A Coherent on Receive X-Band Marine Radar for Ocean Observations

Marine radars are increasingly popular for monitoring meteorological and oceanographic parameters such as ocean surface wind, waves and currents as well as bathymetry and shorelines. Within this paper a coherent on receive marine radar is introduced, which is based on an incoherent off the shelf pulsed X-band radar. The main concept of the coherentization is based on the coherent on receive principle, where the coherence is achieved by measuring the phase of the transmitted pulse from a leak in the radar circulator, which then serves as a reference phase for the transmitted pulse. The Doppler shift frequency can be computed from two consecutive pulse-pairs in the time domain or from the first moment of the Doppler spectrum inferred by means of a short time Fast Fourier Transform. From the Doppler shift frequencies, radial speed maps of the backscatter of the ocean surface are retrieved. The resulting backscatter intensity and Doppler speed maps are presented for horizontal as well as vertical polarization, and discussed with respect to meteorological and oceanographic applications.

High harmonic generation in crystalline silicon irradiated by an intense ultrashort laser pulse

We investigate the high harmonic generation in bulk silicon irradiated by intense near-infrared laser pulses with pulse duration $$\le $$ ≤ 100 fs. For peak field strength of the applied laser is below 1 V/Å, the spectral intensity of the emitted harmonics follows the prediction of perturbative nonlinear optics—the frequency comb consists of a series of discrete peaks at odd harmonic orders. For a pulse duration longer than 30 fs and peak laser field strength exceeding 1 V/Å, non-perturbative effects and generation of even order harmonics occur. The appearance of even harmonics is due to optical rectification of the transmitted pulse, which includes weak quasi-DC component with electric field as low as 3 V/$$\upmu $$ μ m. In the strong coupling regime, when the peak field strength inside vacuum exceeds 1.5 V/Å, the laser creates dense breakdown plasma of electron–hole pairs, which in turn results in severe spectral broadening of the transmitted pulse. The harmonic spectrum superimposes onto a continuous background, the spectral width of individual harmonics is substantially broadened, and their central wavelength undergoes a blue shift that covers the spacing between adjacent harmonic orders. Graphic abstract

Chirplet signal design by FPGA

The ever-expanding growth of the electronics and communications industries present new challenges for researchers. One of these challenges is the generation of the required bandwidth signal over a specific time frame that is used in a variety of contexts, particularly radar systems. To improve the range resolution in the radar along with better SNR, it is necessary to reduce the signal bandwidth and increase the peak power. There are some restrictions for narrowband signals like power limitation, pulse shaping, and the production of unwanted harmonics. So as a solution pulse compression techniques are suggested. Pulse compression is a process that modulating the transmitted pulse to achieve a wideband signal and then at the receiver, the received signal correlates with the transmitted pulse to achieve narrowband representations of data. Chirp is the most common signal used in pulse compression. The chirp signal is produced using linear frequency modulation. In this study, we attempted to add an amplitude modulation to the chirp signal and evaluate its performance by implementation on FPGA. The outcome signal is called chirplet and simulation will show that it enhance target detection and image quality in imaging radars like SAR.

Revisiting the Optimum Receiver Filter Bandwidth for Range-Oversampling Processing

For weather radars, range-oversampling processing was proposed as an effective way either to reduce the variance of radar-variable estimates without increasing scan times or to reduce scan times without increasing the variance of estimates. Range oversampling entails acquiring the received signals at a rate L times as fast as the reciprocal of the pulse width (the conventional rate), where L is referred to as the range-oversampling factor. To accommodate the L-times-as-fast sampling, the original formulation of range-oversampling processing required a receiver filter with a bandwidth L times as wide as that of the matched filter (the conventional receiver filter). In this case, the noise at the output of the receiver filter can still be assumed to be white, resulting in a simplified formulation of the technique but also, and more important, in a more difficult practical implementation since the receiver filter in operational weather radars is typically matched to the transmitted pulse. In this work, we revisit the role of the receiver filter in the performance of range-oversampling processing and show that using a receiver matched filter not only facilitates the implementation of range-oversampling processing but also results in the lowest variance of radar-variable estimates.

Research on a Wind Lidar Without Blind Zone Based on the Technologies of Pseudo Random Code Phase Modulation and Heterodyne Detection

In this measurement method, a wind lidar system with no blind zone based on the technologies of heterodyne detection and pseudorandom code phase modulation is proposed. The simulation results show the feasibility of the method. 15m range resolution and 7.75cm/s line of sight (LOS) wind velocity resolution can be achieved from 0 m to 300m, when the laser transmitted power is 2mW, the transmitted pulse length is 100us, the receiving telescope aperture is 2cm, and an accumulation times of 10. The simulation results also show the positive effect of the transmitted pulse length.

Non-Reciprocal Wave Transmission in a Bilinear Spring-Mass System

Significant amplitude-independent and passive non-reciprocal wave motion can be achieved in a one-dimensional (1D) discrete chain of masses and springs with bilinear elastic stiffness. Some fundamental asymmetric spatial modulations of the bilinear spring stiffness are first examined for their non-reciprocal properties. These are combined as building blocks into more complex configurations with the objective of maximizing non-reciprocal wave behavior. The non-reciprocal property is demonstrated by the significant difference between the transmitted pulse displacement amplitudes and energies for incidence from opposite directions. Extreme non-reciprocity is realized when almost-zero transmission is achieved for the propagation from one direction with a noticeable transmitted pulse for incidence from the other. These models provide the basis for a class of simple 1D non-reciprocal designs and can serve as the building blocks for more complex and higher dimensional non-reciprocal wave systems.

Optimized Parameter Settings of Binary Bat Algorithm for Solving Function Optimization Problems

The bat algorithm (BA) is a new bionic intelligent optimization algorithm to simulate the foraging behavior and the echolocation principle of the bats. The parameter initialization of the discussed binary bat algorithm (BBA) has important influence on the convergence speed, convergence precision, and good global searching ability of the BBA. The convergence speed and algorithm searching precision are determined by the pulse of loudness and pulse rate. The simulation experiments are carried out by using the six typical test functions to discuss this influence. The simulation results show that the convergence speed of the BBA is relatively sensitive to the setting of the algorithm parameters. The convergence precision reduces when increasing the rate of bat transmitted pulse alone and the convergence speed increases the launch loudness alone. The proper combination of BBA parameters (the rate of bat transmitted pulse and the launch loudness) can flexibly improve the algorithm’s convergence velocity and improve the accuracy of the searched solutions.