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\def\A{${A}$}
\def\AV{${A\_v}$}
\def\AH{${A\_h}$}
\def\B{${B}$}
\def\BV{${B\_v}$}
\def\BH{${B\_h}$}
\def\sigmao{$\sigma^o$\ }
\def\sig{$\sigma^o$~}
\def\signo{$\sigma^o$}
\def\sigo {$\sigma^o$}
\def\sigest {$\widehat{\sigma}^o$}
\def\sigstd {$\kappa$}
\def\polrat {$\gamma$}
\def\deg{^\circ}

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\begin{document}

\noindent
{\Large SeaWinds on QuikSCAT}\\
\vskip1in
\noindent
{\huge Special Product: \\
SeaWinds on QuikSCAT Radiometric L2A Product}
\vskip1in
\noindent
{\Large Product Description}\\
\vskip0.5in
\noindent
{\Large 
Version 2.1\\
\vskip0.5in
\noindent
David G. Long}\\
\vskip 10pt
\noindent
Microwave Earth Remote Sensing Laboratory\\
Brigham Young University \\
\vskip0.5in
\noindent
{\Large Linwood Jones} \\
\vskip 10pt
\noindent
Central Florida Remote Sensing Laboratory\\
University of Central Florida\\
\vskip 1in
{\large \date{\today}}

\newpage

\section*{Summary}
Following up on the successful NSCAT mission, the SeaWinds on QuikSCAT
scatterometer (Qscat) will provide \sigmao measurements of the Earth's 
surface at unprecedented coverage and resolution.  In making measurements of
$\sigma^o$, Qscat also measures the system noise power.  These noise
measurements can be converted into measurements of the apparent brightness
temperature of the Earth.   These Ku-band radiometer observations
can be useful in evaluating the location of rain over the ocean and may
have application for land and ice studies.  Designed as a radar, Qscat
is not an optimum radiometer, and its individual brightness measurements
are noisier than desired.  This can be ameliorated by using spatial
averaging.  To this end, a special Qscat radiometric product has been
developed which consists of spatial and temporal averages of the 
brightness temperature-converted noise observations.

\vskip 1in

\tableofcontents

\newpage
\section{Product Overview}

Though cut short by the loss of the host spacecraft, NSCAT was
an unqualified success.  As a follow-up/replacement for the NSCAT mission, 
the SeaWinds on QuikSCAT scatterometer (Qscat) will continue to provide 
high quality \sigmao measurements of the Earth's surface at
unprecedented coverage and resolution.  

In making \sigmao measurements Qscat transmits Ku-band RF pulses
and measures the return echo power.  Since the return echo is contaminated
by radiometric and thermal noise, a separate measurement of the noise power 
is also made and subtracted from the measured echo power to estimate
the return signal power.  Unlike NSCAT, this noise measurement is made
simultaneously with the echo power measurement but over a wider 
(1 MHz versus 180 kHz) bandwidth.  The noise measurement bandwidth 
includes the signal bandwidth, requiring a somewhat more complicated
noise subtraction scheme than used for NSCAT.  Nevertheless, the 
receiver gain is very stable and with careful calibration the noise power
measurements can be converted into apparent brightness temperature 
measurements. 
The observed apparent brightness temperatures are a function of the instrument
noise, the surface emissivity and temperature, and the attenuation/emission
of the intervening atmosphere.
Over the ocean, rain results in higher brightness temperatures than the
ocean alone.  Thus, the brightness temperature measurements can be useful
in locating rain and possibly estimating rain rate.


\section{Product Generation}

This product is generated using information from both L1A and L2B files.
In generating this product the individual noise measurements contained in the
L1A product files are converted to noise-only measurements by subtracting out 
the signal power contained in the slice measurements.  The resulting
noise-only power estimate is converted to effective
brightness temperatures (Tb's) using calibration coefficients determined by
L. Jones \cite{jones}.  

