Description:FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) Title of invention: DICTIONARY BASED TEMPORALLY COMPRESSED SYNTHETIC APERTURE RADAR IMAGE RECONSTRUCTION Applicant Tata Consultancy Services Limited A company Incorporated in India under the Companies Act, 1956 Having address: Nirmal Building, 9th floor, Nariman point, Mumbai 400021, Maharashtra, India Preamble to the description: The following specification particularly describes the invention and the manner in which it is to be performed. TECHNICAL FIELD The disclosure herein generally relates to the field of satellite imaging, and, more particularly, to systems and methods for dictionary based temporally compressed synthetic aperture radar image reconstruction. BACKGROUND Synthetic Aperture Radar (SAR) is an active imaging radar system that is widely used in the field of remote sensing. Electromagnetic waves are illuminated from a transmitter which is mounted on a moving platform and back scattered echoes or signals are captured at different positions for subsequent image reconstruction. Further, SAR is an all-weather friendly imaging paradigm that can provide images under all conditions as opposed to optical systems. Due to this inherent advantage, it finds usage in wide range of applications ranging from infrastructure monitoring, environment monitoring, surveillance, oil spill detection, and the like. A high-resolution SAR imaging system requires more bandwidth and a larger synthetic aperture. On board storage, computing and downlink transmission of data is thus a major bottleneck in such high-resolution SAR systems. Hence, compression techniques are needed to efficiently compress the acquired raw data that can also guarantee robust reconstruction. Acquiring raw data at high sampling rate with wider synthetic aperture and transmitting the compressed SAR image after on-board reconstruction involves huge computational requirement for processing and is often not practical. Compressive Sensing (CS) based compression can be applied in both spatial and range dimensions. Spatial CS can easily be realized by sampling at different random spatial locations, while range CS comes in the scope of analog sub-Nyquist samplers. The state-of-art approaches provide reconstruction with compressed measurements, but it would require chipping sequence at Nyquist sampling rate, thereby requiring specialized hardware along with many additional RF components. SUMMARY Embodiments of the present disclosure present technological improvements as solutions to one or more of the above-mentioned technical problems recognized by the inventors in conventional systems. In an aspect, there is provided 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. In another aspect, there is provided 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. In yet another aspect, there is provided one or more non-transitory machine-readable information storage mediums comprising one or more instructions which when executed by one or more hardware processors cause: 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 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. In accordance with an embodiment of the present disclosure, the one or more hardware 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; wherein each atom of the dictionary H is computed based on a time delay. In accordance with an embodiment of the present disclosure, the formulated optimization problem is represented as: ¦((? ) ^@0)¦(= @0)¦(argmin @?)¦(|r^comp-H?|_2+ ?R(?) @0) where R(?) is the regularizer on the plurality of reflectivity coefficients ?, ? 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. In accordance with an embodiment of the present disclosure, the formulated optimization problem is solved using an Alternating Direction Method of Multipliers (ADMM). In accordance with an embodiment of the present disclosure, the radar is Frequency Modulated Continuous Wave (FMCW) radar. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate exemplary embodiments and, together with the description, serve to explain the disclosed principles: FIG.1 illustrates an exemplary block diagram of a system for dictionary based temporally compressed Synthetic Aperture Radar (SAR) image reconstruction, in accordance with some embodiments of the present disclosure. FIG.2A through FIG.2C illustrate an exemplary flow diagram of a computer implemented method for dictionary based temporally compressed SAR image reconstruction, in accordance with some embodiments of the present disclosure. FIG.3A and FIG.3B illustrate two methods of temporally sampling, at two sub-sampling factors, a pulse compressed signal, in accordance with some embodiments of the FIG.4A through FIG.4F illustrate six synthetic SAR images (scenes) available in the art, used for simulations, in accordance with some embodiments of the present disclosure. FIG.5A illustrates a visual SAR ground truth image which is the FIG.4F reproduced for ease of reference. FIG.5B illustrates visual SAR image reconstruction at SNR of 20dB, peak signal-to-noise ratio (PSNR) being 29.9 for uncompressed