Showing papers on "Pulse repetition frequency published in 2019"
TL;DR: In this article, a single-conversion stepped-frequency continuous-wave (SCSFCW) radar that combines a SFCW radar and a self-injection-locked (SIL) radar to benefit from the range resolution and the Doppler sensitivity of the two radars was proposed.
Abstract: This article proposes a single-conversion stepped-frequency continuous-wave (SCSFCW) radar that combines a stepped-frequency continuous-wave (SFCW) radar and a self-injection-locked (SIL) radar to benefit from the range resolution and the Doppler sensitivity of the two radars. An 8.5–9.5-GHz prototype SCSFCW radar system that comprises a subharmonic up/down converter with a 3–3.5-GHz stepped chirp local-oscillator (LO) signal and a 2.5-GHz SIL IF signal was developed to monitor the vital signs, i.e., respiration rate (RR) and heart rate (HR), of multiple humans. The coherence and range of the developed system were significantly enhanced by using a low pulse repetition frequency (PRF). In the experiment, the minimum distinguishable radial spacing between the vibrating frequencies of the metal plates that are not azimuthally overlapped with one another corresponds to a theoretical range resolution of 15 cm. However, owing to scattering by the human body, the minimum radial spacing for distinguishing between the vital signs of the individuals is three times than that for distinguishing between the metal plates in a similar experimental setup. Accordingly, the monitoring of up to three human vital signs using the developed system was demonstrated with a range-vital-Doppler map.
58 citations
TL;DR: Channel configuration design method based on differential evolution and feasibility rule is proposed to solve the CSOP and obtain the channel configuration for the receiver with the optimal reconstruction performance, and characteristics of channel configuration can be a practical guide for the implementation of multichannel GEO-BiSAR systems.
Abstract: In geosynchronous spaceborne–airborne bistatic synthetic aperture radar (GEO-BiSAR) system, the airborne platform achieves high-resolution imaging by passively receiving the signal from the interested scenario. In this paper, the Doppler characteristics of GEO-BiSAR and the individual contribution of the transmitter and the receiver are first analyzed. The airborne receiver is found to be the dominant contributor for the total Doppler bandwidth, which will lead to Doppler spectrum aliasing regarding the low pulse repetition frequency (PRF) adopted by the GEO-SAR. In order to suppress the Doppler ambiguity without adjusting the PRF of GEO-SAR, azimuth multichannel receiving technique is introduced to the airborne receiver. The multichannel transfer function is derived based on the method of series reversion and the spectrum reconstruction algorithm is then modified for multichannel GEO-BiSAR. Moreover, the reconstruction performance is closely related to the corresponding spacing between each channel (i.e., channel configuration). Therefore, the channel configuration design for GEO-BiSAR aims at optimizing the azimuth ambiguity-to-signal ratio with a satisfactory level of signal-to-noise ratio scaling factor by adjusting the channel configuration. The channel configuration design is modeled as a constrained single objective optimization problem (CSOP). Then, a channel configuration design method based on differential evolution and feasibility rule is proposed to solve the CSOP and obtain the channel configuration for the receiver with the optimal reconstruction performance. Finally, simulations results are presented to verify the effectiveness of the proposed method, and characteristics of channel configuration are analyzed in detail, which can be a practical guide for the implementation of multichannel GEO-BiSAR systems.
52 citations
TL;DR: With the proposed two-step nonlinear chirp scaling (NLCS) method, the well-focused image for GEO BiSAR is obtained and the required number of receiving channels can be decreased, which is beneficial to reducing the system complexity and hardware cost.
