Calibrating a multibeam sonar apparatus
A method of obtaining an acoustic echo signal by a sonar apparatus (receiver array 100, 101 ). The method comprises measuring phases and magnitudes of at least one predetermined acoustic calibration signal received from a plurality of respective directions 109 by the sonar apparatus; determining a complex directional response from the measured phases and magnitudes; receiving an acoustic echo signal; compensating the received acoustic echo signal for the determined complex directional response.
1. A method of obtaining an acoustic echo signal by a sonar apparatus comprising an acoustic array of receiver elements for receiving acoustic signals; and wherein the method comprises:
measuring one or more quantities indicative of phases and magnitudes of at least one predetermined acoustic calibration signal received from a plurality of respective directions by each of the receiver elements;
determining respective complex directional responses for each of the receiver elements from the measured one or more quantities;
receiving an acoustic echo signal by each of the receiver elements;
compensating the received acoustic echo signal for the determined complex directional response for each of the receiver elements.
2. A method according to claim 1 , wherein determining respective complex directional responses comprises determining a vector of complex responses, each corresponding to one of a set of predetermined directions.
3. A method according to claim 1 , wherein compensating comprises calculating a transformation of the received acoustic echo signals weighted by the corresponding determined complex directional responses.
4. A method according to claim 1 , further comprising generating the acoustic calibration signal as a reflected acoustic signal from a predetermined object positioned at respective positions relative to the sonar apparatus.
5. A method according to claim 4 , wherein the object is a sphere of predetermined size and predetermined material.
6. A method according to claim 1 , comprising determining respective complex directional responses from acoustic signals received from a farfield and a nearfiled.
7. A method according to claim 1 , wherein the sonar apparatus is a beamforming sonar apparatus.
8. A method according to claim 1 , wherein measuring comprises measuring one or more quantities indicative of the phases and magnitudes of the at least one predetermined acoustic calibration signal received from a plurality of respective directions, wherein the respective directions are defined by respective angles in a predetermined reference plane.
9. A method according to claim 1 , comprising storing calculated phases and magnitudes in the sonar apparatus for use in the compensation of subsequently measured signals, and using the stored calculated phases and magnitudes in a real-time processing of received signals.
10. A method according to claim 1 , wherein the complex directional response comprises a representation of the complex response of an acoustic receiver as a function of the direction of propagation of the acoustic calibration signal, wherein the complex response is indicative of the magnitude and phase of the response of the acoustic receiver.
11. A sonar apparatus for obtaining an acoustic echo signal, the sonar apparatus comprising:
an acoustic array of receiver elements for receiving acoustic signals;
storage means for storing calibration parameters indicative of a complex directional response determined from measured one or more quantities indicative of phases and magnitudes of at least one predetermined acoustic calibration signal received from a plurality of respective directions by each of the receiver elements;
a signal processor adapted to compensate the received acoustic echo signal for the determined complex directional response for each of the receiver elements.
12. A system for calibrating a sonar apparatus according to claim 11 the system comprising:
means for generating acoustic calibration signals towards the sonar apparatus from respective positions relative to the sonar apparatus;
signal processing means for measuring one or more quantities indicative of phases and magnitudes of the acoustic calibration signals received by each of the receiver elements;
processing means for determining a complex directional response from the measured one or more quantities for each of the receiver elements;
means for storing calibration parameters indicative of the determined complex directional response for each of the receiver elements in the storage means of the sonar apparatus.