Current ultrasonic Local Positioning Systems (LPS) based on an infrastructure of beacons can provide centimeter-level accuracy employing the spread spectrum technique, which also adds robustness against noise. However, the strong attenuation of the acoustic waves at high frequencies, the high directionality of ultrasound transducers, and the Doppler effect caused by moving targets still affect the correct performance of LPS. These phenomena reduce the availability of these systems in weak signal coverage areas, as they are no longer able to distinguish weak arrivals from spurious peaks, failing to calculate the position of the target. In this work, the aforementioned problems are dealt with by transmitting Doppler resilient waveforms together with a validation code based on Complementary Set of Sequences. This validation code is leveraged at the receiver after Doppler compensation to reduce the number of spurious arrival candidates and therefore increase the system availability. Compared to a system with no validation, experimental tests with a moving robot have shown that the proposed system increased the availability in weak coverage areas between 20 and 25%. The robot's average 2D positioning error at rest and in motion was 4.6 cm and 6 cm, respectively.
In relative positioning systems, with the aim of estimating object positions, distances among them are computed in a cooperative way, usually by measuring times-of-flight from the signals that they emit. These emissions are often synchronized with additional signals or suitable hardware that acts as a temporal reference. In this paper, a ranging system is presented where only acoustic emissions are used to compute the distances between objects or nodes. Thus, an organization and operation algorithm is proposed, which provides a temporal reference to the acoustic emissions carried out by every node. In this way, distances are computed by determining the temporal relation between a request of emission from a coordinator node and the corresponding answers emitted by the other nodes. In order to simultaneously detect the acoustic emissions, the signals are encoded with complementary set of sequences allowing multisensory operation and accepting low signal-to-noise conditions. With this measurement scheme, additional signals and high accuracy clocks often used for synchronization can be eliminated, thus reducing hardware complexity, power consumptions, and possible interferences with other systems (i.e. if radio frequency signals are used). The simulation and experimental results show that subcentimeter accuracy can be obtained with the proposed ranging scheme.
This paper raises the design of an ultrasonic array for obstacle detection based on Phased Array (PA) techniques, which steers the acoustic beam through the environment by electronics rather than mechanical means. The transmission of every element in the array has been encoded, according to Code Division for Multiple Access (CDMA), which allows multiple beams to be transmitted simultaneously. All these features together enable a parallel scanning system which does not only improve the image rate but also achieves longer inspection distances in comparison with conventional PA techniques.
The great variability usually found in underwater media makes modeling a challenging task, but helpful for better understanding or predicting the performance of future deployed systems. In this work, an underwater acoustic propagation model is presented. This model obtains the multipath structure by means of the ray tracing technique. Using this model, the behavior of a relative positioning system is presented. One of the main advantages of relative positioning systems is that only the distances between all the buoys are needed to obtain their positions. In order to obtain the distances, the propagation times of acoustic signals coded by Complementary Set of Sequences (CSS) are used. In this case, the arrival instants are obtained by means of correlation processes. The distances are then used to obtain the position of the buoys by means of the Multidimensional Scaling Technique (MDS). As an early example of an application using this relative positioning system, a tracking of the position of the buoys at different times is performed. With this tracking, the surface current of a particular region could be studied. The performance of the system is evaluated in terms of the distance from the real position to the estimated one.
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