Synthetic aperture sonar (SAS) technology has rapidly progressed over the past few years with a number of commercial systems emerging. Such systems are typically based on an autonomous underwater vehicle platform containing multiple along-track receivers and an integrated inertial navigation system (INS) with Doppler velocity log aiding. While producing excellent images, blurring due to INS integration errors and medium fluctuations continues to limit long range, long run, image quality. This is particularly relevant in mine hunting, the main application for SAS, where it is critical to survey the greatest possible area in the shortest possible time, regardless of sea conditions. This thesis presents the simulation, design, construction, and sea trial results for a prototype "active beacon" and remote controller unit, to investigate the potential of such a device for estimating SAS platform motion and medium fluctuations. The beacon is deployed by hand in the area of interest and acts as an active point source with real-time data uploading and control performed by radio link. Operation is tightly integrated with the operation of the Acoustics Research Group KiwiSAS towed SAS, producing one-way and two-way time of flight (TOF) data for every ping by detecting the sonar chirps, time-stamping their arrival using a GPS receiver, and replying back at a different acoustic frequency after a fixed time delay. The high SNR of this reply signal, combined with the knowledge that it is produced by a single point source, provides advantages over passive point-like targets for SAS image processing. Stationary accuracies of < 2 mm RMS have been measured at ranges of up to 36m. This high accuracy allowed the beacon to be used in a separate study to characterise the medium fluctuation statistics in Lyttelton Harbour, New Zealand, using an indoor dive pool as a control. Probability density functions were fitted to the data then incorporated in SAS simulations to observe their effect on image quality. Results from recent sea trials in Lyttelton Harbour show the beacon TOF data, when used in a narrowband motion compensation (MOCOMP) process, provided improvements to the quality of SAS images centred on frequencies of 30 kHz and 100 kHz. This prototype uses simple matched-filtering algorithms for detection and while performing well under stationary conditions, the fluctuations caused by the narrow sonar transmit beam pattern (BP) and changing superposition of seabed multipath often cause dropouts and inaccurate detections during sea trials. An analysis of the BP effects and how the accuracy and robustness of the detection algorithms can be improved is presented. Overcoming these problems reliably is difficult without dedicated large scale testing facilities to allow conditions to be reproduced consistently.

This dissertation thesis was focused on improving the methodology to detect fish with modern sonar systems in lakes and reservoirs. The first part of the thesis is aimed to the vertical beaming acoustics with a key focus on the acoustic dead zone and its practical solution. The second part deals with the fish orientation in reservoir?s open waters and its consequences in horizontal beaming acoustics. The last one dedicates to the DIDSON multi-beam sonar and its reliability in detection and sizing of fish.

Reconstruction of sonar images is an inverse problem, which is normally solved with model-based methods. These methods may introduce undesired artifacts called angular and range leakage into the reconstruction. In this thesis, a method called Learned Primal-Dual Reconstruction, which combines a data-driven and a model-based approach, is used to investigate the use of data-driven methods for reconstruction within sonar imaging. The method uses primal and dual variables inspired by classical optimization methods where parts are replaced by convolutional neural networks to iteratively find a solution to the reconstruction problem. The network is trained and validated with synthetic data on eight models with different architectures and training parameters. The models are evaluated on measurement data and the results are compared with those from a purely model-based method. Reconstructions performed on synthetic data, where a ground truth image is available, show that it is possible to achieve reconstructions with the data-driven method that have less leakage than reconstructions from the model-based method. For reconstructions performed on measurement data where no ground truth is available, some variants of the learned model achieve a good result with less leakage.

Detection of an underwater target with active sonar in shallow waters such as the Baltic sea is a big challenge. This since the sound beams from the sonar will be reflected on the surfaces, sea surface and sea bottom, and the water volume itself which generates reverberation. Reverberation which will be reflected back to the receiver, is strong in intensity which give rise to many false targets in terms of classifying a target in a surveillance area. These false targets are unwanted and a real target might benefit from these miss-classifications in terms of remaining undetected. It is especially hard if the signal-to-noise ratio (SNR) is approaching zero, i.e. the target strength and the reverberation strength are equal in magnitude. The classical approach to a target detection problem is to assign a threshold value to the measurement, and the data point exceeding the threshold is classified as a target. This approach does not hold for low levels of SNR, since a threshold would not have a statistical significance and could lead to neglecting important data. Track-before-detect (TrBD) is a proposed method for low-SNR situations which tracks and detects a target based on unthresholded data. TrBD enables tracking and detecting of weak and/or stealthy targets. Due to the issues with target detection in shallow waters, the hypothesis of this thesis is to investigate the possibility to implement TrBD, and evaluate the performance of it, when applied on a low-SNR target. The TrBD is implemented with a particle filter which is a recursive Bayesian solution to the problem of integrated tracking and detection. The reverberation data was generated by filtering white noise with an Autoregressive filter of order 1. The target is assigned to propagate according to a constant velocity state space model. Two types of TrBD algorithms are implemented, one which is trained on the background and one which is not. The untrained TrBD is able to track and detect the target but only for levels of SNR down to 4dB. Lower SNR leads to the algorithm not being able to distinguish the target signal from the reverberation. The trained TrBD on the other hand, is able to perform very well for levels of SNR down to 0dB, it is able to track and detect the target and neglect the reverberation. For trajectories passing through areas with high reverberation, the target was lost for a short period of time until it could be retracked again. Overall, the TrBD was successfully implemented on the self-generated data and has a good performance for various target trajectories.