back scattered signal. FIG.5C illustrates visual SAR image reconstruction at SNR of 20dB and PSNR of 19.2 with 50% temporal compression for Spatial Decimation. FIG.5D illustrates visual SAR image reconstruction at SNR of 20dB, and PSNR of 23.2 with 50% temporal compression for Range Compressive Sampling (CS). FIG.5E illustrates visual SAR image reconstruction at SNR of 20dB, and PSNR of 24.3 with 50% temporal compression for Compressed Coprime Frequency-Modulated Continuous Wave (FMCW) SAR, in accordance with some embodiments of the present disclosure. FIG.6A illustrates a visual SAR ground truth image which is the FIG.4B reproduced for ease of reference. FIG.6B through FIG.6G illustrate a visual SAR ground truth image reconstruction of the Compressed Coprime FMCW SAR, at 50% temporal compression for different SNR values, in accordance with some embodiments of the present disclosure. FIG.7A through FIG.7F illustrate PSNR and Structural Similarity Index metric (SSIM) comparison for different methods with varying compression ratios at three different SNR values. DETAILED DESCRIPTION OF EMBODIMENTS Exemplary embodiments are described with reference to the accompanying drawings. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. Wherever convenient, the same reference numbers are used throughout the drawings to refer to the same or like parts. While examples and features of disclosed principles are described herein, modifications, adaptations, and other implementations are possible without departing from the scope of the disclosed embodiments. Synthetic Aperture Radar (SAR) is an active imaging radar system that finds wide application in remote sensing for infrastructure monitoring, environment monitoring, surveillance, oil spill detection, and the like. A high-resolution SAR imaging system requires more bandwidth and a larger synthetic aperture. On board storage, computing and downlink transmission of data is thus a major bottleneck in such high-resolution SAR systems. Hence, compression techniques are needed to efficiently compress the acquired raw data that can also guarantee robust reconstruction. Conventional approaches either involve huge computational requirement for processing or require specialized hardware along with many additional Radio Frequency (RF) components. Frequency Modulated Continuous Wave (FMCW) based radars are increasingly being considered for low to midrange SAR imaging due to its power efficiency and better pulse compression. Accordingly, the method and system of the present disclosure uses the FMCW radars for simulation and experimental analysis. For ease of explanation, the present disclosure refers to a sub-Nyquist range compression scheme for SAR as Compressed Coprime Frequency Modulated Continuous Wave SAR (CCF-SAR). However, it will be understood by those skilled in the art, that the method and system of the present disclosure are not limited to the use of FMCW radars alone. The present disclosure provides two approaches for temporally sampling the 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. Referring now to the drawings, and more particularly to FIG. 1 through FIG.7F, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments and these embodiments are described in the context of the following exemplary system and/or method. FIG.1 illustrates an exemplary block diagram of a system for dictionary based temporally compressed Synthetic Aperture Radar (SAR) image reconstruction, in accordance with some embodiments of the present disclosure. In an embodiment, the system 100 includes one or more hardware processors 104, communication interface (s) or input/output (I/O) interface(s) 106, and one or more data storage devices or memory 102 operatively coupled to the one or more hardware processors 104. The one or more hardware processors 104 can be implemented as one or more microprocessors, microcomputers, microcontrollers, digital signal processors, central processing units, state machines, graphics controllers, logic circuitries, and/or any devices that manipulate signals based on operational instructions. Among other capabilities, the processor(s) are configured to fetch and execute computer-readable instructions stored in the memory. In the context of the present disclosure, the expressions ‘processors’ and ‘hardware processors’ may be used interchangeably. In an embodiment, the system 100 can be implemented in a variety of computing systems, such as laptop computers, notebooks, hand-held devices, workstations, mainframe computers, servers, a network cloud and the like. The communication interface (s) 106 can include a variety of software and hardware interfaces, for example, a web interface, a graphical user interface, and the like and can facilitate multiple communications within a wide variety of networks N/W and protocol types, including wired networks, for example, LAN, cable, etc., and wireless networks, such as WLAN, cellular, or satellite. In an embodiment, the I/O interface(s) can include one or more ports for connecting a number of devices to one another or to another server. The memory 