Abstract: Due to the high-altitude illumination and the separation of the receiver and transmitter, geosynchronous (GEO) spaceborne–airborne bistatic synthetic aperture radar (BiSAR) is more flexible and accessible in remote sensing applications. In this paper, the Doppler characteristics of GEO BiSAR with a squint receiver are analyzed. It is found that the Doppler spectrum is generally aliased in GEO BiSAR regarding the low pulse repetition frequency (PRF) adopted by the GEO SAR. In order to suppress the ambiguity without adjusting the PRF of GEO SAR, the azimuth multichannel receiving technique is applied to the receiver and then the multichannel transfer function for GEO BiSAR is derived. However, the whole bandwidth of the imaging scene is much larger than that of the center point, which requires extra receiving channels to suppress the ambiguity and thereby increasing the system complexity. A two-step nonlinear chirp scaling (NLCS) method is proposed to obtain the well-focused image with reduced receiving channels. First, a preprocessing step is conducted to achieve space-variant range cell migration correction. After that, the first-step NLCS processing is applied to equalize the 2-D space-variant Doppler centroid and thereby the Doppler bandwidth is decreased, i.e., the required number of receiving channels for reconstruction is reduced. Then, the unambiguous spectrum is reconstructed based on the proposed multichannel transfer function. Finally, the second-step NLCS processing is carried out to equalize the 2-D space-variant high-order Doppler parameters and obtain the well-focused image. The simulation results validate the effectiveness of the proposed method. With the proposed two-step NLCS method, the well-focused image for GEO BiSAR is obtained and the required number of receiving channels can be decreased, which is beneficial to reducing the system complexity and hardware cost.
48 citations
TL;DR: This paper proposes an improved histogram method for PRI deinterleaving based on pulse correlation to overcome the above-mentioned shortcomings, and introduces the mean filter and interquartile range algorithm to optimize the estimated PRI values.
Abstract: In the electromagnetic space, a single channel radar receiver will often intercept several periodic pulse trains radiating from the surrounding emitters simultaneously. The aim of radar pulse deinterleaving is to sort out the pulses coming from different emitters. Most traditional pulse repetition interval (PRI) deinterleaving methods are easy to sort out the pulses with small PRI fluctuations but difficult in dealing with relatively bigger fluctuation or staggered PRIs. In addition, this searching procedure can easily cause pulse omission phenomenon and even generate false pulses. In this paper, we propose an improved histogram method for PRI deinterleaving based on pulse correlation to overcome the above-mentioned shortcomings. After calculating the multi-level time difference histogram, we introduce the mean filter and interquartile range algorithm to optimize the estimated PRI values. Our method extracts the pulse pairs based on pulse correlation directly instead of searching for the pulses and then determines whether the PRI is staggered or not. The experiments on simulation data show that our method can achieve better performance on both the pulse trains of jittered PRIs and the staggered PRIs.
34 citations
TL;DR: A long-time coherent integration method for a maneuvering target with the first-, second-, and third-order range migrations, and the complex Doppler frequency migration that can eliminate the coupling effects between range and azimuth jointly caused by the radial velocity, radial acceleration, and radial acceleration rate of a moving target.
Abstract: In this paper, we propose a long-time coherent integration method for a maneuvering target with the first-, second-, and third-order range migrations, and the complex Doppler frequency migration. In this method, after range compression, the echo signal is first transformed into the range-frequency and Doppler domain based on series reversion, and then an azimuth matched filtering procedure is implemented in the 2-D frequency domain. It can eliminate the coupling effects between range and azimuth jointly caused by the radial velocity, radial acceleration, and radial acceleration rate of a moving target. Due to the linear transform property, the proposed method can work well under low signal-to-clutter and noise ratio. In addition, the Doppler ambiguities, when target azimuth spectrum either is within a pulse repetition frequency (PRF) or spans over neighboring PRF bands, can be well solved. Both simulated and real synthetic aperture radar data processing results are provided to validate the effectiveness of the proposed algorithm.
32 citations
TL;DR: A dual-wavelength injection-seeded Q-switched Ho:YLF laser for CO2 differential absorption lidar pumped by the thulium-doped fiber laser was demonstrated and the single- frequencies achieved by a pair of highly anti-misalignment corner cubes.
Abstract: A dual-wavelength injection-seeded Q-switched Ho:YLF laser for CO2 differential absorption lidar pumped by the thulium-doped fiber laser was demonstrated. The single-frequency Ho:YLF seed laser was achieved by a pair of highly anti-misalignment corner cubes. The wavelength of an online seed laser was corrected to the P12 CO2 absorption line at 2064.414 nm by using a CO2 absorption cell. At the pulse repetition frequency of 100 Hz, the single-frequency pulsed energy was 16.1 mJ with the pulse width of 221.3 ns after a single-pass amplifier. The full width at half-maximum of a single-frequency pulsed spectrum was about 3.87 MHz, and the fluctuation of center frequency was 2.8 MHz in 30 min.
