Edgetech 2000-TVD, User Hardware Manual

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1-8 1.0: OVERVIEW
2000-TVD COMBINED SONAR 0017424_REV_B
1.4.2 High Signal-to-Noise Ratio
Full Spectrum Chirp technology does not use a conventional matched filter, the correlation filter that is
widely used to compress FM signals, to process wide band signals. Rather it uses proprietary amplitude
and phase weighting functions for the transmitted pulse and a pulse compression filter that maximizes
the SNR of the acoustic images over a wide band of operating frequencies. These functions provide a
significant SNR improvement in the imagery over other pulse and chirp side scan and sub-bottom sonars
with band limited components that are limited in dynamic range.
1.4.3 High Repeatability
The frequency range of operation is determined by the acoustic characteristics of the transmitter and
receiver mounted in the towed vehicle. For the side scan sonar, the transmit frequency is selected based
on the desired range and resolution required. For sub-bottom profiling, the frequency is selected based
on the sub-bottom conditions at the survey site and the type of sub-bottom features that need to be
imaged. The FM pulses are generated by a D/A converter with a wide dynamic range and a transmitter
with linear components. Therefore, the energy, amplitude, and phase characteristics of the acoustic
pulses can be precisely controlled. This precision produces high repeatability and signal definition.
1.4.4 High Resolution
Normally, when using long pulses, resolution is reduced. However, after correlation processing the
received signals, a very sharp wavelet is produced that has a duration equal to the inverse of the sweep
bandwidth. Therefore, the more bandwidth that is used, the sharper this pulse will become. Side scan and
sub-bottom signals received at the surface pass through a software controlled programmable gain
amplifier before being digitized with a 16-bit analog-to-digital (A/D) converter. The FM pulse is then
compressed using a digital compression filter. This correlation process is implemented in real time with
forward and inverse Fast Fourier Transforms (FFTs). The compressed pulse has a time duration
approximately equal to the inverse of the bandwidth of the FM pulse which results in high resolution
across track side scan sonar images and measurements of fine sediment layering in the sub-bottom
profiles, an important factor for sediment classification.
1.4.5 Reduction of Side Lobes
Use of Full Spectrum chirp technology reduces the side lobes in the effective transducer aperture,
enabling high along track resolution and high attenuation of unwanted echoes. The wide bandwidth of
the sweep frequency has the effect of smearing the side lobes of the transducer and thus achieving a
beam pattern with virtually no side lobes. The effective spatial beam width obtained after processing a
full spectrum 2–10 kHz sub bottom signal, for example is 20 degrees measured at the -3db points.