Galileo tests; Rescue role

GPS World, Sept, 2007

The test campaign using the large antenna at Chilbolton Observatory to analyze the navigation signals transmitted by GIOVE-A, the first Galileo satellite, has been success-fully completed, the European Space Agency (ESA) announced in mid-August.

Analysis of the satellite's signals since January 2006, to verify their conformance with the Galileo system specification, has involved the Navigation Laboratory at ESA's European Space Research and Technology Centre (ESTEC) in the Netherlands, the ESA ground station at Redu, Belgium, and the Rutherford Appleton Laboratory (RAL) Chilbolton Observatory in the United Kingdom.

Following signal analysis, operators have made some adjustments, re-programming the spacecraft's navigation signal generation unit to compensate for changes to the signals introduced by the amplifier that boosts them for transmission to Earth and by a filter that protects adjacent frequency bands from interference.

To achieve the correct solution, the calibrated Chilbolton station was used to receive the signals from GIOVE-A. ESA's Navigation Laboratory processed the resulting data. The signal generator manufacturer, TAS (France), calculated the new settings for the unit. Finally, the satellite manufacturer, Surrey Space Technology Limited (SSTL), uploaded the new values to the payload using their ground station at Guildford, in the United Kingdom.

According to ESA, "GIOVE-A is now transmitting optimized signals. Research and testing continues, and manufacturers are using the signals as they develop the receivers that users will need when Galileo enters operational service."

Global Search and Rescue. Once operational sometime after 2012, Galileo will improve the detection of emergency beacons, according to program representatives who attended the annual Joint Committee Meeting of COSPAS-SARSAT, the international program for satellite-aided search and rescue. Galileo satellites will carry transponders to relay distress signals to search and rescue organizations. Galileo partners have committed to developing a search and rescue component as an integral part of Medium Earth Orbit Search And Rescue (MEOSAR), the future worldwide search and rescue satellite system.

COSPAS-SARSAT already has systems operating in low-Earth orbit and geostationary orbit. The low-Earth orbit satellites can determine the location of emergency beacons using the Doppler effect as they pass overhead. However, there is a delay in relaying the distress signal because the satellites can only "see" a part of the Earth's surface at any given time and a beacon is only detected when the satellite passes nearly overhead. Also, the satellites must store the location of the emergency and transmit it to a ground station once one comes into range, creating further delay.

Search and rescue transponders on geostationary satellites can constantly view a large, fixed area of the Earth, eliminating the time delay in detecting distress signals. However, they cannot automatically determine the location of the distress beacon as the low-Earth orbit system does, but must rely on the beacon to use a navigation system to find its position and include it in the distress call.

Emergency beacons require a direct line-of-sight to the geostationary satellites. There are some situations where this is impossible, such as near the Earth's poles, where the satellites are too low in the sky, or when an accident occurs where surrounding terrain obscures the satellite.

Future Improvements. To improve performance of the overall COSPAS-SARSAT system, plans are now being made to fly search and rescue payloads on future navigation satellites. The various navigation satellite constellations will each have about 20 to 30 satellites in medium-Earth orbit, providing global coverage, including at the Earth's poles, and with multiple viewing angles to the satellites, eliminating terrain blocking.

The Galileo search and rescue component will provide two services. The Forward Link Alert Service, backward-compatible with current COSPAS-SARSAT components and interoperable with all other planned MEOSAR elements, detects activated distress beacons and notifies the appropriate rescue body. A new Return Link Service will send a return message to the emergency beacon, notifying the emergency victims that their distress signal has been received and help is on its way.

The Galileo In-Orbit Validation Programme, which will have four satellites fitted with search and rescue transponders, will demonstrate the Galileo MEOSAR services--although its flight timetable has yet to be finalized or announced.