06-04-2011, 04:30 PM
Presented by:
Dr. Jakob van Zyl
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RADAR AND SYNTHETIC APERTURE RADAR BASICS
PRINCIPLES OF RADAR
HOW DOES RADAR WORK?
• RADAR = Radio Detection And Ranging
• Since radar pulses propagate at the speed of light, the difference to the “target” is proportional to the time it takes between the transmit event and reception of the radar echo
PRINCIPLES OF IMAGING RADAR
REAL APERTURE RADAR
• PRINCIPLES OF IMAGING RADAR
THE RADAR EQUATION
• The SNR is derived from the radar equation:
where
Peak transmit power
Antenna gain (one way)
Transmit system loss
Receive system loss
Operating noise figure
Boltzmann’s constant
Noise temperature
Bandwidth
Pulse length
Antenna length
PRINCIPLES OF IMAGING RADAR
THE RADAR EQUATION
• In order to improve the signal-to-noise ratio for a fixed radar frequency, one has (among others) the following options:
– Increase the transmitted power. This is usually limited by the power available from the spacecraft or aircraft.
– Increase the antenna gain. This requires larger antennas, severely affecting the launch mass and volume.
– Increase the pulse length. This means poorer resolution.
– Decrease bandwidth. This also means poorer resolution.
– Fly lower. Increases atmospheric drag, requiring more fuel for orbit maintenance.
• Signal modulation is a way to increase the radar pulse length without decreasing the radar range resolution
– All civilian spaceborne SARs, and most civilian airborne SARs use linear FM chirps as the modulation scheme.
• PRINCIPLES OF RADAR IMAGING
SYNTHETIC APERTURE RADAR
• PRINCIPLES OF RADAR IMAGING
SAR IMAGING COORDINATE SYSTEM
PRINCIPLES OF RADAR IMAGING
POINT TARGET RESPONSE
• The radar system transmits a series of chirp pulses:
• The target will be in view of the radar antenna for a limited time period. During this period, the distance to the target is
• Usually, so that
PRINCIPLES OF RADAR IMAGING
POINT TARGET RESPONSE
• The phase of the returned signal is
• The instantaneous frequency of the transmitted wave is
• This signal has a bandwidth of B centered around fc
PRINCIPLES OF RADAR IMAGING
CORRELATION WITH POINT TARGET RESPONSE
• This signal has an envelope shown on the right that is centered at r(t) and has a 3 dB width of
• This corresponds to a range resolution of
• The phase of the signal, ignoring the carrier term, is
• It is this phase term that provide the interferometric and polarimetric information
PRINCIPLES OF RADAR IMAGING
RANGE-DOPPLER PROCESSING
• The phase of the range compressed signal is
• The last approximation on the right is valid when the antenna beamwidth is very narrow, and is usually a good approximation for most higher frequency airborne SAR systems
• The expression above is that of a chirp signal with a bandwidth of where T is half the time that the target is in the field of view of the antenna
• Note that the bandwidth of the azimuth chirp is a function of the range to the target.
• The range-Doppler processing algorithm uses this fact to first perform matched filter range compression, followed by matched filter azimuth compression
• PRINCIPLES OF RADAR IMAGING
RANGE-DOPPLER PROCESSING
• PRINCIPLES OF RADAR IMAGING
CLASSICAL SAR PROCESSING GEOMETRY
• PRINCIPLES OF IMAGING RADAR
SAR IMAGE PROJECTION
• PRINCIPLES OF IMAGING RADAR
AZIMUTH AMBIGUITIES
• PRINCIPLES OF IMAGING RADAR
RANGE AMBIGUITIES
TYPES OF IMAGING RADARS
SAR POLARIMETRY
SCATTERER AS POLARIZATION TRANSFORMER
• Transverse electromagnetic waves are characterized mathematically as 2-dimensional complex vectors. When a scatterer is illuminated by an electromagnetic wave, electrical currents are generated inside the scatterer. These currents give rise to the scattered waves that are reradiated.
• Mathematically, the scatterer can be characterized by a 2x2 complex scattering matrix that describes how the scatterer transforms the incident vector into the scattered vector.
• The elements of the scattering matrix are functions of frequency and the scattering and illuminating geometries.
SCATTERING MATRIX
• Far-field response from scatterer is fully characterized by four complex numbers
• Scattering matrix is also known as Sinclair matrix or Jones matrix
• Must measure a scattering matrix for every frequency and all incidence angles
POLARIMETER IMPLEMENTATION
POLARIZATION SIGNATURE
• The polarization signature (also known as the polarization response) is a convenient graphical way to display the received power as a function of polarization.
• Usually displayed assuming identical transmit and receive polarizations (co-polarized) or orthogonal transmit and receive polarizations (cross-polarized).
OBSERVED POLARIZATION SIGNATURES:
SAN FRANCISCO
• OBSERVED POLARIZATION SIGNATURES
L-BAND POLARIZATION SIGNATURES OF THE OCEAN
• RADAR INTERFEROMETRY
HOW DOES IT WORK?
• RADAR INTERFEROMETRY
HOW IS IT DONE?
• RADAR INTERFEROMETRY
COMPARISON OF TECHNIQUES
• RADAR INTERFEROMETRY
TRIGONOMETRY
• INTERFEROMETRIC SAR PROCESSING GEOMETRY
• RADAR INTERFEROMETRY
PHASE UNWRAPPING
• RADAR INTERFEROMETRY
HEIGHT ERROR SOURCES
• DIFFERENTIAL INTERFEROMETRY
HOW DOES IT WORK?
DIFFERENTIAL INTERFEROMETRY
ERROR SOURCES
• Uncompensated differential motion
• Atmospheric effects
• Temporal decorrelation
• Layover
EMERGING SAR TECHNIQUES
POLARIMETRIC INTERFEROMETRY
• Polarimetric interferometry is implemented by measuring the full scattering matrix at each end of the interferometric baseline
• Currently there are no single baseline systems that can acquire this type of data
• During the last three days of the second SIR-C/X-SAR mission the system was operated in the repeat-pass interferometric mode, and some fully polarimetric interferometric data were acquired
• Using the full scattering matrix one can now solve for the optimum polarization to maximize the interferometric coherence
• This problem was first analyzed and reported by Cloude and Papathanassiou
• Using interferograms acquired with different polarization combinations, one can also for vector differential interferograms
• These vector differential interferograms have been shown to measure large elevation differences in forested areas, and cm-level elevation differences in agricultural fields
EMERGING SAR TECHNIQUES
POLARIMETRIC INTERFEROMETRY: COHERENCE
• Given two complex radar images, the coherence is defined as
• When the full scattering matrix is measured, the generalized coherence can be written as
• To optimize the coherence, one has to solve this expression for the two complex vectors
