GPS Scatterometry; How Started

Space platforms for GNSS Reflectometry back to 33 years ago.

In 1897, Marconi Company was founded in Chelmsford, England, as the division of defense industry and businesses of its mother: General Electric Company. However, it’s known today as BAE Systems plc, a British aerosystem and defense company with its headquarter based in London and Farnborough, UK. By the end of World War II, Marconi put more focus on radio propagation and its application in radio telegraphy. In 1988, David Hall and Ralph Cordey from the Marconi Space Systems and Marconi Research Centre, respectively, discussed the multistatic scatterometry concept using GPSS signals. Apparently, GPS was used to be called GPSS at that time. It’s interesting to know that Hall and Cordey compared GPS signals features, such as code sequence length and transmitted power, with monostatic criteria of ERS-1 (the first remote sensing mission lunched by ESA) and argued that although GPS reflectometry is conceptually possible, the design of GPS at that time are incapable of solving ambiguities due to the weakness of transmitter power output. We’ll talk about GPS transmitter power later.

In 1991, a french alpha-jet aircraft manufactured by Dassault Electronique Company, conducted an experiment to test GPS receivers ability for real time positioning. Flying at low elevation over Atlantic Sea, the aircraft found the positioning solution disturbed due to multipath effect, which meant that reflected GPS signals could be tracked. Dassault Electronique conducted another study to describe and model multipath signals by defining a flight pattern in which direct and reflected GPS signals could be separately recorded. Jean-Claude Auber and co-authors documented this experiment and characterized GPS multipath over land and sea in 1994. They did this to avoid multipath as a harmful phenomenon in positioning, but you know, one’s signal is another one’s noise, and this research led to introducing a new remote sensing tool: GPS-Reflectometry.

In 1993, Manuel Martín-Neira, who had recently been awarded a fellowship to study on microwave radiometry at ESA, developed the concept of Passive Reflectometry and Interferometric System (PARIS) as a tool for ocean altimetry. This new concept opened a new gate to perform altimetry experiments along directions other than nadir, which was the only possible direction for altimetry at that time, by collecting reflected GPS signals off the ocean from various directions. Explaining the geometry and instrumentation of PARIS, Dr. Martín-Neira explained how one can reach to the vertical accuracy of 70 cm with GPS altimetry. He, for the first time, depicted the patterns of iso-range and iso-Doppler curves formed in GPS altimetry, and how they differ from those in the monostatic SAR case, i.e., how the orientation of iso-range and -Doppler curves may differ from each other depending on the direction of GPS satellites and receiver velocity. Considering patenting of this idea as a turning point, Dr. Martín-Neira has said: “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”

Finally, five years later, Jet Propulsion Laboratory reported that GPS reflections have been observed, for the first time, by the Space-borne Imaging Radar-C (SIR-C) aboard the shuttle. That was the first space-borne observation of GPS signals reflected from the Earth, the Pacific Ocean, giving insights into the expected SNR at the receiver.

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