Enhanced Hydrofobicity of Polymers for Personal Protective Equipment Achieved by Chemical and Physical Modification

The article presents significant results in research on creating superhydrophobic properties of materials which can be used as an interesting material for use in self-cleaning polymer protective gloves and similar applications where the superhydrophobicity plays a significant role. In this work the influence of laser surface modification of MVQ silicone rubber was investigated. The research was conducted using a nanosecond-pulsed laser at 1060 nm wavelength. After a process of laser ablation, the surface condition was examined using a SEM microscope and infrared spectroscopy. During the tests, the contact angle was checked both before and after the laser modification of samples pre-geometrised in the process of their production. The test results presented in the paper indicate that the chemical and physical modifications contribute to the change in the MVQ silicone rubber contact angle. A significant increase (by more than 30°) in the contact angle to 138° was observed. It was confirmed that surface geometrisation is not the only factor contributing to an increase in the contact angle of the analyzed material; other factors include a change in laser texturing parameters, such as mean beam power, pulse duration, scanning speed and pulse repetition frequency.

Enhanced Pre-STAP Beamforming for Range Ambiguous Clutter Separation with Vertical FDA Radar

Medium pulse repetition frequency (MPRF) is an important mode in airborne radar system. Since MPRF mode brings both Doppler and range ambiguities, it causes difficulty for the airborne radar to suppress ground or sea clutter. In recent years, it has been pointed out that the frequency diverse array (FDA) radar is capable of separating the range ambiguous clutter, which is helpful for the airborne radar in detecting weak moving targets originally buried in ambiguous clutter. To further improve the ambiguous clutter separation performance, an enhanced pre-STAP beamforming for range ambiguous clutter suppression is proposed for the vertical FDA planar array in this paper. With consideration of range dependence of the vertical spatial frequency, a series of pre-STAP beamformers are designed using a priori knowledge of platform and radar parameters. The notches of the beamformers are aligned with the ambiguous clutter to extract echoes from desired range region while suppressing clutter from ambiguous range regions. The notches can be widened by using covariance matrix tapering technique and the proposed method can improve the performance of range ambiguous clutter separation with limited degrees-of-freedom (DOFs). Simulation examples show the effectiveness of the proposed method.

Convolutional Neural Networks for Classification of Drones Using Radars

The ability to classify drones using radar signals is a problem of great interest. In this paper, we apply convolutional neural networks (CNNs) to the Short-Time Fourier Transform (STFT) spectrograms of the simulated radar signals reflected from the drones. The drones vary in many ways that impact the STFT spectrograms, including blade length and blade rotation rates. Some of these physical parameters are captured in the Martin and Mulgrew model which was used to produce the datasets. We examine the data under X-band and W-band radar simulation scenarios and show that a CNN approach leads to an F1 score of 0.816±0.011 when trained on data with a signal-to-noise ratio (SNR) of 10 dB. The neural network which was trained on data from an X-band radar with 2 kHz pulse repetition frequency was shown to perform better than the CNN trained on the aforementioned W-band radar. It remained robust to the drone blade pitch and its performance varied directly in a linear fashion with the SNR.

Impact of second-trip echoes for space-borne high-pulse-repetition-frequency nadir-looking W-band cloud radars

The appearance of second-trip echoes generated by mirror images over the ocean and by multiple scattering tails in correspondence with deep convective cores has been investigated for space-borne nadir-looking W-band cloud radar observations. Examples extracted from the CloudSat radar are used to demonstrate the mechanisms of formation and to validate the modelling of such returns. A statistical analysis shows that, for CloudSat observations, second-trip echoes are rare and appear only above 20 km (thus easy to remove). CloudSat climatology is then used to estimate the occurrence of second-trip echoes in the different configurations envisaged for the operations of the EarthCARE radar, which will adopt pulse repetition frequencies much higher than the one used by the CloudSat radar in order to improve its Doppler capabilities. Our findings predict that the presence of such echoes in EarthCARE observations cannot be neglected: in particular, over the ocean, mirror images will tend to populate the EarthCARE sampling window with a maximum frequency at its upper boundary. This will create an additional fake cloud cover in the upper troposphere (of the order of 3 % at the top of the sampling window and steadily decreasing moving downwards), and, in much less frequent instances, it will cause an amplification of signals in areas where clouds are already present. Multiple scattering tails will also produce second-trip echoes but with much lower frequencies: less than 1 profile out of 1000 in the tropics and practically no effects at high latitudes. At the moment, level-2 algorithms of the EarthCARE radar do not account for such occurrences. We recommend to properly remove these second-trip echoes and to correct for reflectivity enhancements, where needed. More generally this work is relevant for the design of future space-borne Doppler W-band radar missions.

