Aerial Synthetic Aperture Research Articles

Double-Directional Time-Spatial Measurement Method Using Synthetic Aperture Antenna

The 5th-generation mobile communication uses multi-element array antennas in not only base stations but also mobile terminals. In order to design multi-element array antennas efficiently, it is important to acquire the characteristics of the direction of arrival (DOA) and direction of departure (DOD), and a highly accurate and simple measurement method is required. This paper proposes a highly accurate and simple method to measure DOA and DOD by applying synthetic aperture (SA) processed at both Rx and Tx sides. It is also shown that the addition of beam scanning to the proposed method can reduce the measurement time while maintaining the peak detection resolution. Moreover, experiments in an anechoic chamber and a shielded room using actual wave sources confirm that DOA and DOD can be detected with high accuracy.

Communications and Robotics Simulation in UAVs: A Case Study on Aerial Synthetic Aperture Antennas

Driven by their versatility and autonomy, advanced unmanned aerial systems have indicated substantial promise along the development of current communication networks, as well as in support of new communication platforms. The resulting integration of robotic and communication systems brings increased levels of complexity, which cannot be modeled accurately by conventional approaches. An example of this are aerial synthetic aperture antennas, in which an aerial robotic swarm coordinates their flight patterns and communication exchanges to produce a synthetic aperture. The accurate representation of such a system mandates the careful simulation of wireless communications, antenna radiation patterns, flight dynamics, and even environmental conditions. This article details an integrated communications and robotics simulation framework, which enables the assessment of these emerging systems. The benefits of this new approach are illustrated for the aerial synthetic aperture antenna example, but extend to a wide array of problems stemming from the union of communications and robotics. This novel modeling platform makes it possible to design and analyze joint flight and communication protocols for unmanned aerial systems. The obtained results provide valuable insights into the operation of swarm-based synthetic aperture antennas. Moreover, the methodology introduced in this work is flexible and can be readily applied to other instances of joint communication and robotic systems.

High-Resolution Wide-Swath SIMO and MIMO SAR Imaging Using Short-Term Shift Orthogonal Code

Conventional single antenna synthetic aperture radar (SAR) has a conflict between wide imaging swath width and fine cross-range resolution. Single input multiple output (SIMO) and multiple input multiple output (MIMO) radars are the two methods to overcome this limitation. In order to separate the signals of MIMO radar, the codes are expected to be orthogonal for all delays. This is impossible in SAR. Due to the special imaging geometry of SAR, short-term shift orthogonal (STSO) codes were proposed to be used in MIMO SAR. This letter compares the SIMO SAR structure and MIMO SAR structure using STSO codes. We draw a conclusion that the number of receivers of MIMO structure is less than that of SIMO structure.

A METHOD OF GENERATING DEM FROM DSM BASED ON AIRBORNE INSAR DATA

Abstract. Traditional methods of terrestrial survey to acquire DEM cannot meet the requirement of acquiring large quantities of data in real time, but the DSM can be quickly obtained by using the dual antenna synthetic aperture radar interferometry and the DEM generated by the DSM is more fast and accurate. Therefore，it is most important to acquire DEM from DSM based on airborne InSAR data. This paper aims to the method that generate DEM from DSM accurately. Two steps in this paper are applied to acquire accurate DEM. First of all, when the DSM is generated by interferometry, unavoidable factors such as overlay and shadow will produce gross errors to affect the data accuracy, so the adaptive threshold segmentation method is adopted to remove the gross errors and the threshold is selected according to the coherence of the interferometry. Secondly DEM will be generated by the progressive triangulated irregular network densification filtering algorithm. Finally, experimental results are compared with the existing high-precision DEM results. The results show that this method can effectively filter out buildings, vegetation and other objects to obtain the high-precision DEM.

