Simultaneously with the launch of additional Galileo satellites, the European Commission (EC) is gearing up to declare the constellation’s “initial services,” which will include a spectacular improvement of search-and-rescue (SAR) capabilities it is hoped will take place gradually starting in December. A victim using a distress beacon could then expect to be located within a few minutes, and an additional feature will eventually be an acknowledgment of receipt, indicating that help is on the way.
The EC’s declaration of initial services does not depend on Ariane 5 launching four more Galileo satellites in November, as the 14 satellites already in orbit are deemed sufficient. Rather, the EC will have to follow the decision made by the Security Accreditation Board (SAB) established within the European GNSS (global navigation satellite system) agency. The SAB, an independent body from the EC and the European Space Agency, was assigned to “verify a high, robust and uniform level of security for EU GNSS.” The target date for the declaration is now Dec. 15 but it has tended, in the recent past, to slip further out.
In a Galileo satellite, the payload that will provide medium-altitude Earth orbit (MEO) SAR (Meosar) services is distinct from the one in charge of navigation. As a result, the best precision will be 100 m (330 ft.)—well below the promised sub-meter performance of Galileo’s positioning system. The counterintuitive choice of making beacon localization independent from a GNSS was made in favor of robustness.
First, to determine a position using navigation satellites, a receiver needs to have its antenna in an appropriate position, notes David Comby, France’s deputy interdepartmental coordinator for Galileo. The antenna is unlikely to be precisely oriented on a crashed aircraft or sinking boat. Second, Galileo-compatible beacons also have to be compatible with Cospas–Sarsat, the existing satellite SAR system, Comby adds.
Like Cospas–Sarsat, which saves an average of six lives a day, Meosar makes the most of Doppler positioning to localize a distress beacon. It uses a combination of time difference of arrival and frequency difference of arrival. MEO local user terminals (Meolut) calculate the beacon location by measuring and processing the differences in the same beacon burst relayed by different satellites.
The enhancement comes from the medium orbit—as opposed to low or geostationary—Galileo satellites use. A medium orbit combines a wide field of view with strong Doppler shift, making it more likely a distress signal can be pinpointed promptly and accurately. The greater number of satellites will help, too. Cospas-Sarsat uses 12, whereas Meosar payloads are planned to operate very soon on 18 Galileo spacecraft and hoped to orbit eventually on 72, including next-generation GPS and Glonass constellations. The user will be visible from more satellites.
Moreover, the signal will be of higher quality, thus enabling a better Doppler measurement, says Comby.
The expected shift in performance will be dramatic for a user in a life-threatening situation. The current average detection time is 45 min., with a typical 5 km (2.7 nm) radius precision, according to McMurdo, the France-headquartered supplier of Meoluts. With an existing beacon and the new Meosar service, this should be improved to 30 sec. for the same radius, five min. for 1 km and 30 min. for 100 m. These specifications are for 95% of occurrences.
A new generation of beacons, which Comby expects to enter service in 2019, should enable localization in 5 min. for a 100-m radius, in a single burst. The frequency will remain at 406 MHz.
Another benefit of Galileo satellites, with new-generation beacons, will be the ability to send a “return link signal” to the beacon. The Cospas-Sarsat mission control center will forward a message to the Galileo constellation operator, who will then “place” a message in Galileo’s L1 navigation signal. It will then be transmitted to the specific distress beacon.
“With this functionality, we could possibly monitor the beacon transmission or even confirm if the emergency signal is a real distress situation or a false alert,” says Alain Bouhet, program coordinator at McMurdo. “But also, a major step in aviation safety could be the possibility to activate from the ground a beacon onboard an aircraft in an abnormal situation, such as the [Flight] MH370 airplane. This can be a huge improvement in determining the location of an airplane potentially in distress and not responding.” The company is also in charge of developing the new beacons. It will lead a consortium including Air France and French space agency CNES.