Metrics to evaluate compressions algorithms for RAW SAR data

Modern synthetic aperture radar (SAR) systems have size, weight, power and cost (SWAP-C) limitations since platforms are becoming smaller, while SAR operating modes are becoming more complex. Due to the computational complexity of the SAR processing required for modern SAR systems, performing the processing on board the platform is not a feasible option. Thus, SAR systems are producing an ever-increasing volume of data that needs to be transmitted to a ground station for processing. Compression algorithms are utilised to reduce the data volume of the raw data. However, these algorithms can cause degradation and losses that may degrade the effectiveness of the SAR mission. This study addresses the lack of standardised quantitative performance metrics to objectively quantify the performance of SAR data-compression algorithms. Therefore, metrics were established in two different domains, namely the data domain and the image domain. The data-domain metrics are used to determine the performance of the quantisation and the associated losses or errors it induces in the raw data samples. The image-domain metrics evaluate the quality of the SAR image after SAR processing has been performed. In this study three well-known SAR compression algorithms were implemented and applied to three real SAR data sets that were obtained from a prototype airborne SAR system. The performance of these algorithms were evaluated using the proposed metrics. Important metrics in the data domain were found to be the compression ratio, the entropy, statistical parameters like the skewness and kurtosis to measure the deviation from the original distributions of the uncompressed data, and the dynamic range. The data histograms are an important visual representation of the effects of the compression algorithm on the data. An important error measure in the data domain is the signal-to-quantisation-noise ratio (SQNR), and the phase error for applications where phase information is required to produce the output. Important metrics in the image domain include the dynamic range, the impulse response function, the image contrast, as well as the error measure, signal-to-distortion-noise ratio (SDNR). The metrics suggested that all three algorithms performed well and are thus well suited for the compression of raw SAR data. The fast Fourier transform block adaptive quantiser (FFT-BAQ) algorithm had the overall best performance, but the analysis of the computational complexity of its compression steps, indicated that it is has the highest level of complexity compared to the other two algorithms. Since different levels of degradation are acceptable for different SAR applications, a trade-off can be made between the data reduction and the degradation caused by the algorithm. Due to SWAP-C limitations, there also remains a trade-off between the performance and the computational complexity of the compression algorithm.Dissertation (MEng)--University of Pretoria, 2019.Electrical, Electronic and Computer EngineeringMEngUnrestricte

Sentinel-1 Imaging Performance Verification with TerraSAR-X

This paper presents dedicated analyses of TerraSAR-X data with respect to the Sentinel-1 TOPS imaging mode. First, the analysis of Doppler centroid behaviour for high azimuth steering angles, as occurs in TOPS imaging, is investigated followed by the analysis and compensation of residual scalloping. Finally, the Flexible-Dynamic BAQ (FD-BAQ) raw data compression algorithm is investigated for the first time with real TerraSAR-X data and its performance is compared to state-of-the-art BAQ algorithms. The presented analyses demonstrate the improvements of the new TOPS imaging mode as well as the new FD-BAQ data compression algorithm for SAR image quality in general and in particular for Sentinel-1

