This document provides a high-level quality assessment of the
CALIPSO
Level 1B Wide Field Camera (WFC) data products, as described in Section 2.4 of
the CALIPSO Data Products Catalog (Version 2.4) (PDF). As
such, it represents the minimum information needed by scientists and
researchers for appropriate and successful use of these data products. We
strongly suggest that all authors, researchers, and reviewers of research
papers review this document for the latest status before publishing any
scientific papers using these data products.
The purpose of these data quality summaries is to inform users of the
accuracy of CALIOP data products as determined by the CALIPSO Science Team and
Wide Field Camera Science Working Group. This document is intended to briefly
summarize key validation results; provide cautions in those areas where users
might easily misinterpret the data; supply links to further information about
the data products and the algorithms used to generate them; and offer
information about planned algorithm revisions and data improvements.
The primary Wide Field Camera Level 1B data products are calibrated radiance
and bidirectional reflectance registered to an Earth-based grid centered on the
Lidar ground track. During the normal operation, the WFC acquires science data
only during the daylight portions of the CALIPSO orbits. For each orbit, three
different data files are produced: 1 km Native Science grid, 125 m Native
Science grid, and 1 km Registered Science grid. The 1 km Native Science grid
covers the full 61 km swath centered on the Lidar track. The 125 m Native
Science grid contains only the central 5 km wide high resolution portion of the
WFC swath. The 1 km Registered Science grid provides WFC data on the identical
grid as the CALIPSO IIR data and is produced to facilitate the use of the WFC
data in the IIR retrievals. In addition to radiance and reflectance grids, the
WFC Level 1 data products include two parameters that quantify the homogeneity
of the cross track image frames: swath homogeneity and track homogeneity. The
major categories of WFC Level 1B data in each file type are:
For the convenience of the user, we describe the contents of each of the
three major file types separately below. Note, in isolated cases satellite
ephemeris and attitude data may be missing for a portion of an orbit. In these
cases, the geolocation process will fail and fill values (-9999) will be
reported in all position and viewing geometry fields. In addition, the Pixel
QC Flag (see below) will also be set appropriately for no geolocation. However,
due to a known software error, the Pixel QC Flag (see below) in this data
release will not always be set appropriately for missing geolocation data.
The 1 km Native Science data product provides WFC radiance and reflectance
measurements across the full 61 km swath at 1 km resolution. The 125 m data in
the central 5 km swath have been interpolated to 1 km resolution to fill in
this portion of the swath. No additional spatial interpolation is performed.
Scan Time
This field reports the International Atomic Time (TAI) for each WFC scan,
in seconds, starting from January 1, 1993.
Scan UTC Time
This field reports the Coordinated Universal Time (UTC) for each WFC scan
with a format 'yymmdd.ffffffff', where 'yy' is the last two digits of year,
'mm' and 'dd' represent month and day, respectively, and 'ffffffff' is the
fractional of the day.
Latitude
This field reports the latitude of the individual 1 km WFC pixel on the
surface.
Longitude
This field reports the longitude of the individual 1 km WFC pixel on the
surface.
Radiance
The band-average spectral radiance of the scene averaged over the
spectral range of the WFC (620-670 nm). Units are
Wm-2sr-1μm-1.
Reflectance
The bi-directional reflectivity of the scene defined as the ratio of the
intensity of the radiation reflected from the surface and atmosphere as
observed by the WFC and the intensity of the incident solar radiation at the
top of the atmosphere. It is has no units.
1 km Homogeneity
The 1 km or swath homogeneity is defined as the variance in radiance over
the full 61 km cross-track swath normalized by the swath mean. The spatial
resolution of the swath homogeneity is 61 km cross-track and 1 km along-track.
Solar Zenith Angle
The angle between the zenith at the WFC pixel footprint on the surface and
the line of sight to the sun.
Solar Azimuth Angle
The azimuth angle measured from north to the line of sight to the sun.
Viewing Zenith Angle
The angle between the WFC viewing vector and the zenith at the WFC pixel
footprint on the surface.
Viewing Azimuth Angle
The azimuth angle measured from north to the WFC viewing vector.
