Stig Synnes

Stig Synnes
Forsvarets forskningsinstitutt | FFI

Doctor of Philosophy

About

21
Publications
2,844
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258
Citations

Publications

Publications (21)
Article
Full-text available
Accurate positioning of autonomous underwater vehicles is a major challenge. The long-term drift is problematic if global position updates are not available, and for applications such as repeat-pass interferometry and coherent change detection, millimeter accuracy is needed. Repeat-pass synthetic aperture sonar (SAS) micronavigation is one potentia...
Article
Synthetic Aperture Sonar (SAS) interferometry is a technique for detailed mapping of the seabed, with the potential of very high resolution and wide swaths simultaneously. There are several specific challenges to overcome for the technique to reach its full potential. These differ from other mapping sensor technologies, e.g., multibeam echosounders...
Article
Synthetic Aperture Sonar (SAS) is a technique which delivers sonarimages of the seabed with both high resolution and large area coverage rate. SAS is therefore a well suited sensor for search of small objects on the seafloor, and is an important tool in many emerging mine countermeasures (MCM) systems. An interferometric SAS system can also resolve...
Conference Paper
High frequency sonar is an important and useful tool for acoustic imaging of the seabed in various applications. There are essentially three different types: sectorscanning sonar which produces a 2D image per transmitted pulse; sidescan sonar (SSS) which operates in a sidelooking geometry on a moving platform, where multiple pulse-echoes are treate...
Article
Wideband and widebeam synthetic aperture sonar (SAS) can provide information on the frequency- and aspect-dependent scattering in a scene. We suggest an approach to predict the quality of the sensor data over the available frequencies and aspect angles. We relate the typical spatial domain quality metrics to their wave number domain (WD) counterpar...
Conference Paper
The adoption of autonomous underwater vehicles equipped with high-resolution side-looking sonars in mine countermeasures (MCM) operations has resulted in a requirement to quantify the effectiveness of these platforms; e. g., by the probability of correctly detecting and classifying mines along with the associated false alarm rate. Recently, the Nor...
Article
Full-text available
Synthetic aperture sonar (SAS) interferometry can provide very high resolution images and topographic maps of the seafloor over large swaths. Processing of interferometric data to retrieve reliable depth estimates of the seafloor is, however, nontrivial. Traditional narrowband interferometry processing relies on advanced phase unwrapping techniques...
Conference Paper
Active sonar operations in shallow waters are important both for autonomous underwater vehicles (AUV) with synthetic aperture sonar (SAS) or sidescan sonar, and for hull-mounted forwardlooking sonar. One of the challenges in shallow water operations is that the direct bottom return is often contaminated by other signal returns, either from scatteri...
Article
Multipath propagation degrades the performance of active, bottom-imaging sonars in shallow-water environments. One way to avoid multipath interference is to use a vertical array with a narrow enough angular response to separate the direct bottom return from the multipath. However, this requires a large array and is often infeasible for practical re...
Article
Full-text available
Synthetic aperture sonar (SAS) is emerging as an imaging technology that can provide centimeter resolution over hundreds-of-meter range on the seafloor. Although the principle of SAS has been known for more than 30 years, SAS systems have only recently become commercially available. The success of SAS is critically dependent on overcoming several c...
Conference Paper
Successful synthetic aperture sonar (SAS) imaging is dependent of several challenges to be overcome. The sonar has to be positioned with accuracy better than a fraction of a wavelength along the entire synthetic aperture. At 100 kHz this equals an accuracy requirement around 1 millimetre along tens of metres of travelled distance. The ocean environ...
Article
Interferometry is used for bathymetric mapping with side scan and synthetic aperture sonars. The principle is based on estimating the time difference between arrivals for two vertically displaced receivers. An important additional feature with interferometric sonar is that an estimate of the signal-to-noise ratio can be derived from the coherence b...
Article
Collection of synthetic aperture sonar (SAS) data along a circular track and forming a circular SAS (CSAS) image has several benefits over traditional stripmap SAS: the area of interest is observed from all aspect angles giving a better perception; the resolution in the image increases and shadow zones are avoided. Navigation requirements however,...
Article
A method is suggested for obtaining a compact and accurate representation of elongated magnetic sources by a series expansion of prolate spheroidal harmonics. While the general solution involves an infinite series of harmonics, practical measurement grids only support discrimination of a limited number of harmonics. In this paper, we present a meth...
Article
The spreading of a proton beam in the upper atmosphere is calculated based onMonte-Carlo simulations. The transport of the atoms is modelled in a magnetic field with dipolestrength. Neuralisation, ionisation and excitation mechanisms of the incoming particles areincluded from collision cross-sections of protons and hydrogen with an effective N2atmo...
Article
Full-text available
It is demonstrated that the structures in the low-energy cross sections for excitation to certain levels of atoms by ions originate from a shift from direct excitation at high and intermediate energies to pronounced two-center molecular mechanisms at lower energies. The molecular mechanisms dominate at small internuclear separations, whereas the di...
Article
Full-text available
The prolate spheroidal harmonics are well suited for describing the magnetic field of a vessel. In this article we discuss several strategies to improve the estimation of the model parameters based on measured field data. In particular we give an analytic expression for the maximum supported set of harmonics for any rectangular grid. This expressio...
Article
The principle of synthetic aperture sonar (SAS) interferometry is to combine the images from two sensors coherently such that the observed differences can be used to retrieve additional information. If the distance between the two sensors forms a baseline perpendicular to the look direction of the sensor, the direction of arrival can be estimated a...

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