NOAA Technical Report NOS CO-OPS 049

The maritime industry depends on accurate bridge clearance or air gap measurements. Knowing
the distance between the lowest point of a bridge (low steel) and the water surface helps to avoid
the risk of a vessel's overhead allision with bridges, which increases as vessel size increases. The potential economic gains to be realized by both increased commerce and the avoidance of allisions are considerable, and as a result, the maritime industry has a clear requirement for air gap information. The potential users of air gap sensor data require continuous, real-time observations of the clearance beneath a bridge with an accuracy of ±15 centimeters (6 inches). CO-OPS further
reduces this accuracy requirement to ±75 millimeters (3 inches) to maintain an initial level of
quality assurance and considers the value to represent two standard deviations of the difference
between the final system output and a reference observation. The system must provide useful observations of air gap every six minutes in real time, corrected for any sensor mounting offset on the bridge from low steel. It must operate in all weather conditions and survive in the hostile bridge environment, where vibration can be a significant issue.