Initial Azimuth Research Articles

Optimized SAR imaging algorithm with SSL constraints for the Azimuth direction

In order to address the low azimuth imaging problem of traditional Range Doppler (RD) algorithm, a high-resolution imaging algorithm with optimal azimuth sampling sequence length (SSL) constraints is hereby proposed. According to this algorithm, efforts are made to design a formula for the calculation of the initial azimuth SSL on the basis of SAR azimuth imaging parameters. Having obtained the initial SSL and the criteria for the evaluation of SAR imaging performance, we continue to figure out the extreme values of the curve within a reasonable range of variation. On the basis of these values, the optimal azimuth SSL can be determined to provide data support for global azimuth imaging. Measurement data from imaging experiments on airborne and spaceborne SARs show that, compared with traditional RD algorithm, the proposed algorithm reveals outstanding advantages as indicated by a number of factors critical for the evaluation of imaging performance, such as resolution, peak sidelobe ratio (PSLR) and integral sidelobe ratio (ISLR). Compared with reference methods with local optimal imaging performance, the proposed algorithm can help to obtain global optimal SAR imaging data.

First Assessment of Bistatic Geometric Calibration and Geolocation Accuracy of Innovative Spaceborne Synthetic Aperture Radar LuTan-1

LuTan-1 (LT-1) is a bistatic synthetic aperture radar (BiSAR) system consisting of two identical L-band SAR satellites. The bistatic mode of LT-1 plays a critical role in generating high-precision digital elevation models (DEMs), which requires precise geometric calibration of initial range and azimuth times for both SARs to ensure the reliability and quality of geolocation. However, existing geometric calibration methods predominantly focus on monostatic SAR systems, with limited literature on slave SAR calibration in bistatic systems. This research addresses this gap by establishing geometric calibration models for both SARs based on signal echo history and the range–Doppler model. The geometric errors are effectively resolved using corner reflector data from Xinjiang, China. Through statistical analysis of LT-1 SAR images acquired between July and November in bistatic mode, this paper has demonstrated range delay accuracy of better than 5 ns and azimuth time accuracy of better than 0.1 ms. This level of precision translates into a positional accuracy better than 0.8 m. The proposed models have been successfully applied to geometric calibration, providing precise geolocation for LT-1, thus enhancing its utility for a wide range of Earth observation applications. This paper is the first endeavor to present the assessment of the geometric calibration and geolocation accuracy of LT-1 and discuss the results of the bistatic geometric calibration of the master and slave SARs in a BiSAR formation.

Facilitating learning adaptive V-tail of a supersonic missile for radar cross-section reduction during interval flight

To study the V-tail inclination of a supersonic missile that meet the requirements of low electromagnetic scattering characteristics in interval flight, an automatic tilt scheme is presented. The improved annealing algorithm is used to determine the optimal solution of the V-tail tilt angle, where multiple scattering of the physical optics plus physical theory of diffraction is used to calculate the radar cross section (RCS) of the target. The results show that the optimal solutions of V-tail inclination are different in various flight intervals of the horizontal observation field. In the forward side flight interval, changing the initial azimuth has less influence on the optimal solution of V-tail tilt angle, while more influence on the missile RCS indicator. In the side tail flight interval, the RCS level of the missile is low, and there is an optimal solution and a few feasible solutions for the V-tail inclination. The feasible solution of V-tail tilt angle in the lateral flight interval is obviously increased. The presented automatic tilt scheme is effective for studying the low scattering performance of the supersonic missile V-tail.

An Efficient 2-D DOA Estimation Method for Unfolded Coprime L-Shaped Array

In this article, a novel two-dimensional direction of arrival (2-D DOA) estimation method using unfolded coprime L-shaped array is presented. First, the signal subspace is divided to establish the relationship between two subarrays. Thus, the 2-D DOA estimation problem is transformed into one-dimensional (1-D) one. Moreover, by introducing the extraction matrix, the nonuniform array can also fit polynomial root finding, from which the initial elevation and azimuth estimates can be obtained. Finally, to further improve the estimation accuracy, we compensate the estimation offsets via least square constructed from the first-order Taylor expansion of the direction matrix. The proposed algorithm outperforms the conventional algorithms in the aspect of computational complexity while achieves preferable estimation performance. Specifically, the complexity of the proposed method is reduced by several orders of magnitude, with RMSE reduced by up to 6 dB.

