EPIRB and SARSAT: Next Generation MEOSAR

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jimh
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EPIRB and SARSAT: Next Generation MEOSAR

Postby jimh » Wed Dec 14, 2016 11:17 am

EPIRB and SARSAT
The Next Generation: MEOSAR


Most boaters are now aware of devices known as emergency position-indicating radio beacon transmitters or EPIRB transmitters or just EPIRB's. These EPIRB devices send emergency alert transmissions to the COSPAS-SARSAT sysetm, a global cooperative system of search and rescue alert detection. The name is formed by two acronyms:

COSPAS is from the Russian COsmicheskaya Sistema Poiska Avariynyh Sudov. which can be translated as space system for the search of vessels in distress.

SARSAT is from Search And Rescue Satellite-Aided Tracking.

The COSPAS-SARSAT was initially a collaboration between the USA, Canada, France, and the former USSR, begun in c.1980, and continuing to this day. As a result of using the system, about 37,000 lives have been saved, which, on average, means about five lives per day. That is an amazing statistic. There are about 1.4-million beacon transmitters that have registered with COSPAR-SARSAT, and more than 40 countries have adopted the system.

The fundamental elements of the COSPAS-SARSAT system have stayed the same for all these years, but there is now underway a significant change to the technology employed. This article will first look at the legacy technology and then describe some of the changes that will be coming. (And from here on I will just refer to the system by its English acronym, SARSAT.)

Understanding SARSAT System Design and Its Technology

There are five major segments to the SARSAT system:

--the User segment: the person having a beacon transmitter and activating it in an emergency or distress situation;

--the Space segment: orbiting satellites with receivers that listen for beacon transmissions and retransmit those signals to ground stations;

--the Ground segment or Local User Terminal (LUT): the receiving sites that monitor the satellites and receive from them the beacon transmissions by relay or recording replay.

--the Mission Control Centers: the network hubs of LUTs that monitor for and evaluate distress alerts received by the system; they send the distress message to the Search and Rescue Point of Contact (SPOC) in their region.

--the Rescue Coordination Centers; the local or regional authorities that can initiate a search and rescue mission to find the person in distress, after checking for false or accidental alerts;

The technology of the User, Space, and Ground segments was designed in the 1970's, and, while no longer state of the art, it still is working well.

User Segment: the beacon transmitters

The beacon transmitters initially used 121.5-MHz and 243.0-MHz, frequencies carried aboard aircraft communication equipment, but later moved to 406-MHz band in c.1979. Since early 2009, only the 406-MHz band signal has been used by the SARSAT system. (The older beacons can still be received by aircraft.) The International Telecommunications Union (ITU) regulates global frequency allocations and has strongly recommended reserving that spectrum on a global basis to exclusive use for earth-to-satellite transmission from mobile satellite service (MSS) ground stations in order to prevent any interference from other and much stronger adjacent-band terrestrial transmitters. Today all EPIRB transmitters being sold operate in the 406-MHz band.

Space Segment: the satellites

The space segment of COPAS-SARSAT consisted of two types of satellites: low-earth-orbit (less than 1,000-km altitude) or LEO satellites, and geostationary-earth-orbit (about 35,800-km altitude) or GEO satellites. The GEO satellites are in orbit on the equator and maintain a stationary view of the Earth, each covering about 38-percent of the Earth. Their coverage is very wide but it decreases above latitude 60-degrees and is not very good in polar regions. While the GEO satellite has good coverage, there is one drawback: since there is no relative motion between the beacon transmitter and the satellite receiver, there is no Doppler Shift modulation on the signal. This means that current GEO SARSAT satellites cannot independently deduce the location of the beacon.

