Full Polarimetric Radar Research Articles

Discrimination Between Dry and Water Ices by Full Polarimetric Radar: Implications for China’s First Martian Exploration

China’s first Mars rover named “Zhurong” has begun its exploration at the surface of the Utopian Plain in the Martian northern hemisphere on May 22, 2021. Mars rover penetrating radar (RoPeR) is one of the key payloads onboard the Zhurong rover and one of its prima scientific objectives is to detect potential water ice and/or dry ice in the subsurface soil at the landing site. The high-frequency channel of RoPeR, which is equipped with a fully-polarized antenna array with a center frequency of 1.3 GHz, can record full polarimetric radar reflections from subsurface anomalies. In this article, a radar system with a polarimetric antenna array the same as RoPeR was set up and carefully calibrated. Laboratory experiments were carried out to test the feasibility of RoPeR in the detection and discrimination of potential dry ice and/or water ice in Martian soils. The experimental results indicate that the reflection signals from the bottom of the dry ice and water ice samples present different polarimetric scattering characterizations. The characteristic polarimetric scattering features of dry ice and water ice revealed in this article would guide the analysis and interpretation of the real data acquired by RoPeR on Mars.

Jensen–Shannon Distance-Based Filter and Unsupervised Evaluation Metrics for Polarimetric Weather Radar Processing

An effective filtering technique is required for rainfall rate measurement by weather radar. A Jensen–Shannon Distance (JSD) based thresholding filter is proposed to mitigate non-meteorological signals, either in clear air or rain situations. This algorithm classifies range-Doppler bins into two classes, hydrometeors and non-hydrometeors, based on spectral polarimetric variable features. The result is a mask to be applied on the spectrograms. The variable selected here is the spectral co-polar correlation coefficient, available in dual-polarization and full polarimetric radars. The algorithm first does a global thresholding by finding an optimized threshold value based on the averaged clear air spectral polarimetric variable distribution. Next, classical filtering steps are carried out like a ground clutter notch filter around 0 ms^-1, a mathematical morphology to fill gaps in the hydrometeor areas and a removal of narrow Doppler power spectra. The second part of this article is the assessment of filtering techniques without ground truth. An assessment without ground-truth is useful to select optimal algorithm configurations from a large solution space. Criteria of good filtering are defined both in the spectral and time domain. Based on those criteria, subjective and objective unsupervised evaluation metrics are derived, with a focus on the objective ones. Data including clear air and rain collected from a full polarimetric Doppler X-band radar in urban area are used. With the proposed unsupervised evaluation metrics, the JSD-based thresholding filter is compared to two spectral polarimetric filters. Overall, the JSD-based filter performs very well considering both the subjective and the objective evaluation metrics.

Clutter Mitigation Based on Spectral Depolarization Ratio for Dual-Polarization Weather Radars

In this article, a spectral polarimetric filter named moving spectral depolarization ratio (MsDR) filter is proposed to mitigate the effects of clutter and noise for dual-polarization weather radar without cross-polar measurements. The filter is based on a spectral polarimetric variable termed as spectral depolarization ratio (sDR). sDR can be estimated by any polarimetric weather radar, making the MsDR filter widely applicable. Taking advantage of the spectral polarimetric features and the range-Doppler continuity of precipitation, the MsDR filter is implemented in the range-Doppler spectrogram. The performance of the MsDR filter is assessed using data collected by 1) an X-band full-polarimetric radar interfered by narrow-band clutter (both stationary and moving) and sidelobe wind turbine clutter; 2) a C-band operational dual-polarization radar affected by radio frequency interference. In addition, the implementation of the MsDR filter is straightforward, and the computational complexity is relatively low. Hence, the MsDR filter has great potential to be applied in real-time for operational radar systems at different frequencies.

Optical Scale Polarimetric Device for Nanotube Forest Measurement: An Opportunity to Anticipate Bistatic Polarimetric SAR Images of Tree Trunk Forests at P-Band

This paper investigates a new polarimetric device that produces reduced scale measurements for bistatic radar. The optical scale device is proposed to overcome the lack of real bistatic and full polarimetric radar acquisitions for forest of trunks. It is based on the scale invariant rule that is deduced from the Maxwell equations. From this principle, the electromagnetic (EM) response is kept at the reduced scale if the illumination wavelength is scaled and if the permittivity remains the same. This last condition needs the use of a carefully chosen material at optical scale. Nanoscale scenes are composed of carbon nanotubes (CNTs). They present several advantages: their arrangement is similar to the structure of a forest of trunks, their shape, and density can be controlled, it is possible to create very large scenes composed of thousands of elements, and finally their permittivity at optical wavelength is about the same order of magnitude than the permittivity of the tree trunks at radar wavelength. They are measured under the view of a microscope objective with a 633-nm wavelength laser playing the role of the EM source. The device images at once the forest polarimetric response, for a given transmitter location, and for the entire set of angular positions of reception. Two samples of forests with different densities are measured. They are analyzed using the Cloude and Pottier decomposition and the Lu and Chipman decomposition. The produced results are unprecedented on forest-like environment and it would be very helpful for the radar community.

