Michael P. Hayes

Altitude control of a quadrotor helicopter using depth map from Microsoft Kinect sensor

Reliable depth estimation is a cornerstone of many autonomous robotic control systems. The Microsoft Kinect is a new, low cost, commodity game controller peripheral that calculates a depth map of the environment with good accuracy and high rate. In this paper we calibrate the Kinect depth and image sensors and then use the depth map to control the altitude of a quadrotor helicopter. This paper presents the first results of using this sensor in a real-time robotics control application.

Synthetic Aperture Sonar: A Review of Current Status

This is a review paper that surveys past work in, and the recent status of, active synthetic aperture sonar (SAS). It covers the early historical development of SAS with its provenance in synthetic aperture radar (SAR) and flows through into what work has been published in the open literature up to early 2007. The list of references is sufficiently complete to include most past and recent SAS publications in the open refereed literature.

Broad-band synthetic aperture sonar

The effects of a broadband signal on the performance of a side-scan synthetic aperture sonar, including detail on the necessary spatial sampling and resolution tradeoffs, are discussed. The mathematical description of the echo signals and the consequent image reconstruction algorithms are shown for a step-and-go scenario that ignores the Doppler effect. Extra modifications for the Doppler effects are introduced with some discussion of their probable impact. Simulated echoes and image reconstructions on hypothetical targets show the limiting performance and some of the effects of narrowband reconstruction approximations on broadband signals. For comparison, images taken from a CTFM 15-30 kHz sonar especially designed as a synthetic aperture sonar are presented. >

A method for estimating the sub-wavelength sway of a sonar towfish

The essentially random motion of the towfish in a side scan sonar system limits the fidelity of the resultant image. Although corrective techniques have been developed to deal with the gross departures from a set of regular amplitude versus range samples along a straight locus as well as the gross rotational errors, sub-wavelength displacement errors are mostly ignored. This paper suggests a technique to estimate the appropriate displacement errors from the statistics of the measured data when there are few or no strong scatterers on an otherwise featureless sea floor. As it happens, the proposed technique is somewhat similar to an existing optical technique but is new to underwater sonar. We show in a series of simulated trials that it is possible to estimate the resultant sub-wavelength horizontal departure from a straight locus under some (hopefully) realistic operating conditions.

Test results using a prototype synthetic aperture sonar

In this paper, the operation is described of a prototype coherent synthetic aperture sonar (SAS) based on continuous transmission frequency modulation that covers a 1‐octave bandwidth. Images calculated from real data (collected from a sonar range in Loch Linnhe, Scotland) show that it is now possible to produce high‐quality images of the seafloor at realistic mapping rates. An air‐filled steel buoy was used as a test target, and several images are shown of this target using a variety of reconstruction algorithms.

Stripmap phase gradient autofocus

Current sonar autofocus techniques for blur removal originate in the radar community but have not provided a complete solution for Synthetic Aperture Sonar (SAS) imagery. The wide-beam, wide-band nature of SAS imagery makes implementation of Synthetic Aperture Radar (SAR) autofocus techniques difficult. This paper describes a generalisation of the standard Phase Gradient Autofocus (PGA) algorithm used in spotlight SAR that allows operation with stripmap SAS geometries. PGA uses prominent points within the target scene to estimate image blurring and phase errors. We show how PGA can be generalised to work with wide-band, wide-beam stripmap geometries. The SPGA method works by employing wave number domain 2D phase estimation techniques. The 2D phase errors are related to aperture position errors using the wave number transform. Robust sway estimates are obtained by using redundancy over a number of target points. We also present an improved Phase Curvature Autofocus (PCA) algorithm using the wave number transform. Preliminary results from the two algorithms (both on field-collected and simulated data sets) are presented and related to those obtained using previous methods. A discussion of SPGAs benefits over traditional algorithms and the limitations of the SPGA algorithm is presented. The SPGA algorithm was found to perform better than 2-D PCA on both simulated and field-collected data sets. Further testing on a variety of target scenes and imagery is required to investigate avenues of autofocus improvement.