29 citations
TL;DR: In this article, a computational investigation of multiple pulses at varying PRF from an atmospheric pressure plasma jet (APPJ) onto a reactive liquid layer is discussed, and three key trends are made clear.
Abstract: The use of plasma jets to treat tissue in the context of plasma medicine often involves a thin intervening liquid layer on top of the tissue. Plasma activated species first transport through and react in the liquid layer prior to reaching the tissue. Of the many parameters that can be used to control this process, pulse repetition frequency (PRF) stands out. Results from a computational investigation of multiple pulses at varying PRF from an atmospheric pressure plasma jet (APPJ) onto a reactive liquid layer are discussed, and three key trends are made clear. First, a high PRF (short time between pulses) enables the gaseous species produced during the previous pulse to remain in the vicinity of the plasma at the onset of the next pulse, thereby increasing the inventory of (H)N x O y and O3 in the gas phase. These species then solvate into the liquid, water in this case, and produce higher densities of aqueous ozone, nitrate, and peroxynitrite. With a lower PRF, reactants produced on a previous pulse are convected away prior to the next discharge pulse with more spatial separation of reactants both above and within the water. As a result, more of the hydroxyl anion (), ozone anion () and nitric oxide (NOaq) reach the tissue beneath the water. The second trend is that the production of H2O2aq and its fluence to the underlying tissue are relatively independent of the PRF. The precursors for H2O2aq are primarily produced by the surface ionization wave (SIW) on the top of the liquid, which then directly solvate into the liquid. Lastly, when the plasma plume touches the liquid, the SIW on the water layer increases the production of all aqueous species compared to configurations where the plasma plume does not touch the liquid. These trends are true for all PRF.
27 citations
TL;DR: In this paper, the authors used an ultra-short pulse laser source in combination with an external array of birefringent crystals to generate near-THz bursts of single-picosecond pulses.
27 citations
TL;DR: A new pulse sequence design and estimation approach, which can increase the maximum detectable velocity in synthetic-aperture (SA) velocity imaging and makes it possible to use longer sequences with better focusing properties, and can also increase the possible interrogation depth for vessels with large velocities.
Abstract: This paper describes a new pulse sequence design and estimation approach, which can increase the maximum detectable velocity in synthetic-aperture (SA) velocity imaging. In SA, $N$ spherical or plane waves are emitted, and the sequence is repeated continuously. The $N$ emissions are combined to form a high-resolution image (HRI). Correlation of HRIs is employed to estimate velocity, and the combination of $N$ emissions lowers the effective pulse repetition frequency by $N$ . Interleaving emission sequences can increase the effective pulse repetition frequency to the actual pulse repetition frequency, thereby increasing the maximum detectable velocity by a factor of $N$ . This makes it possible to use longer sequences with better focusing properties. It can also increase the possible interrogation depth for vessels with large velocities. A new cross-correlation vector flow estimator is also presented, which can further increase the maximum detectable velocity by a factor of 3. It is based on transverse oscillation (TO), a preprocessing stage, and cross-correlation of signals beamformed orthogonal to the ultrasound propagation direction. The estimator is self-calibrating without estimating the lateral TO wavelength. This paper develops the theory behind the two methods. The performance is demonstrated in the accompanying paper for convex and phased array probes connected to the synthetic aperture real-time ultrasound system scanner for parabolic flow for both conventional and SA imaging.
23 citations
TL;DR: An all-fiber CDWL incorporating Golay coding is proposed and demonstrated, which can improve the measuring precision by improving the pulse repetition frequency of the outgoing laser.
Abstract: Generally, the pulse duration of a coherent Doppler wind lidar (CDWL) is shortened to minimize the spatial resolution at the sacrifice of carrier-to-noise ratio, since the peak power of a laser source is limited by the stimulated Brillouin scattering or other nonlinear optical phenomena. To solve this problem, an all-fiber CDWL incorporating Golay coding is proposed and demonstrated. Given the peak power of the laser pulse, the Golay coding method can improve the measuring precision by improving the pulse repetition frequency of the outgoing laser. In the experiment, the Golay coding implementation is optimized by normalizing the intensity of every single pulse of the outgoing laser with a closed-loop feedback, achieving a spatial resolution of 6 m and a temporal resolution of 2 s with a maximum detection range of 552 m. The wind profile in line of sight and the result derived from another noncoding CDWL show good agreement.