In order to Earth-locate the Tb measurements, the lat/lon location of the
corresponding pulse in the L1B file is extracted.  The wind vector cell (wvc)
in which the measurement center falls into is then determined by using
M. Frelich and B. Vanhoff's KEY grid indexing which gives the 
the along-track/cross-track grid index that each measurement falls in.
This insures that the radiometric product grid is identical to the L2B product.  
Note that only Tb measurements
corresponding to `usable' (according the L1B SIS \cite{l1bsis}) 
\sigmao values are retained; the others are discarded.  Frame quality and
other flags are also examined, with measurements discarded which do not
pass very conservative flag checking.

The mean, standard deviation, and count of the Tb measurements (for a
given beam) falling into each wvc are then determined.  This results in
two Tb measurements, one for each polarization or beam, for each wvc
along with the counts and standard deviations.
Note that Tb measurements for both forward-looking and aft-looking azimuth 
directions are averaged into the final product.



\section{Product Format}

This product is stored in HDF form similar to the
L2B standard product.  The Tb, count, and standard deviation arrays are stored
as scientific data sets.  Global attributes are copied from selected L1B global
attributes and are stored in the same manner.  In addition, two new global
attributes are included.  These give the full file names of the L1A and L1B
data files used to create the brightness temperature product.

The 6 scientific data sets in each file are 1624 by 76 arrays containing the
mean Tb values, the counts, and standard deviations for each wvc.  The SDS
names are Tb\_h, Tb\_v, Tb\_hcnt, Tb\_vcnt, Tb\_hstd, and Tb\_vstd.  
Two additional scientific data sets named wvc\_lat and wvc\_lon are
also stored as 1624 by 76 arrays.
The cnt's are stored as 4 byte integers while the other variables are 
stored as floats.

The global attributes (with typical values) are:\\
\begin{tabular}{ll}
Source\_L1A\_file* &                  QS\_S1A00678.19992301242\\
Source\_L1B\_file* &                  QS\_S1B00678.19992301325\\
LongName &  QuikSCAT L2B Radiometer Measurements in 25km Swath Grid\\
ShortName &                        QSCAT\_RadMode\_L2\\
producer\_agency&                   NASA\\
producer\_institution&              Brigham Young University\\
PlatformType&                      spacecraft\\
InstrumentShortName&               SeaWinds\\
PlatformLongName&                  NASA Quick Scatterometer\\
PlatformShortName&                 QuikSCAT \\
project\_id        &                QuikSCAT\\
data\_format\_type   &               NCSA HDF\\
QAPercentOutofBoundsData&          0   \\
QAPercentMissingData    &          0    \\
build\_id                &          QS\_revrad Version 3\\
ProjectionDateTime      &          19992450910\\
RangeBeginningDate      &          1999-217\\
RangeEndingDate         &          1999-218 \\
RangeBeginningTime      &          23:17:50.690\\
RangeEndingTime         &          00:58:54.107\\
rev\_number              &          678\\
\end{tabular}

The global attributes marked with `*' (Source\_L1A\_file and 
Source\_L1A\_file) are new.  They are the file names for the L1A and 
L1B data files used in creating this product.  All other attributes are
identical to those documented in the L1B SIS \cite{l1bsis}.



\section{User Notes}

The Qscat design is optimized for operation as a radar rather than as a
radiometer.  As a radiometer, it design is sub-optimum though good quality 
radiometric measurements are obtained.  The following paragraphs consider 
some of the known limitations of the data.

Because the radiometric and radar measurements are made simultaneously, 
some contamination of the radiometric measurement by the radar signal 
can be expected, particularly for large attitude excursions which might
shift some of the return echo signal outside of the signal measurement 
spectrum. However, very little signal `spill over' has been observed
in the data studied thus far.

