It seems that we are approaching to that time to see non-American GNSS constellations are independently operational for either positioning or reflectometry purposes.
The European Space Agency (ESA) supported GNSS constellation, Galileo, is to improve itself by signing a new contract with their prime operational contractor, Spaceopal. The plan is developing a reference algorithm for those Galileo clients using High-Accuracy Services (HAS) terminals, which is predicted to be conducted in 18 months. The final aim of such services at its second phase is providing a worldwide 20 cm precision for positioning by 2024 by correcting clock, phase and orbit biases as well as correcting ionospheric delay (Haven’t they done it yet?). Here, you can find more information on the scheme designated experiments of HAS.
By employing this innovative algorithm and improving their positioning quality, the Galileo’s team looks forward to compete in general and special GNSS markets, including autonomous vehicles industry, various types of transportation (rail, maritime, etc.), and agriculture. As a reflectometry-oriented user, I hope we’ll be getting stronger contribution from Galileo soon.
A similar new had been heard about the Chinese GNSS constellation, BeiDou, on Dec 2020. Having spent 50 billion USD by the end of 2019, Chinese GNSS industry is to standardize BeiDou technical aspects in four classes of data format, augmentation system, atomic clock and map application. This can be a promising news for sectors in China and around the world which are benefiting from BeiDou’s contribution to GNSS; sectors such as agriculture, urban governance, transportation and public security.
These efforts become much more beneficial and interesting to professional GNSS users when they come with the most recent version of RINEX released on Dec 2020. RINEX 3.05 includes tons of updates in favor of non-US GNSS such as BeiDou. These updates, as announced by the support office of ESA, comprise of a major restructure adding BeiDou tracking code to fully support BDS-1 and BDS-2, two of the “three-step strategy” of the BeiDou systems (read more here), and adding missing flag for the Russian GLONASS.
Two papers were recently published focusing on the effect of surface elevation on terrestrial reflected GNSS signals; one is analyzing the effect of surface roughness as small-scale elevation changes on an air-borne GNSS-reflectometry platform, while the other is taking large-scale elevation changes into account and introducing an on-board constraint for DDMs produced by space-borne GNSS-R systems.
Joan, a Ph.D. candidate with Universitat Politècnica de Catalunya, Spain, working on GNSS-Reflectometry published his most recent paper on Feb 22, 2021, discussing their air-borne GNSS-R experience for retrieving soil moisture. The focus of this paper is solving non-linear reflectivity equation using an artificial neural network based algorithm instead of considering all effective variables and solving the functional model. The rationale behind this strategy is amazing; if any of the variables in the reflectivity model was estimated incorrectly, the soil moisture values retrievals could be dead wrong. For example, the impact of soil moisture on the reflectivity could be as high as ~17 dB, while the surface roughness could change the reflectivity by itself up to ~18 dB over a surface with only 4 cm RMSE in surface elevation changes. Even changes more than this value can be simply seen in farmlands. The effect of surface roughness on reflectivity can be seen in the Figure 9 of the paper.
Except for the surface roughness, other factors affecting the incoherent scattering, as the authors say, are those related to vegetation whose modelling with existing indices, e.g., VOD and NDVI, looks easier than that of the surface roughness effect. In addition, changes in the GPS integration time is an effective factor on the reflectivity, so that the higher the effective integration time is set, the less sensitive to surface roughness are the reflections. However, a bias in reflectivity in the along-track direction is introduced requiring a correction before the NDVI measurements. Moreover, the authors has used statistical metrics of the reflectivity itself, e.g. standard deviation, instead of too much ancillary data as a control for their machine learning approach, and addressed a question regarding the relationship between these statistics and the surface roughness.
The question that I would like to ask Joan and the team is about the relevance of averaging over multiple foot prints. In other words, if we conduct a multi-pass flight (rather than a single-pass one) and define the flight path so that we have multiple specular points over each specific known terrestrial point, is there anyway to cancel or reduce the impact of this small-scale topographies on the reflectivity? This suggestion may be applied in drone-based GNSS-R sensors with high maneuver abilities over farmlands. This idea has been tested and featured in summer 2020 and can be found here.
The second research, which was released on March 9, has been published by Lucinda, who is doing her Ph.D. with the University of Surrey, UK, and investigating the use of GNSS-R for Earth observations. Despite the first paper, this paper focuses on the effect of large-scale surface elevation changes on DDMs obtained from TDS-1 GNSS-R observations and their impact on soil moisture estimation. The authors mentioned that the Earth is approximated by a quasi-spherical model to predict specular points (SP) positions. Although this approximation works good for ocean scatterometry as ocean surface represents a quasi-sphere with small height anomalies smaller than 100 m, it would not always be sufficient for land sensing purposes as terrestrial topography impacts the SPs positioning accuracy. Looking at DDMs obtain from SPs with different altitude, one may see a movement in the peak-power location due to this inconsistency between the quasi-spherical model and real surface topography. An example is shown by the authors in the first figure of their paper.
The authors aimed to establish an on-board algorithm to capture signals reflected from any SP at any surface elevation, which is a huge advance comparing to similar recent studies addressing only limited special conditions. This algorithm is to be tested on a GNSS-R SSTL payload, called ToD-1. The key of the proposed algorithm is transforming SPs locations from a geodetic frame to a local geodetic frame, correcting the height element using a proper DEM, and reversely transforming the SPs coordinates to the initial geodetic frame. Results from tests operated on TDS-1 data were promising; the algorithm performed stunning at re-centering DDMs peak-powers.
Although this paper appears really inspiring for me, as I’m investigating some CYGNSS potential for land observation, a question cropped up regarding the peak-power movements among multiple DDMs assigned to one location but recorded at different times. To be more specific, a clear movement can be seen among DDMs obtained from the central area of Qinghai Lake, Tibet Plato, at different epochs. Three following figures are DDMs related to Oct 7, Oct 29, and Nov 14, 2020, respectively, obtained from CYGNSS micro-satellites; it’s obvious that there are movements along delay axis.
Surprisingly, on Feb 18, 2020, DDMs obtained from two different CYGNSS vehicle have shown different shifts in peak-power from the DDM centre.
Comparing these DDMs to each other, I wonder if there are any other effective factors, rather than topography, on peak-power shifts from/towards the DDM centre. The reason that I ask this question is that Qinghai Lake’s surface can be assumed flat, and in the presence of lake ice, the ice is not so thick to cause a significant topographical movement in the lake surface height.
An English aerospace company has kicked off a GNSS-R project to detect plastics in oceans.
Based in Harwell Village, UK, and as a subsidiary of a Spanish firm, Deimos Space UK Ltd introduced themselves as the pioneer in the use of GPS-Reflectometry to help us in coping with micro-plastic disasters in oceans by starting a one-year project, called GLIMPS. The idea, which is selected and funded by ESA, is applying bistatic radar to detect micro-plastics in the ocean as plastic particles reduce ocean roughness and increase the bistatic reflection towards GNSS antennas. The main goal of this project is to produce global maps for micro-plastics concentration in oceans using machine learning algorithms.
Although the idea looks fantastic, there are serious questions regarding its applicability as the GNSS-R scatterometry micro-satellites have only shown potential for mesoscale ocean eddies measurement so far (e.g. look this). How micro-plastics are going to increase the signal power reflected from ocean with relatively large crests and troughs? We are looking forward to seeing more updates from this new ESA-funded project submitted to Open Space Innovation Platform (OSIP).
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.