The LEO SAR satellites are launched into polar orbits (inclined almost 90-degrees) and have an orbital period of about 100-minutes. They overfly high-latitude and polar regions every 100-minutes, but because of the low orbit altitude each satellite covers only about 3-percent of the Earth's surface at any time. The polar and high-latitude coverage complements the GEO satellite coverage. And, since they are in motion with respect to the beacon transmitter, the source of the signals the LEO satellites receive can be located with Doppler Shift (Frequency of Arrival) measurement techniques. The drawback of LEO SAR satellite is the very limited Earth view they have, which also means they are not in view of a ground station at all times. These satellites must therefore store distress signals for later playback when they are in view of a ground station. This adds delay in reporting of the distress event to the COPAS-SARSAT ground segment. (The LEO SAR satellites actually carry two types of transponders. A Search and Rescue Repeater or SARR immediately relays the received signal to a ground station, if in view, which can then analyze the data in the signal. A Search and Rescue Processor or SARP receives the signal, performs its own analysis of its Doppler Shift and other properties, stores the data, and forwards it to a ground station when the ground station comes into view.)

MEOSAR: the next generation SARSAT

The next generation of the SARSAT space segment will use transponders on satellites in medium Earth orbit (MEO). Search and rescue (SAR) satellites in MEO orbit are called MEOSAR satellites and will be in orbits between 19,000 and 24,000-km. MEOSAR satellites will have the advantages of the combination of LEO and GEO. The satellites that will carry the MEOSAR transponders will be global navigation satellite system (GNSS) satellites that will be part of the GPS, GLONASS, and GALILEO constellations.

In the USA, the GPS system will begin to carry MEOSAR SARSAT transponders on the future-generation GPS-III-F satellites. However, since 2001 the USA's GPS satellites have been carrying an experimental transponder called GPS-DASS (GPS Distress Alert Satellite System). The DASS transponders use an S-band downlink frequency. As of June 2015, there are 17 operational transponders DASS in the GPS system. The first operational payload with the new MEOSAR transponders should be carried on GPS-III-F SV-9, which won't be launched for several years.

In the GLONASS constellation there were two MEOSAR transponders as of June 2015.

In GALILEO as of June 2015 there were six transponders. GALILEO has been expanding rapidly since then.

The new MEOSAR system will be able to work with existing SARSAT beacon transmitters and with newer second-generation beacon signals. This capability is obtained because the satellites will not be receivers but frequency translators, and will preserve the original modulation of the signal.

MEOSAR will produce enhanced performance with first-generation beacon transmitters by improving availability of the satellites and improving independent location accuracy. But many more enhancement will be available when SARSAT beacon transmitters transition to a second-generation design intended specifically to work with the MEOSAR satellites.

A first-generation beacon transmitting to the MEOSAR system should produce an independent location accuracy of 5-km with 95-percent probability, assuming a 90-percent probability of locating the beacon within 10-minutes of activation. The independent location deduction will be done with Time-of-Arrival and Frequency-of-Arrival (Doppler Shift) methods.

Second-generation beacons transmitting to MEOSAR should produce better results. The transmitting frequency of second-generation emergency beacons will be more precisely controlled. This will give more accurate Doppler Shift measurements. The independent location accuracy is expected to be:

-- 5-km accuracy 95-percent of the time within 30-seconds after activation

-- 1-km accuracy 95-percent of the time within 5-minutes after activation

-- 100-meter accuracy 95-percent of the time within 30-minutes after activation

Second generation beacons will also have a return-link capability, allowing them to receive messages from the MEOSAR satellites. The return link can be used to send a confirmation message from the system, giving the user assurance his distress alert has been received. Or, in the event of a missing ship, plane, or person, the return link can be used to force activation of the beacon transmitter.

Going forward, it is expected that more beacon transmitters will have their own position locating device using GNSS technology, and will be able to encode into their distress alert beacon transmission their own position. However, there is no guarantee that a beacon will always have its own GNSS position fix data to send, so independent position determination by the SARSAT system remains an important component.

In the transition to MEOSAR SARSAT I suspect there will also be changes to the ground segment technology, but these are outside the scope of this short article.

