Electrical and Computer Engineering

College of Engineering and Applied Science

UW STAT Information

The University of Wyoming Source Tracking Array Testbed, UW STAT [1], was specifically developed to perform experimental evaluation of high-resolution source tracking algorithms. The array is a six element uniformly spaced linear array. UW STAT is a compact testbed and allows for precision source motion. The motion is controlled by a motor and the angular position is recorded by an encoder, thus allowing for a direct comparison of the tracking algorithm's performance with the true position recorded by the encoder.
In the past decade, many advanced algorithms have been developed to track moving sources using an array of sensors. Most of these algorithms rely on an updating of the signal or noise subspaces and the use of a high-resolution direction finding algorithm. Although a significant amount of research has been performed in this area, very few experimental tracking results have been reported in the open literature for actual sensor array systems.
The UW STAT allows for a variety of experimental scenarios with precise control of source motion. The tracking array is a compact ultrasonic array. The compactness of the testbed and the precisely controlled environment allows for detailed studies of cause and effect. Source motion is controlled using a precision motor to turn the shaft of a radial arm. An important aspect of the system is the true source position being precisely measured using an encoder located on the shaft. Tracking algorithm performance is therefore compared to the actual position recorded by the encoder.
Many of the new tracking algorithms use high-resolution source localization methods, which estimate the direction-of-arrival of a source at one instant in time. These high-resolution algorithms are very computationally intensive because they involve the separation of the received signal into a signal subspace and a noise subspace. Much of the recent research therefore has concentrated on updating these subspaces after each snapshot of array data is received. These algorithms can usually be classified into one of two groups -- adaptive or recursive. For adaptive tracking algorithms the update of the subspace will asymptotically approach the true subspace if the data is wide sense stationary. For recursive tracking algorithms the update of the exact subspace, using a new data vector, results in the exact subspace for the given modified data matrix. An excellent discussion and bibliography of many subspace tracking algorithms are given in [2]. These algorithms have been investigated with some theoretical analysis and simulation studies, but little has been reported on the experimental capabilities and limitations of the methods using actual array data.
A front view of the mechanical system for UW STAT is shown in Figure 1. The mechanical system, where signal transmission and reception take place, is contained in an echoless anechoic chamber and consists of the following: two transmitting sources capable of independent movement in the same arc about the linear receiving sensor array, a DC motor for moving one of the sources, and an encoder for precise position knowledge of the moving source. The remainder of the testbed is placed outside the anechoic chamber and consists of the following: transmission hardware, reception hardware, carrier signal generation hardware, message signal generation devices (white noise generators and arbitrary function generators), and a PC/data acquisition system.
Figure 1
A doublexi-sideband suppressed-carrier (DSBSC) modulated signal is generated by the transmission hardware. Message signals with an acoustic frequency range of 0-2 kHz from the message signal devices can be used for modulation with a carrier signal. The carrier signal is generated at a frequency of 40 kHz so that piezoelectric transducers with natural frequencies of 40 kHz can be utilized for signal transmission and reception. This ultrasonic carrier frequency allows for the physical dimensions to be practical for a testbed. The approximate narrowband plane wave generated by the transmitting transducer has a wavelength of l = 8.275 mm. Due to the physical size of the receiving transducers, the element spacing of the linear array is 2.11. This sensor spacing limits the field of view for a spatially non-aliased sector from -13.5 to 13.5 degrees. This non-aliased range for the angle-of-arrival of a transmitting source corresponds well to the limited range over which the transducers can be considered omnidirectional.
Movement of one of the transmitting sources is controlled with a DC motor geared to the sources radial arm shaft as shown in Figure 1. This allows for motion control and experimental repeatability of movement for one of the sources. In addition to the DC motor control, this source also has an incremental pulse encoder coupled to its shaft as illustrated in Figure 1. The encoder allows for the position of the source to be known at all times as it moves through the sensor array field-of-view. Source tracking algorithm performance can be evaluated against the precisely known position of this source. The second transmitting source is able to move independently from this source at approximately the same arc length, but it does not have motor control or encoder position knowledge. This second source can be used as an interfering source to see how well source tracking algorithms perform when two sources occupying the same temporal frequency band are near each other, or it could be used to create multipath signal effects
The system is approximately modeled as far field. Gain and phase characteristics of the transmitted DSBSC signal are affected by differences in the characteristics of the six receiving transducers. The reception hardware performs the in-phase and quadrature demodulation of the received DSBSC signal. This gives both the real and imaginary components of the message signal so that the recovered message signal information is complete.
After quadrature demodulation of the signal received at the sensor array, a 12 bit data acquisition card is utilized to transform the recovered analog data into digital data for storage in a PC. The data acquisition system has 16 channels. Since it is desirable to sample the recovered demodulated signals from the six sensors in the array at the same time, simultaneous sample and hold circuitry is utilized in conjunction with the data acquisition card. The data acquisition card also acquires data from the encoder to allow for the precise position knowledge of one of the transmitting sources. One of the transmitted message signals may also be sampled. The digital data is then stored in the PC for processing at a later time. The array is calibrated for gain, phase, and mutual coupling errors using the method described in [3]. Thirty one measured direction vectors were used to estimate the gain, phase, and mutual coupling calibration matrix for the system.
[1]J.W. Pierre, E.D. Scott, and M.P. Hays, "A Sensor Array Testbed for Source Tracking Algorithms," Proc. of ICASSP '97, Munich, Germany, April 1997.
[2]E.M. Dowling, L.P. Ammann, and R.D. DeGroat, "ATQR-iteration based adaptive SVD for real time angle and frequency tracking," IEEE Trans. on Signal Processing, vol. 42, April 1994.
[3]J.W. Pierre and M. Kaveh, "Experimental evaluation of high resolution direction-finding algorithms using a calibrated sensor array testbed," Digital Signal Processing: A Review Journal, October 1995.
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