KAPRI - the Polarimetric and Interferometric Bistatic Real- Aperture Ku-Band Radar for Environmental Information Retrieval
Creators: name: Mas I Sanz, Esther
name: Štefko, Marcel
name: Hajnsek, Irena
Publisher: name: ETH Zurich
Year: 2023

description: Radar observations that combine polarimetry and interferometry allow an enhanced characterisation of the natural environment and its processes. Fully polarimetric imaging provides data on scattering mechanisms present in the resolution cell and thus on the structure of the natural medium. Furthermore, differential interferometry allows the monitoring of displacements which occur on the line-of-sight component, focusing on changes caused by movements of the natural environment.

Another key factor in radar measurements for characterization of natural processes is the revisit time: monitoring some of the Earth’s dynamic processes requires dense temporal sampling and long time series durations. For this reason, ground-based radar systems are a suitable choice for applications with high temporal sampling requirements as they can provide repetition times on the order of minutes and virtually unlimited time series [1]. In contrast, current spaceborne synthetic aperture radar (SAR) Earth Observation systems cover large areas in a single pass but their revisit time is, at best, within the region of several hours, making them unsuitable.

KAPRI is a ground-based portable, frequency-modulated continuous-wave (FMCW) , Ku-band polarimetric radar interferometer. It is able to acquire both monostatic and bistatic full-polarimetric measurements with high spatial and temporal resolution. The bistatic configuration makes use of two independent KAPRI devices, one operating as the primary transmitter/receiver and the other as the secondary receiver. A new processing pipeline as well as a calibration method were developed to address these differences between bistatic and monostatic configurations [2].

The bistatic configuration is of special interest as less extensive research of its applications in the context of natural processes has been done if compared to the monostatic case, in part, due to the added challenges coming from separating the transmitter and the receiver [3].

KAPRI’s bistatic regime sets the scene for new investigations given that had not been possible by employing the monostatic mode, such as the measurement of 3-D deformation vector fields or the investigation of possible presence of unique scattering events exclusively detectable in bistatic configurations. These bistatic capabilities of KAPRI were recently leveraged for the first characterization of the coherent backscatter opposition effect (CBOE) in dry snow using a terrestrial bistatic radar system [4].

A better understanding of bistatic radar measurements, which can be efficiently provided by ground-based systems, lays the foundations for future SAR space missions and pushes forward the present knowledge on snow landscape areas and their role on climate.

[1] 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

[2] Stefko, M., Frey, O., Werner, C., & Hajnsek, I. (2022). 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

[3] Moccia, G. Salzillo, M. D’Errico, G. Rufino, and G. Alberti, “Performance of spaceborne bistatic synthetic aperture radar,” IEEETrans. Aerosp. Electron. Syst., vol. 41, no. 4, pp. 1383–1395, Oct. 2005

[4] Stefko, M., Leinss, S., Frey, O., & Hajnsek, I. (2022). 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