102 may include any computer-readable medium known in the art including, for example, volatile memory, such as static random access memory (SRAM) and dynamic random access memory (DRAM), and/or non-volatile memory, such as read only memory (ROM), erasable programmable ROM, flash memories, hard disks, optical disks, and magnetic tapes. In an embodiment, one or more modules (not shown) of the system 100 can be stored in the memory 102. For ease of explanation, an FMCW signal model is explained herein, as an exemplary radar to be utilized in the method and system of the present disclosure. Equation (1) below is a representation of a transmitted signal by the FMCW Radar which is a chirp signal, in which the frequency increases or decreases with reference to time. s(t)=e^(i(?_0 t+at^2)) ? (1) where ?_0 denotes the carrier frequency, chirp rate a=B/TP, B and TP denote the chirp bandwidth and a time period respectively. In SAR imaging, the radar collects measurements at different positions. Once the data acquisition is done, then the data collected from each SAR position is combined to form a better quality SAR image. Equation (2) below represents a received signal acquired at k^th position, k={1,2,….,K}, where K denotes the number of observations. (r_k ) ~(t)=?¦??(x)s(t-t?(x))dx ? (2) where x represents a region of interest for imaging and ?(·) represents an associated reflectivity coefficient. t(x) is the delay represented as 2R(x)/c, where R(·) represents a range of distance from the radar to the region of interest and c is propagation velocity of light. The received signal is first deramped by mixing it with s^* (t), reference (conjugate of transmitted) signal, which is expressed in the form of Equation (3) provided below. r_k (t)=(r_k ) ~(t) s^* (t)=?¦??(x)s(t-t?(x))s^* (t)dx+?_k (t) ? (3) where ?_k (t) denotes an additive white Gaussian noise. By using the transmitted signal of Equation (1) in Equation (3), an intermediate frequency (IF) signal as shown in Equation (4) below is obtained. r_k (t)=?¦?(x) e^(-i(?_0 t(x)+2att(x)-at(x)^2)) dx+?_k (t)? (4) The scene to be imaged and represented by x, the region of interest is discretized into smaller cells, say into L cells, {x_1,x_(2,…..,) x_L }. The Equation (4) is then expressed as Equation (5) given below. r_k (t)=?_(l=1)^L¦?(x_l ) e^(-i(?_0 t(x_l )+2att(x_l )-at(x_l )^2))+?_k (t) ? (5) The Compressive Sensing (CS) based architectures known in the art are multi-channel architectures and require pulse spreading functions whose chipping rate must be at Nyquist sampling rate. Hence, the existing state-of-art CS based SAR compression schemes are hardware intensive. The present disclosure provides simple yet efficient approaches for compressed acquisition of r_k (t). FIG.2A through FIG.2C illustrate an exemplary flow diagram of a computer implemented method 200 for dictionary based temporally compressed SAR image reconstruction, in accordance with some embodiments of the present disclosure. In an embodiment, the system 100 includes the memory 102 operatively coupled to the one or more hardware processors 104 and is configured to store instructions configured for execution of steps of the method 200 by the one or more hardware processors 104. The steps of the method 200 will now be explained in detail with reference to the components of the system 100 of FIG.1. Although process steps, method steps, techniques or the like may be described in a sequential order, such processes, methods and techniques may be configured to work in alternate orders. In other words, any sequence or order of steps that may be described does not necessarily indicate a requirement that the steps be performed in that order. The steps of processes described herein may be performed in any order practical. Further, some steps may be performed simultaneously. In accordance with the present disclosure, the one or more hardware processors 104, are configured to receive at step 202, a back scattered signal via a radar, from a region of interest for imaging. In an embodiment, the radar is an FMCW radar. Electromagnetic waves are illuminated from a transmitter which is mounted on a moving platform and the back scattered signal (echo) is captured at different positions for subsequent image reconstruction. In accordance with the present disclosure, the one or more hardware processors 104, are configured to mix, at step 204, the received back scattered signal with a reference signal to obtain a pulse compressed signal. In an embodiment, the reference signal is a conjugate of a transmitted signal by the radar. In the event that the radar is the FMCW radar, the step of mixing comprises deramping as explained above. In accordance with the present disclosure, the one or more hardware processors 104, are configured to temporally sample, at step 206, 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. The step of temporally sampling to acquire the temporally compressed sampled signals at sub-sampling rate comprises one of two methods provided herein below. FIG.3A and FIG.3B illustrate the two methods (approaches) of temporally sampling