Scatterometer Climate Record Pathfinder
QuikSCAT Enhanced Resolution Image Product Users Notes
This page is designed as a dynamic
repository of relevant information to aid users of QuikSCAT enhanced
resolution image products. Additional information is available here as a pdf document. Low resolution
Sigma-0 Browse product information is available here as a pdf document.
SeaWinds-on-QuikSCAT (by JPL convention, termed "QuikSCAT") began
operations in 1999. The sensor is identical to the SeaWinds sensor
aboard ADEOS-II (by JPL convention, termed "SeaWinds"). This document applies to both.
Enhanced resolution images made from QuikSCAT data use two
different forms of the single-variate Scatterometer Image
Reconstruction (SIR) algorithm: for egg sigma-0 measurements the SIR
algorithm is used while for slice sigma-0 measurements, the SIR with
filtering (SIRF) algorithm is used. For egg measurements the full
antenna/processing spatial response is used while a simplified
spatial response function in which the spatial response is assumed
to be 1 over the footprint and 0 elsewhere is used for slice
measurements. Unlike other instruments, QuikSCAT observations are at
(essentially) a single incidence angle for each beam so no incidence
angle dependence (B) estimates are made. The SIR algorithm with eggs
makes images of A (in this case, sigma-0 at the measurement
incidence angle) on an 4.5 km pixel grid. The effective resolution
varies depending on region and sampling conditions but is estimated
to be 8-10 km in most areas. The SIRF algorithm with slices makes A
images at 2.225 km pixel spacing with an estimated effective
resolution of 6-8 km. Slice measurements are much noisier than egg
measurements. Multiple passes of the spacecraft are combined to
produce a higher spatial resolution (at a cost of reduced temporal
resolution).
QuikSCAT is a dual-pencil-beam conically scanning scatterometer
with the outer beam V pol and the inner beam H pol. The operation
frequency is 13.4 GHz. In combining the multiple passes, sigma-0 is
assumed to be independent of azimuth angle. While true for most
areas, some azimuth dependence in sigma-0 has been observed in
Antarctic firn, presumably due to sastrugi or snow dunes.
QuikSCAT data was obtained from QuikSCAT L1B data archived at the PO.DAAC.
No recalibration has been applied.
Images are produced in the BYU .SIR file
format, using the standard naming
scheme. The files are gzipped to minimize storage and transfer
requirements. The standard images are designed for land and ice
observation and so are landmasked. However, .SIR format land mask
files (containing 0 for ocean and 1 for land) are available for each
standard region. .SIR format images containing "images" of the
latitude and longitude of each pixel for each region are also
available.
Due to its polar orbit, the local-time-of-day of the QuikSCAT
observations varies with latitude and direction of the orbit. At mid-
to low-latitudes, most areas are imaged at two different local
times-of-day -- one for the ascending (north-bound) pass and one for
the descending (south-bound) pass. Thus, by separating the data into
ascending and descending passes, images corresponding to two different
local times-of-day are created. In the polar regions, particular
locations are observed at multiple local times-of-day and so a
different method is required. Local time-of-day is considered in more
detail in MERS research report on
improving temporal resolution for QuikSCAT/SeaWinds at the poles
(pdf) and
improving temporal resolution for Tandem SeaWinds at the poles
(pdf).
|