For readers interesting in more details for the user and space segments of the impending transition to MEOSAR for SARSAT, I recommend the following articles:

Innovation: The Distress Alert Satellite System
Taking the Search out of Search and Rescue

By David W. Affens, Roy Dreibelbis, James E. Mentall, and George Theodorakos
Published at: http://gpsworld.com/innovation-the-distress-alerting-satellite-system-10883/

MEOSAR
New GNSS Role in Search & Rescue

By Yoam Gregoire, Ana Petcu, Thibaud Calmetttes, Michel Monnerate, Lionel Ries, and Eric Luvisutto
Available from: http://www.insidegnss.com/auto/novdec14-WP_0.pdf

MEOSAR Overview
by Jesse Reich
Available from: https://www.dco.uscg.mil/Portals/9/CG-5R/EmergencyBeacons/2012SarsatConf/Presentations/SAR2012_Feb16_MEOSAR_Overview_Reich.pdf

MEOSAR & GPS
By Lisa Mazzuca
Available from:
http://www.gps.gov/multimedia/presentations/2014/11/ICG/mazzuca.pdf

jimh
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Re: EPIRB and SARSAT: Next Generation MEOSAR

Postby jimh » Sat Jan 01, 2022 12:49 pm

Five years have passed since I published the initial article in this topic, and there have been some changes to be mentioned.

GALILEO CONSTELLATION
The foremost change that has occurred since 2015 is the completion of the GALILEO GNSS constellation. As of January 2022, there are now 19 GALILEO FOC (full operational capacity) satellites, which means the satellite also includes the SARSAT 406-MHz Forward Link Service (FLS). The FLS operates as a bent pipe transponder and relays the 406-MHz signal to ground stations and then onto the Rescue Coordination Center(RCC). With 19 satellites in Medium Earth Orbit (MEO) now participating from GALILEO, there is excellent coverage of the entire Earth between latitude 60-North and 60-South.

RETURN LINK SERVICE
A further change is also being provided by the GALILEO system, an entirely new feature called Return Link Service (RLS). The RLS system allows a signal to be sent to the EPIRB device that initiated the emergency alert, permitting the person in distress to receive an acknowledgement that their distress signal has been received. Of course, in order for this to occur, the EPIRB beacon must be of the latest design and support the Return Link Service feature. The RLS went into full operation just this year, in January 2020.

There are now EPIRB and other distress beacons available that support the COSPAS-SARSAT Return Link Service (RLS), and at this writing, there are 34 nations or territories that have indicated to COSPAS-SARSAT the will allow these devices to be registered by their authorities.

Unfortunately, the United States has not yet authorized registration of distress beacons that can use the Return Link Service. In the manner of big bureaucracies, all that has occurred so far is a posting in the Federal Register in February 2021 of a public notice and request for comments by NOAA regarding permitting use of the RLS in the USA. The period for comments closed about seven weeks later on April 30, 2021. (The page view counter for the public notice showed only 276 views at the time I am writing this, which seems like a woefully small number of people have even read the announcement.)

One consideration regarding the authorization to register distress beacons that can use the Return Link Service is that this feature is currently only supported by the the GALILEO full-operational-capability (FOC) satellites. None of the older space segment assets of SARSAT support the RLS feature. But with 19 GALILEO FOC already in orbit, and with the very large coverage area that is provided by their medium-earth orbit (MEO) position, there should be excellent global coverage. The European Space Agency (ESA) mentions that the RLS is unique to GALILEO, which could be inferred to mean it might not be supported on the GPS-III-F satellites.

A great deal of information about the Return Link Service and the ground-to-satellite-communication providers or Return Link Service Providers (RLSP) that are needed to implement the return message link is given in

Section 3.2, SAR/GALILEO RETURN LINK SERVICE

on page 15 of the

SAR/GALILEO SERVICE DEFINITION DOCUMENT
https://www.gsc-europa.eu/sites/default/files/sites/all/files/Galileo-SAR-SDD.pdf

