9 research outputs found
Microwave Photonic Imaging Radar with a Millimeter-level Resolution
Microwave photonic radars enable fast or even real-time high-resolution
imaging thanks to its broad bandwidth. Nevertheless, the frequency range of the
radars usually overlaps with other existed radio-frequency (RF) applications,
and only a centimeter-level imaging resolution has been reported, making them
insufficient for civilian applications. Here, we propose a microwave photonic
imaging radar with a millimeter-level resolution by introducing a
frequency-stepped chirp signal based on an optical frequency shifting loop. As
compared with the conventional linear-frequency modulated (LFM) signal, the
frequency-stepped chirp signal can bring the system excellent capability of
anti-interference. In an experiment, a frequency-stepped chirp signal with a
total bandwidth of 18.2 GHz (16.9 to 35.1 GHz) is generated. Postprocessing the
radar echo, radar imaging with a two-dimensional imaging resolution of ~8.5
mm~8.3 mm is achieved. An auto-regressive algorithm is used to
reconstruct the disturbed signal when a frequency interference exists, and the
high-resolution imaging is sustained
A Beam-Segmenting Polar Format Algorithm Based on Double PCS for Video SAR Persistent Imaging
Video synthetic aperture radar (SAR) is attracting more attention in recent
years due to its abilities of high resolution, high frame rate and advantages
in continuous observation. Generally, the polar format algorithm (PFA) is an
efficient algorithm for spotlight mode video SAR. However, in the process of
PFA, the wavefront curvature error (WCE) limits the imaging scene size and the
2-D interpolation affects the efficiency. To solve the aforementioned problems,
a beam-segmenting PFA based on principle of chirp scaling (PCS), called
BS-PCS-PFA, is proposed for video SAR imaging, which has the capability of
persistent imaging for different carrier frequencies video SAR. Firstly, an
improved PCS applicable to video SAR PFA is proposed to replace the 2-D
interpolation and the coarse image in the ground output coordinate system
(GOCS) is obtained. As for the distortion or defocus existing in the coarse
image, a novel sub-block imaging method based on beam-segmenting fast filtering
is proposed to segment the image into multiple sub-beam data, whose distortion
and defocus can be ignored when the equivalent size of sub-block is smaller
than the distortion negligible region. Through processing the sub-beam data and
mosaicking the refocused subimages, the full image in GOCS without distortion
and defocus is obtained. Moreover, a three-step MoCo method is applied to the
algorithm for the adaptability to the actual irregular trajectories. The
proposed method can significantly expand the effective scene size of PFA, and
the better operational efficiency makes it more suitable for video SAR imaging.
The feasibility of the algorithm is verified by the experimental data
Signal Processing for Digital Beamforming FMCW SAR
According to the limitations of single channel Frequency Modulation Continuous Wave (FMCW) Synthetic Aperture Radar (SAR), Digital Beamforming (DBF) technology is introduced to improve system performance. Combined with multiple receive apertures, DBF FMCW SAR can obtain high resolution in low pulse repetition frequency, which can increase the processing gain and decrease the sampling frequency. The received signal model of DBF FMCW SAR is derived. The continuous antenna motion which is the main characteristic of FMCW SAR received signal is taken into account in the whole signal processing. The detailed imaging diagram of DBF FMCW SAR is given. A reference system is also demonstrated in the paper by comparing with a single channel FMCW SAR. The validity of the presented diagram is demonstrated with a point target simulation results
Innovative Adaptive Techniques for Multi Channel Spaceborne SAR Systems
Synthetic Aperture Radar (SAR) is a well-known technology which allows to coherently combine
multiple returns from (typically) ground-based targets from a moving radar mounted either on an airborne
or on a space-borne vehicle. The relative motion between the targets on ground and the platform
causes a Doppler effect, which is exploited to discriminate along-track positions of targets themselves.
In addition, as most of conventional radar, a pulsed wide-band waveform is transmitted periodically,
thus allowing even a radar discrimination capability in the range direction (i.e. in distance).
For side-looking acquisition geometries, the along-track and the range directions are almost
orthogonal, so that the two dimensional target discrimination capabiliy results in the possibility to
produce images of the illuminated area on ground. A side-looking geometry consists in the radar
antenna to be, either mechanically or electronically, oriented perpendicular to the observed area.
Nowadays technology allows discrimination capability (also referred to as resolution) in both alongtrack
and range directions in the order of few tenths of centimeters.
Since the SAR is a microwave active sensor, this technology assure the possibility to produce images
of the terrain independently of the sunlight illumination and/or weather conditions. This makes the SAR
a very useful instrument for monitoring and mapping both the natural and the artificial activities over
the Earth’s surface. Among all the limitations of a single-channel SAR system, this work focuses over some of them
which are briefly listed below:
a) the performance achievable in terms of resolution are usually paid in terms of system
complexity, dimension, mass and cost;
b) since the SAR is a coherent active sensor, it is vulnerable to both intentionally and unintentionally
radio-frequency interferences which might limit normal system operability;
c) since the Doppler effect it is used to discriminate targets (assumed to be stationary) on the
ground, this causes an intrinsic ambiguity in the interpretation of backscattered returns from
moving targets.