19 citations
TL;DR: In this article, a 4.2 W double-pulse 532 nm picosecond laser at a pulse repetition frequency of 1 kHz was achieved at the Shanghai satellite laser ranging (SLR) station.
TL;DR: In this article, the thermal burnout effect of a gallium arsenide (GaAs) pseudomorphic high electron mobility transistor (PHEMT) low-noise amplifier (LNA) caused by repetitive microwave pulses is studied by theoretical analyses, simulations, and experiments.
Abstract: The thermal burnout effect of a gallium arsenide (GaAs) pseudomorphic high electron mobility transistor (PHEMT) low-noise amplifier (LNA) caused by repetitive microwave pulses is studied by theoretical analyses, simulations, and experiments. The theoretical model for thermal burnout under a single microwave pulse injection is first acquired by analyzing the power absorption in the electrical procedure and the heat distribution in the thermal procedure. By adopting two new assumptions and using the linear superposition theorem, the theoretical model for thermal burnout under a repetitive microwave pulse injection is acquired by further extension. Through derivation, the analytical relationship among the thermal burnout power threshold, the pulsewidth in a cycle, the pulse repetition frequency (PRF) and the pulse number is acquired. Because some assumptions and approximations are adopted, both the pulsewidth in a cycle and the total repetitive microwave pulselength must be between 10-ns scale and 1-μs scale. It shows that the theoretical results agree well with the simulation and experimental results. A minimum of two sets of data by experiment or simulation are needed to fit the analytical relationship. Therefore, experimental or simulation costs can be substantially reduced, and a helpful reference for
TL;DR: In this article, an acousto-optic modulator (AOM) was employed in the cavity of a fluoride fiber laser co-doped with holmium and praseodymium to generate sub-5 ps pulses with 10 nJ of energy at a center wavelength of 2.86 µm and a pulse repetition frequency of 30.1 MHz.
Abstract: We demonstrate ultrashort pulse generation from a fluoride fiber laser co-doped with holmium and praseodymium. To date the majority of work focused on short pulse generation from this class of fiber laser has employed loss modulators in the cavity, both real and artificial. In this work we alternatively employ a frequency shifting element: an acousto-optic modulator (AOM) in the cavity. This results in mode-locked output of sub-5 ps pulses with 10 nJ of energy at a center wavelength of 2.86 {\mu}m, and a pulse repetition frequency of 30.1 MHz, equating to a peak power of 1.9 kW. Additional experimental investigation of the relationship between frequency shift and cavity round trip offer insight into the complex underlying dynamics. As a complementary mode-locking technique to conventional loss modulation, this method of pulse formation may greatly expand the design flexibility of pulsed mid-infrared fiber lasers.
01 Jan 2019
TL;DR: In this article, an inverse deconvolution technique was developed, evaluated, and used to determine the temporal evolution of the temperature and volume of ignition kernels initiated by NPHFD and single pulse discharges.
Abstract: Nanosecond pulsed high frequency discharges (NPHFD) have attracted interest as a means of enhancing ignition probability and ignition kernel growth rates. Previous studies have shown that increasing the pulse repetition frequency (PRF) of a NPHFD burst can result in enhanced probability of ignition relative to lower values of PRF. The cause of this enhanced ignition probability is not well understood, in part, because the temperature evolution of ignition kernels have not been measured. In this work, an inverse deconvolution technique was developed, evaluated, and used to determine the temporal evolution of the temperature and volume of ignition kernels initiated by NPHFD and single pulse discharges. Radiation intensity measurements of ignition kernels were compared to modeled values to determine path-averaged temperatures. Comparisons of values derived using the deconvolution technique and measured temperatures above a McKenna burner typically agreed within 4%. When the technique was applied to ignition kernels initiated by NPHFD over a range of PRF, it was observed that increasing the PRF of a discharge sequence increased the initial rate of change of the temperature as compared to lower PRF. Additionally, kernels generated with high PRF discharges achieved higher initial peak temperatures than those generated by lower PRF discharges. A relaxation in temperature was observed following the peak, which was followed by a subsequent temperature increase in all cases generated with PRF > 10 kHz. PRF above 5 kHz result in larger initial kernel volumes and volume rates of change compared to those observed for lower PRF values. This is attributed to pulse-to-pulse coupling during the energy deposition process. The observed relationship between kernel temperature, volume, and PRF help to explain ignition probability measurements reported previously. Findings from the effort illustrate how the rate of energy deposition through pulse-to-pulse coupling can significantly enhance ignition through elevated temperatures.
