DATASONICS SIS-1500 System Manual page 81

SIS-1500 Seafloor Imaging System
connects to the pitch, roll, and heading sensors through an RS-232 serial
port, and to the optional cesium magnetometer, pressure sensor, and
responder through three additional RS-232 serial ports. The Microcontroller
board, upon power up, initializes the sonar electronics and invokes a ping
cycle four times a second, which is the default repetition rate. The repetition
rate, along with the Chirp pulse widths, transmitter on and off, receiver gain,
and diagnostics, is controlled with the downlink commands from the topside
processor.
The Burst board generates the Chirp transmit waveforms and the transmit
gate for each channel. The sonar transmit signal frequencies sweep in the
190 kHz to 210 kHz band with the port and starboard sonars sweeping in
opposite directions. This technique minimizes crosstalk between the two
channels. The transmit gates, which control the pulse width of the transmitted
signals, are output to the Transmitter boards and are automatically set in
accordance with the range setting. The range is operator settable and is
controlled by the Microcontroller board in accordance with the corresponding
downlink commands. The transmit waveforms are also output to the Receiver
board and function as the diagnostic signals to assist in fault isolation.
The Transmitter board is a dual channel transmitter that amplifies the transmit
signals that are input from the Burst board and drives the corresponding port
and starboard transducer arrays through two independent impedance
matching transformers and transmit/receive (T/R) networks. The T/R
networks allow the transducer arrays to both transmit and receive acoustic
signals, where otherwise separate transmit and receive transducer arrays
would be required for each channel.
The Receiver board amplifies and filters the signals received by the port and
starboard transducer arrays, and it inputs the port and starboard diagnostic
signals from the Burst board. Both fixed gain and time varying gain are
applied. The level of the fixed gain is operator adjustable and is controlled by
the Microcontroller board in accordance with the corresponding downlink
command. The time varying gain, which is not operator adjustable,
compresses the dynamic range of the received signals, which can be as high
as 120 dB, to that which can be processed with 16 bits, or about 96 dB. This
is done by increasing the receiver gain with time over that of the fixed gain at
a rate that compensates for losses in the transmitted signals due to the
effects of spreading loss and grazing angle. Although the rate at which the
time varying gain increases is fixed, the length of time it is applied varies with
the repetition rate, decreasing proportionately with increasing repetition rate.
The gain increases from -20 dB at the beginning of a ping cycle to a maximum
SECTION 4 Theory of Operation
4-13
Subsea Electronics