Motion compensation for convolutional SAR algorithms

• US 20060028370A1
• Filed: 08/04/2004
• Published: 02/09/2006
• Est. Priority Date: 08/04/2004
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: Lawrence, Michael E., Hansen, Charles T.
• Granted Patent: US 7,126,524 B2
• Time in Patent Office: Days
• US Class Current: 342/25.A
• CPC Class Codes: G01S 13/9019 Auto-focussing of the SAR signals
• CPC Class Codes: G01S 13/904 SAR modes
• CPC Class Codes: G01S 13/9088 Circular SAR [CSAR, C-SAR]

Abstract

A synthetic aperture image of a scene is acquired using a radar system. The scene has one or more radar scatterers located on a horizontal flat {circumflex over (x)},ŷ plane. The radar system is mounted on a moving platform moving at an angle θ_tilt with respect to the {circumflex over (x)},ŷ plane, with a component of motion in a perpendicular {circumflex over (z)} direction. The synthetic aperture image acquisition requires digitizing radar returns having a phase returned from scatterers in the scene, adjusting the phase of the radar returns in response to θ_tilt to generate phase adjusted returns, then computing the synthetic aperture image from said phase adjusted returns. The phase adjustment takes into account θ_tilt platform motion with respect to the scene.

Claims

1. A radar for acquiring a synthetic aperture image of a scene, said scene having one or more radar scatterers located on a horizontal flat {circumflex over (x)},ŷ

plane, said radar system mounted on a moving platform moving at an angle θ

_tilt with respect to said {circumflex over (x)},ŷ

plane, with a component of motion in a z direction, said z direction perpendicular to said {circumflex over (x)},ŷ

plane, said radar comprising;

a radar receiver for digitizing radar returns having a phase from said scene, and a computer for adjusting said phase of said radar returns in response to said θ

_tilt to generate phase adjusted returns and computing said synthetic aperture image from said phase adjusted returns.

2. A radar system as described in claim 1 wherein said phase of said radar returns is adjusted by computing a nominal aircraft track x_ac,o(s) given by

x_ac,o(s)=s cos(θ

_tilt){circumflex over (x)}+y_ac,oŷ

+z_ac,o{circumflex over (z)}+s sin(θ

_tilt){circumflex over (z)}

an aircraft position parameter s given by

$s=\frac{1}{\cos <br><br>\left({\mathrm{\theta <br><br>}}_{\mathrm{tilt}}\right)}<br><br>\left[x_o+\frac{x_{a<br><br>\text{\hspace{1em}<br><br>}<br><br>c}-x_o}{y_{a<br><br>\text{\hspace{1em}<br><br>}<br><br>c}-y_o}<br><br>\left(y_{a<br><br>\text{\hspace{1em}<br><br>}<br><br>c,o}-y_o\right)\right]$

a Fast Time Scale Factor SF_RI given by

${\mathrm{SF}}_{\mathrm{RI}}=\frac{y_{\mathrm{ac},o}-y_o}{y_{\mathrm{ac}}-y_o}·<br><br>\frac{<br><br>x_{\mathrm{ac}}-x_o<br><br>}{<br><br>x_{\mathrm{ac},o}<br><br>\left(s\right)-x_o<br><br>};<br><br>$

a Range Offset R_P given by

$R_p=<br><br>x_{\mathrm{ac}}-x_o<br><br>-\frac{1}{{\mathrm{SF}}_{\mathrm{RI}}}<br><br>\left[<br><br>x_{\mathrm{ac},o}<br><br>\left(s\right)-x_o<br><br>-R_s\right];<br><br>$

a phase adjustment given by

$r<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(x_{\mathrm{ac},o}<br><br>\left(s\right),x_T\right)={\mathrm{e<br><br>}}^{\mathrm{i<br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{SF}}_{\mathrm{RI}}\right)<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\varphi <br><br>}}_{\mathrm{real}}<br><br>\left(x_{\mathrm{ac}},x_T\right)}<br><br>{\mathrm{e<br><br>}}^{-\mathrm{i<br><br>}<br><br>\frac{4<br><br>\mathrm{\pi <br><br>}}{\mathrm{\lambda <br><br>}}<br><br>R_p};<br><br>$

where

x_ac,o y_ac,o and z_ac,o show an aircraft position,

x_o y_o and z_o show an initial aircraft position in the {circumflex over (x)}, ŷ

, {circumflex over (z)} direction respectively, and

λ

is an operating wavelength of said radar, XT is radar target location and R, is an initial range.

3. A method for acquiring a synthetic aperture image of a scene using a radar system, said scene having one or more radar scatterers located on a horizontal flat {circumflex over (x)},ŷ

plane, said radar system mounted on a moving platform moving at an angle θ

_tilt with respect to said {circumflex over (x)},ŷ

plane, with a component of motion in a z direction, said {circumflex over (z)} direction perpendicular to said {circumflex over (x)}, ŷ

plane, said method comprising the steps of;

digitizing radar returns having a phase from said scene;

adjusting said phase of said radar returns in response to said θ

_tilt to generate phase adjusted returns;

computing said synthetic aperture image from said phase adjusted returns.

4. A method as described in claim 3 wherein said phase of said radar returns is adjusted by computing a nominal aircraft track x_ac,o(s) given by

x_ac,o(s)=s cos(θ

_tilt){circumflex over (x)}+y_ac,oŷ

+z_ac,o{circumflex over (z)}+s sin(θ

_tilt){circumflex over (z)}

an aircraft position parameter s given by

$s=\frac{1}{\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\theta <br><br>}}_{\mathrm{tilt}}\right)}<br><br>\left[x_o+\frac{x_{\mathrm{ac}}-x_o}{y_{\mathrm{ac}}-y_o}<br><br>\left(y_{\mathrm{ac},o}-y_o\right)\right];<br><br>$

a Fast Time Scale Factor SFRI given by

${\mathrm{SF}}_{\mathrm{RI}}=\frac{y_{\mathrm{ac},o}-y_o}{y_{\mathrm{ac}}-y_o}·<br><br>\frac{<br><br>x_{\mathrm{ac}}-x_o<br><br>}{<br><br>x_{\mathrm{ac},o}<br><br>\left(s\right)-x_o<br><br>};<br><br>$

a Range Offset R, given by

$R_p=<br><br>x_{\mathrm{ac}}-x_o<br><br>-\frac{1}{{\mathrm{SF}}_{\mathrm{RI}}}<br><br>\left[<br><br>x_{\mathrm{ac},o}<br><br>\left(s\right)-x_o<br><br>-R_s\right];<br><br>$

a phase adjustment given by

r(x_ac,o(s), x_T)=e<sup>i(SFRI)Φ

real(xac,xT)</sup>e<sup>−

i4π

/λ

Rp</sup>

where

x_ac,o y_ac,o and z_ac,o show an aircraft position,

x_o y_o and z_o show an initial aircraft position in the {circumflex over (x)}, ŷ

, {circumflex over (z)} direction respectively, and

λ

is an operating wavelength of said radar, x_T is radar target location and R_s is an initial range.