GPS-III-F TIMELINE FOR SARSAT MEO
A further clarification seems necessary about future U.S. Space Force GPS support for SARSAT: the capability for GPS-III satellites to receive and relay the 406-MHz signal will not occur until the GPS-III-follow-on or GPS-III-F satellites are in orbit. As of January 2022 there are no GPS-III-F satellites in orbit. The funding and awarding of a contract to build GPS-III-F satellites was not completed until September 2018. The first completed GPS-III-F satellite is anticipated to be available for launch in 2026. So at best, GPS is still four years away from providing MEO SARSAT capabilities as the new system is designed. There are ten GPS-III-F satellites planned to be launched, which may take several years or more. And even when all ten GPS-III-F satellites are in orbit, the usual GPS constellation is 30 to 32 satellites, so only about one-third of the constellation will support the new SARSAT MEO ground station system,

However, an earlier proof-of-concept (POC) implementation of a similar system, the Distress Alerting Satellite System or DASS, continues to operate. The principal difference is the frequency of the downlink was in a different microwave band. Nine GPS-II-R satellites and 12 GPS-II-F satellites were equipped with DASS POC transponders. These older DASS POC satellites used a downlink in S-band (2226-MHz right-hand circular polarization). The newer SARSAT MEO plan calls for the downlink frequency to be in L-band (1544.1-MHz left-hand-circular polarization).

The flight hardware for the GPS-III-F space vehicles to support the SARSAT 406-MHz system will be provided by the government of Canada. A CA$39-million contract for an initial ten devices has been given to the Canadian firm Macdonald, Dettwiler and Associates Corporation (MDA), to design, construct, and deliver the necessary hardware to be integrated onto the GPS-III-F space vehicles. MDA may not be a familiar name, but their products are very well-known: they provided the Canadarm used on 90 missions of the NASA Space Shuttle, and the Canadarm used on the International Space Station.

SARSAT MEO POSITION FINDING
Perhaps also needing clarification is the position finding capabilities of the SARSAT MEO system. While some modern EPIRB devices may be capable of sending their own position location as part of their distress alert message, the SARSAT MEO system will continue to use Doppler Frequency Shift and Time of Arrival techniques to calculate its own determination of the location of the EPIRB transmitting. This is an essential element of the SARSAT system, as many legacy devices do not have the capability of encoding their own position into their transmission, and even devices that do have their own GNSS receiver to deduce their position cannot be certain that the GNSS receiver will always be capable of receiving signals and finding its own position, particularly in the case of marine use where the EPIRB might be afloat in high seas and not be able to acquire its own position fix from a GNSS system.

porthole
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Re: EPIRB and SARSAT: Next Generation MEOSAR

Postby porthole » Mon Jan 03, 2022 1:38 pm

jimh wrote:...in the event of a missing ship, plane, or person, the return link can be used to force activation of the beacon transmitter.
How is this possible if the beacons are not activated until a float free event (sinking) or manually turned on?
Thanks,
Duane
2016 World Cat 230DC
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1980 42 Post
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jimh
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Re: EPIRB and SARSAT: Next Generation MEOSAR

Postby jimh » Mon Jan 03, 2022 4:11 pm

The Return Link Service only works with emergency beacons that incorporate a receiver that listens for signals from the GALILEO satellites. A particular emergency beacon that listens for Return Link Service (RLS) would only process messages that were intended for it to receive.

At the moment only a few message types are supported in the Return Link Service, mainly just the type that acknowledges to the beacon device that a satellite received its distress alert or that a rescue coordination center has received the distress alert. The mention of future message types that could be implemented does reflect the current service capabilities.

The protocol for Return Link Service could be expanded in the future to provide other message types, including perhaps messages to command a beacon to begin transmitting. Such a message would likely be sent to Emergency Locator Transponder (ELT) devices on aircraft that have gone missing. Whether other beacons like a PLB and an EPIRB will ever support this feature remains to be seen. Any feature added to the Return Link Service must, of course, be supported by the distress beacon device to which the message will be sent. Because of the enormous number of distress beacon devices already in use, the implementation of new features--such as even the RLS itself--will be a slow process.

