FTS RSS Distance Test, John C. Stennis Space Center, Dec 2015

Two identical Radar Stage Sensors from Forest Technology Systems, were evaluated to determine if they are suitable for U.S. Geological Survey (USGS) hydrologic data collection. The sensors were evaluated in laboratory conditions to evaluate the distance accuracy of the sensor over the manufacturer’s specified operating temperatures and distance to water ranges. Laboratory results were compared to the manufacturer’s accuracy specification of ±0.007 foot (ft) and the USGS Office of Surface Water (OSW) policy requirement that water level sensors have a measurement uncertainty of no more than 0.01 ft or 0.20 percent of the indicated reading. Both of the sensors tested were within the OSW policy requirement in both laboratory tests and within the manufacturer’s specification in the distance to water test over tested distances from 3 to 15 ft. Both radars were mounted to a plate at the end of a movable arm above the water. The water level was measured by using a float system attached to a Sutron Model 56-0540 SDI-12 shaft encoder. The arm level was measured with a separate Sutron Model 56-0540 shaft encoder. The reference distance from the arm to the water was computed based on the arm encoder reading and the water level encoder reading. The arm was initially set so that the radar was approximately 3 ft above the water. By using an attached motor controller, the arm was raised in 3.00-ft increments to a maximum air gap of 15 ft. At each 3-ft set point, both radars were given approximately 4 minutes to adjust to the new level before recording a water level measurement. At each step, the change in the RSS water level measurement was compared to the change in air gap. A total of five cycles from 3 to 15 ft and back to 3 ft were completed.