ROBusTNEss And ACCurACyThAT's whAT EsA wANTs FROM sPACE

by Giuseppe Nucera

Performing an in-orbit demonstration to prove that the LEO-PnT satellite navigation constellation and the new Genesis earth mapping satellite are planned to deliver robustness and accuracy.

Does your navigation system get stuck in a tunnel, in a city, or under a mountain? ESA's LEO-PNT constellation represents novel and more robust navigation techniques designed to overcome such issues. The Global Navigation Satellite System (GNSS) provides signals from space that transmit positioning and timing information to be used to determine a destination and the time of arrival. The GNSS service is based on the U.S. GPS and Europe's Galileo navigation systems, delivering the signal to about 6.5 billion receivers worldwide. It is an essential service that must address ever more particular user needs: from sustainable mobility to smart cities, from the Internet of Things to indoor positioning.

The purpose of ESA’s mission to perform an in-orbit demonstration of LEO-PNT constellation aims to meet all these future user demands. The PNT acronym stands for Positioning, Navigation, and Timing services delivered to users, while LEO speaks of low Earth orbit constellations. Unlike traditional satellite-based navigation systems from MEO only, above 20,000 km altitude, the LEO-PNT constellation would incorporate signals from low-Earth orbit satellites at altitudes of less than 2,000 km. Combining LEO-MEO systems will ensure a more robust and efficient PNT delivery, with many more satellites providing coverage also where there is none today.

"Low-orbit satellites move faster ensuring better functioning of the precise positioning algorithms," said Mauro Cardone, Head of ASI Navigation Programs Office, "and being closer to Earth, they can be more powerful and their signals stronger and able to over- come interference." In addition, they do not require an atomic clock on board, being able to lock onto that of the MEO satellites, thus implementing a multilayer system-of-systems’ approach. Unlike MEO satellites, LEO satellites will be much smaller entailing an economy of scale with multiple launches. All this will translate into lower development costs for European companies.

The GNSS system can be a powerful tool for scientific applications provided that it delivers positioning data with errors that do not exceed the sub-centimeter level of precision.

The GGOS (Global Geodetic Observing System) program targets a global reference system accuracy of around 1 mm with a long-term stability of 0.1 mm/y by combining all four space geodetic techniques (GNSS, DORIS, SLR-LLR, VLBI). For the first time ever they will be co-located on a single space platform with Genesis, a mission approved by the European Space Agency during the Council at the Ministerial Level in November 2022.

GENESIS, to be launched in 2026 at an altitude of 6,000 km, aims to achieve the goal set by GGOS. "By combining the four geodetic techniques on a single space platform, with a well-calculated orbit, I can reduce to a set level their systematic errors," said Francesco Vespe, Program Manager of Space Geodesy Activities at ASI Center of Matera. This is a challenge that sees the Italian Space Agency, ASI, at the forefront: the Matera Space Center is one of the few ground stations where the four techniques are co-located together.

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