Inside GNSS Media & Research

MAY-JUN 2018

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36 Inside GNSS M A Y / J U N E 2 0 1 8 GRICAS mission duration), ELT(DT) also have now to comply with recent aeronautical standards (DO 160, ED 162) and qual- ity standards: REACH, ROHS, EMC and ESD including documentation, being understood that those require- ments have to be fulfilled without any compromise on product availability and reliability, offering ELT(DT) with higher performance requirements. e Cospas-Sarsat ADT solution has been defined and specified in aeronau- tical standardization bodies for Europe (EU RO C A E) a nd Nor t h A mer ic a (RTCA) through documents ED-62B and Doc10054, under consolidation and with an expected closure date by June 2018. Currently, it is the only ADT sys- tem for which such specification exists, which should ease the support to its adoption. MEOSAR: Enhancing Location Performance e MEOSAR system will be interop- erable w it h t he current 1.9 million deployed First Generation Beacons. W h i le t he L EOSA R a nd GEOSA R systems have been used for decades, the MEOSAR system just entered in early operations capability in Decem- ber 2016. e evolution towards MEO orbits is supported by GNSS constella- tions (Galileo, GPS and GLONASS and future plans for BeiDou) equipped with payloads that relay the distress signals towards ground stations (MEOLUT). MEOSAR uses a different approach compared to LEOSAR. While LEO- SAR relies on a multiple burst location method, exclusively based on accurate frequency measurements, MEOSAR relies on instantaneous spatial diversity (reception of the same burst by multiple satellites) and uses both time and fre- quency measurements (TOA: Time Of Arrival, FOA: Frequency Of Arrival) to compute a location. en, compared to LEOSAR, the new location method used in MEOSAR allows: • Localizing a beacon with a single burst • Localizing a beacon that is fast mov- ing This transition will increase the quality of service of the Cospas-Sarsat service, with several key added values compared to LEOSAR and GEOSAR system: • Worldwide real time detection • Instantaneous independent local- ization (localization without reli- ance on the possible GNSS receiver of the beacon, but based instead on the characteristics of reception of the beacon signal by the SAR system itself) with improved accuracy • Opportunity to implement the Sec- ond Generation Beacon (SGB) The MEOSAR system opens many new possibilities: • The beacon may be activated and independently localized in-f light, even in single-burst and even when fast-moving and at high altitude. • e beacon is continuously tracked aer activation, during the complete descent phase, so that rescue teams may be continuously aware of the trajectory evolution. e last burst, just before crash, is also known and localized, which is crucial for the evaluation of its position. In addition and in parallel of the MEOSAR evolution, the Galileo sys- tem introduces the Return Link Service which offers a new feature: the possibil- ity to acknowledge to the user of a bea- con the reception of the distress message transmitted and therefore inform them that the alert and its location are pro- cessed. is is called the Type-1 RLM. Although Type-1 RLM is the only use case that has been agreed yet by SAR authorities, the Return-Link capability opens several other possibilities to con- tribute to SAR operations improvement that will be analyzed within a EURO- CAE Working Group 98 in 2018, such as: • A dialog between the ground systems and the beacon, to adapt the trans- mission periods, the beacon message, and to create system monitoring solutions. A particular dialog case is the switch-off aer cancellation, thanks to acknowledgement logic. This switch-off is very important to create autonomous cancellation capacity, as it avoids maintaining alerts if the situation has gone back to normal, and therefore reduces the used bandwidth and prevents from system overcapacity. • The Return-Link may be used for remote activation. This is particu- larly useful if the aircra disappears from other communication systems as the beacon is autonomous (works if everything else in the aircraft is off). Such remote activation could in particular have been used for MH370 flight disappearance. C o s p a s - S a r s a t o p e r a t o r s h ave expressed their requirements in terms of SGB performance. ese requirements are described in document Cospas-Sar- sat G.008 "Operational Requirements for Cospas-Sarsat Second-Generation 406-MHz Beacons" that is accessible on the Cospas-Sarsat website. A detailed presentation of SGB specifications and performance can be found in Additional References, and the key improvements are recalled here: • Better detection performance thanks to a stronger error correcting code • Increased message content with flex- ible structure allowing the transmis- sion of information through multiple burst • Increased TOA measurement accu- racy thanks to the use of a spread spectrum modu lation, a l low ing more accurate locations In particular, the improvement of TOA accuracy compared with FGB characteristics allows reaching the same location accuracy for static and moving beacons. In other words, while TOA and FOA measurements have to be used jointly to estimate the location of an FGB in order to get sufficient location accu- racy, the use of TOA only is sufficient for SGB and typically improves the location accuracy by a factor of 10. Since TOA is not affected by the beacon speed, the SGB independent location, which main- ly relies on a significantly improved TOA measurement, is also independent of the beacon speed, which is a great asset for independent location of ELT(DT). With

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