Specifications

BATS AND PORTABLE BATS PRINCIPLES OF OPERATION
4-1
4. PRINCIPLES OF OPERATION
BATS (Broadband Acoustic Tracking System) measures and analyzes sound waves sent from an
acoustic sound source that is attached to a target. The signal is received by a hydrophone to de-
termine target position relative to the tracking ship. The hydrophone amplifies the received acous-
tic signals and sends them to the Transceiver and then to the PC for further processing by the
Trackman Software.
The BATS uses an ultra-short baseline, phased-array hydrophone containing sound transducing el-
ements arranged at the vertices of an equilateral triangle. Each element is spaced less than half a
wavelength away from each of the other elements in the array. When a sound wave impinges up-
on the array it strikes one element first, then one of its neighbors, and finally the remaining ele-
ment. The sound wave, as sensed by any element in the array, appears out of phase with the
wave as sensed by the other elements in the array. The phase differences between the elements
in the array provide the information required to calculate a three-dimensional vector from the array
to the sound source.
Three types of sound sources are used with the BATS: pingers, transponders and responders. A
pinger emits a sound pulse at a predetermined frequency, repeated at a set interval. The hydro-
phone receives the pulse and determines bearing and depression angle. Transit time of the pulse
is unknown.
A transponder emits a sound pulse in response to a specific acoustic interrogation pulse from the
controller transmitter. A responder emits a sound pulse via a wire connecting the responder and
the Signal Interface Module (SIM) when electrically triggered by the SIM. The system now has
timing information to calculate directly the slant range of the target, assuming the average velocity
of sound in the water column and the turnaround time of the transponder or responder are known.
In either pinger or transponder/responder mode, the relative bearing and depression angle of the
target being tracked are determined by measuring the phase differences of the sound waves for
each hydrophone element. The bearing of the target is measured in degrees relative to the hydro-
phone heading. The depression angle is formed by the intersection of a horizontal plane and a line
drawn from the vessel reference point to the target. Once these values are determined, the rest of
the positioning parameters can be calculated.
NOTE: The term "vessel reference point" denotes the point relative to which all measure-
ments are calculated. The actual position of the vessel reference point is determined
by hydrophone offsets entered into the system (see Figure 4-1). If no offsets are en-
tered, the hydrophone itself is considered the vessel reference point. This is the case
in Figure 4-2, which illustrates the positioning parameters that the system calculates.
The slant range is the straight line distance from the vessel reference point to the target. When a
sound source other than a pinger is used, the travel time of the sound wave between the sound