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QuikSCAT

NASA's Quick Scatterometer (QuikSCAT) is a satellite that was lofted into space from Vandenberg Airforce Base in California on Saturday, June 19, 1999.

Fig. 1. The QuikSCAT satellite.

Courtesy of JPL, NASA.

QuikSCAT operates in a near polar orbit. Its angle of inclination is about 98 degrees, which means that according to a compass it is flying north by north-west at a directional angle of 352 degrees when it is ascending (i.e. south to north). It flies in a circular orbit at an altitude of approximately 800 km above sea level. It completes a full orbit in about 101 minutes, which translates to a little more than 14 orbits per day.

The Instrument. SeaWinds is the main instrument on the QuikSCAT satellite. SeaWinds is an active radar scatterometer. This scatterometer operates by transmitting high-frequency microwave pulses to the ocean surface and measuring the echoed radar pulses bounced back to the satellite. The scatterometer estimates wind speed and direction over the Earth's oceans at 10 m above the surface of the water by analyzing the backscatter from the small wind-caused ripples, called cat's paws, on the surface of the water. When the microwave pulses strike the ocean surface, it causes a scattering affect referred to as backscatter. A rough ocean surface returns a stronger signal because the waves reflect more of the radar energy back toward the scatterometer antenna. A smooth ocean surface returns a weaker signal because less of the energy is reflected. QuikSCAT can acquire hundreds of times more observations of surface wind velocity each day than can ships and buoys, and can provide continuous, accurate and high-resolution measurements of both wind speeds and direction regardless of weather conditions.

SeaWinds uses a rotating dish antenna with two spot beams that sweep in a circular pattern. It actively transmits 13.4 GHz microwave pulses (Ku-band radar) at a rate of 1 pulse every 5.4 ms. The pulses are alternately polarized, vertical and horizontal. It uses the same parabolic antenna for both pulses, with different feeds. The antenna rotates at a rate of 18 rotations per minute. The feeds on the antenna are set up so that the vertical polarized beam has an elevation angle of 45 degrees and the horizontal polarized beam an elevation of 39 degrees. This creates an outer and an inner beam.

Fig. 2. Details of SeaWinds ground track.

As the antenna rotates, the instrument pulses form a circular footprint on the ground. The outer beam covers a circle on the surface of 1800 km diameter while the inner beam covers about 1400 km. When the circular rotation of the antenna is combined with satellite movement of approximately 25 km per rotation, a helical shape is traced out on the ground. This type of coverage pattern allows the instrument to cover 90 percent of the earth every day. The instrument is currently collecting data over ocean, land, and ice in a continuous, 1,800-kilometer-wide band that results in twice per day coverage. Because of the twice per day coverage, one dayÕs worth of data can be displayed on a global map for the ascending paths separately from the descending paths.

Fig. 3. Ascending paths of QuikSCAT on 9/13/1999. Hurricane Floyd can be observed to the east of Florida.

Image courtesy of the Marine Observing Systems Team, National Oceanic & Atmospheric Administration, Ocean Surface Winds, QuikSCAT Data Archive. QuikSCAT is a JPL/NASA program. Data processing and distribution performed by NOAA/NESDIS.

 

Fig. 4. Descending paths of QuikSCAT on 9/13/1999. Hurricane Floyd can be observed to the east of Florida.
Image courtesy of the Marine Observing Systems Team, National Oceanic & Atmospheric Administration, Ocean Surface Winds, QuikSCAT Data Archive. QuikSCAT is a JPL/NASA program. Data processing and distribution performed by NOAA/NESDIS.

QuikSCAT data can be displayed in several different ways. The winds speeds may be shown as a background color palette with wind direction indicated as arrows overlain. An example of this kind of display is shown below.

Fig. 5. Wind speed and direction near South Georgia Island on 13-Sept-99, as measured by QuikSCAT.

Image courtesy of the Jet Propulsion Laboratory, NASA.

Alternatively, the wind speed and direction can be indicated by wind barbs. Wind barbs are lines drawn in the direction the wind is blowing, with shorter line segments ("feathers", or "barbs") or triangles ("flags") attached at the end. The feathers and flags represent increments of wind speed, and are summed up to give the total wind speed. Wind barbs may be viewed as arrows flying with the wind; with feathers or flags at the end of the arrows, and with the point of the arrows missing. The following convention is following concerning the meaning of feathers and flags:

half feather/barb = 5 units of speed

full feather/barb = 10 units of speed

flag = 50 units of speed

Fig. 6. Wind barb description in the northern hemisphere.

Image courtesy of Research Application Program, NCAR.

The feathers and flags most often represent wind speed in knots, but sometimes other units are used; here, the winds are in m/s. Examples of wind barbs corresponding to various wind speeds are below:

Fig. 7. Wind speed associated with feathers and flags on wind barbs using units of m/s.

Image courtesy of Leslie M. Hartten of the NOAA Aeronomy Laboratory.

Often, QuikSCAT data is displayed with wind barbs that are color coded. In the example below, the color of the wind barb assigns it to a particular 5 knot interval of wind speed. Summing up the feathers and flags on individual wind barbs will provide a more precise wind speed.

Fig. 8. QuikSCAT data showing the cyclonic surface winds of a
typhoon approaching Japan.

Image courtesy of the Marine Observing Systems Team, National Oceanic & Atmospheric
Administration, Ocean Surface Winds, QuikSCAT Data Archive. QuikSCAT is a
JPL/NASA program. Data processing and distribution by NOAA/NESDIS.

Alternatively, both wind speed and direction can be indicated with a vector. The arrow point of the vector shows which way the wind is blowing. The length of vector indicates the wind speed. This kind of representation can be useful when QuikSCAT data is combined with data from other satellites and sensors. For example, below, vectors showing wind speed and direction (red arrows) are combined with data from TRMM TMI, for a hurricane.

Fig. 9. QuikSCAT data and TRMM TMI data for Hurricane Cindy
on August 25, 2000.

Image courtesy of Seaflux, Jet Propulsion Laboratory, NASA.