Acoustic-microwave water level sensor comparisons in an estuarine environment

Microwave water level sensors offer certain advantages over the acoustic sensor, the present standard for water level measurements obtained in U.S. coastal areas by the National Oceanic and Atmospheric Administration (NOAA). These include high reflectivity of microwave radiation from the target medium (water), low sensitivity to variations in air temperature and humidity, and open-beam transmission eliminating any contact between the device and the water. The latter feature has raised the question of possible interaction between time-of-flight microwave measurements and wind wave motion at the air-water interface. A field comparison between a microwave sensor and the NOAA acoustic water level sensor at Yorktown, Virginia revealed close agreement between sensor measurements in an operational setting and produced no evidence of an dasiaoffsetpsila in the presence of irregular surface gravity waves. However, unlike the acoustic sensor which has a mechanical filter (stilling well) to eliminate wave motion above a fixed dasiacutoffpsila frequency, microwave sensors operate without a stilling well and require numerical filtering to obtain water level measurements in the frequency range of interest; i.e., tidal and sub-tidal frequencies for the classic dasiatide stationpsila. Numerical methods now offer greater choice in deciding where to make the cutoff while reducing measurement error.