DATASONICS SIS-1500, System Manual

Page: 28 / 110

1-14 DATASONICS
Volume I System Manual June 1998
Chirp Pulse Transmission and Reception
The sequence of steps in the transmission and reception of the Chirp pulses
are summarized as follows:
1. A linear FM pulse is generated at fixed intervals.
2. The signal is sent to power amplifiers that drive the two sidescan sonar
transducer arrays. The sidescan’s port transducer array sweeps from a
low to a high frequency, while the starboard array sweeps from a high to
a low frequency. The counter-directional sweeps minimize cross-talk
between the sidescan channels.
3. The sidescan sonar transducer arrays transmit the pulse and the same
transducer arrays detect the reflected energy.
4. Reflections at the receiver array are amplified by a computer-controlled
amplifier and a user-programmable gain stage. In addition, the sidescan
receiver applies time varying gain to enhance attenuated signals
returning from greater distances.
5. The received sonar signals are digitized with a 16-bit A/D converter.
6. On the surface a dual channel digital signal processor in the workstation
de-Chirps (compresses the FM reflections using the matched-filter) the
return signals from the sidescan sonar. This correlates the received
reflections with a compensated replica of the outgoing pulse. Signals that
do not resemble the outgoing pulses are attenuated by this type of
processing. Compressed returns are further processed to correct for
amplitude losses from attenuation and absorption by the water.