Certificates are the foundation of secure communication over the internet as of today. While certificates can be issued with long validity periods, there is always a risk of having them compromised during their lifetime. A good practice is therefore to use shorter validity periods. However, this limits the certificate lifetime and gives less flexibility in the timing of certificate replacements. In this thesis, we use publicly available network logs from Rapid7's Project Sonar to provide an overview of the current state of certificate usage behavior. Specifically, we look at the Let's Encrypt mass revocation event in March 2020, where millions of certificates were revoked with just five days notice. In general, we show how this kind of datasets can be used, and as a deeper exploration we analyze certificate validity, lifetime and use of certificates with overlapping validity periods, as well as discuss how our findings relate to industry standard and current security trends. Specifically, we isolate automated certificate services such as Let's Encrypt and cPanel to see how their certificates differ in characteristics from other certificates in general. Based on our findings, we propose a set of rules to help improve the trust in certificate usage and strengthen security online, introducing an Always secure policy aligning certificate validity with revocation time limits in order to replace revocation requirements and overcoming the fact that mobile devices today ignore this very important security feature. To round things off, we provide some ideas for further research based on our findings and what we see possible with datasets such as the one researched in this thesis.

Thesis: S.M. in Oceanographic Engineering, Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2017. / Thesis: S.M., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science; and the Woods Hole Oceanographic Institution), 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 885-86). / A newly discovered double ducted acoustic environment present throughout much of the Beaufort Sea in the Arctic has a major effect on active acoustic transmissions. This work performs an in depth analysis of how the lower duct impacts the propagation of various active signals used commonly for acoustic communications or active sonar. First, this thesis performs a thorough modal analysis of the effect of the double ducted environment on long range propagation of a 300 Hz and 3500 Hz pulse. Signal excess is determined for the two different source pulses to quantify the effect of the lower duct on noise and SNR. Finally, channel capacity is calculated for the two frequency bands to evaluate operational impacts of the lower duct on acoustic communication systems in the Arctic. / by Scott Adams Carper / S.M. in Oceanographic Engineering / S.M.

The internet is one of the most widely used mediums for communication in modern society and it has become an everyday necessity for many. It is therefore of utmost importance that it remains as secure as possible. SSL and TLS are the backbones of internet security and an integral part of these technologies are the certificates used. Certificate authorities (CAs) can issue certificates that validate that domains are who they claim to be. If a user trusts a CA they can in turn also trust domains that have been validated by them. CAs can in turn trust other CAs and this, in turn, creates a chain of trust called a certificate chain. In this thesis, the structure of these certificate chains is analysed and a longitudinal dataset is created. The analysis looks at how the certificate chains have changed over time and puts extra focus on the domains of big tech companies. The dataset created can also be used for further analysis in the future and will be a useful tool in the examination of historical certificate chains. Our findings show that the certificate chains of the domains studied do change over time; both their structure and the lengths of them vary noticeably. Most of the observed domains show a decrease in average chain length between the years of 2013 and 2020 and the structure of the chains vary significantly over the years.

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<p>Small, low cost Autonomous underwater vehicles (AUVs) provide ideal platforms for shallow water survey, as they are capable of unmanned navigation and can be programmed to acquire data at constant depth, or constant altitude above the seabed. AUVs can be deployed under most sea states and are unaffected by vessel motions that often degrade sonar and magnetometer data quality. The integration of sonar and magnetometer sensors on AUV’s is challenging, however, due to limited payload and strong magnetic fields produced by the vehicle motor.</p> <p>In this study, a Marine Magnetics Explorer Overhauser magnetometer was mated to a portable AUV (OceanServer Iver2) creating the first practical AUV- deployed magnetic survey system. To eliminate magnetic interference from the AUV, the magnetometer was tethered to the AUV with a 5 m tow cable, as determined by static and dynamic instrument testing. The results of the magnetic tests are presented, along with field data from a shallow water test area in Lake Ontario near Toronto, Canada. AUV-acquired magnetic survey data were compared directly with a conventional boat-towed magnetic survey of the same area. The AUV magnetic data were of superior quality despite being collected in rough weather conditions that would have made conventional survey impossible. The resulting high-resolution total magnetic intensity and analytic signal maps clearly identify several buried and surface ferrometallic targets that were verified in 500-kHz side- scan sonar imaging and visual inspection by divers.</p> / Master of Science (MSc)

You do not want your drone to crash. Therefore, safety systems should be put in place to prevent such an event, and obstacle avoidance is a major part of this. Today, the most successful techniques use cameras or light detection and ranging (LIDAR) to find and avoid obstacles; but to improve resiliency, multiple systems should be used. This thesis proposes to use microphones, listening to the drone’s own noise, to estimate the distance to surrounding obstacles. An obstacle echolocation solution for multi-rotor aerial vehicles (MAVs) using ego-noise is developed. The MAV’s noise is captured and auto-correlated to detect echoes at different time delays. This signal is whitened to remove structured measurement noise resulting from the narrow-band components of the MAV’s noise. By recording the MAV’s noise using multiple microphones, a time of arrival (TOA) estimate of the obstacle position is achieved. A beamforming-based solution is used to calculate this estimate. A series of simplified proof-of-concept experiments show that ego-noise echolocation is possible and that the developed solution works in a controlled environment. A prototype implementation of a realistic system is also created. Four signal fusion alternatives are compared, though no best alternative is found for all situations. More work is needed to apply the findings of this work in a robust way, but the principle is shown to work.