1. Field of the Invention
This invention relates to the field of sonar, and more particularly to the field of demultiplexer circuits used in demultiplexing the multiplexed signal information obtained from DIFAR sonobouys.
DIFAR sonobouys are dropped in numbers by intelligence gathering aircraft into ocean areas that are to be subjected to short term sonar monitoring. As shown in FIGS. 1a and 1b respectively, the DIFAR sonobouys typically have a first dipole directional sonar hydrophones that respond to sound along a sonobouy (Y) axis, a second dipole directional hydrophones that respond to sound along a sonobouy (X) axis, and a non-directional or omni hydrophone that responds to sound with uniform sensitivity on all level radials from the instrument.
The DIFAR sonobouy is typically characterized to float at a predetermined depth for a predetermined duration. The (X) and (Y) axes are orthogonal and are kept float level by operation of the hydrostatic design of the sonobouy. The sonobouy is equipped with a fluxgate compass. No means is provided to orientate the sonobouy once it is in the water.
Sounds received by the hydrophones in the sonobouy are multiplexed in accordance with a predetermined DIFAR format for transmission to an aircraft sent to record the received data.
The DIFAR format requires that the following conventions be adopted. Using a top view of the sonobouy, angle Phi is measured clockwise from magnetic north to the +Y axis. Angle Theta is measured clockwise from the +Y axis to the target location. When dropped into the ocean, the angle phi can be any angle from 0 to 360 degrees. The sonobouy orientation can continually change due to wave action. The relative amplitude and polarity of target signals from the Y axis hydrophones varies as the cosine of angle theta. The relative amplitude and polarity of target signals from the X axis hydrophones varies as the Sine of angle theta. The omni hydrophone signal is positive in all quadrants of angle theta.
The sonobouy circuitry provides a 15.0 kilohertz phase reference signal. The 15.0 kilohertz phase reference signal is phase shifted or delayed in time by an interval proportional to the magnetic heading angle phi to generate a signal referred to as the phase pilot signal.
The Y axis hydrophone information is referred to as cosine channel information. The X axis channel information is referred to as sine channel information. The cosine channel information is multiplied by a reference carrier signal having a frequency of 15.0 kilohertz. The reference carrier signal leads the 15.0 Kilohertz phase reference by 90 degrees. All delays are measured with respect to the phase reference signal. The sine channel information is multiplied by a quadrature reference carrier signal having a frequency of 15.0 kilohertz and is phased shifted to lead the reference carrier of the cosine channel by 90 degrees.
The sonobouy also provides a constant amplitude 7.5 Kilohertz frequency pilot signal. The frequency pilot signal is by definition exactly one half the frequency of the phase reference signal.
The omni signal is not multiplied by a carrier. The bandpass of the omni signal information is typically under 2.5 kilohertz. An RF carrier is modulated with the sum of the frequency pilot, the phase pilot, the cosine channel, sine channel and the omni signal information and transmitted by the DIFAR sonobouy to the sampling aircraft to be recorded on board the aircraft or for retransmittal to a receiving station as a single channel signal for demultiplexing by the invention DIFAR demultiplexing circuit.
2. Prior Art
Previous DIFAR demultiplexer circuits used two or more phase locked servos. The invention DIFAR demultiplexer circuit uses one phase locked servo loop with a digital correction in 45 degree steps and analog adjustment of the quadrature phase signal to the phase pilot.