Range Dividing MIMO Waveform for Improving Tracking Performance

A multiple-input multiple-output (MIMO) method that shares the same frequency band can efficiently increase radar performance. An essential element of a MIMO radar is the orthogonality of the waveform. Typically, orthogonality is obtained by spreading different signals into divided domains such as in time-domain multiplexing, frequency-domain multiplexing, and code domain multiplexing. This paper proposes a method of spreading the interference signals outside the range bins of interest for pulse doppler radars. This is achieved by changing the pulse repetition frequency under certain constraints, and an additional gain can be obtained by doppler processing. This method is very effective for improving the angular accuracy of the MIMO radar for a small number of air targets, although it may have limitations in use for many targets or in high clutter environments.

Autocorrelation Measurement of Picosecond Pulses with Two-Photon Absorption

In this work, an autocorrelation measurement method is proposed to obtain the key information of picosecond pulses using the two-photon absorption (TPA) effect. The autocorrelation measurement process is simulated with a linear tuning of the pulse repetition frequency (PRF). Given the dispersion of picosecond pulses, the profile of the autocorrelation signal is broadened symmetrically. Moreover, the dispersive distribution in time-frequency domain of picosecond pulses and the different bandwidth of the TPA spectrum of materials should bring in sub pulses in the autocorrelation signal with the relative different delay. As shown in simulations, with an ideal broadband two-photon response spectrum, only the broadening of autocorrelation trace appears. But the detection with a narrowband two-photon response spectrum displays the greater sensitivity for pulse dispersion of the edge of the pulse, benefiting from the more sub pulses. Detections of picosecond pulses within the space wireless communication band region generally employ the photoconductive antenna and electro-optic effect in free space. However, with respect to the TPA effect in the specific materials, we could build an extremely compact autocorrelation measurement configuration for the key information extraction of picosecond pulses in space wireless communication and astronomical measurement, which would provide the same information as conventional detections about the autocorrelation signal of picosecond pulses.

Influence of the cathode region preionization on the operating parameters of the eptron

Investigations of the operating parameters of a plasma-cathode switch based on a capillary discharge in helium and neon in the burst mode are presented. An increase in the efficiency of the switch is demonstrated when an additional preionization pulse is applied at a low pulse repetition frequency (5 kHz). The compression ratio of voltage pulses more than 300 is achieved at a pulse repetition frequencies less than 40 kHz.

Developments of Space Debris Laser Ranging Technology Including the Applications of Picosecond Lasers

Debris laser ranging (DLR) is receiving considerable attention as an accurate and effective method of determining and predicting the orbits of space debris. This paper reports some technologies of DLR, such as the high pulse repetition frequency (PRF) laser pulse, large-aperture telescope, telescope array, multi-static stations receiving signals. DLR with a picosecond laser at the Shanghai Astronomical Observatory is also presented. A few hundred laps of space debris laser-ranging measurements have been made. A double-pulse picosecond laser with an average power of 4.2 W, a PRF of 1 kHz, and a wavelength of 532 nm has been implemented successfully in DLR, it’s the first time that DLR technology has reached a ranging precision at the sub-decimeter level. In addition, the characteristics of the picosecond-pulse-width laser transmission with the advantages of transmission in laser ranging were analyzed. With a mode of the pulse-burst picosecond laser having high average power, the DLR system has tracked small debris with a radar cross-section (RCS) of 0.91 m2 at a ranging distance up to 1726.8 km, corresponding to an RCS of 0.1 m2 at a distance of 1000 km. These works are expected to provide new technologies to further improve the performance of DLR.

Design and Processing Method for Doppler-Tolerant Stepped-Frequency Waveform Using Staggered PRF

Stepped-frequency waveform may be used to synthesize a wideband signal with several narrow-band pulses and achieve a high-resolution range profile without increasing the instantaneous bandwidth. Nevertheless, the conventional stepped-frequency waveform is Doppler sensitive, which greatly limits its application to moving targets. For this reason, this paper proposes a waveform design method using a staggered pulse repetition frequency to improve the Doppler tolerance effectively. First, a generalized echo model of the stepped-frequency waveform is constructed in order to analyze the Doppler sensitivity. Then, waveform design is carried out in the stepped-frequency waveform by using a staggered pulse repetition frequency so as to eliminate the high-order phase component that is caused by the target’s velocity. Further, the waveform design method is extended to the sparse stepped-frequency waveform, and we also propose corresponding methods for high-resolution range profile synthesis and motion compensation. Finally, experiments with electromagnetic data verify the high Doppler tolerance of the proposed waveform.