2024-07-21 13:25:00
Par Jean Maréchal, National Center for Space Studies (CNES)
Less known than the GPS American, Galileo, the European satellite navigation system, has been providing operational services since 2016 and is now massively used by smartphones, vehicles, drones and all consumer applications. This is no coincidence. While the constellation of satellites was still incomplete, the performance of the European system very quickly proved to be excellent, offering one meter of precision, twice as good as GPS. To take advantage of this new performance, navigation chips have very naturally evolved in the space of a few years, to integrate the new Galileo signals, thus improving the availability and precision of the position for the user.
27 satellites at an altitude of over 20,000 km
Galileo is of course a constellation of satellites. It is organized on three orbital planes at an altitude of 23,200 km, each of the planes containing 8 nominal satellites and one or more redundant satellites. These redundant satellites are active and ready to be moved into orbit to replace a satellite that fails. In total, the constellation therefore contains 24 nominal satellites and at least 3 redundant satellites. And we are almost there! After the 13th launch which will take place in September 2024, the challenge will have been met and the constellation will reach 27 satellites.
However efficient they may be, the Galileo satellites cannot provide the services alone. They need navigation data calculated and transmitted by the ground segment, which plays a decisive role. The ground segment is made up of transmitting and receiving stations deployed at fifteen European sites around the globe, particularly in the French and Dutch overseas territories, and at various centres in several European countries.
The processing carried out on the ground mainly consists of determining in real time the precise orbits of the satellites to within 20 cm, resynchronizing the atomic clocks of each of the satellites to the nanosecond (billionth of a second) and then transmitting this valuable data to the satellites so that they can rebroadcast them to users in the navigation signals.
Unparalleled precision
It should be remembered that the four navigation constellations, Galileo, GPS (Global Positioning Systemthe American system), Beidou (the Chinese system) and Glonass (the Russian system) are interoperable: their signals are assembled in the navigation chip to produce a single position. Let us also remember that this calculation is carried out locally and autonomously by the chip without exchanging information with the satellite. The signals are in fact simply broadcast by the satellites to their billions of users, just as an FM radio transmitter transmits signals without knowing the listeners who are listening to it.
If Galileo is so precise, it is because the European engineers who designed it in the early 2000s were able to observe and analyze how the GPS of the time worked. However, racing for precision with GPS is not the fundamental reason for the European system. Galileo, a major space infrastructure, was first wanted 20 years ago by the Member States of the European Union to meet their sovereignty requirements. The aim was to satisfy both the daily needs of 450 million citizens and more than 20 million companies, and to provide governments and their armed forces with protected and robust services, completely independent of the American military GPS system and the Chinese and Russian systems.
A navigation system with many services
Galileo is a civil system that has been designed since its inception to provide free better services to citizens. The first of these, the positioning service, therefore delivers very high-quality performance worldwide. In addition, since January 2023 Galileo has also been transmitting a very precise positioning service intended for professionals, the High Accuracy ServiceThis service provides a precision of 20 cm used for example in agriculture or construction.
Galileo also provides two authentication services to combat spoofing, i.e. the transmission of false satellite signals to deceive the user about his position. The first, OSNMA (Open Service Navigation Message Authentication), intended for consumer applications, is currently being tested, and the Signal Authentication Servicea second, even more robust service, will enable highly demanding professional uses from 2026. This ability to authenticate satellites protects the user against malicious threats. It also helps combat fraud, when the position is essential to control an activity such as fishing or transport. The first regulatory use of Galileo authentication will be to confirm the positions calculated in tachographs, position and speed recorders for heavy goods vehicles.
Protection, assistance and alerts
Protection and assistance to people are also services with a special place. Each satellite carries specific equipment that relays the signals emitted by distress beacons when they are activated in the event of an air accident, shipwreck or personal accident in isolated areas. This capacity for global and instantaneous retransmission of distress signals by the Galileo constellation has revolutionized the speed of calculating the position of beacons, allowing almost instantaneous location of distress, whereas it used to be necessary to wait several hours to locate them. Galileo is also capable of confirming to the beacons that the distress message has been received. This return path to the beacons will soon allow rescue teams to communicate with the people to be rescued in order to optimize the organization of operations.
To complete the range of services provided to citizens, we must finally mention another innovation, the emergency alert to populations, which will be accessible to the civil security authorities of the EU Member States. At their request, Galileo will be able to transmit alert messages in its navigation signal, directly to smartphones and public billboards. Targeted at areas in emergency situations such as forest fires, floods or industrial accidents, these messages will contain information and recommendations for the populations concerned. The service will have the decisive advantage of being able to operate anywhere and in the absence of a telephone network. Currently being demonstrated on the constellation, this service should become operational at the end of 2025.
Towards a second generation of satellites
This long list of services may surprise you with its diversity, but the story is only just beginning. What will Galileo’s new services look like in 2050? It is difficult to predict, and yet it is already necessary to build the 2nd generation satellites that will soon take over from the first satellites launched thirteen years ago. Twelve satellites are currently under construction. They will also be flexible. When the first of them are in orbit in 2027, they will first be programmed to transmit current signals to ensure continuity of service. And then, in 7 to 10 years, once deployed in sufficient numbers, they will also transmit new innovative signals to meet Europe’s future needs.
Made possible by the determination of the EU Member States who together wanted to equip themselves with this sovereign means, Galileo is now available in almost every pocket and in every recent vehicle in Europe and the world. Galileo is here, and for a long time!
This article was co-written by Jean Maréchal and Damien Bellier, delegated interministerial coordinator for Galileo.
Jean MaréchalNavigation and localization program manager, National Center for Space Studies (CNES)
This article is republished from The Conversation sous licence Creative Commons. Lire l’article original.
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