Often the specified measurable depth by ALB systems refers to a factor in relation to depth. The method of measuring water clarity. depth in Rio de Janeiro is determined by the use of a circular 20cm diameter plate with alternating black and white quadrants. The disk is attached to a measuring line or pole, which is lowered into the water and the depth at which the disk disappears from view, is noted. The disk is further lowered, and then raised until it comes into view again. The depth is again noted. The depth is the average of these two depths. Acquisition geometry includes the swath width and the footprint size. The swath width is the across track distance covered by each single scan line. The footprint size is the area on the sea bed which is covered by each individual laser pulse. The scan angle is the angle from nadir to maximum scanned angle. In ALB the swath width and the footprint size are dependent on the scan angle. The footprint size of the laser increases with depth due to scattering and refraction in the water column, however this is not nearly to the same degree as in MBES. The small changes in ALB footprint size over varying depths means that the swath width can remain constant without adversely affecting the point density. If the altitude of the aircraft needs to be changed to be able to fly below clouds or turbulent airspace, the scan angle can be widened to maintain the same swath width and footprint size. Advantages& Disadvantages of ALB. A. Advantages: The ability to survey the combined features and constructions, both above and below the waterline in a single scan. In addition, detailed digital images are recorded to allow for visual analysis and use with digital terrain models. The ability to survey closes to shore in areas where it would be dangerous or inaccessible to boats. The capacity to perform missions at short notice and in limited windows of time. This can be due to environmental conditions such as extreme weather or ice coverage. This capacity is also useful when the need to survey damage caused by weather and natural disasters occurs. The ability to survey large areas in relatively short time frames in a cost effective manner.B. Disadvantages: The amount of energy required to transmit a green laser pulse is much higher than that of an infrared laser pulse. The higher power laser pulses result in a lower frequency of generated pulses for green laser (1 kHz) than infrared laser (8 kHz), and therefore a lower point density is achieved. To safely operate a system using green laser, the scan beam needs to be expanded to make it eye safe at the surface. This reduces the amount of energy which returns to the sensor. This factor in addition to the scattering in the water column limits the maximum achievable depth. Reliable measurements are dependent on water clarity. Some surveys may need to be repeated if water clarity is not suitable. Waves cause surface foam which can make water penetration difficult due to the added refraction. Under these conditions additional flight lines need to be scanned.
Posted on: Sat, 14 Sep 2013 22:07:47 +0000
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