Differential interferometry is a powerful remote sensing technique that allows radar systems to capture very small displacements from a far observational position. Most systems are monostatic, meaning that they can only capture displacement changes in the line-of-sight (LOS). The simplicity of monostatic systems also restricts their suitability to either displacements with a single principal motion direction or with a known direction in space. Adding a passive receiver (bistatic systems) provides an additional LOS of information, allows to reconstruct more complex vector fields and reduces the assumptions on the displacement direction. This study presents a set of differential interferometric observations retrieved in Aletsch glacier during a winter campaign in March 2022, using KAPRI, a Ku-Band a ground-based radar interferometer. We investigate the glacier motion based on the datasets corresponding to each of KAPRI’s configurations and propose two independent approaches to solve the displacement field: monostatic (1 LOS) and monostatic-bistatic (2 LOS). As the monostatic and bistatic datasets were retrieved simultaneously, it is possible to do a direct comparison of the resulting displacements and of the respective uncertainties. Results show that the accumulated displacement for the 09:01:30 h to 10:13:30 h at the near-range region (ROI 1) is of 3.83mm in the monostatic case and 4.42mm in the monostatic-bistatic case. In the far-range region (ROI 3) the monostatic displacements result in 8.54mm while according to the bistatic approach 17.82 mm. Assumptions used for the monostatic resolution of the glacier displacements hold for the flatter areas corresponding to the main glacier flow but fail in the steep terrain. Monostatic acquisitions present high SNR which maintains a low uncertainty (below 0.5mm for all ROIs) throughout the scene compared to the bistatic dataset. Monostatic-bistatic derived displacements are susceptible to the increased noise in the far range, but yield a more robust (direction and magnitude) and smoother displacement field in the near-mid range. These comparisons provide valuable insights on the advantages and shortcomings of employing monostatic and bistatic configurations for 3D displacement retrieval and serve as proxy for upcoming satellite missions which plan to use bistatic radar configurations.

We present two novel experimental setups for automated measurement of coherent backscatter enhancement (also known as the coherent backscatter opposition effect – CBOE) of Ku-band radio waves in seasonal snow cover. The experimental setups were developed in order to enable fast and repeatable measurements of the effect’s properties, through quickly altering the bistatic baseline of the radar system between zero and eight meters. The experimental setups were tested in December 2023 at the High Altitude Research Station Jungfraujoch in Switzerland, yielding a novel dataset exploring the properties of the early-winter snow cover on top of the Great Aletsch Glacier, as well as allowing a comparison of the benefits and drawbacks of the two experimental setups. We present preliminary results of these experiments, as well as discuss the potential of such measurements for improving our understanding of radio wave scattering in snow.

We present the results of recent observations of terrestrial snow cover with KAPRI, a Ku-band polarimetric-interferometric radar system with bistatic capabilities. The bistatic configuration allows observation of phenomena which are not observable in the monostatic regime. Observations of polarization phase differences of snow cover on top of the Great Aletsch Glacier revealed that the behaviour of these parameters markedly varies between different times of year, as well as between the monostatic and bistatic acquisition modes. The bistatic capabilities of KAPRI were also recently used to characterize the coherent backscatter opposition effect (CBOE) in seasonal snow. We discuss the potential of such bistatic observations to observe quantities which are unobservable in the monostatic regime, and which could serve as new inputs for snow parameter retrieval methods.

Synthetic aperture radar (SAR) is an advanced measurement technique, which is often the subject of discussions between remote sensing experts, as well as other specialists and members of the general public. These discussions can however be hindered by misunderstandings caused by the rather complex principles of SAR acquisitions. We present the implementation of SAR2, an augmented-reality app for the Microsoft HoloLens, which aims to enhance understanding of synthetic aperture radar through visualization of the SAR acquisition procedure. Its distinguishing feature is real-time simulation of the radar echo, combined with real-life SAR processing algorithms, which ensure verisimilitude and flexibility of the visualization. The virtual environment allows users to directly interact with the satellite and the observed scene, modify acquisition parameters, and visualize data flows during a simulated SAR acquisition. The ultimate aim of development of this app is to serve as an educational outreach tool for experts, educators, and the public alike.

The Ku-band provides opportunities for investigations of snow morphology through radar observations, since it exhibits a relatively high amount of scattering even from snow layers of limited depth, while maintaining low absorption. Due to technological and practical challenges, the bistatic parameter space of Ku-band radar observations of natural media such as snow, has been relatively unexplored. We present radar measurements of snow cover obtained with KAPRI, a bistatic polarimetric Ku-band radar system. In August 2021 and March 2022, we carried out time series observations of the Aletsch glacier in the Swiss Alps, acquiring a fully-polarimetric interferometric time series of both monostatic and simultaneous bistatic observations of the glacier's accumulation zone. This dataset will serve as a test-bed to investigate new snow parameter inversion methods based on bistatic Ku-band radar data. The bistatic polarimetric measurement configuration, as well as preliminary results of the analysis of radar backscatter, are presented.

This article presents the bistatic operation mode and the performance analysis of KAPRI, a terrestrial frequency-modulated continuous-wave (FMCW) Ku-band polarimetric radar interferometer capable of acquiring bistatic full-polarimetric datasets with high spatial and temporal resolution. In the bistatic configuration, the system is composed of two independently operating KAPRI devices, one serving as a primary transmitter and receiver and the other as a secondary receiver. The secondary bistatic dataset is affected by possible offsets between the two devices’ reference clocks, as well as distortions arising from the bistatic geometry. To correct for this, we present a two-chirp bistatic FMCW signal model, which accounts for the distortions, and a reference chirp transmission procedure, which allows correcting the clock offsets in the deramped signal time domain. The second challenge of operation of a bistatic polarimetric system is polarimetric calibration since it is not possible to employ purely monostatic targets such as corner reflectors. For this purpose, we developed a novel active calibration device Variable-Signature Polarimetric Active Radar Calibrator (VSPARC), designed for monostatic and bistatic calibration of all polarimetric channels. VSPARC and its associated novel polarimetric calibration method were then used to achieve full calibration of both KAPRI devices with polarimetric phase calibration accuracy of 20° and 30-dB polarization purity in field conditions. This article thus presents a complete measurement configuration and data processing pipeline necessary for synchronization, coregistration, and polarimetric calibration of bistatic and monostatic datasets acquired by a real-aperture FMCW radar.

In this submission the bistatic operation mode of KAPRI, a polarimetric-interferometric, ground-based, real-aperture, Ku-band, FMCW radar is presented. Its possible configurations and the synchronization procedures needed to achieve distortion-free single-look complex bistatic images are described. The challenges of polarimetric calibration in the bistatic regime are outlined, and a novel active calibration device and the associated polarimetric calibration method which are used for bistatic calibration of KAPRI are presented. First results of observations of natural environments (vegetation, snow) using bistatic KAPRI are shown.