Estimation and Correction of Azimuth and Attitude Errors of Attitude Tracking for Vehicular Spacecraft

Shortening the ground preparation time of vehicular spacecraft is an important topic in the aerospace field. The time is hard to be significantly shortened using the traditional methods, because initial alignment and spacecraft erection must be fulfilled in order. To solve the problem, an estimation and correction method for azimuth and attitude errors of attitude tracking is proposed in this article. We propose a real-time error estimation model to reveal the effect of the initial azimuth, attitude errors and the gyro biases of the spacecraft inertial measurement unit (SIMU) on the current azimuth and attitude errors during attitude tracking, making it possible to post-compensate the initial coarse azimuth error and the gyro biases of the SIMU at the end of attitude tracking. This model is the key and the theoretical basis of our proposed method. Detailed estimation and correction steps are designed, to achieve the purpose of fulfilling the spacecraft erection and the fine-alignment of the vehicular inertial measurement unit in parallel. The results of the simulations and experiments show that the preparation time is remarkably shortened using the proposed method, without decreasing the final azimuth and attitude accuracy of the SIMU. This proposed method detaches the inherent viewpoint that fine-alignment must be completed before attitude tracking and spacecraft erection in the traditional horizontal collimation scheme. It provides a completely new way to shorten the ground preparation time of vehicular spacecraft.

Auto Sweptback Wing Based on Low Scattering Demand for an Unmanned Aerial Vehicle in Phase Flight

In order to study the optimal sweepback angle when a variant unmanned aerial vehicle (UAV) exhibits a low radar cross-section (RCS) indicator during phase flight, an auto sweep scheme based on electromagnetic scattering evaluation and an improved particle swarm optimization algorithm was presented in this article. An aircraft model with variable swept wings was built, and high-precision grids were used to discretize the target surface. The results showed that the optimal sweep angle did not change with the increase in the initial azimuth angle when the observation field was horizontal and the ending azimuth was 90°. While the increase in the elevation angle affected the optimal sweepback angle of the aircraft under the given conditions, when the observation initial azimuth angle was 90°, the auto sweep scheme could reduce the mean and some minima of the RCS indicator curve of the aircraft and could provide the aircraft with an optimal sweep angle under different observation conditions. The presented method was effective in learning the optimal sweep angle of the aircraft when low scattering characteristics were required during the phase flight.

Resistance characteristics of broad-leaf crop canopy in air-assisted spray field and their effects on droplet deposition.

Air-assisted spray technology is widely applied in high-efficiency pesticide applications. The resistance characteristics of the crop canopy reflect its energy dissipation effect on the assisted airflow, connecting the structure of the crop canopy, assisted airflow velocity, and droplet deposition effect. Using a common broad-leaf crop canopy as the research object, the resistance characteristics of the crop canopy in the air-assisted field were investigated in this study by performing theoretical analysis and wind tunnel tests. Further, the feasibility of using the resistance characteristics of the crop canopy was assessed to evaluate its droplet deposition effect. The results showed that under the conditions of different number of leaf layers and initial leaf azimuth angles, the canopy pressure drop experiences a non-linear increasing trend with increasing assisted airflow velocity and that its regression function conforms to the Darcy–Forchheimer function. Moreover, when the initial azimuth angles of single- and multi-layer leaves were 90°–270°, the change rate of the canopy pressure drop with airflow velocity was 7–9 m/s, and there was a critical wind speed. However, with an increasing number of leaf layers in the crop canopy and changes in the initial leaf azimuth angle, the corresponding changes between the maximum canopy pressure drop and resistance coefficient were non-linear. Thus, it is proposed that the resistance characteristics of multi-layer leaves cannot be quantified as the results of the linear superposition of the resistance characteristics of several single-layer leaves—that is, it should be regarded as a whole research object. Combined with the analysis of the influence of the crop canopy resistance on droplet deposition, it is considered that when the crop canopy has multiple leaf layers in the airflow direction, the existing air-assisted spray technology cannot guarantee droplet deposition and canopy penetration simultaneously.

Tight focusing of high-order polarized anomalous vortex beam

Based on the vector Debye theory, the tight focusing properties of a high-order polarized anomalous vortex (HPAV) beam are studied. The corresponding mathematical expressions of the HPAV beam are derived theoretically. We accomplish the inner and outer gear shapes of the focusing intensity where the number of the gear tooth can be modulated by polarization order. The results show that the focusing gear intensity can be flexibly modulated by initial polarization azimuth which may determine the trapping effects. Various charming focusing field patterns can be used to capture two kinds of different refractive indices particles simultaneously. The compactness of the intensity distribution can be freely adjusted by the HPAV beam topological charges and polarization order. The focal spot size, which is far beyond the Rayleigh diffraction limitation can be achieved. It may be expected to have potential applications in optical microscopy, imaging, optical telecommunication and other fields.