the pulse compressed signal, at two sub-sampling factors, in accordance with some embodiments of the present disclosure. In accordance with a first approach referred to as step 206a, sub-Nyquist sampling is performed using the two sub-sampling factors (d_1 and d_2) which are co-primes. Referring to FIG.3A, the pulse compressed signal referred as FMCW IF Signal r_k (t) is temporally sampled at two different sub-Nyquist rates using two sub-sampling factors (d_1 and d_2). The measurements that are collected at the two sampling rates are then concatenated to form a received measurement vector r. T represents a Nyquist sampling period. In accordance with a second approach referred to as step 206b, the pulse compressed signal at one of the two sub-sampling factors is delayed and then the sub-Nyquist sampling is performed using a sub-sampling factor d_3, on (i) the delayed pulse compressed signal and (ii) the pulse compressed signal. In accordance with the present disclosure, a delay factor (C_1) is employed which is not a multiple of the sub-sampling factor d_3. In an embodiment, the sub-sampling factor d_3 maybe same as either of the two sub-sampling factors (d_1 and d_2). Referring to FIG.3B, the pulse compressed signal referred as FMCW IF Signal r_k (t) and its delayed version are sampled at same rate using the same sub-sampling factor (d_3). In accordance with the present disclosure, C_1 and d_3 need to satisfy the following conditions: C_1 is an integer C_1?nd_3, where n is an integer For any d_3, C_1=1 is valid. In the second approach of step 206b, only one clock is sufficient, since the sampling is performed at the same rate. There are no common measurements between the sampler’s output and hence no redundancy which in turn increases the number of measurements. Unlike in the first approach of step 206a, where the two sub-sampling factors (d_1 and d_2) are coprime, in the second approach, the sub-sampling factor d_3 can be any number and C_1 is accordingly selected. In accordance with the present disclosure, the one or more hardware processors 104, are configured to construct, at step 208, a dictionary H based on the temporally compressed sampled signals obtained at step 206. Let ?r_k?^(?(d?_p)) (n), p={1,2} represent the two sub-sampling factors sampled signal which is expressed as given below. ?r_k?^(?(d?_p)) (n)=?_(l=1)^L¦?(x_l ) e^(-i(?_0 t(x_l )+2ad_p nt(x_l )-at(x_l )^2))+??_k?^(d_p ) (n). Let r=[¦(?r_k?^(d_1 )&?r_k?^(d_2 ) )], where ?r_k?^(d_p )=[¦(?r_1?^(d_p ),&?r_2?^(d_p ) ),…?r_K?^(d_p ) ]·?r_k?^(d_p ) denotes a collection of ?r_k?^(d_p ) (n) for all time samples. Equation (6) provided below represents a forward model equation from which a plurality of reflectivity coefficients ? of the region of interest are estimated using the received measurement vector r^comp=vec{r} and the dictionary H. r^comp=H?+? ? (6) In an embodiment, the step of constructing the dictionary H 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 as represented in Equation (7a) below 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 as represented in Equations (7b1 and 7b2) below. Let N denote number of time samples, then in an embodiment, the dictionary H=[H_(d_1 ),H_(d_2 ) ]^T, where H_(d_p ) is a NK×L dimension matrix, whose ?(u,w)?^th atom is expressed as Equation (7a) given below. ?[H_(d_p ) ]_(u,w)=e?^(-i(?_0 t_u (x_w )+2ad_p wnt_u (x_w )-at_u (x_w )^2)) ? (7a) where t_u (x_w )=2R_u (x_u)/c, which represents a range from u^th radar location to x_w^(th ) position in an image space. . In another embodiment, the dictionary H=[H_(d_3 ),H_(?(d?_(3-delayed))) ]^T. For? d?_3, ?r_k?^(?(d?_3)) (n)=?_(l=1)^L¦?(x_l ) e^(-i(?_0 t(x_l )+2ad_3 nt(x_l )-at(x_l )^2))+??_k?^(d_3 ) (n) and ?[H_(d_3 ) ]_(u,w)=e?^(-i(?_0 t_u (x_w )+2ad_3 wnt_u (x_w )-at_u (x_w )^2)) ?(7b1) For d_(3-delayed), the pulse compressed signal gets delayed by C_1. ?r_k?^(?(d?_(3-delayed))) (n-C_1 )=?_(l=1)^L¦?(x_l ) e^(-i(?_0 t(x_l )+2a?(d?_(3-delayed))(n-C_1)t(x_l )-at(x_l )^2))+??_k?^(?(d?_(3-delayed))) (n-C_1) and ?[H_(?(d?_(3-delayed))) ]_(u,w)=e?^(-i(?_0 t_u (x_v )+2a?(d?_(3-delayed) )w(n-C_1 ) t_u (x_w )-at_u (x_w )^2)) ?(7b2) In accordance with the present disclosure, the one or more hardware processors 104, are configured to formulate, at step 210, an optimization problem comprising the temporally compressed sampled signals, the constructed dictionary H and a regularizer. Further, in accordance with the present disclosure, the one or more hardware processors 104, are configured to estimate, at step 212, the plurality of reflectivity coefficients of the region of interest by solving the formulated optimization problem. ?=[?(x_1 ),?(x_2 ),……?(x_L )]^T denotes a vector of the plurality of reflectivity coefficients. It may be noted that rows of H_(d_1 )and H_(d_2 ) at row position, aLCM(d_1,d_2 ),a ? Z are common (duplicate). In accordance with the present disclosure, the duplicate rows and corresponding measurements are removed by retaining only one row and the corresponding measurement, hence the actual dimension of H will be M×L, where M=(2NK-¦|aLCM(d_1,d_2 ):aLCM(d_1,d_2 )