CCD Temperature
Temperature of the focal plane of the WFC CCD array. The temperature of
the WFC detector is actively controlled by a TEC. The set point is
0o C and the nominal range is about +/- 0.5o. Larger
excursions have been observed with no impact on the data quality. However, if
excursions exceed more than about +/-5o, the data should be used
with caution.
Base Plate Temperature
Temperature of the feet of the WFC housing. Typically ranges between
10o C and 20o C. Primarily used as a diagnostic tool.
Reflectance Bins
Statistics on the observed WFC reflectance are produced for each orbit
and reported here. The data are sorted into 5o solar zenith angle
bins (0-5o, 5-10o, 10-15o, etc). There are
72 reflectance bins within each solar zenith bin and the total number of
occurrences per orbit is reported in each bin.
Pixel QC Flag
This is a 32-bit integer to identify potential data quality issues. Only
the first 5 bits are used as described below. Most data will have QC Flag
values of zero; however, such as in the case of missing satellite ephemeris
and attitude data, this will not always be true. If the QC Flag value is
greater than 1, the data should be used with caution. If the QC Flag value is
greater than 3, the data should not be used. Note, due to a known software
error, bit 3 may not always be set properly in this data release. However,
pixels with no geolocation will be identified with fill values in the
position fields.
The 125 m Native Science data product provides WFC radiance and reflectance
measurements across just the central 5 km swath at 125 m resolution. No spatial
interpolation is performed.
Scan Time
This field reports the International Atomic Time (TAI) for each WFC scan,
in seconds, starting from January 1, 1993.
Scan UTC Time
This field reports the Coordinated Universal Time (UTC) for each WFC scan
with a format 'yymmdd.ffffffff', where 'yy' is the last two digits of year,
'mm' and 'dd' represent month and day, respectively, and 'ffffffff' is the
fractional of the day.
Latitude
This field reports the latitude of the individual 125 m WFC pixel on the
surface.
Longitude
This field reports the longitude of the individual 125 m WFC pixel on the
surface.
Radiance
The band-average spectral radiance of the scene averaged over the
spectral range of the WFC (620-670 nm). Units are
Wm-2sr-1μm-1.
Reflectance
The bi-directional reflectivity of the scene defined as the ratio of the
intensity of the radiation reflected from the surface and atmosphere as
observed by the WFC and the intensity of the incident solar radiation at the
top of the atmosphere. It is has no units.
125 m Homogeneity
The 125 m or track homogeneity is simply as the standard deviation in
radiance over the central 5 km high-resolution portion of the WFC image frame
normalized by the mean.
Reflectance Bins 125 m
Statistics on the observed WFC reflectance are produced for each orbit
and reported here. The data are sorted into 5o solar zenith angle
bins (0-5o, 5-10o, 10-15o, etc). There are
72 reflectance bins within each solar zenith bin and the total number of
occurrences per orbit is reported in each bin.
Pixel QC Flag
This is a 32-bit integer to identify potential data quality issues. Only
the first 5 bits are used as described below. Most data will have QC Flag
values of zero; however, such as in the case of missing satellite ephemeris
and attitude data, this will not always be true. If the QC Flag value is
greater than 1, the data should be used with caution. If the QC Flag value is
greater than 3, the data should not be used. Note, due to a known software
error, bit 3 may not always be set properly in this data release. However,
pixels with no geolocation will be identified with fill values in the
position fields.
To facilitate the use of the WFC data in IIR retrievals, the WFC radiometric
data is also registered to the same Earth-based geometric grid as the IIR data.
This grid projection has been defined as follows:
Grid lines are orthogonal to the lidar track
Center point in each grid line is aligned with the lidar track
Center point is registered with a lidar shot
Grid lines are separated by about 1 km, but exact sampling is determined
by translation of sub-satellite point during a time Δt equivalent to 3
lidar shots (i.e. ~148 ms)
The WFC data are registered to the IIR grid by interpolation of the
"native grid" data using a bilinear interpolation scheme.
Lidar Shot Time
This field reports the International Atomic Time (TAI) for each Lidar shot
that defines a grid line, in seconds, starting from January 1, 1993.
