Dictionary based temporally compressed synthetic aperture radar image reconstruction

Patent metadata

Field	Value
Publication number	US-2024077604-A1
Application number	US-202318346500-A
Country	US
Kind code	A1
Filing date	Jul 3, 2023
Priority date	Sep 7, 2022
Publication date	Mar 7, 2024
Grant date	—

Abstract

This disclosure relates generally to Synthetic Aperture Radar (SAR) reconstruction and finds wide application in remote sensing. Conventional approaches either involve huge computational requirement for processing or require specialized hardware along with many additional Radio Frequency (RF) components. The present disclosure provides two approaches for temporally sampling a received pulse compressed signal at two sub-sampling factors, wherein both methods involve frugal hardware implementation. Reconstruction approach of the art is based on the principle of difference ruler and is not suitable for SAR image reconstruction due to the large measurements and image dimensions. In accordance with the present disclosure, the reconstruction problem is framed as an inverse imaging problem by suitably using a forward model and employing an approach like Alternating Direction Method of Multipliers (ADMM) for solving this model which allows use of readily available Plug and Play (PnP) priors.

First claim

What is claimed is:

1 . A processor implemented method comprising:
receiving via a radar, a back scattered signal from a region of interest for imaging; mixing, via one or more hardware processors, the received back scattered signal with a reference signal to obtain a pulse compressed signal, wherein the reference signal is a conjugate of a transmitted signal by the radar; temporally sampling, via the one or more hardware processors, the pulse compressed signal at two sub-sampling factors, to obtain temporally compressed sampled signals, a signal from each of the two sub-sampling factors, wherein the temporally sampling comprises one of:
sub-Nyquist sampling using the two sub-sampling factors (d 1 and d 2 ) which are co-primes; and
delaying the pulse compressed signal at one of the two sub-sampling factors and performing the sub-Nyquist sampling, using a sub-sampling factor d 3 on (i) the delayed pulse compressed signal, wherein an associated delay factor (C 1 ) is not a multiple of the sub-sampling factor d 3 and (ii) the pulse compressed signal;
constructing, via the one or more hardware processors, a dictionary H based on the temporally compressed sampled signals; formulating, via the one or more hardware processors, an optimization problem comprising the temporally compressed sampled signals, the constructed dictionary and a regularizer; estimating, via the one or more hardware processors, a plurality of reflectivity coefficients of the region of interest by solving the formulated optimization problem; and imaging, via the one or more hardware processors, the region of interest by reconstructing a Synthetic Aperture Radar (SAR) image thereof, based on the temporally compressed sampled signals, using the estimated plurality of reflectivity coefficients.

2 . The processor implemented method of claim 1 , wherein the step of constructing a dictionary H based on the sampled signals comprises one of (i) a union of measurement matrices H d 1 and H d 2 corresponding to the two sub-sampling factors d 1 and d 2 respectively and (ii) a union of measurement matrices H d 3 and H d 3-delayed corresponding to the sub-sampling factor d 3 on the pulse compressed signal and the delayed pulse compressed signal respectively; and wherein each atom of the dictionary H is computed based on a time delay.

3 . The processor implemented method of claim 1 , wherein the formulated optimization problem is represented as:

ρ
^

=

argmin
ρ

⁢

❘
"\[LeftBracketingBar]"

r
comp

-

H
⁢
ρ

❘
"\[RightBracketingBar]"

2

+

λ
⁢

R
⁡
(
ρ
)

where R(ρ) is the regularizer on the plurality of reflectivity coefficients ρ, λ is a penalty term which controls the amount of regularization and r comp =vec{r}, where r is a collection of temporally compressed sampled signals.

4 . The processor implemented method of claim 3 , wherein the formulated optimization problem is solved using an Alternating Direction Method of Multipliers (ADMM).

5 . The processor implemented method of claim 1 , wherein the radar is a Frequency Modulated Continuous Wave (FMCW) radar.

6 . A system comprising:
a memory storing instructions; one or more communication interfaces; and one or more hardware processors coupled to the memory via the one or more communication interfaces, wherein the one or more hardware processors are configured by the instructions to: receive via a radar, a back scattered signal from a region of interest for imaging; mix, the received back scattered signal with a reference signal to obtain a pulse compressed signal, wherein the reference signal is a conjugate of a transmitted signal by the radar; temporally sample, the pulse compressed signal at two sub-sampling factors, to obtain temporally compressed sampled signals, a signal from each of the two sub-sampling factors, wherein the temporally sampling comprises one of:
sub-Nyquist sampling using the two sub-sampling factors (d 1 and d 2 ) which are co-primes; and
delaying the pulse compressed signal at one of the two sub-sampling factors and performing the sub-Nyquist sampling, using a sub-sampling factor d 3 on (i) the delayed pulse compressed signal, wherein an associated delay factor (C 1 ) is not a multiple of the sub-sampling factor d 3 and (ii) the pulse compressed signal;
construct, a dictionary based on the temporally compressed sampled signals; formulate, an optimization problem comprising the temporally compressed sampled signals, the constructed dictionary and a regularizer; estimate, a plurality of reflectivity coefficients of the region of interest by solving the formulated optimization problem; and image, the region of interest by reconstructing a Synthetic Aperture Radar (SAR) image thereof, based on the temporally compressed sampled signals, using the estimated plurality of reflectivity coefficients.

7 . The system of claim 6 , wherein the one or more processors are configured to construct a dictionary based on the sampled signals by one of (i) a union of measurement matrices H d 1 and H d 2 corresponding to the two sub-sampling factors d 1 and d 2 respectively and (ii) a union of measurement matrices H d 3 and H d 3-delayed corresponding to the sub-sampling factor d 3 on the pulse compressed signal and the delayed pulse compressed signal respectively; and wherein each atom of the dictionary H is computed based on a time delay.

8 . The system of claim 6 , wherein the formulated optimization problem is represented as:

ρ
^

=

argmin
ρ

⁢

❘
"\[LeftBracketingBar]"

r
comp

-

H
⁢
ρ

❘
"\[RightBracketingBar]"

2

+

λ

Assignees

• Tata Consultancy Services Ltd

Inventors

• Rokkam Krishna Kanth
• Gigie Andrew
• Kuchibhotla Aditi
• Kumar Achanna Anil
• Chakravarty Tapas
• Purushothaman Balamuralidhar
• Reddy Kancham Pavan Kumar

Classifications

• G01S13/9004 Primary

  SAR image acquisition techniques · CPC title

• G06V10/25

  Determination of region of interest [ROI] or a volume of interest [VOI] · CPC title

• H04N19/17

  the unit being an image region, e.g. an object · CPC title

• H04N19/167

  Position within a video image, e.g. region of interest [ROI] · CPC title

• H04N19/132

  Sampling, masking or truncation of coding units, e.g. adaptive resampling, frame skipping, frame interpolation or high-frequency transform coefficient masking · CPC title