Thursday, May 20, 2010

Radar for accurate draft measurements

Here at CCOM, we have been testing a small radar for use as an accurate tide gauge out in the field. The WaterLog Series radar has an accuracy of +/- 3mm and and typical report time of 800 milliseconds, which is pretty snazzy.

(Radar mounted above CCOM's wave tank)

I was thinking it would be pretty sweet if we could mount one of these radars on the sonar mount of the R/V Coastal Surveyor, CCOM's main survey ship. Right now we have to take draft measurements by holding a yard stick up to a clear tube and measuring the distance between the meniscus of the water in the tube and the top of the Inertial Motion Unit (IMU). This leaves A LOT of room for human error.

(R/V Coastal Surveyor, with the hydraulic ram out of the water)

On the Surveyor, sonars are mounted to the bottom of a hydraulic ram that moves up and down. Every morning prior to surveying, surveyors measure the height of the ram above a specific point on the deck of the ship. This is then used to calculate the depth of the sonar below the IMU since you know the total length of the ram, and you know the height of the IMU in relation to the point on the deck that you measure the ram height to. This means, each day, we would also know the height of the radar in reference to the sonar as well. We could simply take a running average of say, a minute of data, and get a much more accurate draft than we could ever measure with a yard stick. Furthermore, we could get continuous draft measurements throughout the day, which is something you cannot readily do now, barring someone taking several yard sticks measurements in a row and hoping that the average somehow actually represents the real draft. Plus, trying to measure a meniscus in a tube with a yard stick, on a small moving vessel that is rocking in waves is not something I particularly want to do.

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