Lidar Shot UTC Time
This field reports the Coordinated Universal Time (UTC) for each Lidar
shot that defines a grid line, with a format 'yymmdd.ffffffff', where 'yy' is
the last two digits of year, 'mm' and 'dd' represent month and day,
respectively, and 'ffffffff' is the fractional of the day.
Latitude
This field reports the latitude of the individual 1 km WFC grid points on
the surface.
Longitude
This field reports the longitude of the individual 1 km WFC grid points
on the surface.
Radiance
The band-average spectral radiance of the scene averaged over the
spectral range of the WFC (620-670 nm). Units are
Wm-2sr-1μm-1.
Reflectance
The bi-directional reflectivity of the scene defined as the ratio of the
intensity of the radiation reflected from the surface and atmosphere as
observed by the WFC and the intensity of the incident solar radiation at the
top of the atmosphere. It is has no units.
1 km Homogeneity
The 1 km or swath homogeneity is defined as the variance in radiance over
the full 61 km cross-track swath normalized by the swath mean. The spatial
resolution of the swath homogeneity is 61 km cross-track and 1 km
along-track.
Solar Zenith Angle
The angle between the zenith at the WFC grid point on the surface and the
line of sight to the sun.
Solar Azimuth Angle
The azimuth angle measured from north to the line of sight to the sun.
Viewing Zenith Angle
The angle between the WFC viewing vector and the zenith at the WFC pixel
footprint on the surface.
Viewing Azimuth Angle
The azimuth angle measured from north to the WFC viewing vector.
Pixel QC Flag
This is a 32-bit integer to identify potential data quality issues. Only
the first 5 bits are used as described below. Most data will have QC Flag
values of zero; however, such as in the case of missing satellite ephemeris
and attitude data, this will not always be true. If the QC Flag value is
greater than 1, the data should be used with caution. If the QC Flag value is
greater than 3, the data should not be used. Note, due to a known software
error, bit 3 may not always be set properly in this data release. However,
pixels with no geolocation will be identified with fill values in the
position fields.
The WFC is currently fully functional and operating nominally. To date, the
WFC data quality assessments have been focused on two primary areas:
geolocation and radiometric accuracy. Post-launch checks of the WFC geolocation
identified both along-track and cross-track biases in the reported WFC pixel
locations. These systematic offsets were on the order of several 100 meters and
were attributed to a small, uncharacterized misalignment of the WFC relative to
the spacecraft platform. Geolocation corrections have been implemented in the
Level 1 ground processing to eliminate these biases. WFC geolocation accuracy
for the V1.10 data release is estimated to be better than 50 m. There is no
on-orbit radiometric calibration capability for the WFC. Therefore, we must
rely on vicarious approaches to verify and monitor the WFC radiometric
calibration. Since the WFC bandpass is matched to the well-calibrated Aqua
MODIS Channel 1, direct comparisons with nearly coincident MODIS Channel 1
measurements provide an excellent means of assessing the WFC radiometric
performance. Preliminary comparisons of WFC and Aqua MODIS channel radiance
measurements indicated that the WFC radiometric measurements were biased high
relative to MODIS by about 10%. Further investigation revealed that an offset
in the reported WFC exposure time was the likely cause of this bias. A review
of pre-launch ground test data and results from diagnostic experiments
performed on-orbit confirmed that the true WFC exposure time is about 0.4 ms
longer than reported. Accounting for this exposure time offset results in about
a 9% reduction in the magnitude of the WFC radiance values. This correction has
been implemented in the Level 1 processing for the V1.10 data release. A more
detailed assessment of the WFC on-orbit performance has been performed based on
analysis of WFC and MODIS data from the first twelve months of the CALIPSO
mission. Using deep convective clouds as vicarious calibration targets, direct
comparisons of WFC and MODIS radiance measurements indicate that the WFC
radiance tracks the MODIS data very closely with daily mean differences never
exceeding 1.2%. Analysis of WFC and MODIS deep convective cloud reflectance
distributions also indicate that the WFC has exhibited excellent radiometric
stability during the first year of operation with no apparent drift relative to
MODIS. For more details, please see
Pitts et al. (2007) (PDF).
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