CYGNSS and Spatial Interpolation; a Review

“Why talk to others when you can talk to yourself?” This is how Clara Chew captioned her fantastic YouTube video in LinkedIn to start suggesting a spatial interpolation method for CYGNSS data.

First of all, I should say what a great idea form one of my most inspiring and favorite researchers. Establishing a YouTube channel to talk to ourselves about GNSS reflectometry is what we’d need, and to be honest, that’s the main encouragement for myself to kick off this website. Although I am currently the only writer here, but this platform is going to become an online magazine for GNSS-R researchers to freely read and write others’ opinions. It’s yet to be widely advertised, but the core idea is the same: let’s just talk to ourselves : )

About the video, let’s start from the end; this method is not going to be used for every application. “If you are trying to do some sort of analysis where you need exact knowledge of reflectivity at a certain point,” Clara clarifies “you might not want to use this method”. It sounds very true for my current research focusing on Qinghai Lake, Tibet Plateau, where I restricted Fresnel Zones to only the central region of the lake to make sure that I am not receiving reflections from the surrounding lands or even the shore lines. So, no interpolation is required at any level. But aside from the Qinghai’s research, it’s been a few months that I’ve been thinking about expanding my CYGNSS explorations to a wider region, say central desert of Iran, and meanwhile, one of my greatest concerns has been how to do data interpolation over a large area. Clara has also mentioned it somewhere in her video that “trust me, it won’t be too frustrating”, but it is for me :)) because I haven’t started yet.

But when I’m saying my concern is on how to do it, I wouldn’t say that choosing the interpolation method is not a big concern, as it really is, but it is not the biggest issue for me, because I had practiced a method years ago. The story took place in 2013-2015, when I was working on geodynamics of the Earth, and my specific research at that time was calculating strain tensors. I found out that routine methods for interpolating crustal movement values, e.g. IDW and Spline, are way off the road. I spent couple of months on that, and realized that “deterministic interpolation methods” do not perfectly work for those problems which require “geo-statistical interpolation methods”, such as Kriging. That was the point.

In short, despite deterministic methods which only consider distance as the effective parameter in weighting procedure, geo-statistical interpolation methods also take the correlation into account as a weighting function. This is exactly what Clara suggested: “correlation coefficients“. In Kriging, for example, a covariance matrix is created among node points based on statistical constraints, and then, that matrix plays a key role in establishing the weighting function. The same idea can be seen in another powerful interpolation method called Least-Square Collocation, which is widely used in geoid problems and potential field anomaly interpolations.

I employed Ordinary and Universal Kriging interpolation techniques for estimating strain matrix components, and improved the accuracy up to 70%. As Clara has remained the discussion open at the end of her video, I would invite her to a collaboration in order to explore the ability of geo-statistical methods of interpolation in CYGNSS cases.

Animated Video: GPS-Reflectometry for Soil Moisture Monitoring

To clarify a subject to non-specialists, creating a video is a much more efficient way compared to giving lectures, preparing graphs, and even writing plain language articles.

Gradflix is a competition held each year at the University of Waterloo encouraging grad students to create 1-minute videos to explain their research topics. Then, based on a set of predefined criteria, three videos are chosen as the winners.

I participated last year (2020) and made a very simple video to explain the very basic idea of GPS-Reflectometry. Surprisingly, I was chosen as a finalist. Although I did not place among the first three winners, it was a great inspiration for me to walk on the red carpet as a finalist.

This year (2021), I spent more time and created a more professional video to explain our research on the GNSS-Reflectometry capability of soil water content monitoring. After a month, I was waiting for another confirmation email, like the one that I received last year, saying that I have been chosen again as a finalist for this year, but I received a rejection email saying “although your video is an excellent number one fantastic phenomenon piece of art work, BUT your video was not qualified to be chosen among finalists.” Such a retrogression :)) I can’t deny that I got disappointed when I received that rejection email, but when I got back to my video and showed it to a couple of animation specialists, they confirmed that it was just a bit better than awful =))

By the way, I would say that this video could still be beneficial for those who want to get more familiar with daily applications of GNSS-Reflectometry.