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 406.1-MHz in c.1979. Since early 2009, only the 406.1-MHz 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 23,500-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 next-generation GPS-III 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 a S-band frequency 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 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 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/
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
by Jesse Reich
MEOSAR & GPS
By Lisa Mazzuca
VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
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