Image analysis – Image transformation or preprocessing
Reexamination Certificate
1998-02-19
2003-09-30
Mehta, Bhavesh M. (Department: 2721)
Image analysis
Image transformation or preprocessing
C382S277000, C382S280000, C382S281000
Reexamination Certificate
active
06628844
ABSTRACT:
DESCRIPTION
1. Technical Field
The invention relates to the field of signal processing, and in particular to a signal processing apparatus and method which processes data captured from a sensor such as a synthetic aperture radar to provide a high resolution image.
2. Background of the Invention
Synthetic aperture radar (SAR) is a well known technique for imaging stationary objects. SAR is an all weather imaging radar system which provides a high resolution image in both the range dimension and the cross range dimension. Range resolution is achieved in a well known manner by using either a high bandwidth fixed frequency transmit pulse or a frequency modulated (FM) transmit pulse. Resolution in the cross range dimension is achieved by synthesizing a large antenna aperture.
In a conventional non-synthetic aperture radar system, resolution in the cross range dimension is:
&dgr;
cr
R&thgr;
B
(Eq. 1)
where:
&dgr;
cr
=cross range
R=range
&thgr;
B
=beamwidth of the transmitted signal in radians
Therefore, to improve the cross range resolution &dgr;
cr
the beamwidth &thgr;
B
must be decreased. &thgr;
B
is defined as:
&thgr;
B
=(
k
&lgr;)/
D
(Eq. 2)
where:
k=constant
&lgr;=wavelength of the transmitted signal (i.e., c/f
c
)
D=antenna width
c=speed of light
f
c
=carrier frequency
Substituting Eq. 2 into Eq. 1, one can see that for improved cross range resolution &dgr;
cr
the radar designer can either increase the antenna width D or decrease the wavelength &lgr; of the transmitted signal. However, there are clearly limits on how large the antenna width D can get (especially on an airborne platform) to achieve cross range resolution satisfactory for imaging. Similarly, the wavelength &lgr; can be decreased only so far before it becomes so short that the radar performance becomes degraded in foul weather conditions (e.g., rain, snow and sleet), or the system becomes impractical because of the bandwidth requirement. SAR solves this problem by employing signal processing techniques which allow a larger antenna of width D′ to be synthesized using the motion of the radar platform (e.g., an antenna mounted on an aircraft). That is, SAR achieves cross range resolution by using the motion of the vehicle carrying the radar to generate a synthesized antenna of size D′ sequentially, rather than simultaneously as in the case with a real antenna of the same size.
The key to SAR is the data processing of stored reflected return data, and the amplitude weighting, phase shifting and coherently summing of the data to form the synthetic aperture radar antenna of width D′. For an overview of SAR see “
An Introduction to Synthetic Aperture Radar
” by W. M. Brown and L. J. Porcelli, IEEE Spectrum (September, 1969) pages 52-62.
An airborne SAR system is typically used to map or image a specific ground terrain (also referred to herein as a SAR scene). As an example,
FIG. 1
illustrates a SAR equipped aircraft
20
flying along a flight path
22
monitoring a certain SAR scene
24
. The SAR equipped aircraft
20
transmits a series of RF pulses towards the SAR scene
24
and receives backscattered RF energy whose information content is indicative of the terrain and other reflecting objects on the terrain (e.g., buildings, trucks, cars, ships, planes . . . ). A short time later the aircraft
20
is located at a second location
28
along the flight path
22
and again transmits RF energy towards the SAR scene
24
. As known, the distance traveled by the aircraft between pulse transmissions should be less than one-half the illuminating aperture size when the radar's line of sight is perpendicular to the platforms velocity vector. The received RF energy at the second location
28
is again indicative of the SAR scene but this time it is taken from a different view. Since radar signals travel at the speed of light, it is known precisely when a return signal is likely to come from SAR scene
24
at a given range from the aircraft
20
. Accordingly, for each transmitted RF pulse there will be a plurality of return signals corresponding to the various scatterers within the SAR scene located at various ranges from the aircraft. These returns can be processed in real-time or off-line to create an image of the SAR scene
24
and stationary objects therein using the Doppler history of the objects. That is, each return signal contains the radar carrier frequency signal f
c
component with a Doppler shift in frequency (f
c
±f
d
) which in reality is the phase of the backscattered signal as a function of time with respect to the phase of the transmitted signal.
Referring to
FIG. 2
, an SAR system
30
includes an antenna
32
which transmits pulsed RF energy (e.g., X or Ku band) and receives backscattered RF energy from the illuminated SAR scene
24
(FIG.
1
). The radar system
30
includes an exciter
34
and an amplifier
36
which generate and provide an uncompressed pulse of RF energy signal on a line
38
which is coupled to the antenna
32
.
To obtain fine range resolution, a linear FM waveform is used in which frequency value f
c
is changed linearly from a frequency value f
1
to a value f
2
over the transmitted pulse length &tgr;. This allows the radar to utilize a long pulse to achieve a large amount radiated energy while retaining the range resolution associated with a shorter pulse. Other known pulse compression techniques include nonlinear FM, discrete frequency shift, polyphase codes, phase coded pulse compression, compound Barker codes, coding sequences, complementary codes, pulse burst and stretch.
During receive mode, each antenna
32
receives backscattered RF energy data indicative of the SAR scene
24
(
FIG. 1
) being imaged and provides a received signal on a line
42
to a receiver
44
. The receiver
44
coherently processes the received signal data and provides a received signal on a line
46
containing both in-phase(I) and quadrature(Q) data to a signal processor
48
. A coherent reference signal is generally required for the signal processing since an azimuth angle measurement is a measurement of phase from spatially separate positions. That is, the coherent radar remembers the phase difference from transmission of a pulse to reception of the backscattered energy from the pulse. The received signals contain the carrier signal f
c
with a Doppler shift f
d
in frequency, which in reality is its phase versus time.
Each backscattered RF signal is often converted to a digital signal format as early as possible in the signal processing sequence due to the greater degree of design flexibility inherent in the discrete time domain. This often occurs after the RF received signal has been bandshifted to an intermediate frequency (IF) and then to a video signal having both an in-phase(I) and quadrature(Q) component. The sampling rate of the analog-to-digital converter (ADC) (not shown) must be fast enough to meet the well known Nyquist sampling criteria to prevent alaising. Once sampled and digitized, the received video signal containing the I and Q signal components can be processed by the signal processor
48
to image objects within the SAR scene. A radar processor/controller
50
controls the operation of the radar system based upon inputs received from an operator control panel/interface
52
and the current operating condition of the radar system. Images formed by the signal processor are presented on a display
54
. The system also includes a memory storage
56
wherein received data can be stored for subsequent, non-realtime processing.
FIG. 3
illustrates a top-level functional block diagram of signal processing routines
60
performed either in real-time or off-line to image stationary object within the SAR scene
24
(FIG.
1
). To implement the routines in real-time, one skilled in the art will appreciate that the signal processor
48
requires a large amount of data storage and processing power.
The signal processor
48
executes a data calibration routine
62
which recei
Massachusetts Institute of Technology
Mehta Bhavesh M.
Patel Kanji
Samuels , Gauthier & Stevens, LLP
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