Condor-E Spacecraft with a Synthetic Aperture Antenna and Its Capabilities

The main parameters of a small spacecraft (SSC) (Condor-E) and its onboard radar with a synthetic aperture antenna (SAR) are presented in the paper. Two such SSCs were launched in Russia into a 500-km orbit in June 2013 and in December 2014. The structure of the radar with synthetic aperture antenna and the basic parameters of three standard imaging modes: detailed Spotlight (DSL), detailed Stripmap (DSM), and Scan-SAR are also presented. All three modes are accompanied by radar images generated during flight tests. In this paper it is examined whether it is possible to introduce new modes (an interferometric assessment of terrain relief in the case of an “oblique” survey and the selection of moving targets) and the relevant pictures are provided. Condor-E SAR and TerraSAR are compared as typical space radars in their design and experimental modes. We conclude that the SARs perform similarly and give hope for the further augmentation of both SAR capabilities, including new modes without serious hardware modifications.

Dual-filter transfer alignment for airborne distributed POS based on PVAM

Distributed array antenna synthetic aperture radar (SAR), flexible baseline interferometric SAR (InSAR) or integrated multi-task imaging sensors are the most attractive development directions of aerial survey and remote sensing system, and urgently demand a distributed Position and Orientation System (DPOS) to accurately measure multi-nodes time-spatial reference information. However, the traditional transfer alignment (TA) method can't meet the requirements of some high precision interferometric imaging task. To solve the problem, a dual-filter TA method based on position-velocity-attitude matching (PVAM) was proposed. Firstly, the TA method based on attitude matching (AM) is conducted in filter-1 to estimate the flexible angles and the derivatives. Then the TA method based on position-velocity matching (PVM) is conducted in filter-2, and the position and velocity measurements are compensated by the flexible angles and the derivatives estimated by filter-1. Finally, the strapdown solutions of slave IMU are corrected by the corrections from both filter-1 and filter-2. A semi-physical simulation based on airborne DPOS flight experiment has been conducted, verifying that the baseline error has been reduced from 0.0240 m to 0.0083 m and the computation time has been decrease by 3.41%.

Multisensor Time Synchronization Error Modeling and Compensation Method for Distributed POS

An airborne distributed position and orientation system (POS) is high-precision measurement equipment that can accurately provide multinode time-spatial reference for novel remote sensing system as multitask imaging sensors and array antenna synthetic aperture radar. However, it is difficult for multisensor to precisely acquire information at the same moment and result in data fusion error. Thus, the measurement precision is severely degraded. To solve the problem, a multisensor time synchronization error modeling and compensation method is proposed. Based on the component and operation principles of distributed POS, the time synchronization mechanism is analyzed. Multisensor time synchronization error models that include time delay error, random error, and time-varying error are established. A time synchronization error compensation method of the distributed POS is proposed. The experiment results show that the proposed method can accurately calibrate and compensate for the time synchronization error, and improve the measurement precision of the distributed POS. It verified the validity of the proposed method.

Experimental Check of the Coherence of Radiolocation Signals from Objects with Nonlinear Electrical Properties

We propose a basis for the arrangement of an experimental installation for the investigation of the possibility of coherent accumulation of echo signals from objects with nonlinear electrical properties (ONEPs) in a synthetic aperture antenna or synthetic aperture radar (SAR). We established experimentally the dependence of the signal-to-noise ratio (SNR) on the accumulation time, confirming the possibility of coherent accumulation of signals.

AN ACCURATE IMAGING ALGORITHM FOR MILLIMETER WAVE SYNTHETIC APERTURE IMAGING RADIOMETER IN NEAR-FIELD

Due to the fact that the imaging distance is similar to the dimension of synthetic aperture antenna in near-fleld, the Fourier imaging theory used in the traditional synthetic aperture imaging radiometer (SAIR), which is based on the far-fleld approximation, is invalid for near-fleld synthetic aperture imaging. This paper is devoted to establishing an accurate imaging algorithm for near- fleld millimeter wave SAIR. Firstly, the near-fleld synthetic aperture imaging theory is deduced and its relationship to the far-fleld imaging theory analyzed. Then, an accurate imaging algorithm based on the near-fleld imaging theory is established. In this method, the quadratic phase item and antenna pattern are taken into consideration, and the image reconstruction is performed by minimizing the Total- Variation norm of brightness temperature image, which reduces the in∞uence of the visibility observation error and improves imaging precision. Finally, the efiectiveness of the proposed imaging algorithm has been tested by means of several simulation experiments, and the superiority is also demonstrated by the comparison between it and the existing Fourier transform methods. The results demonstrate that the proposed method is an e-cient, feasible imaging algorithm for near- fleld millimeter wave SAIR.

