Beam phase modulation for improved synthetic aperture detection and estimation

• US 20090066562A1
• Filed: 03/07/2007
• Published: 03/12/2009
• Est. Priority Date: 03/07/2007
• Status: Active Grant

• Current Assignee: Chirp Corporation
• Original Assignee: Chirp Corporation
• Inventors: Altes, Richard Alan
• Granted Patent: US 7,701,380 B2
• Time in Patent Office: Days
• US Class Current: 342/25.F
• CPC Class Codes: G01S 13/9029 specially adapted for moving target detection within a single SAR image or within multiple SAR images taken at the same time
• CPC Class Codes: G01S 13/9054 Stripmap mode

Abstract

Phase modulated beam patterns are substituted for the constant-phase versions that have been used in prior synthetic aperture systems. Relative movement between a radar/sonar/ultrasound platform and a point target causes a sequence of echoes from the point target to be phase and amplitude modulated by the beam pattern, as well as by the usual quadratic phase variation caused by range changes. Azimuth, range rate, and azimuth rate estimation, as well as detection in clutter, are substantially improved by appropriate beam pattern phase modulation, which is applied to the transmitter and/or receiver beam patterns. Phase modulated beam patterns are synthesized with array element weighting functions that are designed for high ambiguity function peak-to-sidelobe level, reduction of unwanted ambiguity ridge lines, and adequate spatial sampling. Two dimensional beam pattern phase modulation is useful when the relative motion between a transmit-receive array and multiple targets has both azimuth and elevation components.

Claims

1. Beam pattern phase modulation or coding for improved estimation/detection performance of a synthetic aperture system (SAR, SAS, ISAR, ISAS, monostatic, bistatic, and multistatic) comprising:

a transmit, receive, or combined transmit/receive beam pattern that changes phase in a nonlinear manner as an observer moves across the beam in one direction (azimuth), or in two directions (azimuth and elevation);

a phased array element weighting (shading) function with nonlinear phase variation, such that the shading function induces nonlinear phase modulation of the beam pattern in one direction (azimuth) or in two directions (azimuth and elevation), independent of element phase shifts that are used for beam steering and independent of phase corrections necessitated by a non-homogeneous propagation environment;

a method for constructing generalized target phase history models that incorporate beam-induced phase modulation (in addition to quadratic phase variation caused by range changes) as the beam pattern is moved across the target or the target is moved across the beam pattern, and that incorporate unknown (hypothesized) azimuth, range rate, and azimuth rate parameters when relative motion between an array and the target environment is only in the azimuth direction, and that include hypothesized elevation and elevation rate parameters when relative motion is in both the azimuth and elevation directions;

a method for correlating echo data with the generalized target phase history models so as to create a generalized synthetic aperture map that represents target strength as a function of range, azimuth, range rate, and azimuth rate for relative motion in the azimuth direction, and that represents target strength as a function of range, azimuth, elevation, range rate, azimuth rate and elevation rate for relative motion in both the azimuth and elevation directions;

a method for depicting the generalized synthetic aperture map for the benefit of a human observer; and

a method for depicting a difference image constructed from a generalized data map and a generalized reference map for the benefit of a human observer.

2. Beam pattern phase modulation or coding defined in claim 1, wherein the relative motion between a transmit-receive array and the target environment is in one dimension (azimuth), and the specific phased array element weighting function in the azimuth (x) direction has Hann (half-wave, squared-cosine) amplitude and phase modulation:

p_trans,az(x)=cos²[π

x/(2dM)]exp{jγ

cos²[π

x/(2dM)]},−

dM≦

x≦

dM

where d is the element spacing, the planar, rectangular array contains 2M−

1 rows of elements with nonzero weighting in the x-direction (and two virtual element rows at the ends that have zero weighting), γ

is a phase modulation factor with absolute value less than or equal to 2M, and the same array element weighting function is used for transmission and reception. The complete azimuth-elevation element weighting function is p_tans(x,y)=p_tans,az(x)p_trans,el(y), where p_trans,el(y) is a smooth shading function without nonlinear phase modulation, e.g.,

p_tans,el(Y)=cos²[π

y/(2dN)],−

dN≦

y≦

dN.

It will be understood that various substitutions with respect to particular complex beam shading functions (e.g., Hamming amplitude functions with phase functions specified by phase-hop codes, frequency-hop codes, or continuous nonlinear phase modulation functions) and non-rectangular, non-planar array configurations may be made by those skilled in the art, without departing from the spirit of the invention.

3. Beam pattern phase modulation or coding defined in claim 1, wherein the relative motion between the transmit-receive array and the target environment is in two dimensions (azimuth x and elevation y), and the specific two-dimensional phased array element weighting function for transmission can be written as the product

p_trans(x,y)=p_trans,az(x)p_trans,el(y)

for a planar, rectangular array, where

p_trans,az(x)=cos²[π

x/(2dM)]exp{jγ

cos²[π

x/(2dM)]},−

dM≦

x≦

dM

and

p_trans,el(y)=cos²[π

y/(2dM)]exp{−

jγ

cos²[π

y/(2dM)]},−

dM≦

y≦

dM.

The receiving array has the same element weighting function as the transmitting array. The element spacing is d in both x and y directions, and the array dimensions are (2M−

1)×

(2M−

1) when only elements with nonzero weights are considered. The phase modulation factor γ

has absolute value less than or equal to 2M. It will again be understood that various substitutions with respect to particular complex beam shading functions and non-rectangular, non-planar array configurations may be made by those skilled in the art, without departing from the spirit of the invention.