France: A rain of navigation signals falls constantly upon the Earth from GPS and the initial satellites in Europe’s Galileo system, enabling an ever-increasing number of positioning and guidance services.
Afterwards these microwave beams bounce back to space – where a proposed ESA mission aims to harness them as a scientific resource and explore their potential for terrestrial remote sensing applications.
Intercepting reflected satnav signals and calculating their travel time is a way to track the changing contours of the Earth’s ocean, land and ice surfaces. And because this detector technology – known as the Passive Reflectometry and Interferometry System (PARIS) – can track multiple signal bounces at once, it can build up a rapid picture of fast-moving mid-sized phenomena such as storms, current eddies and high waves.
“At the heart of the mission is a double-sided antenna made up of many small elements that can track multiple signals from above and below in parallel” explains PARIS project researcher Manuel Martín-Neira of ESA. “The phase difference of original and bounced-back signals originating from the same satellite are then compared to reveal the difference in their travel time and obtain ranging information. The principle has been proven with airborne demonstrators, but the high-gain satellite-tracking antenna we require for maximum ranging accuracy can only be tested in space.”
The proposed PARIS In-Orbit Demonstrator mission would include a 1.1-m antenna to prove the concept, with any future operational mission being equipped with a larger 2.4-m antenna.
PARIS is a passive variation on an existing instrument called a radar altimeter, currently flying on ESA’s Envisat and other Earth-observing satellites. These altimeters fire 1800 microwave pulses downward per second then measures their echo return time to calculate sea surface height. As ocean currents and eddies form height undulations on the sea surface of the order of tens of centimetres, altimetry results gradually yield to a rich and scientifically valuable map of ocean circulation. Current altimeters sample only one point at a time, while PARIS would measure multiple points with a faster revisit time, yielding new data on rapidly-changing features.