Satellite Reflectometry - an Earth Observation byproduct from Navigation Satellites
Your phone or satnav receiver routinely picks up signals from navigation satellites in order to tell you precisely where you are. But have you ever thought what happens to those satnav signals afterwards? A foresighted ESA inventor had the idea of using them as a tool for observing the Earth. 1)
There are currently more than 120 satellite navigation satellites in orbit, making up multiple constellations including Europe’s system, sending down a continuous rain of satnav signals for the benefit of users worldwide. Just like visible light, these microwave signals go on to reflect off Earth’s land and sea surfaces.
The traditional attitude to these reflected signals is to see them as something of a nuisance – known in the trade as ‘multipath’, they can confuse satnav receivers and reduce their overall accuracy.
But back in 1993 – at the same time as the US GPS satnav system reached its full constellation of 24 satellites – a young ESA microwave engineer, named Manuel Martin-Neira, came up with the idea of treating these satnav reflections as a scientific resource instead. 2) 3) 4)
“My head of division asked me to come up with a budget-friendly way of increasing the overall sampling rate to build up a fuller picture of ‘mesoscale’ phenomena, and that led me to start looking into making use of additional signals of opportunity, chiefly satnav signals.
“The initial reaction was mixed, because the forecast accuracy was not as precise as the ERS-1 altimeter of ESA could deliver – but on the plus side there would be a lot of these signals to make use of, and the performance has improved a lot since those early days.”
Inspiration from reflection
The basic idea of what Manuel christened PARIS (Passive Reflectometry and Interferometry System) comes down to a two-sided antenna. As the topmost side picks up a satnav signal from the satellites in orbit, the other side picks up the version of the signal bounced back from Earth.
By comparing this initial, overhead signal with its reflected equivalent using a process called interferometry – measuring tiny differences in signal phases – the extra travel time of this reflected beam can be determined, down to an accuracy of less than five centimeters, determining sea height and sea ice thickness.
Additional ‘amplitude waveform’ processing can deliver further data on wind and wave measurements over the ocean, and soil moisture and biomass over land.
‘Satellite reflectometry’ has since grown into a thriving field. This summer Manuel attended the latest international workshop on the method he first devised 26 years ago.
Reflectometry reaches space
“It’s been fantastic to have experimental evidence, and that’s really been made possible by the growing availability of smaller satellites,” explains Manuel. “Because satellite reflectometry is a passive form of remote sensing, it makes for an attractive potential payload because it doesn’t need a lot of power to operate. Then one of the results is meteorology data that private companies intend to make money with by delivering to public agencies.”
Back in 2003, the UK-DMC microsatellite was the first mission to fly a reflectometry payload, followed in recent years by, for example, the UK’s TechDemoSat-1, NASA’s CyGNSS constellation to monitor hurricanes and the Spire Global constellation of commercial nanosatellites.
“These satellites have really given the reflectometry community a wealth of signals, demonstrating what reflections look like over different surfaces including sea ice, forests, and even inland water bodies such as the Amazon River and its tributaries.
“In parts of the ocean near continental masses and within atolls we are seeing reflected signals from very calm waters which resembled a mirror, giving us very high precision down to 1 cm level. Such measurements could potentially complement current altimetry missions, by for instance measuring sea level rise.”
ESA activities taking flight
ESA meanwhile is active on reflectometry in various ways, having developed and tested a steerable airborne antenna called the SPIR (Software PARIS Interferometric Receiver), capable of steering separate antenna beams to build up a rapid surface picture, and differentiating between different signal sources, such as GPS from Galileo.
Manuel adds: “ESA’s GNSS Science Support Center, based at the Agency’s European Space Astronomy Center near Madrid, has been taking a keen interest in these activities.”
Missions are also in development, including a dedicated 3U CubeSat with RUAG-Austria and the University of Graz called PRETTY (Passive REflecTomeTry and dosimetry), which would also carry a radiation detector), and a small satellite pair called FSSCat from Spain’s Universitat Politècnica de Catalunya (Barcelona), backed through the Copernicus Masters competition, seen as a prototype for a future reflectometry constellation.
For GNSS reflectometry, the reflected signal is typically correlated with a clean replica generated on-board of the spacecraft. The ESA PRETTY CubeSat mission, however, will correlate the received reflected signal with the received direct signal. This technique is known as the interferometric approach. The main advantage for the interferometric approach is, that one is not bound to use known signals but can exploit signals with unknown data modulation, opening up the possibility to use more generic signals for Earth observation. PRETTY will focus on low elevation angles, whereby the direct and reflected signal will be received via the same antenna. 5)
ESA’s Directorate of Telecommunications and Integrated Applications is also working with the Spire Global company of San Francisco to fly enhanced reflectometry instruments, starting at the end of this year. Each Lemur nanosatellite carries an AIS (Automatic Identification System), GNSS-RO (GNSS-Radio Occultation), and an ADS-B (Automatic Dependent Surveillance-Broadcast) receiver.
When it comes to the thriving state of today’s reflectometry community, Manuel recalls the patenting of his idea as a turning point: ‘Having had this idea, which was not particularly well received, the proposal by ESA’s Patents Group to patent it made all the difference. It gave me a feeling of confidence, that somebody else at least saw the potential of this idea – and the rest is history.”
1) ”New maps of Earth from reflected satnav, invented at ESA,” ESA, 20 September 2019, URL: http://www.esa.int/spaceinimages/Images/2019/09/Manuel_Martin-Neira
2) Manuel Martin-Neira, ”A Passive Reflectometry and Interferometry System (PARIS): Application to Ocean Altimetry,” ESA Journal, Volume 17, No 4, pp: 331-335, January 1993, URL: https://esamultimedia.esa.int/docs/technology/ESA_Journal_1993.pdf
3) Manuel Martin-Neira's original 1993 presentation to the Patents Group, 7 September 1993, URL: https://esamultimedia.esa.int/docs/technology/Presentation_to_Patents_Board_7-Sep-1993.pdf
4) Manuel Martin-Neira, Thesis, “Application of the Global Navigation Satellite Systems to Spacecraft Landing, Attitude Determination and Earth Observation Constellations”, presented at the Polytechnic University of Catalonia, Barcelona (Spain), 5 April 1996.
5) Michael Meindl, Giorgio Savastano, Andreas Dielacher, Fernando Martin Porqueras, Jacob Christensen,”Space Service Volume II,” 7th international colloquium on scientific and fundamental aspects of GNSS, Zürich, Switzerland, 4-6 September 2019, URL: https://atpi.eventsair.com/QuickEventWebsitePortal/19a07---7th-gnss-colloquium
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).