On October 9th, 2023, the European Space Agency (ESA) PRETTY mission was launched into space aboard the Vega rocket from French Guiana and will soon capture satellite navigation signals (GNSS). A team from the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC) participates in the scientific consortium of this nanosatellite that will test innovative and cost-effective ways to monitor sea and ice surface levels from space.

The PRETTY (Passive REflecTometry and dosimeTrY) is a CubeSat 3U nanosatellite, a miniature cuboidal-shaped satellite, equipped with a pair of forward-facing antennas to capture signals from navigation satellites close to the horizon. The antenna on its forward face will pick up the signal from a GNSS satellite that travels through two ray-paths: a direct path directly from the GNSS and a path that has first bounced off the Earth’s cryosphere or the oceans. By innovative processing and analysis of these signals, the mission aims to demonstrate that the technique can derive ice and sea height to an accuracy of at least 50 cm from 550 km orbital altitude.

PRETTY CubeSat with two patch antennas. Credits: ESA.

GNSS signals reflected in the sea make it possible to accurately measure the topography of the sea at very low angles. The ICE-CSIC Earth Observation research group, led by Estel Cardellach, including Serni Ribó and Weiqiang Li –all of them also IEEC members– has been developing altimetry techniques based on GNSS signals since the end of the 1990s. The team participates in the scientific consortium coordinated by the Norwegian University of Science and Technology (NTNU) in partnership with ICE-CSIC, IEEC, the GFZ German Research Centre for Geosciences and German Aerospace Center (DLR-SO).

The ICE-CSIC and IEEC team has helped defining parameters of the GNSS receiver onboard, and will be in charge, together with DLR-SO and GFZ, of generating the first altimetric solutions from PRETTY’s collected signals during the initial phases of the satellite mission

GNSS signals, a new approach to reflectometry

“In PRETTY, we will try to demonstrate different aspects of GNSS altimetry at very low angles. It is a technique with great potential but it also presents complicated challenges,” says Estel Cardellach, ICE-CSIC and IEEC researcher. “In our group, we have the responsibility of providing the first altimetric results during the initial phases after the launch,” she adds.

“In addition, PRETTY is making use of a new GNSS frequency for the first time, employing the longer-wavelength E5/L5 as transmitted by European Galileo and US GPS satellites. The PRETTY team made this change late in mission development, guided by ground testing that showed superior accuracy using this frequency band.”, explains ESA engineer Manuel Martín Neira. 

The ground experiment was led by ICE-CSIC and IEEC in 2021, with the participation of Mediterranean Institute for Advanced Studies (IMEDEA-CSIC), and funded by ESA, to investigate reflectometry at grazing angles. Researchers Estel Cardellach and Serni Ribó set up a navigation satellite receiver at the top of Puig Major in Mallorca. The campaign and the data analysed were useful to explore a clearer way of monitoring sea level, giving proper advice for the PRETTY mission, as well as for future missions using cost-effective altimetry techniques and GNSS signals.

Exploring the shape of the sea surface, its height and the processes that alter it will allow better applications at a scientific and social level in climate research, ocean currents prediction, cyclones prediction, fisheries management, monitoring of debris and plastics, etc.

The PRETTY mission, funded through ESA’s General Support Technology Programme by Austria, has been developed by an Austrian consortium composed by Beyond Gravity Austria, Seibersdorf Laboratories and Technical University of Graz.