Application of Linear Prediction Technique to Passive Synthetic Aperture Processing
© Yunshan Hou et al. 2010
Received: 10 October 2009
Accepted: 15 February 2010
Published: 24 March 2010
A method for the synthesis of an aperture with improved angular resolution and array gain is described. The proposed method explores the merit of linear prediction technique to improve the performance of conventional ETAM (extended towed array measurements) method. Previous efforts to improve the ETAM method generally focused on how to get more accurate estimation of overlap correlator, with an aim to reduce bearing estimation variance. In this paper, however, we discuss how to further improve the angular resolution when the effective synthetic aperture is rather limited. We resort to linear prediction technique to further extend the synthetic aperture obtained by ETAM, which produces a much longer virtual aperture. Results from simulations and lake experiment showed that the proposed LP-ETAM method achieved better angular resolution than ETAM.
Line arrays are widely used in underwater acoustics to measure the spatial field of propagating acoustic waves. When employing conventional beamforming techniques, the angular resolution and the signal gain are proportional to the ratio of aperture length to signal wavelength. High bearing resolution becomes more important and more difficult to achieve as the frequency regime is made lower in order to increase the detection range. Because increased low-frequency bearing resolution means longer hydrophone arrays, it is not practical to increase the physical length of line arrays in order to match the coherence length of signals. An alternative approach in this case is to employ coherent synthesis of subapertures, namely, synthetic aperture technique.
Application of synthetic aperture technique to passive sonar has received a sustained interest for more than twenty years. This technique exploits the array motion and the temporal coherence of narrow band received signals to build a large synthetic array, resulting in an improvement in beamforming performance compared with that of the short physical array.
As it is widely known, there are four typical methods for passive synthetic array, namely, the method introduced by Yen and Carey , Extended towed-array measurements (ETAM) , Maximum Likelihood method (ML) , and FFT Synthetic Aperture processing method (FFTSA) . The method introduced by Yen and Carey is a processing technique in the beam domain which provides coherent processing of subapertures by proper selection of a phase term based on knowledge or the use of a velocity filter concept for the source-receiver relative speed. In the ETAM algorithm, developed by Stergiopoulos and Sullivan, the successive subaperture signals are coherently synthesized in the aperture domain into an extended aperture size by using a phase correction factor, which is derived by cross-correlating overlapping space samples of the acoustic signal received at successive moments by the moving array. The ML method requires the acquisition of very long hydrophone time series over an interval which is required by the moving towed array to travel a distance equivalent to the desired length of the synthetic aperture. It performs a two-dimensional search in equivalent phase and frequency domains to yield the best estimates of the unknown parameters. FFTSA method is like the method introduced by Yen and Carey, but it does not need the velocity information. It performs coherent processing of subaperture signals at successive time intervals in the beam domain via FFT transformations. Theoretical studies and sea experiments showed that ETAM method yielded the best performance among them [5, 6].
So far, there have emerged many improved versions of ETAM method, most of which endeavor to improve the bearing estimation precision by working on the overlap correlator [7, 8]. But in this paper, we try to improve the angular resolution of ETAM method. As well known, in real sea environment, the effective synthetic aperture is limited by the coherence length of underwater signals. In some adverse sea environments , the effective synthetic aperture size is only 1.5 times of the physical array. Thus, how to further improve the bearing resolution within the effective synthetic aperture is a meaningful problem.
Linear prediction (LP) is a technique widely used in the data processing field. It predicts the time series in the future or in the past according to obtained time series. In recent years linear prediction technique has found extensive applications in the fields of voice identification, image processing, and signal frequency estimation [10–12]. This paper incorporates spatial linear prediction technique into the ETAM method to propose a new hybrid technique called LP-ETAM method for the bearing estimation of multiple coherent underwater targets. The proposed method employs spatial linear prediction technique to do extrapolation on the synthetic aperture obtained by ETAM to further enlarge the aperture size, which is the main reason leading to superior bearing resolution to ETAM method. Results from simulations and applications of the ETAM and LP-ETAM method on real data with pure tone CW signals showed that the proposed LP-ETAM method achieved better angular resolution than ETAM method.
2. Review of Conventional ETAM Method
In this section we describe the conventional ETAM method for underwater towed arrays.
