 243MHz frequencies generated a high number of false alerts (the false alert rate for the 121.5MHz units was an astonishing 97%).
It became all but impossible to uniquely identify such systems. So, in 2009, the COSPAS-SARSAT stopped receiving signals from 121.5 and 243MHz, and just processed digital signals on 406MHz.
However, many ELTs do still include a 121.5MHz transmitter, which can be received by local search crews, using simple direction-finding equipment. Beacon technology has evolved since these transmitters were first used in 1982. At that time, positional information was calculated by ground stations based on Doppler shift calculations. Now, GPS technology is ubiquitous, and many beacons incorporate GPS receivers.
Registration and Testing
Beacons have to be registered to be used on the COSPAS-SARSAT system, using a hexadecimal identifier, unique to each beacon. In the event of an alert being received, the authorities can first try to contact the owner, to rule out a false alert. Most beacons also provide a switch allowing for testing. Even a very short transmission with a beacon in ‘distress’ mode will cause a distress alert to be sent. Therefore, it is very important that test mode is engaged when and where appropriate.
In the case of EPIRBs automatically activated on immersion in water, if this happens inadvertently, it is important for the beacon’s owner to contact one of the relevant co-ordination centres in order to have the distress alert cancelled.
The RNLI has a useful page about EPIRB and PLBs including details of how to register your beacon:
Can I Listen to 406MHz Transmissions?
You can! Of course, the signals are meant to be decoded by satellite. Unless you are in an elevated position close to the sea, or in a very remote part of the world, it is, perhaps, less likely that you will hear anything.
Intriguingly, the AOR AR-DV10 (RadioUser, February 2019) has an option for a COSPAS-SARSAT decoder. This decodes the GPS information from the digital transmission and also listens on 121.500MHz for any analogue transmission. You can see a very brief video of this in action here: https://youtu.be/xAlXjd5waC0
COAA, who produce an interesting range of software including PlanePlotter and ShipPlotter, which readers may be familiar with, has a program called EPIRBPlotter. As you might expect, this decodes transmissions from any EPIRBs that you can hear. You need a UHF scanner tuned to 406.025 or 406.0275MHz, with the audio output of the scanner fed into a computer’s soundcard
EPIRBPlotter is marketed for those wishing to test their EPIRBs – and there are cautionary words on the website about doing this and taking up important airtime. More details are here: https://www.coaa.co.uk/epirbplotter.htm
You may also enjoy a video of receiving the 1.5GHz transponder of COSPAS-SARSAT signals, using a simple WorldSpace patch antenna and an RTL-SDR type receiver. You can see, among other things, data bursts from EPIRBs, which are repeated from the 406MHz band: https://www.youtube.com/watch?v=o5yWZcjReb8
I hope you have found this initial column interesting – I certainly enjoyed researching the subject! Join me again next month or another aspect of Emergency Communications.
This article was featured in the April 2019 issue of Radio User