Line array doppler processing
View Patent ↗A method is taught to extract more information about the motion of an acoustic wave emitter moving relative to a line array of hydrophones that are part of a sonar system by determining the variation in Doppler shift across the entire line array of hydrophones together with the a range measurement of the emitter to calculate the emitter's velocity.
1. A method of determining the velocity of an underwater object moving parallel to an underwater line array of a plurality of N equally spaced acoustic sensors that are part of an active sonar system comprising:
making contact with the underwater object with a narrow band continuous wave pulse from the active sonar system at a frequency f 0 ;
determining the range R of the object from the line array with a shorter pulse from the active sonar system centered at a frequency f 0 , wherein the time duration of the pulse is shorter than the time duration of the first narrow band continuous wave pulse;
receiving a frequency f N at every acoustic sensor of the line array of N acoustic sensors;
calculating the frequency shift (T N −f 0 ) for each of the N individual acoustic sensors of the line array of N acoustic sensors;
plotting a line of frequency shift versus acoustic sensor for the measured Doppler shifts of a plurality of interior acoustic sensors to allow for confirmation of any difference in frequency using a linear least squares fit of the frequency shifts and to determine whether there is a measurable variation in the Doppler shift of the entire line array;
calculating the variation in Doppler shift of the entire line array of N acoustic sensors, based on the frequency shift of the first acoustic sensor and the Nth acoustic sensor of the line array of N acoustic sensors, expressed as
Δ
f
N
-
Δ
f
1
f
0
;
and
determining the velocity of the underwater object according to the equation
Δ
f
2
-
Δ
f
1
f
0
≅
v
Δ
θ
c
,
wherein c is the speed of sound in water and wherein Δθ is equal to the length of the line array of N acoustic sensors divided by the range R of the underwater object.
2. The method of claim 1 further comprising the step of forming a synthetic aperture as the line array of N acoustic sensors moves to get more aperture to increase the measurement accuracy relative to a line array of N acoustic sensors by increasing the measurable difference in the variation in Doppler shifts.
3. The method in claim 1 , further comprising a method to determine the variation of the Doppler shift due to the motion of the object normal to the line array using the equation
Δ
f
2
-
Δ
f
1
≅
v
2
c
[
θ
1
2
-
θ
2
2
]
.
4. The method in claim 3 , further comprising the removal of any frequency shifts due to the motion of the object normal to the line array by fitting the difference in frequency to a line with a least squares fit, thereby more accurately determining the motion of the object parallel to the line array.