Synthetic aperture controlled source electromagnetics

Controlled‐source electromagnetics (CSEM) has been used as a de‐risking tool in the hydrocarbon exploration industry. Although there have been successful applications of CSEM, this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs CSEM can detect. In this paper, we apply the concept of synthetic aperture to CSEM data. Synthetic aperture allows us to design sources with specific radiation patterns for different purposes. The ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna. Consequently, the types of hydrocarbon reservoirs that CSEM can detect are significantly extended. Because synthetic apertures are constructed as a data processing step, there is no additional cost for the CSEM acquisition. Synthetic aperture has potential for simplifying and reducing the cost of CSEM acquisition. We show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture CSEM source.

Analysis of depth-of-focus of synthetic aperture antenna arrays

Interferometric synthetic aperture arrays (SAA) can be refocused at any range via digital processing of the raw data, called the visibility function. Such systems are sensitive, however, to ranging errors in digital refocusing. They, therefore, exhibit a limited depth of focus, because these errors are equivalent to introducing defocus in the system. We derive an analytical expression for the Strehl ratio of synthetic aperture arrays, which accounts for both the antenna voltage pattern and the focal shift factor encountered in imaging systems with a low Fresnel number. Assessment of the depth of focus of short-range imaging arrays is allowed and is illustrated for common array designs.

A Velocity Estimation Algorithm of Moving Targets using Single Antenna SAR

A new algorithm is proposed for velocity estimation of moving targets in single antenna synthetic aperture radar (SAR). Based on the fact that different velocity vectors cause different geometrical figures of the two-dimensional (2-D) signature in the range-Doppler (RD) domain, this algorithm estimates the azimuth and range velocities by a 2-D search such that the range cell migration correction (RCMC) and the second range compression (SRC) are correctly performed. It is shown that, using the proposed algorithm, the Doppler ambiguity problem can be avoided and satisfactory accurate velocity estimation can be obtained in high signal-to-clutter ratio (SCR) scenarios.

Novel concepts for surface movement radar design

Traditional millimeter-wave surface movement radar (SMR) has been designed and tested. A novel concept for the design of a new SMR is suggested based on synthetic aperture antenna and noise radar technology. It enables designing SMR without mechanical rotation of an antenna, but applying several nonmoving synthetic aperture antennas instead. Application of noise radar technology is also considered in more detail to improve the expected performance of a new SMR.

Ka-band bistatic ground-based noise waveform SAR for short-range applications

The designing and testing of a new Ka-band ground-based noise waveform synthetic aperture radar (GB NW-SAR) are presented. Its design is based upon new synthetic aperture antennas and noise radar technology. The authors present preliminary results of its indoor trials. The GB NW-SAR designed may be reconfigured for operation in mono-, bi- and multistatic (or MIMO) modes in both CW and pulse regimes using single- and dual-frequency modes to enhance range resolution via expanding the frequency bandwidth of the signal. Potential resolution of the GB NW-SAR is 15 cm both in range and azimuth. Results of indoor trials are presented for both SAR imaging and differential interferometry measurements. Designed and tested Ka-band GB NW-SAR is applicable for precise remote monitoring of various manmade or natural objects, such as ceilings and roofs of big halls and hangars, dams, bridges, TV towers etc.

Imaging Simulation for MMW Synthetic Aperture Radiometer

A synthetic aperture radiometer is a new type of passive remote sensing instrument that can reach high spatial resolution. Restricted by antenna element size and operating wavelength, however, the system design is difficult at MMW band. In this paper, main technical parameters optimization of a synthetic aperture radiometer is discussed. Based on low-redundancy linear arrays, a millimeter wave synthetic aperture radiometric imaging scheme is presented. The simulation experimental results show that the system imaging effect depends on the synthetic aperture antenna pattern.