Measurements of acoustic phase stability in Loch Linnhe, Scotland

The temporal phase stability of the acoustic environment is vital to the formation of a synthetic aperture. Acoustic phase stability measurements were made of the water in Loch Linnhe, Scotland over a period of several days. The phase stability was estimated from a sequence of phase measurements made over periods of 1 and 15 min. Over periods of a minute, it was found that the standard deviation of the fluctuations around the mean phase was 10°. Moreover, for frequencies in the band 15–30 kHz, the standard deviation was independent of frequency. This implies that the standard deviation measured is limited by the measurement accuracy of the experiment rather than a measure of the actual acoustic stability of the medium. Consequently, it is concluded that the acoustic phase stability is better than 10° for all frequencies and that the path‐length variation due to medium fluctuations is smaller than 6 mm in a total acoustic path of 130 m.

Statistical autofocus of synthetic aperture sonar images using image contrast optimisation

We present results of a technique for autofocus of stripmap synthetic aperture sonar (SAS) imagery. The technique of contrast optimisation is posed in a general Bayesian framework. The algorithm uses standard optimisation techniques on towfish motion parameters to minimise a cost function. The cost function consists of an image contrast measure, approximating the image likelihood, and a path likelihood measure, obtained from the towfish motion statistics. A number of simple image contrast measures are compared, and the optimal method for parameterising the towfish path is presented. Results showing the cost-function surface for simulated and real data are given. These show the technique of contrast optimisation is viable, and demonstrate the need for some form of regularisation in the cost function, justifying the Bayesian approach. Results on simulated data, with known towfish motion, show a high degree of accuracy is possible when minimising this cost function, but can depend on the image quality measure used. Local gradient optimisation algorithms are shown to be inadequate for real data, due to the high level of noise on the contrast surface. Preliminary results from a multiple-frequency interferometric SAS system, built by the Acoustics Research Group at the University of Canterbury are presented and ways to combine this extra information for autofocus purposes are discussed. When combined with a bulk-error removal method, or INS measurements, this technique shows promise for high-quality autofocus of regions of interest.

Motion-Compensation Improvement for Widebeam, Multiple-Receiver SAS Systems

The effect that uncompensated motion errors have on synthetic aperture sonar (SAS) imagery can be severe. Time-domain beamforming SAS reconstruction is able to compensate arbitrary track errors, but the more efficient frequency-domain reconstruction algorithms, such as the range-Doppler, chirp-scaling, and wave number (aka range migration or Stolt-mapping) algorithms do not allow for simple compensation, especially for widebeam sonars. Data processed via these block algorithms is usually compensated before azimuth compression in a computationally inexpensive preprocessing step. Unfortunately, this compensation assumes a narrowbeam geometry, causing blurring in high-resolution images collected with widebeam sonars. In this paper, we demonstrate a new technique for compensation of large, but known, motion errors in data collected with widebeam geometry sonars. The technique relies on obtaining angle-of-arrival information from the multiple-receiver array configuration typical in high-resolution SAS systems. The new method of compensating for motion errors was found to significantly outperform the previous techniques in a simulation of point-reflector imagery.

Test results from a multi-frequency bathymetric synthetic aperture sonar

Describes the implementation of a bathymetric synthetic aperture sonar and presents preliminary results from sea trials of the sonar. The sonar is designed for high resolution seafloor imaging in a shallow water environment. This is achieved through coherent summation of successive echo signals to synthesise an aperture many times longer than the towfish. Provided the motion of the towfish is accurately estimated and compensated, the application of aperture synthesis can result in a range independent resolution over the operating swath. While high resolution is desirable for high quality imagery, it is not always sufficient for the discrimination of targets of interest from general seafloor clutter. Therefore, the authors have attempted to use bathymetric techniques to discriminate targets from the seafloor on the basis of height. The sonar is configured to use an array of three vertically spaced hydrophones from which three independent synthetic aperture images are reconstructed. Using knowledge of the sonar geometry, the relative heights of the seafloor scatterers are estimated using the phase differences resulting from the slight range difference of a scatterer from each hydrophone. Preliminary results are presented from trials in a shallow water (nominally 16 m) harbour environments. Results are shown for two different frequency bands: the sonar transmits two simultaneous linear FM chirp signals; one covering the frequency band of 20-40 kHz and the other covering 90-110 kHz. Due to the use of a neutrally-buoyant, high-drag towfish, reasonable reconstructed imagery is obtained without autofocusing at the low frequency range. However, autofocusing is required for the high frequency data even though the synthetic apertures are shorter.