TL;DR: Analyzing the consistencies of reflectivity spectra using the three modes and the influence of coherent integration and pulse compression, developing an algorithm for Doppler spectral density data quality control (QC) and merging based on multiple-mode observation data yielded a comprehensive and accurate depiction of most of the clouds and precipitation in the vertical column above the radar.
Abstract: The Chinese Ka-band solid-state transmitter cloud radar (CR) can operate in three different work modes with different pulse widths and coherent integration and non-coherent integration numbers to meet the requirement for long-term cloud measurements. The CR was used to observe cloud and precipitation data in southern China in 2016. In order to resolve the data quality problems caused by coherent integration and pulse compression, which are used to detect weak cloud in the cloud radar, this study focuses on analyzing the consistencies of reflectivity spectra using the three modes and the influence of coherent integration and pulse compression, developing an algorithm for Doppler spectral density data quality control (QC) and merging based on multiple-mode observation data. After dealiasing Doppler velocity and artefact removal, the three types of Doppler spectral density data were merged. Then, Doppler moments such as reflectivity, radial velocity, and spectral width were recalculated from the merged reflectivity spectra. Performance of the merging algorithm was evaluated. Three conclusions were drawn. Firstly, four rounds of coherent integration with a pulse repetition frequency (PRF) of 8333 Hz underestimated the reflectivity spectra for Doppler velocities exceeding 2 m·s−1, causing a large negative bias in the reflectivity and radial velocity when large drops were present. In contrast, two rounds of coherent integration affected the reflectivity spectra to a lesser extent. The reflectivity spectra were underestimated for low signal-to-noise ratios in the low-sensitivity mode. Secondly, pulse compression improved the radar sensitivity and air vertical speed observation, whereas the precipitation mode and coherent integration led to an underestimation of the number concentration of big raindrops and an overestimation of the number concentration of small drops. Thirdly, a comparison of the individual spectra with the merged reflectivity spectra showed that the Doppler moments filled in the gaps in the individual spectra during weak cloud periods, reduced the effects of coherent integration and pulse compression in liquid precipitation, mitigated the aliasing of Doppler velocity, and removed the artefacts, yielding a comprehensive and accurate depiction of most of the clouds and precipitation in the vertical column above the radar. The recalculated moments of the Doppler spectra had better quality than those merged from raw data.
TL;DR: An 806 nm laser diode end-pumped continuous-wave (CW) and actively Q-switched (AQS) orthogonally-polarized dual-wavelength Nd:YLF laser operating at 1314 and 1321 nm is demonstrated.
Abstract: We demonstrate an 806 nm laser diode end-pumped continuous-wave (CW) and actively Q-switched (AQS) orthogonally-polarized dual-wavelength Nd:YLF laser operating at 1314 and 1321 nm. Benefitting from the small difference of the stimulated emission cross sections at 1314 and 1321 nm in Nd:YLF crystal, the power equalized emissions at both wavelengths were achieved by simply titling the output coupler. A maximum CW output power of 9.2 W was obtained with the incident pump power of 32.5 W, giving an optical-to-optical conversion efficiency of approximately 28% and a slope efficiency of approximately 33%. Furthermore, active Q-switching was realized by inserting a Brewster-cut acousto optic modulator. For an incident pump power of 30 W, this oscillator delivered an average power of 6.5 W at a pulse repetition frequency (PRF) of 20 kHz, and a pulse energy of 2.6 mJ with a peak power of approximately 72 kW at a PRF of 1 kHz.
TL;DR: By simultaneously inserting MoS2 and WS2 saturable absorbers (SAs) into laser resonator, a doubly passively Q-switched Tm:YAP laser at 2.5μm was realized as discussed by the authors.
TL;DR: In this article, an all-fiber bidirectional ultrafast thulium-doped fiber laser at 2 μm with a single-wall carbon nanotube presented as saturable absorber was demonstrated.