These drawbacks can be easily overcome by resorting to a Multi-cannel SAR (M-SAR) system
Innovative Adaptive Techniques for Multi Channel Spaceborne SAR Systems
Synthetic Aperture Radar (SAR) is a well-known technology which allows to coherently combine
multiple returns from (typically) ground-based targets from a moving radar mounted either on an airborne
or on a space-borne vehicle. The relative motion between the targets on ground and the platform
causes a Doppler effect, which is exploited to discriminate along-track positions of targets themselves.
In addition, as most of conventional radar, a pulsed wide-band waveform is transmitted periodically,
thus allowing even a radar discrimination capability in the range direction (i.e. in distance).
For side-looking acquisition geometries, the along-track and the range directions are almost
orthogonal, so that the two dimensional target discrimination capabiliy results in the possibility to
produce images of the illuminated area on ground. A side-looking geometry consists in the radar
antenna to be, either mechanically or electronically, oriented perpendicular to the observed area.
Nowadays technology allows discrimination capability (also referred to as resolution) in both alongtrack
and range directions in the order of few tenths of centimeters.
Since the SAR is a microwave active sensor, this technology assure the possibility to produce images
of the terrain independently of the sunlight illumination and/or weather conditions. This makes the SAR
a very useful instrument for monitoring and mapping both the natural and the artificial activities over
the Earth’s surface. Among all the limitations of a single-channel SAR system, this work focuses over some of them
which are briefly listed below:
a) the performance achievable in terms of resolution are usually paid in terms of system
complexity, dimension, mass and cost;
b) since the SAR is a coherent active sensor, it is vulnerable to both intentionally and unintentionally
radio-frequency interferences which might limit normal system operability;
c) since the Doppler effect it is used to discriminate targets (assumed to be stationary) on the
ground, this causes an intrinsic ambiguity in the interpretation of backscattered returns from
moving targets.
These drawbacks can be easily overcome by resorting to a Multi-cannel SAR (M-SAR) system
Frequency-modulated continuous-wave synthetic-aperture radar: improvements in signal processing
With the advance of solid state devices, frequency-modulated continuous-wave (FMCW) designs have recently been used in synthetic-aperture radar (SAR) to decrease cost, size, weight and power consumption, making it deployable on smaller mobile plat-forms, including small (< 25 kg) unmanned aerial vehicle(s) (UAV). To foster its mobile uses, several SAR capabilities were studied: moving target indication (MTI) for increased situational awareness, bistatic operation, e.g. in UAV formation flights, for increased range, and signal processing algorithms for faster real-time performance.
Most off-the-shelf SAR systems for small mobile platforms are commercial proprie-tary and/or military (ITAR, International Trades in Arms Regulations) restricted. As such, it necessitated the design and build of a prototype FMCW SAR system at the early stage to serve as a research tool. This enabled unrestricted hardware and software modifica-tions and experimentation.
A model to analyze the triangularly modulated (TM) linear frequency modulated (LFM) waveform as one signal was established and used to develop a MTI algorithm which is effective for slow moving targets detection. Experimental field data collected by the prototyped FMCW SAR was then used to validate and demonstrate the effectiveness of the proposed MTI method.
A bistatic FMCW SAR model was next introduced: Bistatic configuration is a poten-tial technique to overcome the power leakage problem in monostatic FMCW SAR. By mounting the transmitter and receiver on spatially separate mobile (UAV) platforms in formation deployment, the operation range of a bistatic FMCW SAR can be significantly improved. The proposed approximation algorithm established a signal model for bistatic FMCW SAR by using the Fresnel approximation. This model allows the existing signal processing algorithms to be used in bistatic FMCW SAR image generation without sig-nificant modification simplifying bistatic FMCW SAR signal processing.
The proposed range migration algorithm is a versatile and efficient FMCW SAR sig-nal processing algorithm which requires less memory and computational load than the traditional RMA. This imaging algorithm can be employed for real-time image genera-tion by the FMCW SAR system on mobile platforms. Simulation results verified the pro-posed spectral model and experimental data demonstrated the effectiveness of the modi-fied RMA
Spotlight SAR Data Processing Using the Frequency Scaling Algorithm
The paper presents a new processing algorithm for spotlight SAR data processing. The algorithm performs the range cell mnigration correction for non-chirped raw data without interpolation by using a novel frequecy scaling operation. The azimuth processing is based on a spectral analysis approach wich is made highly accurate by azimuth scaling. In almost all processing stages, a subaperture approach is introduced. The paper presents the complete derivation of the algorithm. Processing results using data from the E-SAR-System of DLR are presented