The RLS message is sent by GALILEO FOC satellites as part of the E1 signal used for position determination. If a distress beacon in its inactive state is provided with external power, it could already be receiving the GALILEO E1 signal to deduce its current position. If a RLS message were sent to activate the beacon, it could begin its periodic burst transmissions on 406-MHz. Distress beacons will tend more and more to have their own GNSS receivers; this is now a recommended feature.

From the COPAS-SARSAT HANDBOOK ON BEACON REGULATIONS and in regard to EPIRB design in that publication at

ANNEX
RECOMMENDATION ON PERFORMANCE STANDARDS FOR
FLOAT-FREE RELEASE AND ACTIVATION ARRANGEMENTS FOR
EMERGENCY RADIO EQUIPMENT

the use of external power or data connections to an EPRIB are mentioned:
For radio equipment requiring external power or data connection, or both, the means of
connection should not inhibit the release or activation of the radio apparatus.


Distress beacons do not continuously transmit their 406-MHz distress alert signal. To conserve battery power, the 5-Watt distress alert transmission occurs as a half-second burst made at intervals of about 50-seconds. Non-continuous transmission prevents one beacon from obscuring other beacons. The system has been designed to prevent the collision of two beacons transmitting simultaneously by randomizing the repetition interval. Also, the new beacons use very precise frequency standards, allowing for multiple channels to be available in the reserved SAR band at 460 to 460.1-MHz for use by the beacons. The greater precision of the transmit frequency required in newer distress alerting beacons also improves the accuracy of the position solution using the trilateralization method of Doppler frequency shift and Time-of-Arrival of signal to the orbiting satellites.

Upon activation, and EPIRB must be capable of maintaining the burst signal power output for 24-hours at an operating temperature of minus 20-degrees-Celsius (minus-4-degrees-F), with a power of 5-Watts ± 2 dB (or roughly between 3.2 and 7.9-Watts).

According to the SARSAT recommendations:
An EPIRB should be...be provided with a 121.5 MHz beacon primarily for homing by aircraft....

Source: Op. cit.

The transmitter power of the 121.5-MHz homing beacon is very low, specified to be maintain an effective peak output power of just 0.05-Watts, or one-hundredth the power of the 406-MHz signal. This signal should not have less than a 50-percent duty cycle, ON for 1.125-seconds then OFF for 1.125-seconds. A higher duty cycle, if used, should favor more ON time.

We can estimate the power consumption of the 121.5-MHz transmission. A continuous power drain of 0.05-Watts for 1-hour is a drain of 0.05-Watt-hours. With a duty cycle of only 50-percent, the power consumed by the 121.5-MHz transmitter is then only 0.025-Watt-hours.

We can estimate the power consumption of the 406-MHz transmission. Transmitting at intervals of 50-seconds in 1-hour produces 72-transmission. Each transmission lasts 0.5-seconds, so total transmitting time is then 36-seconds every hour. In one hour there are 3600-seconds, so the total transmitting time is then 36-seconds/3600-second or 0.01 hour. The total power consumption is then 5-Watts times 0.01-hour or 0.05-Watt-hours.

The surprising result is the 406-MHz transmitter is only consuming about twice the power of the 121.5-MHz transmitter, even though its output power is 100-times greater.

The new MEOSTAR satellite payloads no longer carry receivers to listen for the 121.5-MHz or the 243-MHz homing signals. These signals are only useful to aircraft searching in the area of the beacon's location or to radio-direction-finding searchers on the ground or on the sea very close by the beacon.

Many modern EPIRB devices no longer contain a 121.5-MHz beacon transmitter. The effect of elimination of the 121.5-MHz transmitter is to reduce total transmitter power consumption. The combined consumption was estimated as being 0.05-Watt-hours plus 0.025-Watt-hours, for a total of 0.075-Watt-hours. Reducing to just 0.05-Watt hours is a 33-percent reduction. Assuming a constant amount of battery power available in each case, a 406-MHz-only distress beacon would be able operate its single transmitter about 50-percent longer than a dual-transmitter beacon.