Proc. SPIE. 7800, Image Reconstruction from Incomplete Data VI
KEYWORDS: Statistical analysis, Data modeling, In situ metrology, Error analysis, Data processing, Velocity measurements, Atmospheric modeling, Wind measurement, Global Positioning System, Data analysis
The problem of estimating the wind velocity from measurement of limited flight data from a sailplane flight in
atmospheric mountain waves is considered. A Sailplane are often equipped with a flight recorder that records
position, and sometimes other information, at regular intervals during the flight. These data contain information
on the state of the atmosphere during the flight. A maximum likelihood method is developed for estimating
wind fields using such sailplane flight data. The methods are evaluated by application to simulated flight data.
Over the past few years, ultrasonic phased arrays have shown good potential for non-destructive testing (NDT), thanks to
high resolution imaging algorithms that allow the characterization of defects in a structure. Many algorithms are based
on the full matrix capture, obtained by firing each element of an ultrasonic array independently, while collecting the data
with all elements. Because of the finite sound velocity in the specimen, two consecutive firings must be separated by a
minimum time interval. Therefore, more elements in the array require longer data acquisition times. Moreover, if the
array has N elements, then the full matrix contains N<sup>2</sup> temporal signals to be processed. Because of the limited
calculation speed of current computers, a large matrix of data can result in rather long post-processing times. In an
industrial context where real-time imaging is desirable, it is crucial to reduce acquisition and/or post-processing times.
This paper investigates methods designed to reduce acquisition and post-processing times for the TFM and wavenumber algorithms. To reduce data capture and post-processing, limited transmission cycles are used. Post-processing times is also further reduced by demodulating the data to baseband, which allows reducing the sampling rate of signals. Results are presented so that a compromise can be made between acquisition time, post-processing time and image quality. Possible improvement of images quality, using the effective aperture theory, is discussed. This has been implemented for the TFM but it still has to be developed for the wavenumber algorithm.
If sparse arrays are attached to structures for the purposes of structural health monitoring it is likely that there will be
variation in the placement of the sensors, resulting in deviation from the assumed locations. In addition, poor knowledge
of the material through which the signals are propagating can result in the use of incorrect velocities, or failing to take
account of delays inherent in the equipment. These deviations will result in reduced performance in terms of defect
detectability and characterisation. This paper outlines an autofocus approach whereby the transducer locations and
material properties can be estimated from the experimental data to ensure the highest levels of defect detectability. The approach is validated using both models and a more complex real world structure. The performance of the approach is considered across a range of potential operating conditions to demonstrate its robustness. Finally limitations and potential solutions to these are addressed.
This paper presents an algorithm for autofocusing imagery obtained from a flexible ultrasonic array with unknown
geometry. The relative positions of the array elements are parameterised using a polynomial function.
The polynomial coefficients are estimated by iterative maximisation of the SAFT image contrast via simulated
annealing. The estimate can be refined in the final stages of iteration using the full 3-D matrix of echo data via
the total focusing method. The resultant polynomial gives an estimate of the array geometry and the profile of
the surface that it has conformed to, providing a well-focused, high quality image. The algorithm is demonstrated
on experimental data obtained using a flexible array prototype.