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The ASCAT wind retrieval processing system used by NOAA was developed by the KNMI scatterometer team and adapted to the NOAA environment. The sigma-0 measurements are related to the ocean wind vectors and the radar measurement parameters via a geophysical model function (GMF). The GMF utilized in the ASCAT wind retrieval is CMOD5, and the wind retrieval is performed using the maximum likelihood estimation (MLE) principle. The harmonic characteristic of the GMF results in solutions is not unique which generally results in several ambiguities being found. The NOAA products currently include four ambiguity solutions. The NOAA Ice and Ultra High Resolution (UHR) processing algorithms were developed by Professor David Long, Brigham Young University.
The fan-beam ASCAT scatterometer provides measurements radar backscatter at 6 azimuth angles and a
diversity of incidence angles. The wide swath and frequent overflights permit generation of a wide
variety of ASCAT Image products. For highest possible spatial resolution, multiple orbit passes are
combined. These are the 'all pass' images.
Enhanced resolution images made from ASCAT data are created from SZF files using the multi-variate
the Scatterometer Image Reconstruction (SIR) algorithm with filtering (SIRF). Like previous fan-beam
scatterometers, ASCAT collects measurements at multiple incidence angles and up to 6 azimuth angles.
The effective image resolution varies depending on region and sampling conditions but is estimated
to be 12-15 km in most areas. The SIRF algorithm makes "A" (sigma-0 normalized to a 40 deg incidence
angle) images at 4.45 km pixel spacing for each beam. Multiple passes of the spacecraft are combined
to produce a higher spatial resolution (at a cost of reduced temporal resolution). For ice imaging,
the multiple beams are combined to create denser sampling, which reduces the noise level and
provides the best spatial resolution. The resulting images show the normalized radar cross-section
(sigma-0) in dB (10log10) at 40 degree incidence angle. No recalibration of the ASCAT data has been
applied in this processing.
For Ultra High Resolution (UHR) wind retrieval the ASCAT SZF measurements are processed using the AVE algorithm (the first iteration of the SIR algorithm) from single passes, separately for each beam. The geometry information is retained. Then, the standard KNMI wind retrieval algorithm is applied at each pixel of the set of multi-azimuth images to estimate the wind speed and direction and each 4.45 km wind vector cell (pixel). The UHR winds are noisier than conventional 25 km winds, but have higher spatial resolution. Near the coastline, the estimated winds can be contaminated by proximity to land, causing the retrieved wind speeds to appear excessively high. Note that UHR winds are not currently rain flagged. Rain generally causes the wind speed to appear higher than it should but it can also cause the wind speed to appear reduced.
MetOp-A, the first of three satellites developed under a joint program being carried out by the European Space Agency and the European Meteorological Satellite Organization (EUMESAT), was successfully launched from Baikonur, Kazakhstan on October 16, 2006. The satellite is now under the control of ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany. MetOp-B was successfully launched in September 06, 2012. MetOp-A/B forms the space segment of the EUMETSAT Polar System (EPS), designed to collect atmospheric and environmental data to complement the hemispheric survey conducted from geostationary orbit by the Meteosat system. EPS will be operated in coordination with the US Polar Operational Environmental Satellite (POES) system managed by the National Oceanic and Atmospheric Administration. The satellite was released into a circular orbit at an altitude of 837km over the Kerguelen archipelago in the South Indian Ocean. With a slightly retrograde 98.7° inclination, this orbit enables MetOp-A to circle the globe from pole to pole while always crossing the equator at the same local time, i.e. 9:30 am. Known as 'sun-synchronous', this type of orbit allows revisits to almost each point of the Earth's surface under similar solar illumination conditions on an approximately daily basis. While NOAA satellites are deployed in an 'afternoon' orbit (i.e. crossing the equator in the afternoon, local time), Europe's MetOp provides its service in a 'morning' orbit.
The ASCAT scatterometer transmits linear frequency-modulated pulses with a duration of around 10 msec at a peak power of 120 W. Echo signal is received by the instrument and may be thought of as a large number of superimposed echo pulses arriving over a range of times corresponding to the width of the instrument swath. The received echo is mixed with a suitably delayed pulse, which is a frequency-modulated replica of the transmitted signal. Because of the linear frequency modulation, the pulses are offset from the local oscillator pulse for the duration of the echo by frequencies f1 and f2, respectively. These frequency differences allow the discrimination of signals originating at different ranges. For the real received echo, the mixer output is a spectrum of signals, which is present for a time corresponding to the time that the transmit pulse is present within the swath. By sampling the mixer output in time, and Fourier-transforming the time series into a frequency series, the frequency spectrum may be extracted from the data and the height and frequency of its spectral lines may be interpreted in terms of power and range, respectively.
In addition to the processing of echo signals, the instrument also performs an internal calibration process within each pulse repetition interval. This consists of a measurement of the output power during the transmit pulse, the injection into the receiver of a signal proportional to the transmit pulse at a point in the inter-pulse period, and monitoring of the magnitude of this signal at the receiver output. This gives an indication of how the combination of transmitter power and receiver gain is varying and allows changes in these parameters to be compensated.
Also contained within the pulse repetition interval is a period after all echoes have decayed, during which the receiver noise output is monitored. This enables the contribution of the receiver noise to the radar measurement to be estimated and a correction performed.