Particulate Projectiles Driven by Cavitation Bubbles.

The removal of surface-attached particles with cavitation bubbles is usually attributed to the jetting or shear stresses when bubbles collapse. In this Letter, we report an unexpected phenomenon that millimeter-sized spherical particles made of heavy metals (e.g., stainless steel), when initially resting on a fixed rigid substrate, are suddenly accelerated like projectiles through the production of nearby laser-induced cavitation bubbles of similar sizes. We show experimentally and theoretically that the motion of a particle with radius R_{p} is determined by the maximum bubble radius R_{b,max}, the initial distance from the laser focus to the center of the particle L_{0}, and the initial azimuth angle φ_{0}. We identify two dominant regimes for the particle's sudden acceleration, namely, the unsteady liquid inertia dominated regime and the bubble contact dominated regime, determined by R_{b,max}R_{p}/L_{0}^{2}. We find the nondimensional maximum vertical displacement of the particle follows the fourth power and the square power scaling laws for respective regimes, which is consistent with the experimental results. Our findings can be applied to nonintrusive particle manipulation from solid substrates in a liquid.

Computational studies of the rotors aerodynamic characteristics of multirotor drones

The article presents the results of computational studies of aerodynamic characteristics for unmanned lift-generating multi-rotor drones of various configurations. The distinctive features of rotors flow were characterized. The rotor interaction was evaluated. The computations were based on the nonlinear rotor blade vortex theory in a non-stationary arrangement. The combinations of four, eight (four coaxial) and fourteen two-bladed rotors at velocity V = 100, 150, 200 km/h were considered. Semi-empirical methods were employed to select the rotor angles of attack, rotation speed, blade installation angles and geometric parameters at the given take-off weight for each combination of rotors and flight airspeed. The computations showed that for a four-rotor lift-generating design (quad-rotor), two rotors installed downstream, depending on the velocity due to the mutual effect, have values of the thrust coefficients ≈10...20% less than those of the rotors located upstream. For a coaxial quad-copter, the effect of the upper front rotor on the upper rear rotor is similar to the effect of the front rotors on the rear ones in a four-rotor lift-generating design. The effect of the upper front rotor on the lower rear rotor does not vary in terms of the average thrust value, and variations are only local in nature. The interaction of other rotors is identical to that of the four-rotor version. A fourteen-rotor lift-generating multi-rotor drone has a complex flow pattern, which generates deviance in the thrust coefficients variation with respect to time. Depending on the mode and rotors location, the average rotor thrust coefficient can vary approximately twice. The computations showed that with the similar geometric parameters and kinematics characteristics, rotors thrust is substantially subject to variation, which causes destabilizing moments to a significant degree without additional control input. Thrust pulsations and, respectively, vibrations grow in intensity as the flight airspeed increases. Probably, the right choice of the rotor configuration and the automatic control system can counterbalance thrust surge by so-called "phasing", i.e. selecting an initial azimuth angle for each rotor.

The future of autonomous underwater vehicle control

The future of autonomous underwater vehicle control

Ground moving target indication and parameter estimation algorithm using deramp space-time adaptive processing

Deramp space-time adaptive processing (Deramp-STAP) algorithm has been validated in its effectiveness in reducing the required channels to enable ground moving target indication (GMTI) for multi-channel synthetic aperture radar (SAR) systems. However, the parameter estimation for the moving target, including the critical radial velocity, is not accurate enough, and the radial velocity is always approximated by the equivalent velocity obtained by the Deramp operation. For the indication and imaging of ground moving targets, the accurate estimation of the radial velocity is of importance to improve the signal-clutter-noise-ratio of STAP and estimate the initial azimuth position of the moving target. A method for GMTI and the corresponding parameter estimation using Deramp-STAP is proposed. Compared with the traditional Deramp-STAP algorithms, the proposed algorithm utilizes the equivalent velocity estimated by Radon transform to remove the linear range migration before STAP and figure out where the moving target is located in the azimuth frequency domain. To estimate the radial velocity more accurately, the fast Fourier transform along the spatial direction is performed on the pixel where the moving target is located in the azimuth frequency domain after the Deramp operation, and then the radial velocity can be obtained through the spatial spectrum analysis. Finally, the estimated radial velocity is applied to construct the azimuth compression function for the moving target imaging, and then the initial azimuth position can be estimated. Several simulation experiments have been implemented to validate the effectiveness of the proposed algorithm.