The absolute calibration of the measurements is somewhat uncertain since
no provisions for absolute radiometric calibration of the Qscat receiver 
are incorporated into the design or operation since they are not needed for
radar operation.  The short-term (hours to days) relative calibration
is considered to be accurate to a few tens of Kelvin.  Long-term
calibration will be established by comparison to other satellite radiometer
systems (e.g., SSM/I and TRMM-TMI). The radiometric
precision ($\Delta T$) for a single pulse observation is approximately 25 K.
This is reduced by averaging multiple pulses with an improvement being
1/sqrt(number of pulses averaged). 

In producing this temporal/spatial average product note that, unlike
the L2B wind product which reports wvc lat/lon values as the average of the
lat/lon's of the individual \sigmao measurements used in the particular
wvc, the reported lat/lon values are the nominal wvc (ideal wvc center location)
lat/lon values.


\section{Product file name format}

The standardized data file naming scheme is:
$$
QS\_XTbap2A{\bf RRRRR}.{\bf YYYYDDDHHMM}
$$
where {\bf RRRRR} is the 5 digit rev number, and the 11 character extension 
contains the production date where {\bf YYYY} is a four digit year code,  
{\bf DDD} is a three digit Julian day code, {\bf HH} is a two digit hour, 
and {\bf MM} is a two digit minute.   One file is produced for each orbit 
rev. Files are typically just under 3 MB in size.


\begin{thebibliography}{1}

\bibitem{l1bsis}
B. Weiss, ``Level 1B data Software Interface Specification,'' JPL Internal Document
D-16077, May 1999.

\bibitem{jones}
W.L. Jones and D.G. Long, "QuickSCAT Ratiometer
Processing", QuikSCAT Cal/Val meeting minutes, Nov. 1999


\end{thebibliography}

\newpage

\section{Points of Contact}

Questions concerning data distribution should be be directed to the PO.DAAC.  
Issues related to data quality or processing should be directed to David Long 
or Linwood Jones.  Please
note that e-mail is the preferred means of communication.
%\vskip 1in

\subsection{PO.DAAC: Data Distribution Issues - Contact information}

JPL PO.DAAC User Services Office\\
Jet Propulsion Laboratory \\
Mail Stop Raytheon-299\\
4800 Oak Grove Dr. \\
Pasadena, CA\ \ \ 91109 \ \ \ USA
\vskip 10pt
\noindent
e-mail: {\tt qscat@podaac.jpl.nasa.gov}
Telephone: (626) 744-5508\\
FAX: (626) 744-5506\\
Homepage: {\tt http://podaac.jpl.nasa.gov/quikscat}\\
FTP: {\tt ftp://podaac.jpl.nasa.gov}\\

%\vskip 1in

\subsection{Technical/Algorithm Issues - Contact information}

Prof. David G. Long\\
Microwave Earth Remote Sensing (MERS) Laboratory\\
Electrical and Computer Engineering\\
Brigham Young University\\
459 Clyde Building\\
Provo, UT\ \ \ 84602\ \ \ USA
\vskip 10pt
\noindent
e-mail: {\tt long@ee.byu.edu}\\
Telephone: (801) 378-4383\\
FAX: (801) 378-4383\\
Homepage: {\tt http://www.ee.byu.edu/ee/mers}\\
FTP: {\tt ftp://ftp.ee.byu.edu/mers}\\
\vskip 0.25in
\noindent
Prof. W. Linwood Jones\\
Central Florida Remote Sensing Laboratory\\
Dept. of Electrical and Computer Engineering\\
University of Central Florida\\
P.O. Box 162450\\
Orlando, FL\ \ \ 32816-2450\ \ \ USA
\vskip 10pt
\noindent
e-mail: {\tt ljones@iu.net}\\
Telephone: (407) 823-6603\\
FAX: (407) 823-5835\\
%Homepage: {\tt http://www.ee.byu.edu/ee/mers}\\
%FTP: {\tt ftp://ftp.ee.byu.edu/mers}\\


\end{document}\end