Wideband sonar receiver and sonar signal processing algorithms
A wideband sonar receiver is provided that includes: a selectable bandpass filter adapted to filter a received sonar signal to produce a filtered signal and a correlator adapted to correlate the baseband samples with baseband replica samples to provide a correlated signal. In addition, the wideband sonar receiver may include a shaping filter to shape unshaped received pulses. Finally, a variety of sonar processing algorithms are described with regard to reducing clutter and interference, target detection, and bottom detection.
1. A wideband sonar receiver, comprising:
a digital basebanding and decimation stage adapted to baseband and decimate digitized samples of a received sonar signal to produce baseband samples of the received sonar signal; and
a correlator adapted to correlate the baseband samples with shaped baseband replica samples, wherein the correlator comprises a digital signal processor (DSP) configured to divide a frequency-domain version of the shaped baseband replica samples (FSRS) by a frequency-domain version of the baseband samples (FS) to provide a shaping filter response, to multiply the shaping filter response by a conjugate of the FSRS to provide a combined correlation and shaping response, and to multiply the FS with the correlation and shaping response to produce a correlated and shaped signal.
2. The wideband sonar receiver of claim 1 , further comprising:
a selectable bandpass filter adapted to filter the received sonar signal to produce a filtered signal; and
an analog-to-digital converter for converting a version of the filtered signal to provide the digitized samples of the received sonar signal.
3. The wideband sonar receiver of claim 2 , further comprising:
a variable gain amplifier adapted to apply a time-varying gain to the filtered signal from the selectable bandpass filter to produce a variable-gain-amplified version of the filtered signal to provide to the analog-to-digital converter as the version of the filtered signal.
4. The wideband sonar receiver of claim 2 , wherein the analog-to-digital converter converts according to a subharmonic sampling rate.
5. The wideband sonar receiver of claim 2 , wherein the analog-to-digital converter converts according to a rate of four times a center frequency for the received sonar signal.
6. The wideband sonar receiver of claim 1 , wherein the correlator comprises a variable-length correlator and/or the shaped baseband replica samples correspond to a replica signal shape different from that of the received sonar signal.
7. The wideband sonar receiver of claim 1 , wherein the received sonar signal comprises a frequency-modulated signal, and wherein the wideband sonar receiver further comprises:
a complex-to-magnitude stage configured to receive the correlated and shaped signal and extract a power of complex correlation results of the correlated and shaped signal; and
a logarithm stage configured to provide a detected echo signal output based on the extracted power of the complex correlation results.
8. A method of processing a received wideband sonar signal, the method comprising:
digitizing a filtered wideband sonar signal to produce digitized samples; and
correlating the digitized samples with shaped replica samples, wherein the correlating comprises dividing a frequency-domain version of the shaped replica samples (FSRS) by a frequency-domain version of the digitized samples (FS) to provide a shaping filter response, multiplying the shaping filter response by a conjugate of the FSRS to provide a combined correlation and shaping response, and multiplying the FS with the correlation and shaping response to produce a correlated and shaped signal.
9. The method of claim 8 , further comprising:
selecting from a plurality of filter bands for a selectable bandpass filter based upon a center frequency for the received wideband sonar signal; and
filtering the received wideband sonar signal using the selectable bandpass filter according to the selected filter band before digitizing the wideband sonar signal.
10. The method of claim 9 , further comprising applying a time-varying gain to the filtered wideband sonar signal before digitizing the wideband sonar signal.
11. The method of claim 8 , wherein the digitizing occurs according to a subharmonic sampling rate or according to a rate of four times a center frequency for the received wideband sonar signal.
12. The method of claim 8 , further comprising:
generating digital samples of a complex local oscillator signal;
multiplying the digital samples from the complex local oscillator signal with the digitized samples to produce mixed samples; and
low pass filtering the mixed samples to produce a basebanded version of the digitized samples before correlating the basebanded version of the digitized samples with a basebanded version of the shaped replica samples.
13. A method of rejecting sonar interference in the received wideband sonar signal processed according to the method of claim 8 , comprising:
detecting echoes corresponding to a series of transmitted sonar pulses, wherein each detected echo is represented by a series of time samples, and wherein one of the detected echoes corresponds to the received sonar signal;
for each detected echo, comparing a series of comparison time samples in the one detected echo to corresponding time samples in a preceding echo and a subsequent echo to determine whether detection levels in the comparison time samples exceed the set threshold and exceed the corresponding time samples by a set margin; and
if the comparison time samples exceed the set threshold and the set margin, replacing the comparison samples with alternative sample values determined from the comparison time samples and/or the corresponding time samples in the adjacent echoes.
14. The method of claim 13 , wherein replacing the comparison time samples comprises replacing each comparison time sample with an average, a mean, or a minimum of the compared time sample and/or the corresponding time samples in the adjacent echoes.
15. A sonar system for shaping a received sonar signal according to a shaped replica pulse, comprising:
a digital signal processor (DSP) configured to divide a frequency-domain version of the shaped replica pulse (FSRP) by a frequency-domain version of the received sonar signal (FS) to provide a shaping filter response, to multiply the shaping filter response by a conjugate of the FSRP to provide a combined correlation and shaping response, and to multiply the FS with the correlation and shaping response to produce a correlated and shaped signal.
16. The sonar system of claim 15 , wherein the DSP is further configured to limit an extent of the correlation and shaping response to be no more than two or three times an extent of the FSRP.
17. The sonar system of claim 15 , wherein the DSP is further configured to taper either end of the correlation and shaping response, and/or wherein the shaped replica pulse comprises a replica pulse shape different from that of the received sonar chirp pulse.
18. The sonar system of claim 15 , wherein the received sonar pulse comprises an unshaped pulse, a chirp pulse, and/or a symmetric chirp pulse.
19. A method, comprising:
within a sonar system, dividing a frequency-domain version of a shaped replica pulse (FSRP) by a frequency-domain version of the received sonar signal (FS) to provide a shaping filter response;
multiplying the shaping filter response by a conjugate of the FSRP to provide a combined correlation and shaping response; and
multiplying the FS with the correlation and shaping response to produce a correlated and shaped signal.
20. The method of claim 19 , wherein the received sonar signal comprises an unshaped pulse, a chirp pulse, and/or a symmetric chirp pulse, the method further comprising detecting a target within the correlated and shaped signal.