Using Microwave Range Sensors for Long Term Remote Sensing of Ocean Surface Dynamics

Over the past two hundred years, water level observations in coastal areas have been used to help mariners navigate oceans and estuaries, cartographers develop nautical charts, government agencies regulate boundaries, and scientists gain a better understanding of various physical processes in the ocean. Water level observations can capture a range of physical processes occurring at the ocean surface with varying time scales, including gravity waves, storm surge, and astronomical tides. An important mission of the National Oceanic and Atmospheric Administration (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS) is to support those who depend upon these data by providing the most up-to-date water level products and services available. This involves maintaining a network of over 200 observatories throughout the United States coastal regions and continually exploring new and improved oceanographic measurement technology.

Recently developed microwave range sensors offer the opportunity to overcome one of the largest disadvantages of NOAA's currently-used water level sensors by avoiding contact with the harsh marine environment. They can be deployed on structures in coastal areas, such as piers or pilings, in order to measure the ocean surface remotely. As a result, many typical problems related to obtaining long term ocean observations with sub-surface sensors can be avoided, such as biological fouling, corrosion, and high deployment and maintenance costs. NOAA/COOPS has been conducting a series of laboratory and field tests of four different types microwave sensors in order to understand sensor functioning and performance capabilities and to assess the suitability for incorporation of these sensors into the NOAA National Water Level Observation Network.

Recently, a two day controlled laboratory test was conducted on several different microwave sensors at the Naval Surface Warfare Center (NSWC) Maneuvering and Sea Keeping Basin (MASK) in Carderock, MD. The MASK facility includes a 110 m (360 ft) long, 73 m (240 ft) wide tank, with the capability to generate waves from two different sides of the tank. The basic design of the experiment involved collecting measurements over a range of different sensor heights (sensor to water surface range) while generating a series of different surface wave conditions. Results showed that the effect of surface gravity waves on microwave signals depended on two parameters: 1) the microwave sensor footprint width on the water surface and 2) the wavelength of the gravity waves present on the surface.

Recent field testing has involved deploying a suite of four different sensors at several different locations including the United States Army Corps of Engineers Field Research Facility (ACE FRF) in Duck, NC. ACE FRF is an internationally-recognized coastal observatory, which includes a 560 m long pier, extending through the open ocean surf zone. The facility is equipped with a suite of environmental instruments that provide constant high resolution measurements of many oceanographic and meteorological parameters. The pier's open ocean location on North Carolina's Outer Banks often experiences high wave conditions and storm surges, representing a very challenging environment for a microwave sensor to operate in. The test sensors at ACE FRF have been collecting 1 Hz water level measurements over a period of several months, and records have captured a variety of surface gravity wave conditions, storm surges, and tidal signals.

An overview of NOAA's microwave sensor test plans and objectives will be presented followed by results from the NSWC wave tank laboratory test and field measurements collected at the ACE FRF pier, which show examples of various ocean surface dynamics measured remotely by microwave range sensors.