Three-Baseline InSAR Estimation of Forest Height

In this letter we propose a three-baseline approach to the extraction of forest tree height from synthetic aperture radar data. Three polarimetric–interferometric pairs are used to constrain a physical model that relates forest parameters to multiple repeat-pass coherence observations. The observations may be performed by a dual, compact or full polarimetric radar, and may be affected by distinct levels of temporal decorrelation. Here, we present the theoretical framework based on the random-motion-over-ground model, and describe an algorithm to extract tree height from the data. The performance of the algorithm is illustrated with L-band airborne data collected by the German Aerospace Center in the frame of the BIOSAR2008 campaign. The proposed method provides height estimates in good agreement with lidar measurements and can be applied to data to be collected by forthcoming polarimetric–interferometric spaceborne missions.

Suppression of Cross-Channel Interference Based on the Fractional Fourier Transform in Polarimetric Radar

A new cross-channel interference suppression method based on the fractional Fourier transform (FRFT) is proposed to eliminate the effect due to cross-channel interference when a full-polarimetric radar adopts the opposite-slope linear frequency modulation transmit signals and de-ramping processing in the simultaneous measurement mode. The FRFT with an appropriate order is operated on the signals in the cross-interference interval. Such order is obtained via the calculation of the frequency-modulation slope of transmitted signals. The cross-channel interference is changed into sinc functions that are detected by a designed threshold and filtered out in the FRFT domain to suppress the cross-channel interference. The simulations show that the proposed method can obviously reduce the peak sidelobe levels and the integrated sidelobe levels of targets in range profiles compared with the traditional method, which is benefit to improve the detection and measurement performances of weak polarimetric scattering components. The proposed method is suitable for stable or slow-moving targets.

Reconfigurable digital receiver design and application for instantaneous polarimetric measurement

This paper presents the development of a reconfigurable receiver to undertake challenging signal processing tasks for a novel polarimetric radar system. The field-programmable gate arrays (FPGAs)-based digital receiver samples incoming signals at intermediate frequency (IF) and processes signals digitally instead of using conventional analog approaches. It offers more robust system stability and avoids unnecessary multichannel calibrations of analog circuits for a full polarimetric radar. Two kinds of dual-orthogonal signals together with corresponding processing algorithms have been investigated; the digital implementation architectures for all algorithms are then presented. Processing algorithms implemented in FPGA chips can be reconfigured adaptively regarding to different transmitted waveforms without modification of hardware. The successful development of such reconfigurable receiver extends our radar capacity and thus yields tremendous experimental flexibility for atmospheric remote sensing and polarimetric studies of ground-based targets.

Power Optimization for Polarimetric Bistatic Random Mechanisms

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The aim of this paper is to investigate the optimal polarizations (i.e., emitted and received polarizations for which the received power is maximum or minimum) in the general case of random mechanisms observed by a bistatic full-polarimetric radar. We are particularly interested in the methods leading to analytical solution as far as this is possible. It is the reason why the Kennaugh matrix is taken as the starting point of the optimizations. The optimal polarizations corresponding to the monostatic and bistatic deterministic cases are found again as particular cases. </para>

Calibration of the MERIC full-polarimetric radar: theory and implementation

This paper describes the architecture and calibration design of the experimental ground based radar station MERIC. This full-polarimetric radar is conceived for the analysis and the recognition of non cooperative aircraft in flight. We carefully study how the full-polarimetric capability is obtained for a simultaneous transmit (simultaneous transmission of two linear FM with opposite slopes) radar system, using analogue deramping with a replica. The phase distortions of the signal propagating in the four polarimetric channels are carefully estimated. We define a phase calibrating method compatible with the outdoor measurements conditions and few constraints on reference targets. We show the phase accuracy obtained with the proposed calibration method on real measurements.

Real-time and full polarimetric FM-CW radar and its application to the classification of targets

The Sinclair scattering matrix plays a decisive role in radar polarimetry. For realization of a real-time and full polarimetric radar, there are some technical problems associated with hardware implementations. Since the FM-CW radar system is suitable for short-range sensing applications, we modified an FW-CW radar to a real-time and polarimetric system by adding a PIN diode-switching circuitry for antenna polarization changes. A parallel operation scheme in the signal processing and the radar control is employed in a personal computer, which enables 44 snapshots for acquiring scattering matrices and displays the polarimetric target classification results along 43 radar ranges per second. In this paper, we present the hardware design of the system and its fundamental application to the classification of targets using the three-component decomposition theorem.