Numerical study of the polarization proprieties in a ring cavity with total polarization control

We report the numerical study of the polarization properties of a net anomalous dispersion ring cavity with total polarization control. As a mode locker, we use the nonlinear polarization rotation (NPR) method. The initial ellipticity and azimuth were fixed to 13° and 0°, respectively. The simulations, made by the split-step Fourier method, show changes in polarization with respect to initial conditions, after the mode-lock process such as previously reported in the literature. It was found that polarization changes are caused by non-linear birefringence. The pulsed laser produces solitonic pulses with 0.4 ps FWHM and 240 W peak power.

Thrust pulsation of coaxial main rotor, caused by the blades relative position

The influence of reciprocal position of the upper rotor blades in respect to the lower rotor blades is characteristic for coaxial main rotor. It is established that the initial azimuth of the blade, for example, of the upper rotor’s which does not coincide with the initial azimuth of the lower rotor blades, affects the level of vibrations caused by the rotors thrust pulsations, the level of noise, generated mainly by coaxial rotor. This paper presents numerical studies which assess the effect of the initial azimuth of the upper rotor blades (phasing) on the helicopter coaxial rotor thrust force pulsation. The research was carried out applying the calculation method based on the nonlinear vortex theory in a non-stationary formulation. The results of the helicopter coaxial rotor with different initial azimuths of the upper rotor blade relatively to the azimuth of the lower rotor blade flow around numerical simulation are presented. The influence of the blades phasing on the rotor thrust coefficient change and thrust force pulsation magnitude is shown. The flow of a six-bladed coaxial main rotor (two rotors with 3 blades) was simulated in the oblique flow mode at speeds of 51.25 m/s and 71.75 m/s at the rotor angles of attack– 5 0 and – 12 0 , respectively. The change in the coefficient of the main rotor thrust per revolution at different values of phasing was studied. The coaxial rotor thrust coefficient is determined by summing the lower and upper rotors thrust coefficients respectively. Thus, at some phasing the thrust coefficient of the lower and upper rotors increase intensifies the thrust pulsations, and at others, the peaks of the upper and lower rotors pulsations are displaced and the total coaxial rotor thrust coefficient changes per one revolution with smaller amplitude. It is established what phasing produce the maximum values of thrust pulsation, and at which-a minimum of thrust pulsation.

The 12 December 2017 Baumgarten Gas Hub Explosion: A Case Study on Understanding the Occurrence of a Large Infrasound Azimuth Residual and a Lack of Seismic Observations

The Baumgarten explosion occurred on 12 December 2017 at a gas storage site about 30 km east of Vienna, Austria. Acoustic arrivals from this accidental surface explosion were detected at dozens of stations of the AlpArray seismic network to distances up to 150 km, mainly in easterly directions. Thus it was expected that the Hungarian infrasound array PSZI located about 230 km to the east-southeast of Baumgarten would detect this acoustic wave as well. Standard progressive multichannel correlation processing and frequency-wavenumber analysis identified a signal emerging at 7:57:55 UTC from an azimuth of 296°–300° and with trace-velocity > 400 m/s. The extraordinarily high trace-velocity and excessive backazimuth residual, relative to the explosion site direction of 282°, however cast strong doubts on the arrival’s connection to the Baumgarten event. Accounting for the effect of non-planar geometry of the infrasound array results in a reduction of the azimuth residual by half. Additionally, 2D and 3D raytracing methods are used including the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric model to further explain the remaining azimuth residual as well as to elucidate the large trace velocity estimates. The prevailing stratospheric winds in excess of 150 m/s are identified as the underlying cause. Including both factors the initial azimuth residual of up to 18° decreases to ~ 4°, allowing to associate the infrasound signal at PSZI with the Baumgarten event. Finally, the data from a seismic station at 30 km range is re-investigated for magnitude estimation. The local magnitude of ML < 1.0 explains well the scarcity of seismic observations within 50 km range, where three or four stations show signals, mainly consisting of Rg-type surface waves, but no body waves.

