DEM generation and cryospheric investigations with KAPRI, a ground-based fully polarimetric interferometric radar
Creators: name: Mas I Sanz, Esther
name: Štefko, Marcel
name: Hajnsek, Irena
Publisher: name: ETH Zurich
Year: 2023

description: Radar remote sensing is a commonly used technique for Earth Observation and studying the natural environment. Mainly, two techniques are used to extract the information contained in the radar signal: polarimetry and interferometry. Polarimetry provides information on the different scattering mechanisms that take place within the resolution cell and describe how the signal interacts with the natural medium. On the other hand, interferometry exploits the information resulting from the phase differences between two signals. This might be the phase differences from two acquisitions retrieving data at the same time slightly from different observation angles (single-pass interferometry) or the phase difference between observations across time (differential interferometry). In the first case, the resulting phase can be converted to height and is commonly used for digital elevation model (DEM) generation. The latter allows to monitor displacements in the direction of the line-of-sight (LOS) of the radar, proving to be specially useful when targeting dynamic processes in nature which involve the motion of the terrain, such as soil settlements, glacier displacements, thermal dilation, etc.

Radar systems are extremely versatile and can be installed in a wide range of platforms, varying from ground-based sensors to air and space-borne ones. While radar sensors used in satellites offer vast coverage swaths, they are only capable of retrieving new information in a few days span, at best. In case of aircraft-based sensors, revisit times can be shortened to tens of minutes but still the time series span is limited. In contrast, ground-based systems offer the advantage of high-density repetition times for prolonged durations, hence, providing extended time series while reducing time decorrelation. It must be noted, at the expense of reduced coverage. This is particularly useful when observing natural phenomena within a limited spatial extent that presents variations in a matter of hours or even minutes, for instance, in glacier monitoring, the focus of our current investigations. A common approach in glacier monitoring is obtaining DEMs at different seasons and comparing the height differences to assess the volume loss of the glacier (Gardelle 2012). However, variations in the snow properties resulting from, for example, a snowfall or an increase in water content due to a temperature rise, have a direct impact on the received radar signal changing its penetration depth and eventually yielding different heights on the DEMs (Guneriussen 2001). How can  these variations be accounted when generating DEMs? Furthermore, based on these alterations of the interferometric phase, is it possible to retrieve the properties of the snowpack by combining with other techniques such as polarimetry and tomography?

To address these questions, KAPRI (Ku-band Advanced Polariemtric Radar Interferometer), a ground-based  portable, frequency-modulated continuous-wave (FMCW) radar is used. This radar is based originally on the GPRI (Gamma Portable Radar Interferometer) and modified to be able to acquire fully-polarimetric measurements (Baffelli, 2018). Moreover, the system was further upgraded together with the development of a new processing pipeline to operate in bistatic mode by making use of two independent KAPRI devices, one functioning as the primary transmitter/receiver and the other as the secondary receiver (Stefko, 2022a). KAPRI’s bistatic capabilities were recently applied to characterize the coherent backscatter opposition effect (CBOE) in dry snow (Stefko, 2022b). 

In this poster, preliminary results of the DEMs from the monostatic data at two different seasons (summer of 2021 and winter of 2022) as well as bistatic acquisitions are presented. The current research with KAPRI focuses on DEM generation at the Aletsch Glacier (Switzerland) using the seasonal monostatic data. The aim is combining the monostatic data with the bistatic acquisitions in order to reconstruct 3D deformation vectors that describe the glacier flow. Moreover, the data of these campaigns (in addition to future ones) constitutes a seasonal dataset that will be further investigated to characterise snow parameters by exploring scattering phenomena in the bistatic configuration featured on the polarisation matrix as non-symmetries.



REFERENCES 

Baffelli, S., Frey, O., Werner, C., & Hajnsek, I. (2018). Polarimetric calibration of the ku-band advanced polarimetric radar interferometer. IEEE Transactions on Geoscience and Remote Sensing, 56(4), 2295–2311. https://doi.org/10.1109/TGRS.2017.2778049

Gardelle, J., Berthier, E., & Arnaud, Y. (2012). Impact of resolution and radar penetration on glacier elevation changes computed from DEM differencing. Journal of Glaciology, 58(208), 419–422. https://doi.org/10.3189/2012JOG11J175

Guneriussen, T., HɈgda, K. A., Johnsen, H., & Lauknes, I. (2001). InSAR for estimation of changes in snow water equivalent of dry snow. IEEE Transactions on Geoscience and Remote Sensing, 39(10), 2101–2108. https://doi.org/10.1109/36.957273

Stefko, M., Frey, O., Werner, C., & Hajnsek, I. (2022a). Calibration and Operation of a Bistatic Real-Aperture Polarimetric-Interferometric Ku-Band Radar. IEEE Transactions on Geoscience and Remote Sensing, 60, 1–19. https://doi.org/10.1109/TGRS.2021.3121466

Stefko, M., Leinss, S., Frey, O., & Hajnsek, I. (2022b). Coherent backscatter enhancement in bistatic Ku- and X-band radar observations of dry snow. Cryosphere, 16(7), 2859–2879. https://doi.org/10.5194/tc-16-2859-2022