Monitoring the constantly changing shape of the sea surface is important for scientific and societal applications such as ocean current forecasting, climate research, ship routing, cable laying and debris tracking.
A project supported by the Discovery element of ESA’s Basic Activities recently investigated a technique to precisely measure sea-surface topography. The project was based on an idea submitted by the Institute for Space Studies of Catalonia (IEEC) through the Open Space Innovation Platform (OSIP) — ESA’s place for your space ideas.
The technique involves GNSS reflectometry — signals that have been reflected off of the sea surface at very low angles. At these “grazing” angles, waves and surface roughness have little impact on the reflection process; the sea surface acts as a very smooth mirror.
“In a mirror-like reflection the phase of the signal can be tracked — it is continuous,” explained IEEC’s Estel Cardellach, principal investigator and submitter of the OSIP idea. “Different surface heights result in different phase measurements. It gives a very precise measurement of the surface altitude at a few centimetres’ precision.”
Balearic Islands Project
The ESA-funded activity involved developing a GNSS receiver and setting up an experiment in the Balearic Islands to collect GNSS signals reflected off the sea surface. The team — made up of IEEC, imedea, SOCIB and DLR — then processed the signals for optimized measurements of the shape of the sea surface.
“Thanks to OSIP and ESA Discovery we have been able to conduct this experiment on grazing GNSS reflectometry under monitored conditions,” said Manuel Martin-Neira, ESA technical officer for the project. “We have linked the coherence of the reflected signals to wave height and the elevation angle of GNSS satellites. These results have been very useful for preparing the PRETTY mission.”
ESA’s PRETTY (Passive REflecTomeTry and dosimetry) CubeSat mission is a small satellite that will carry out grazing angle GNSS altimetry from orbit. It is due to launch later this year.
A new concept that involves mounting an instrument on the International Space Station and taking advantage of signals from navigation satellites could provide measurements of sea-surface height and information about features related to ocean currents, benefiting science and ocean forecasting.
We have all seen the beautiful photographs of our planet taken by astronauts, but orbiting Earth 16 times a day just 400 km above, the Space Station also offers a platform from which to measure certain variables related to climate change.
So, in 2011 the European Space Agency (ESA) called for proposals to explore how the Space Station could be used to make scientifically valid observations of Earth. After reviewing and assessing numerous proposals, the result is to further develop the GEROS-ISS mission concept.
Jason Hatton, GEROS-ISS project coordinator, said, “The concept is still going through feasibility studies, but the aim is to launch the experiment towards the end of 2019. It would be carried to the Space Station on a cargo vehicle and installed on ESA’s Columbus space laboratory using a robotic arm, after which GEROS-ISS would run for at least a year.”
GEROS-ISS stands for GNSS reflectometry, radio occultation and scatterometry on board the ISS. GPS and Galileo satellites send a continual stream of microwave signals to Earth for navigation purposes, but these signals also bounce off the surface and back into space.
The idea is to install an instrument with an antenna on the Space Station that would capture signals directly from these satellites as well as signals that are reflected or scattered from Earth. This process could be used to calculate the height of the sea surface, and to measure waves — or “roughness” — that can then be used to work out the speed of surface winds.
Variations in sea-surface height (cm) obtained by merging multiple altimeter measurements. GEROS-ISS would be able to observe this variability so that maps covering latitudes 51° N to 51° S can be produced every four days. (Photo: ESA)
GEROS-ISS is primarily an experiment to demonstrate new ways of observing Earth. However, if taken beyond the testing phase this new approach would complement measurements from satellites carrying altimeters such as CryoSat and Sentinel-3, and satellites carrying wind scatterometers such as MetOp.
Importantly, it is the first concept to assess the potential of spaceborne GNSS reflectometry to determine and map ocean height at scales of 10–100 km or longer in less than four days. Current satellite altimeters, in comparison, offer global maps at scales of around 80 km, which are produced from multiple datasets every 10 days.
A system based on GEROS-ISS would, therefore, complement existing satellite systems, helping to map ocean variability at finer spatial and temporal scales over a range of seas in tropical and temperate regions. It would also refine our understanding of how well the concept would work for measuring the roughness of the ocean surface.
In this respect, the development of GEROS-ISS benefits from experience gained with the UK’s TechDemoSat-1, which also measures ocean-surface roughness using a similar technique. It is also hoped that NASA’s upcoming CYGNSS constellation of mini satellites will help pave the way for GEROS-ISS.
In addition, GEROS-ISS uses a technique called radio occultation whereby the antenna receives signals that are refracted as they pass through the atmosphere. This can be used to generate vertical profiles of atmospheric humidity, pressure and temperature, as does the GRAS instrument on the MetOp satellites, for example.
GEROS-ISS will be installed on the upper balcony of ESA’s Columbus space laboratory, which provides mechanical interface plates as well as power, command and data links to the ISS systems. (Photo: ESA, taken by ESA astronaut Luca Parmitano during his spacewalk on July 9, 2013. )
“It is very flexible, combining different mission concepts and applications in one: GNSS-reflectometry to determine sea-surface height, scatterometry to measure sea-surface roughness and radio occultation for atmospheric studies,” said Jens Wickert who leads the science team that proposed GEROS-ISS.
ESA engineer Manuel Martin-Neira noted, “The original concept actually goes back over 20 years and has matured considerably through numerous studies and campaigns, however, it has never been duly tested from space.”
“Being able to use the International Space Station in this way means that we can quickly validate innovative observing techniques without having to build an entire satellite, and we expect this to lead to new opportunities for science,” added Michael Kern, ESA’s GEROS-ISS mission scientist.
The GEROS-ISS feasibility studies are being carried out through ESA’s General Studies Programme.
Editor’s Note:GPS World discussed the use of GPS for radio occultation in itsMarch 1994 Innovation column, “Monitoring the Earth’s Atmosphere with GPS,” by Rob Kursinski.