Abstract: In this paper, we demonstrated an all-fiber bidirectional ultrafast thulium-doped fiber laser at 2 μm with a single-wall carbon nanotube presented as saturable absorber. We successfully obtained bidirectional mode-locking operations with pulse repetition frequency adjustable from 35 to 122 MHz by shortening the cavity length. Meanwhile, with the reduction of the intracavity dispersion, the number of Kelly sidebands was significantly decreased and the output energy was more concentrated on the soliton pulse. Besides, by manipulating the pump power and polarization controller, the two results of bidirectional mode-locked pulses with the same repetition frequency and differential repetition frequency were observed. We found that the repetition rate difference between the clockwise and counterclockwise pulse trains was adjustable by changing the pump power or controlling the intracavity polarization. When the pulse repetition frequency was 35 MHz and 122 MHz, the adjustment range of repetition rate difference was 764–922 Hz and 540–2000 Hz, respectively. It is believed that this novel laser source can support free-running dual optical comb spectroscopy to detect important air gases like H2O or CO2 in the future.
TL;DR: Heuristic algorithms for the interleaved pulse scheduling problems, which are shown to be NP-hard, are presented to produce feasible solutions in a computationally tractable manner and are validated by a complexity analysis and numerical simulations.
Abstract: This paper addresses interleaved pulse scheduling in multitarget tracking with a pulse Doppler phased array radar that can process multiple simultaneous receive beams (MSRB), using element or subarray level digital beamforming (DBF) architectures Key constraints in this pulse scheduling such as consistent selection of pulse repetition frequency in an interleaved group of pulses are identified; the scheduling problems of both DBF levels are then formulated as an integer program that minimizes the total tracking time under these constraints The formulation particularly takes into account the relief of nonoverlapping constraints on received beams by the MSRB processing, which facilitates pulse interleaving with reduced overall track occupancy While the integer program formulation applies to both levels of beamforming architectures, an additional selection problem, termed disk selection, is presented for the subarray level beamforming case in order to consider a geometric limitation on the directions of received beams in digital postprocessing, called resteering Heuristic algorithms for the interleaved pulse scheduling problems, which are shown to be NP-hard, are presented to produce feasible solutions in a computationally tractable manner The practicality of the algorithms, in terms of performance and computation time, is validated by a complexity analysis and numerical simulations
TL;DR: In this article, the authors developed the modeling of pulsed laser micromachining of polymers having high absorption at the irradiation wavelength and provided a comprehensive model for various types of nanosecond pulsed lasers.
TL;DR: In this paper, an 879-nm laser diode was directly pumped passively Q-switched Nd:GdTaOO4 laser at 1066-nm, achieving an optical-to-optical efficiency of 43.8%.
TL;DR: An accurate estimation of the waveform parameters including frequency hopping sequence, hop duration, frequency hopping interval and carrier frequency is realized and the effectiveness and robustness under various signal-noise-ratio conditions are verified.
Abstract: Robust estimation of frequency hopping radar signal parameters is the key to achieving stable jam in frequency hopping communication. In order to deal with the poor noise resistance performance of conventional time-frequency analysis based parameter estimation method under low signal-noise-ratio conditions. A robust method is proposed in this paper for frequency hopping radar waveform parameters estimation. The method is performed in time domain. Firstly, the pulse repetition interval of the received radar signal is extracted by matching filtering, which is used as the window length to divide the signal into multiple frames. The coherent integration process is performed on the multi-frame signal to increase the signal-noise-ratio, which effectively ensures the robustness of the algorithm under low signal-noise-ratio conditions. Secondly, the short-time Fourier transform is used to realize the rough estimation of the frequency hopping number, which reduces the range of subsequent hopping counts and reduces the amount of algorithm calculation. Finally, an accurate estimation of the waveform parameters including frequency hopping sequence, hop duration, frequency hopping interval and carrier frequency is realized. Simulation experiments verify the effectiveness of the algorithm and the robustness under various signal-noise-ratio conditions.
TL;DR: A new Doppler ambiguity tolerated method is proposed to focus and detect maneuvering targets with jerk motions and the computational complexity of the proposed method is remarkably reduced in comparison with that of multidimensional parameter searching methods.