Metrics to evaluate compressions algorithms for RAW SAR data

Modern synthetic aperture radar (SAR) systems have size, weight, power and cost (SWAP-C) limitations since platforms are becoming smaller, while SAR operating modes are becoming more complex. Due to the computational complexity of the SAR processing required for modern SAR systems, performing the processing on board the platform is not a feasible option. Thus, SAR systems are producing an ever-increasing volume of data that needs to be transmitted to a ground station for processing. Compression algorithms are utilised to reduce the data volume of the raw data. However, these algorithms can cause degradation and losses that may degrade the effectiveness of the SAR mission. This study addresses the lack of standardised quantitative performance metrics to objectively quantify the performance of SAR data-compression algorithms. Therefore, metrics were established in two different domains, namely the data domain and the image domain. The data-domain metrics are used to determine the performance of the quantisation and the associated losses or errors it induces in the raw data samples. The image-domain metrics evaluate the quality of the SAR image after SAR processing has been performed. In this study three well-known SAR compression algorithms were implemented and applied to three real SAR data sets that were obtained from a prototype airborne SAR system. The performance of these algorithms were evaluated using the proposed metrics. Important metrics in the data domain were found to be the compression ratio, the entropy, statistical parameters like the skewness and kurtosis to measure the deviation from the original distributions of the uncompressed data, and the dynamic range. The data histograms are an important visual representation of the effects of the compression algorithm on the data. An important error measure in the data domain is the signal-to-quantisation-noise ratio (SQNR), and the phase error for applications where phase information is required to produce the output. Important metrics in the image domain include the dynamic range, the impulse response function, the image contrast, as well as the error measure, signal-to-distortion-noise ratio (SDNR). The metrics suggested that all three algorithms performed well and are thus well suited for the compression of raw SAR data. The fast Fourier transform block adaptive quantiser (FFT-BAQ) algorithm had the overall best performance, but the analysis of the computational complexity of its compression steps, indicated that it is has the highest level of complexity compared to the other two algorithms. Since different levels of degradation are acceptable for different SAR applications, a trade-off can be made between the data reduction and the degradation caused by the algorithm. Due to SWAP-C limitations, there also remains a trade-off between the performance and the computational complexity of the compression algorithm.Dissertation (MEng)--University of Pretoria, 2019.TM2019Electrical, Electronic and Computer EngineeringMEngUnrestricte

Adaptive On-Board Signal Compression for SAR Using Machine Learning Methods

Satellites with synthetic aperture radar (SAR) payloads are growing in popularity, with a number of new institutional missions and commercial constellations launched or in planning. As an active instrument operating in the microwave region of the electromagnetic spectrum, SAR provides a number of unique advantages over passive optical instruments, in that it can image in all weather conditions and at night. This allows dense time-series to be built up over areas of interest, that are useful in a variety of Earth observation applications. The polarisation and phase information that can be captured also allows for unique applications not possible in optical frequencies. The data volume of SAR captures is growing due to developments in modern high-resolution multi-modal SAR. Instruments with higher spatial resolution, wider swaths, multiple beams, multiple frequencies and more polarization channels are being launched. Miniaturization and the deployment of SAR constellations is bringing improved revisit times. All of these developments drive an increase in the operational cost due to the increase in data downlink required. These factors will make on-board data compression more crucial to overall system performance, especially in large scale constellations. The current deployed state-of-the-art of on-board compression in SAR space-borne payloads is Block Adaptive Quantization (BAQ) and variations such as Flexible BAQ, Entropy Constrained BAQ and Flexible Dynamic BAQ. Craft Prospect is working on an evolution of these techniques where machine learning will be used to identify signals based on dynamics and features of the received signal, with this edge processing allowing the tagging of raw data. These tags can then be used to better adjust the compression parameters to fit the local optimum in the acquired data. We present the results of a survey of available raw SAR data which was used to inform a selection of applications and frequencies for further study. Following this, we present a comparison of a number of SAR compression algorithms downselected using trade-off metrics such as the bands/applications they can be applied to and various complexity measures. We then show an assessment of AI/ML feasibility and capabilities, with the improvements assessed on mission examples characterised by the SAR modes and architecture for specific SAR applications. Finally, future hardware feasibility and capability is assessed, targeting a Smallsat SAR mission, with a high level roadmap developed to progress the concept toward this goal

FDBAQ A NOVEL ENCODING SCHEME FOR SENTINEL-1

Modern operational and/or high resolution SAR satellite missions impose stringent requirements on on-board data compression such as a higher data reduction ratio, more flexibility, and faster data throughput. A novel approach is Flexible Dynamic Block Adaptive Quantization (FDBAQ). This method outperforms currently used Block Adaptive Quantization with respect to Signal-to-Noise-Ratio related to the compression ratio. The FDBAQ method allows bit rate programmability with non-integer rates. This allows the SAR information throughput to be optimized for different types of targets and down-link scenarios using a tradeoff between thermal and quantization noise