DATASONICS SIS-1500, System Manual

Page: 26 / 110

1-12 DATASONICS
Volume I System Manual June 1998
Sonar Electronics
Contained within the electronics housing in the tow vehicle is the sonar
electronics, which controls the tow vehicle’s functions, generates and
transmits the Chirp waveforms, and processes the received signals. Included
with the sonar electronics in the electronics housing are the pitch, roll, and
heading sensors. The sonar electronics receives power, downlink
commands, and the sonar and responder keys from the Chirplink II Digital
Multiplexer, and transmits the sonar data, the tow vehicle status information,
and the sensor data to the digital multiplexer.
Optional Sensors
Spare RS-232 and analog inputs are available for connecting optional
sensors and oceanographic instruments, including a temperature sensor, a
pressure sensor, and a cesium magnetometer. In addition, a responder input
is included, which allows for the installation of a responder for tow vehicle
positioning.
Chirp Technology
Chirp technology uses digitally produced linear FM acoustic transmissions to
produce high resolution images of seafloor contours. In all sonar systems,
higher frequency content is invariably associated with an increase in
resolution, and in the case of a sidescan sonar, a decrease in range. Chirp
technology, as implemented in the SIS-1500 Seafloor Imaging System,
reduces this trade-off, providing
both high resolution and extended range.
Across-Track Resolution
The resolution of a sidescan sonar is measured by its ability to discern closely
spaced objects that lie in the direction that is 90 degrees to the path of the
tow vehicle. This resolution is referred to as the across-track resolution. A
sidescan sonar system with an across-track resolution of 10 cm will detect
individual objects that are at least 10 cm apart. Objects spaced closer than
10 cm will be resolved by the sonar as a single object. In a conventional
sidescan sonar system, the limit of its resolution is determined by the pulse
length of the transmitted waveform. In the SIS-1500 Seafloor Imaging
System, which is a Chirp sidescan sonar system, it is the bandwidth of the
transmitted pulse that sets the system's theoretical resolution. The theoretical