Abstract: Maneuvering target detection is a challenging task for modern radar sensor applications. Range migration (RM) and Doppler frequency migration (DFM) lead to serious coherent integration loss, which affects the performance of maneuvering target focusing and detection. Moreover, Doppler ambiguity can easily appear due to the limitation of pulse repetition frequency; thus, maneuvering targets become difficult to focus correctly. In this paper, a new Doppler ambiguity tolerated method is proposed to focus and detect maneuvering targets with jerk motions. First, the order of RM and DFM is reduced by multiplying the signals before and after the time reversal operation. Second, generalized modified keystone transform is proposed to eliminate residual RM and DFM simultaneously. Subsequently, a well-focused result is obtained. In comparison with the traditional keystone transform (KT) or second-order KT based methods, the proposed method works well in situations wherein Doppler center blur and spectrum ambiguity appear. Moreover, the computational complexity of the proposed method is remarkably reduced in comparison with that of multidimensional parameter searching methods. These advantages show that the proposed method achieves balance between detection performance and computational load. The experiment results validate the effectiveness of the proposed method.
TL;DR: In this article, an all-fiber erbium ultrashort pulsed laser in a distributed fiber temperature sensor as a source of probing pulses has been performed and the results reveal the factors that limit the effective length of the temperature sensor, such as a high pulse repetition frequency and intrinsic laser noises.
Abstract: The testing of an all-fiber erbium ultrashort pulsed laser in a distributed fiber temperature sensor as a source of probing pulses has been performed. Among the prospects of such an approach are an improved signal–noise ratio in the receiving system and a better spatial resolution of the temperature sensor. The experiments have revealed the factors that limit the effective length of the fiber temperature sensor, such as a high pulse repetition frequency and intrinsic laser noises. As a result of the performed work, the distributed fiber optical temperature sensor with a near-room-temperature resolution of ~1.5 K, an effective length of ~3 m, and a spatial resolution of ~10 cm has been developed.
TL;DR: The results show that the image resulting from beamforming features better azimuth ambiguity-to-signal ratio and the proposed approach for PRF selection allows one to individuate a subset of the available images leading to uniform distribution of samples which can be used to support FF-SAR processing.
Abstract: Formation-flying synthetic aperture radar (FF-SAR) enables new working modes and can achieve very high performance through a series of very compact, low-weight, satellite platforms thanks to passive operations of conveniently distributed formation-flying receivers. System timing is a crucial aspect of FF-SAR. The manuscript presents a novel approach to pulse repetition frequency (PRF) selection in order to obtain a uniform distribution of samples at given platform positions. A digital beamforming algorithm is applied on a stack of monostatic repeat-pass images collected by the Sentinel-1 system to test the validity of the PRF selection method. Processed images were thus properly selected to achieve the best merit index measuring the quality of samples distribution. The results show that: (a) the image resulting from beamforming features better azimuth ambiguity-to-signal ratio and (b) the proposed approach for PRF selection allows one to individuate a subset of the available images leading to uniform distribution of samples which can be used to support FF-SAR processing.
TL;DR: In this paper, an optical parametric oscillator (OPO) based on a ZnGeP2 (ZGP) crystal was developed for converting a 2.1-μm pump laser into mid-IR.
Abstract: We report a mid-infrared (mid-IR or 3.5 to 5.0 μm) laser source for testing jamming code effectiveness against heat-seeking missiles in open-field conditions. We developed an optical parametric oscillator (OPO) based on a ZnGeP2 (ZGP) crystal for converting a 2.1-μm pump laser into mid-IR. The pump laser is an acousto-optically Q -switched Ho:YAG laser pumped by a continuous-wave (cw) Tm:fiber laser. The maximum possible average mid-IR power obtained is 6.5 W at a pump power of 20 W with a 33% power conversion efficiency. The pump pulses resulting from the Q -switched operation of the Ho:YAG laser have a pulse width of 30 ns (FWHM) at a pulse repetition frequency (PRF) of 50 kHz. We characterized the output beam in terms of its output power and wavelength spectrum. The output power of the pump laser can be modulated in a pattern of on/off pulses with a smaller frequency than the PRF. The frequency and the duty cycle of these on/off pulses can be adjusted. Identical on/off pulse modulation appears on the mid-IR output beam after the OPO. It is possible to create jamming patterns and codes in mid-IR band using this feature. We have presented examples of on/off modulation of the OPO output at arbitrary pattern frequencies and duty cycles.
Patent•
29 Jul 2019
TL;DR: In this paper, a high voltage pulser with an output that provides pulses with an amplitude greater than 1 kV, a pulse width greater than about 1 μs, and a pulse repetition frequency greater than 20 kHz is described.
