Scanning synthetic aperture sonar system
A synthetic aperture sonar (SAS) system utilizes a novel timing and pointing method to illuminate and process data from multiple receive channels over one or more elevation swaths. The system further utilizes cross-track interferometry to improve accuracy of three-dimensional mapping.
1. A synthetic aperture sonar apparatus comprising:
a transmitter;
at least one transducer receive array including a plurality of transducers and a multi-channel digital receiver; and
a processor associated with the transmitter and the at least one transducer receive array,
said processor configured to control said transmitter to transmit a first pulse train at a first pulse rate interval and a second pulse train at a second pulse rate interval, the first pulse train and second pulse train being interleaved with one another, the first pulse train illuminating a first elevation swath and the second pulse train illuminating a second elevation swath, and
said processor further configured to receive timeshared data from reflections of the first and second pulse trains received by said at least one transducer receive array across said first and second elevation swaths and to generate at least 2D image data therefrom for said first and second elevation swaths,
wherein the at least one transducer array comprises first and second transducer arrays providing first and second scanning tracks, the receiver is configured to receive different phases from the first and second scanning tracks, and the processer is further configured to perform cross-track interferometric height mapping in response to the received different phases, said height mapping not being dependent on a target having higher contrast than a background.
2. The synthetic aperture sonar apparatus of claim 1 wherein:
said at least one transducer receive array comprises a first transducer receive array including a first plurality of transducers, a second transducer receive array including a second plurality of transducers, and a multi-channel digital receiver, said first and second transducer receive arrays being spaced vertically by a baseline distance B,
said processor being configured to receive timeshared data received by the digital receiver from said first transducer receive array and said second transducer receive array across said first and second elevation swaths, to apply interferometric processing to said received data to thereby triangulate between first transducer receive array measurements and second transducer receive array measurements, and to generate 3D topographic image data based at least in part on the triangulation.
3. The synthetic aperture sonar apparatus of claim 1 wherein the first elevation swath and the second elevation swath have different elevation beam widths.
4. The synthetic aperture sonar apparatus of claim 1 wherein the first elevation swath and the second elevation swath have the same elevation beam width.
5. The synthetic aperture sonar apparatus of claim 1 wherein said at least one transducer receive array is not longer than 25 cm.
6. The synthetic aperture sonar apparatus of claim 1 wherein said at least one transducer receive array is not wider than 6 cm.
7. The synthetic aperture sonar apparatus of claim 1 wherein said at least one transducer receive array comprises a conformable material configured to acoustically couple to water.
8. A method of operating a synthetic aperture sonar apparatus comprising:
controlling a transmitter to transmit interleaved first and second pulse trains, the first pulse train having a first pulse rate interval, the second pulse train having a second pulse rate interval different from the first pulse rate interval, and to illuminate a first elevation swath with the first pulse train at the first pulse rate interval and to illuminate a second elevation swath with the second pulse train at the second pulse rate interval;
receiving time shared data from at least one transducer array and an associated multi-channel receiver across said first and second elevation swaths, based on received reflections of the interleaved first and second pulse trains from the first and second elevation swaths; and
generating data representing at least 2D images of said first and second elevation swaths from the received timeshared data,
wherein the at least one transducer array comprises first and second transducer arrays providing first and second scanning tracks, the receiver is configured to receive different phases from the first and second scanning tracks, and the method further comprises cross-track interferometric height mapping in response to the received different phases, said height mapping not being dependent on a target having higher contrast than a background.
9. The method of claim 8 further comprising:
receiving timeshared data from a first transducer array and a second transducer array across said first and second elevation swaths;
applying interferometric processing to said timeshared data received from said first and second transducer receive arrays to thereby triangulate between first transducer array measurements and second transducer array measurements; and
generating 3D topographic image data based on the triangulation.
10. The method of claim 8 wherein the first elevation swath and the second elevation swath have different elevation beam widths.
11. The method of claim 8 wherein the first elevation swath and the second elevation swath have the same elevation beam width.
12. The method of claim 9 wherein the first elevation swath and the second elevation swath have different elevation beam widths.
13. The method of claim 9 wherein the first elevation swath and the second elevation swath have the same elevation beam width.
14. The synthetic aperture sonar apparatus of claim 1 wherein the transmitter is configured to emit acoustical signals with a center frequency of 450 kHz at the first and second pulse rate intervals.
15. The method of claim 8 further including the transmitter emitting acoustical signals with a center frequency of 450 kHz at the first and second pulse rate intervals.
16. Apparatus comprising:
a transmitter configured to transmit a first pulse train at a first pulse rate interval and a second pulse train at a second pulse rate interval different from the first pulse rate interval, the first and second pulse trains being interleaved, the first pulse train illuminating a first elevation swath and the second pulse train illuminating a second elevation swath closer to the transmitter than the first swath;
a transducer receive array including a plurality of receive transducers;
a multi-channel digital receiver connected to the transducer receive array; and
a processor connected to the multi-channel digital receiver, the processor configured to receive and process timeshared data the multi-channel digital receiver derives from reflections of the transmitted first and second pulse trains received by the transducer receive array, the processor further configured to generate at least 2D image data of the first elevation swath and at least 2D image data of the second elevation swath,
wherein the transducer receive array comprises first and second transducer arrays providing first and second scanning tracks, the receiver is configured to receive different phases from the first and second scanning tracks, and the processer is further configured to perform cross-track interferometric height mapping in response to the received different phases, said height mapping not being dependent on a target having higher contrast than a background.
17. The apparatus of claim 16 wherein the transmitter ensures that, during operation, at least two pulses are in the water at any given time.
18. The apparatus of claim 16 wherein the second pulse train comprises at least some pulses of the first pulse train plus additional pulses that are not in the first pulse train.
19. The apparatus of claim 16 wherein the transmitter is beamforming and the apparatus is configured to maintain range ambiguities.
20. The synthetic aperture sonar apparatus of claim 1 wherein the received timeshared data includes data that is timeshared among the first and second elevation swaths, the timeshared data comprising reflections of the first and second interleaved pulse trains.
21. The synthetic aperture sonar apparatus of claim 1 wherein the multi-channel digital receiver is configured to perform elevation scanning of returned versions of the first and second pulse trains from different elevation swaths to thereby increase azimuth resolution relative to a single swath.