Multiple Model Algorithm for Single-Beacon Navigation of Autonomous Underwater Vehicle without Its A Priori Position. Part 1. Mathematical Formulation

The article considers a problem of autonomous underwater vehicle (AUV) positioning with acoustic measurements of range and possibly radial velocity relative to a single beacon, velocity components and coordinate increments from an inertial navigation system, and the data from a water-speed log or a ground log. Interruptions in acoustic measurements reception are acceptable. Availability of a priori information on relative position of AUV and the beacon is not assumed. In order to solve the problem, the author proposes a multiple model algorithm based on a bank of extended Kalman filters which independently estimate the initial horizontal range and errors of the used data under different hypotheses about a value of AUV initial azimuth relative to the beacon. Current coordinates of AUV are determined by the outputs of filters, taking into account the a posteriori probabilities of the corresponding hypotheses. The algorithm is rather simple for programming and does not require much computational power.

Long-term evolution and lifetime analysis of geostationary transfer orbits with solar radiation pressure

The dynamical evolution of geostationary transfer orbits (GTOs) is a fundamental issue for the space debris mitigation in this orbit region. In this paper, the long-term dynamical evolution and orbital lifetime of low-inclination GTOs are investigated based on the Semi-analytic Tool for End of Life Analysis software (STELA) with the solar radiation pressure (SRP) and Earth's shadow taken into account. We carry out numerical investigations of long-term orbital evolution of objects with area-to-mass ratio (A/m) between 0.02 m2/kg and 1 m2/kg in the GTO region. Their dynamical orbital lifetime is recorded and presented in the Ω–ω (right ascension of ascending node–argument of perigee) plane to investigate its dependence on the initial conditions and physical parameters, particularly the initial azimuth of the Sun with respect to the apsidal line and the value of A/m. Based on these results, we further analyze the effect of SRP on the long-term dynamical evolution, as well as the solar apsidal resonances. The results are helpful to enrich the knowledge about long-term evolution and orbital lifetimes of GTO objects with relatively high A/m as well as to promote uncatalogued objects observation and identification.

Effect of East Gyro Drift and Initial Azimuth Error on the Compass Azimuth Alignment Convergence Time

The effect of gyro constant drift and initial azimuth error on the convergence time of compass azimuth is analyzed in this article. Using our designed compass azimuth alignment system, we obtain the responses of gyro constant drift and initial azimuth error in the frequency domain. The corresponding response function in the time domain is derived using the inverse Laplace transform, and its convergence time is then analyzed. The analysis results demonstrate that the convergence time of compass azimuth alignment is related to the second-order damping oscillation period, the gyro constant drift, and the initial azimuth error. In this study, the error band is set to 0.01° to determine convergence. When the gyro drift is less than 0.05°/h, compass azimuth alignment can converge within 0.9 damping oscillation periods. When the initial azimuth error is less than 5°, compass azimuth alignment can converge within 1.4 damping oscillation periods. When both conditions are met, the initial error plays a major role in convergence, while gyro drift has a smaller effect on convergence time. Finally, the validity of our method is verified using simulations.

Azimuth Relocation for Multichannel SAR Ground Moving Targets via Noncoregistrated Inteferometry

A ground moving target usually shifts its initial azimuth location in the synthetic aperture radar (SAR) image due to the radial motion. To address this problem, a new azimuth relocation method is proposed for a multichannel SAR in this letter. Based on the signal analysis in the range-compressed domain, it is found that the zeroth-order term of the interferometric phase without coregistration is determined by the moving target's initial azimuth location. A new azimuth relocation method utilizing the noncoregistrated interferometric phase is then proposed. First, the moving target is detected after clutter suppression in the range-compressed domain. Then, the interferometric phase along the slow time without coregistration can be extracted for parameter estimation via the least-squares method. Compared with the existing radial velocity-based relocation methods, the proposed method has higher accuracy, and there is no azimuth location ambiguity problem even when the target has a large radial velocity. Finally, both the simulated and real data are provided to demonstrate the effectiveness of the proposed method.

Method to determine initial aiming azimuth accuracy using acceleration, gyroscope, and geomagnetic sensors

Abstract Initial aiming azimuth measurement using a geomagnetic sensor is very common, but difficult to use, because the accuracy cannot be determined even in places where geomagnetic disturbance can be compensated. In this letter, we propose a novel algorithm that employs acceleration, gyroscope, and geomagnetic sensor data to determine the accuracy of the initial oriented azimuth. Decision accuracy is 75% for various experiment locations. In the case of a false alarm, it is necessary to move to another location and remeasure, and ultimately the decision accuracy could reach 100%.