Abstract: Some embodiments include a high voltage pulsing power supply. A high voltage pulsing power supply may include: a high voltage pulser having an output that provides pulses with an amplitude greater than about 1 kV, a pulse width greater than about 1 μs, and a pulse repetition frequency greater than about 20 kHz; a plasma chamber; and an electrode disposed within the plasma chamber that is electrically coupled with the output of the high voltage pulser to produce a pulsing an electric field within the chamber.
TL;DR: A novel approach for a joint platform of radar and communication systems to increase the throughput of communications without affecting the radar operation, using two generalized sidelobe cancellers to enable communication during the whole pulse repetition interval.
Abstract: In this paper, we develop a novel approach for a joint platform of radar and communication systems. The previous approaches for dual-function radar-communication systems focused only on the simultaneous transmission of radar and communications in radar active mode. To increase the throughput of communications without affecting the radar operation, we enable communication during the whole pulse repetition interval by using two generalized sidelobe cancellers. We propose two different operational modes of the dual-function radar-communications system. In active mode, the radar function is achieved through the mainlobe, and communications are achieved through the sidelobes. In active mode, only one generalized sidelobe canceller is functional. In rest mode, both the generalized sidelobe cancellers are functional. The first generalized sidelobe canceller has the mainlobe and sidelobe levels as in active mode, and the second generalized sidelobe canceller has the same mainlobe level but double the power in the sidelobes. The output is the difference between the two, in which the mainlobe is canceled, while the sidelobes have the same power, as in the case of active mode. The effectiveness of the proposed scheme is investigated in terms of the bit error rate. Moreover, the proposed system allows communication during the whole pulse repetition interval, thus, enhancing the throughput tremendously.
TL;DR: It is determined that despite the fact that at higher PRFs the noise in the estimation of geophysical parameters is reduced, significant sea-state-dependent biases are also introduced during the retracking process, which are particularly relevant for sea surface height and significant wave height.
Abstract: In this paper, we revisit the pulse-to-pulse correlation properties of nadir-looking pulse-limited altimeters, with the objective of determining the effect of the partial correlation of radar echoes transmitted at much higher rate than the conventional pulse repetition frequency (PRF). This is particularly relevant for the Sentinel-6/Jason-CS mission. The pulse-to-pulse echo power autocorrelation shows much shorter decorrelation times toward the trailing edge of the waveform than those observed for range gates close to the leading edge. At high PRFs this creates a significant variability in the statistical properties of the range gates in the 20-Hz multilooked waveforms. By processing an extensive data set of CryoSat-2 Synthetic Aperture Radar mode data in a pseudo-low resolution mode fashion, we determined that despite the fact that at higher PRFs the noise in the estimation of geophysical parameters is reduced, significant sea-state-dependent biases are also introduced during the retracking process, which are particularly relevant for sea surface height and significant wave height. Those biases will need to be appropriately accounted for when integrating Sentinel-6/Jason-CS data in a climatological data record.
TL;DR: In this article, an in-band PQS self-Raman laser with a c-cut Nd:YVO4 composite crystal was demonstrated for the first time to the knowledge.
Abstract: An in-band pumped passively Q-switched (PQS) self-Raman laser with a c-cut Nd:YVO4 composite crystal is demonstrated for the first time to our knowledge. A Cr4+:YAG composite crystal was utilized to improve the stability and misalignment sensitivity of the resonator. The PQS self-Raman output performance was systematically investigated with different initial transmissions (T0) of the Cr4+:YAG and different transmissions (TS) of the Raman output coupler. In addition, the focus position of the incident pump beam and cavity length have been optimized during the experiment. With Cr4+:YAG of T0=90% and output coupler of TS=10%, the maximum average output power at 1178 nm was up to 2.53 W under the absorbed pump power of 20.6 W, corresponding to an optical-to-optical conversion efficiency of 12.3%. The pulse repetition frequency and pulse width were measured to be 39.1 kHz and 20.4 ns, respectively, and the corresponding pulse energy and peak power were calculated to be 64.7 μJ and 3.2 kW. At the full output power, the fluctuation of Raman output power was less than ±0.5% within one hour. To the best of our knowledge, we have improved the PQS self-Raman laser about 3.2 times.