DATASONICS SIS-1500, System Manual

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SIS-1500 Seafloor Imaging System 1-13
SECTION 1 SIS-1500 Overview Chirp Technology
across-track resolution of a conventional system is calculated by multiplying
the length of the pulse by the speed of sound, and then dividing the product
by 2 to account for the pulse’s round trip.
across-track resolution = pulse length x speed of sound / 2
In comparison the pulse length equivalent of a de-Chirped swept frequency
pulse equals the inverse of the bandwidth.
pulse length = 1 / bandwidth
For example, the duration of a pulse with a bandwidth of 20 kHz, which is the
bandwidth of a system configured to operate between 190 and 210 kHz, is
approximately 50
µ
s (1/20000 Hz = 0.00005 sec). Travelling at about
1500 m/sec, the sound will traverse approximately 7.5 cm in 50
µ
s. Allowing
for the round trip, this results is a one-way distance of 3.75 cm, which is the
across-track resolution.
Along-Track Resolution
In addition to the frequency and bandwidth of the transmitted sonar beam,
which affect the across-track resolution, the horizontal width of the beam
affects the sidescan sonar’s resolution in the direction that is parallel to the
path of the tow vehicle. This resolution is referred to as the along-track
resolution. To attain a high along-track resolution, narrow beam widths are
required. However, narrow beams produce side lobes, which contain energy
that produces undesired echoes from reflections not located in the primary
beam. In conventional sidescan sonar systems resolution is lost due to the
presence of these echoes. With Chirp technology the side lobes are greatly
reduced through matched-filter correlation processing, which attenuates
echoes that do not correlate well with the transmitted pulse.
Signal-to-Noise Ratio
Another factor that affects the image quality is the signal-to-noise ratio. As the
transmitted pulse travels through the water, its amplitude becomes
attenuated and falls below the noise level. The matched-filter correlation
processing used in the SIS-1500 Seafloor Imaging System improves the
signal-to-noise ratio, and hence the quality of the sonar images.