3. Basic Concepts for Linear Prediction Technique
Linear prediction (LP) is a technique widely used in the data processing field. It predicts the time series in the future or in the past according to obtained time series, which are called forward prediction and backward prediction, respectively. Assuming that the signal output is the linear combination of samples before time and the coefficients maintain the same value from one sample to the next, this is called forward prediction. Next, we outline the process of the linear prediction technique by taking forward prediction as an example.
Obviously, the calculation of the prediction coefficients for the th order filter is of paramount importance. The classical method to calculate prediction coefficient is to solve (13) directly. In practice, since is usually unknown, (14) is estimated by obtained sample series
4. LP-ETAM Method
According to linear prediction theory, the delayed sample points in the time domain correspond to the array sensors in the spatial domain. For the -sensor uniform line array, the -order forward prediction filter in the spatial domain uses the data of the first sensors to estimate the data of the th sensor, while the -order backward prediction filter in the spatial domain uses the data of the last sensors to estimate the data of the sensor before them.
After we get the prediction coefficients, we can use them to extrapolate the array sensors. The array is extended as a sensor at one time. The forward prediction coefficients are used to extend the array forwardly. The backward prediction coefficients are used to extend the array backwardly.
To extend the -sensor synthetic array to -sensor virtual array by forward and backward prediction filter. In this process, sensors are extrapolated to the left and right of the synthetic array, respectively.
5. Performance Analysis
5.1. Angular Resolution
The performance of the LP-ETAM method has been tested with synthetic data; the relevant results are presented in this section. The far-field acoustic pressure measurements of a towed array have been simulated according to (1). In the numerical experiment, an 8-hydrophone towed array is considered with 1 m spacing moving with 5-knot speed. The received signal is assumed to be a narrow-band CW at 330 Hz. The at the hydrophone is . In our simulations, the performance of the LP-ETAM method is compared with conventional beamforming method (CBF) and ETAM method. The integration time used to synthesize the aperture is 24 , and the 8-hydrophone towed array is extended to a 64-hydrophone synthetic aperture by ETAM and a 192-hydrophone virtual aperture by LP-ETAM. The values of other parameters are , and
5.2. Array Gain
The next step is to calculate the array gain for the physical and the synthetic apertures by using (24) and the values of the coefficients shown in Figures 8 and 9. The expected array gain estimates for an 8-hydrophone, a 64-hydrophone, and a 192-hydrophone physical arrays are 9 dB, 18 dB, and 22.8 dB, respectively. The experimental array gain estimates for the 8-hydrophone physical array, the 64-hydrophone and the 192-hydrophone extended aperture are 6.9 dB, 15.8 dB, and 20.6 dB, respectively.
6. Lake Experiments
The lake experiment was designed to evaluate the angular resolution provided by the LP-ETAM method and the ETAM method in comparison with that provided by the physical array.
The measurement procedure in this experiment included one tow ship and two stationary projectors. The tow ship was used to tow a receiving array along a straight line course at 5 knots with 8 hydrophones spaced at 1 m and at 20 m depth. The experiments were carried out in a lake in south China. The water depth was 40 m. The data acquisition and control system included amplification of the hydrophone signals, bandpass filtering, digitization, and continuous recording on a high performance digital recorder for offline processing of the time series. Two projectors transmitting CW signals at 650 Hz were placed 400 meters apart, and they were moored at 20 m depth approximately 10 km from the towed array.
This paper focuses on the development of a new passive synthetic aperture technique that explores the merits of both linear prediction technique and ETAM method. The proposed LP-ETAM method employs spatial linear prediction technique to do extrapolation on the synthetic aperture obtained by ETAM to further enlarge the aperture size, which leads to superior angular resolution to conventional ETAM method. Simulation analysis shows that the proposed LP-ETAM method yields narrower beams and higher array gain than ETAM method. Results from application of ETAM method and the proposed LP-ETAM method on real data with very stable CW signals have shown that the angular resolution provided by LP-ETAM resolves two closely spaced sources that are unresolvable by the ETAM method.
This work was supported by the National Natural Science Foundation of China (60972152), Aeronautical Science Foundation of China (2009ZC53031), and the Basic Research Foundation of Northwestern Polytechnical University (NPU-FFR-W018102).
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