Презентация Synthetic-Aperture Radar (SAR) Image Formation Processing онлайн
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Презентации » Физика » Synthetic-Aperture Radar (SAR) Image Formation Processing
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- Тип файла:ppt / pptx (powerpoint)
- Всего слайдов:48 слайдов
- Для класса:1,2,3,4,5,6,7,8,9,10,11
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Слайды и текст к этой презентации:
№2 слайд
![Outline Raw SAR image](/documents_6/4a3ecea9401810c0c664908c57397cef/img1.jpg)
Содержание слайда: Outline
Raw SAR image characteristics
Algorithm basics
Range compression
Range cell migration correction
Azimuth compression
Motion compensation
Types of algorithms
Range Doppler algorithm
Chirp scaling algorithm
Frequency-wavenumber algorithm (-k or f-k)
Comparison of algorithms
Processing errors, Computational load, Pros and cons
Autofocus techniques
№10 слайд
![Demodulated baseband SAR](/documents_6/4a3ecea9401810c0c664908c57397cef/img9.jpg)
Содержание слайда: Demodulated baseband SAR signal
[from Digital processing of synthetic aperture radar data, by Cumming and Wong, 2005]
Time domain representation
After removing the radar carrier cos(fo) from the received signal, the demodulated, complex, baseband signal from a single point target can be represented as
№12 слайд
![SAR signal spectrum from](/documents_6/4a3ecea9401810c0c664908c57397cef/img11.jpg)
Содержание слайда: SAR signal spectrum
[from Digital processing of synthetic aperture radar data, by Cumming and Wong, 2005]
Frequency-domain represention
For reasons of efficiency, many SAR processing algorithms operate in the frequency domain.
For the low-squint case, the two-dimensional frequency spectrum of the received SAR signal is
№13 слайд
![SAR signal spectrum Also f](/documents_6/4a3ecea9401810c0c664908c57397cef/img12.jpg)
Содержание слайда: SAR signal spectrum
Also
f : range frequency, Hz, where –Fr /2 f Fr /2
Fr : range sampling frequency, Hz
f : azimuth (Doppler) frequency, Hz
fc : absolute Doppler centroid frequency, Hz
Wr(f ) : envelope of the radar data’s range spectrum
Wa(f ) : envelope of the antenna’s beam pattern Doppler spectrum
The relationship between azimuth time to frequency is
where
№15 слайд
![Matched filter processing](/documents_6/4a3ecea9401810c0c664908c57397cef/img14.jpg)
Содержание слайда: Matched filter processing
Given an understanding of the characteristics of the ideal SAR signal, an ideal matched-filter can be applied using correlation to produce a bandwidth limited impulse response.
However this process has limitations as the characteristics of the ideal matched-filter varies with the target’s position in range and azimuth.
So while such correlation processing is theoretically possible, it is not computationally efficient and is not appropriate when large-scale image-formation processing is required, e.g., from a spaceborne SAR system.
№24 слайд
![Chirp scaling algorithm The](/documents_6/4a3ecea9401810c0c664908c57397cef/img23.jpg)
Содержание слайда: Chirp scaling algorithm
The range-Doppler algorithm was the first digital algorithm developed for civilian satellite SAR processing and is still the most widely used.
However disadvantages (high computational load, limited accuracy secondary-range compression in high-squint and wide-aperture cases) prompted the development of the chirp-scaling algorithm to eliminate interpolation from the range-cell migration compensation step.
As the name implies it uses a scaling principle whereby a frequency modulation is applied to a chirp-encoded signal to achieve a shift or scaling of the signal.
№31 слайд
![Omega-K algorithm WKA The](/documents_6/4a3ecea9401810c0c664908c57397cef/img30.jpg)
Содержание слайда: Omega-K algorithm (WKA)
The chirp-scaling algorithm assumes a specific form of the SAR signal in the range Doppler domain, which involves approximations that may become invalid for wide apertures or high squint angles.
The Omega-K algorithm uses a special operation in the two-dimensional frequency domain to correct range dependent range-azimuth coupling and azimuth frequency dependence.
The WKA uses a focusing step wherein a reference function is multiplied to provide focusing of a selected range. Targets at the reference range are correctly focused while targets at other ranges are partially focused.
Stolt interpolation is used to focus the remainder of the targets.
№32 слайд
![Omega-K algorithm WKA](/documents_6/4a3ecea9401810c0c664908c57397cef/img31.jpg)
Содержание слайда: Omega-K algorithm (WKA)
Illustration of the range/azimuth cross coupling using the raw phase history from a point target.
Range-cell migration introduces a phase change into the azimuth samples in addition to the normal phase encoding.
The RCM cross coupling creates an additional azimuth phase term which affects the azimuth FM rate.
№38 слайд
![Motion compensation Imperfect](/documents_6/4a3ecea9401810c0c664908c57397cef/img37.jpg)
Содержание слайда: Motion compensation
Imperfect trajectories during SAR data collection will distort the data set resulting in degraded images unless these imperfections are removed.
Removal of the effects of these imperfections is called motion compensation.
Motion compensation requires precise knowledge of the antenna’s phase center over the entire aperture.
For example vertical velocity will introduce an additional Doppler shift into the data that, if uncompensated, will corrupt along-track processing.
Similarly a variable ground speed will result in non-periodic along-track sampling that, if uncompensated, will also corrupt along-track processing.
Knowledge of the antenna’s attitude (roll, pitch, yaw angles) is also important as these factors may affect the illumination pattern as well as the position of the antenna’s phase center.
№41 слайд
![Motion compensation In](/documents_6/4a3ecea9401810c0c664908c57397cef/img40.jpg)
Содержание слайда: Motion compensation
In addition to position and attitude knowledge acquired from various external sensors and systems, the radar signal itself can provide information useful in motion compensation.
The Doppler spectrum can be used to detect antenna pointing errors.
The nadir echo can be used to detect vertical velocity (at least over level terrain).
№42 слайд
![Autofocus Just as non-ideal](/documents_6/4a3ecea9401810c0c664908c57397cef/img41.jpg)
Содержание слайда: Autofocus
Just as non-ideal motion corrupts the SAR’s phase history, the received signal can also reveal the effects of these motion imperfections and subsequently cancel them.
This process is called autofocus.
Various autofocus algorithms are available
Map drift
Phase difference
Inverse filtering
Phase-gradient autofocus
Prominent point processing
Many of these techniques exploit the availability of a high-contrast point target in the scene.
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