Radio holographic principle for observing natural processes in the atmosphere and retrieving meteorological parameters from radio occultation data

The radio holographic principle is briefly described and tested by using radio occultation data of the GPS/MET and MIR/GEO experiments. Sub-Fresnel spatial resolution ∼12 m/pixel was achieved using focused synthetic aperture radio holographic approach, and direct evidence of multibeam propagation effects in the atmosphere was obtained. The achieved instrumental accuracy in angular distance measurements was near 0.004 milliradian/pixel, and observed angular distance between different rays was equal to 0.3 milliradians. The angular resolution of the radio holographic method depends on the wavelength as λ1 compared to λ1/2 in conventional methods. In general case the principal limit of the vertical resolution may be determined using focused synthetic aperture antenna theory and may achieve a value ∼20–40 m under assumptions of spherical symmetry and quiet atmospheric conditions. Wave structures were discovered in the altitude distribution of the gradient electron density at a height interval of 60–95 km with spatial period 1–2 km and vertical resolution 300–500 m. Good correspondence was found between the temperature profiles revealed by radio holographic analysis and those obtained by traditional retrieval using UCAR GPS/MET data.

A radiation center representation of antenna radiation patterns on a complex platform

A sparse model of the antenna radiation pattern on a complex platform is presented. This representation is based on a point radiator model that describes the radiation pattern by a collection of radiation centers on the platform. The methodology for obtaining the radiation center model is presented. It entails first generating the three-dimensional (3-D) antenna synthetic aperture radar (ASAR) imagery of the platform and then parameterizing the resulting image by a collection of point radiators via the CLEAN algorithm. It is shown that once such a representation is obtained, we can reconstruct and extrapolate antenna radiation patterns over frequencies and aspects with good fidelity, thus achieving high data compression ratio. Furthermore, it is shown that the resulting radiation center information can be used to pinpoint cause-and-effect in platform scattering and provide important guidelines for reducing platform effects.

On extracting the radiation center representation of antenna radiation patterns on a complex platform

Using full-wave computational electromagnetics data, we demonstrate that the radiated field from an antenna on a complex platform can be sparsely represented by a radiation center model. An extraction scheme based on the matching-pursuit algorithm is devised to extract the model parameters from multifrequency, multiaspect radiation data. To accelerate the parameterization procedure, the previously developed antenna synthetic aperture radar imaging algorithm is used to provide an initial estimate of the radiation center position during the search. It is shown that the resulting radiation center model provides a sparse, physical representation of the antenna–platform interaction. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 4–7, 2000.

ASAR-antenna synthetic aperture radar imaging

The antenna synthetic aperture radar (ASAR) imaging concept is introduced. We present the ASAR imaging algorithm to pinpoint the locations of secondary scattering off a platform from antenna radiation data. It is shown that a three-dimensional (3-D) ASAR image of the platform can be formed by inverse Fourier transforming the multifrequency, multiaspect far-field radiation data from an antenna mounted on the platform. This concept is demonstrated using the computed radiation data from the code Apatch, which employs the shooting and bouncing ray (SBR) technique. Furthermore, we develop a fast ASAR algorithm specially tailored for the SBR approach. By taking advantage of the ray tracing information within the SBR engine, we demonstrate that the fast approach can result in the same quality of image as the frequency-aspect algorithm at only a fraction of the computation time.

Active sonic boom control

A theory and simulation code are developed to study non-steady sources as means to control sonic booms of supersonic aircraft. A key result is that the source of sonic boom pressure is not confined to the length of the aircraft but occupies an extensive segment of the flight path. An aircraft in non-steady flight functions as a synthetic aperture antenna, generating complex acoustic waves with no simple relation to instantaneous volume or lift distributions.The theory applies linear acoustics to slender non-steady sources but requires no far-field approximation. The solution for pressure contains a term not seen in Whitham's theory for sonic booms of distant supersonic aircraft. The term describes a pressure field that decays algebraically behind the Mach cone and, in the case of steady flight, integrates to a ground load equal to the weight of the aircraft. The algebraic term is separate from those that describe the sonic boom.Two non-steady source phenomena are evaluated: periodic velocity changes (surge), and periodic longitudinal lift redistribution (slosh). Surge can attenuate a sonic boom and covert it into prolonged weak reverberation, but accelerations needed to produce the phenomenon seem too large for practical use. Slosh may be practical and can alter sonic booms but does not, on average, result in boom attenuation. The conclusion is that active sonic boom abatement is possible in theory but maybe not practical.