Billions of receivers use Galileo signals for tracking purposes. But this open, mass service is not the only one available. Alongside, scientists use other Galileo resources, although this use is not part of the system specifications. Today, 20,000 fixed GNSS stations are operated worldwide for scientific purposes.
Hijacking the potential of GPS first, then of Galileo today, allows some major technological breakthroughs for the scientific world
Félix Perosanz, Head of the Terre-Solide program at CNES.
Ever better known earthquakes and volcanoes
Not an active seismic fault or a volcano now escapes GNSS monitoring. “By using specific receivers and algorithms, GNSS makes it possible to observe the deformation of the Earth with millimeter precision! All tectonic faults, active volcanoes are today monitored by GNSS ».
Model the phenomenon and measure the accumulation of stresses, precursors of major events, today help to obtain detailed information on the level of risk. In Mayotte, since the beginning of 2018, daily monitoring of GNSS stations (IGS and private REGINA) has shown that the island is bending towards the east while sinking several tens of centimeters into the sea.
The appearance of an underwater volcano deformed the earth’s crust and led to numerous earthquakes. “Galileo has accumulated observation and contributed to the calculations of the coordinates of these stations, to measure the deformation of the island with increased precision” specifies Félix Perosanz, also president of the International GNSS Service, IGS.
“It is a collaborative scientific service based on GNSS that has been operating for almost 30 years”! Around 100 organizations from 50 countries are participating. On the subject, CNES contributes to the IGS by providing access to data from its global network of REGINA stations and by providing precise orbit products.
Meteorology and oceanography at developer
Meteorologists are used to using multiple resources for their modeling. And when they are interested in the troposphere, an area of the atmosphere sensitive to pressure, temperature, humidity, Galileo is a powerful complementary source to classical meteorological tools. “Arpège has been aggregating GNSS data (which is sensitive to these weather parameters) from a European network for at least 15 years.
This is also the case for data from on-board receivers: Galileo makes it possible to carry out soundings by radio-occultation (deviated signal from a satellite located behind the Earth)” says Felix Perosanz. GNSS-RO uses GNSS measurements received by a satellite in low Earth orbit to “profile” the Earth’s atmosphere with high vertical resolution and global coverage.
Thanks to this approach, intense events could be better anticipated, such as the Cévennes phenomena. “Publications show that the exploitation of such data makes it possible to carry out atmospheric tomography to identify the masses of precipitable water which accumulate. This will probably be exploited soon” explains Felix Perosanz. Especially since the lack of stations in the Mediterranean Sea, necessary to improve accuracy, can be filled with mobile GNSS stations, on buoys or boats.
The oceanographic field represents another immense field of experimentation, where Galileo provides scientists with strategic measurements for understanding climate change.
If GNSS make it possible to categorize the data recorded by tide gauges – the sea rises or the earth’s crust sinks – their contributions go beyond that. Galileo notably brings increased precision to the calibration/validation of space altimeters, which are essential tools for studying the oceans and climate change.
Better remote sensing, with reflectometry
The science of GNSS reflectometry, known as “GNSS – R” – is developing at high speed, and for good reason: signals from satellites are reflected on water, ice and the ground. Captured via an antenna on the ground or on board a satellite in low orbit, these make it possible to determine the height of the antenna in relation to the reflecting surface or to measure a large number of physical variables of this surface. “The technology makes it possible, for example, to characterize the height (snow, river, coastal ocean, etc.) of an area, the roughness of the ocean and observe the formation of cyclones or soil humidity. This remote sensing technique is in full development » says Felix Perosanz.
The latest ITRF benchmark, powered by Galileo data
The international terrestrial reference frame (ITRF), the unit of the ruler in a way, is used for all ! Without it, Earth dynamics would be a mystery. Geodesists define this benchmark, using GNSS data in particular.
An international service, IERS, broadcasts the international convention of the reference frame and rotation of the Earth. The last, made in 2020, was broadcast in May 2022. Galileo aligns itself back on it. However, this is not the case with GPS systems, from Glonass or Beidou.
Improve the real-time accuracy of satellites
“Thalès equipped its large satellite platforms with its TopStar 3000 GPS receiver in the 1990s” recalls Thomas Junique, specialist in on-board GNSS receivers within the radionavigation department at CNES. The advent of nanosatellites has helped to democratize space GNSS receivers.
On-board receivers meet on-board time synchronization needs as well as orbit restitution, either by post-processing but also in real time, all with the need for very high compactness
Thomas Junique, specialist in on-board GNSS receivers in the CNES radionavigation department.
Thanks to Galileo, the multi-constellation and multi-frequency dynamic initiated multiplies the advantage of having space-based GNSS receivers: the availability of service increases through the better visibility generated, and the quantity of data available allows greater precision on the position .
Towards new capacities for NanoSats
It remains to properly assess the needs: the channels necessary for frequency processing potentially involve greater computing power in flight.
Manufacturers now want have a generic on-board GNSS receiver for their platform, capable of carrying out a wide range of missions. But the design of a flight GNSS receiver is subject to different constraints from a ground receiver: “Technical constraints (consumption, speed, antennas, etc.) and related to the space environment can limit their performance. The navigators on board help to “filter” the data in terms of real-time position, including if there is not enough GNSS satellite in visibility” says Thomas Junique.
CNES is actively working on the subject, as the example of N-SPHERE reminds us. Developed by Syrlinks, the receiver was developed with the support of CNES: this receiver will allow contribute to precise multi-constellation (GPS, Galileo) and multi-frequency GNSS orbit determinationit will be able to provide a excellent on-board real-time accuracy… less than one meter.
N-SPHERE will be used for the next GOMX-5 mission organized by ESA and Gomspace A/S. The receiver, to which a navigator is associated, is integrated into a 12U Nano/Cubesat platform with the objective of demonstrating new nanosatellite capabilities for future generations of constellations in low orbit.