This document provides a high-level quality assessment of the
CALIPSO Level
1B Wide Field Camera (WFC) data products, as described in Section 2.3 of the
CALIPSO Data Products Catalog (Version 3.2) (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.
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 averaged to the 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 0° C
and the nominal range is about +/- 0.5°. Larger excursions have been
observed with no impact on the data quality. However, if excursions exceed
more than about +/-5°, the data should be used with caution.
Base Plate Temperature
Temperature of the feet of the WFC housing. Typically ranges between
10° C and 20° 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 5° solar zenith angle bins
(0-5°, 5-10°, 10-15°, 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 0, the data should be used with caution. If the QC Flag value is
greater than 3, the data should not be used.
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 5° solar zenith angle bins
(0-5°, 5-10°, 10-15°, 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 0, the data should be used with caution. If the QC Flag value is
greater than 3, the data should not be used.
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 0, the data should be used with caution. If the QC Flag value is
greater than 3, the data should not be used.
an 80-byte (max) character string specifying the data product name. The values
for the Wide Field Camera data products will be "WFC_Native_125m",
"WFC_Native_1Km", and "WFC_IIR_Registered_1km".
Date Time at Granule Start
a 27-byte character string that reports the date and time at the start of the
file orbit segment (i.e., granule). The format is yyyy-mm-ddThh:mm:ss.ffffffZ.
Date Time at Granule End
a 27-byte character string that reports the date and time at the end of the
file orbit segment (i.e., granule). The format is yyyy-mm-ddThh:mm:ss.ffffffZ.
Date Time at Granule Production
This is a 27-byte character string that defines the date at granule production.
The format is yyyy-mm-ddThh:mm:ss.ffffffZ.
Number of Good 125 m Records
This field reports the number of good 125m records.
Number of Bad 125 m Records
This field reports the number of bad 125m records.
Number of Good 1 km Records
This field reports the number of good 1 Km records.
Number of Bad 1 km Records
This field reports the number of bad 1 Km records.
This field reports the orbit number at the granule start time.
Orbit Number at Granule End
This field reports the orbit number at the granule stop time.
Orbit Number Change Time
This field reports the time at which the
orbit number changes in the granule.
Path Number at Granule Start
This field reports the path number at the granule start time.
Path Number at Granule End
This field reports the path number at the granule stop time.
Path Number Change Time
This field reports the time at which the path number changes in the granule.
Ephemeris Files Used
This is a 160-byte character that reports a maximum of two ephemeris files
used in processing the spacecraft position and velocity.
Attitude Files Used
This is a 160-byte character that reports a maximum of two attitude files
used in processing the spacecraft attitude and attitude rate.
Vicarious Calibration File Used
This is an 80-byte character that reports the calibration file that contains
the dark current offset, relative responsitivity (calibration coefficients),
quaternion rotations, and bad pixel map that is used in the processing of
the data.
1km Radiance Calibration Coefficients
This is a 61 element array that contains the coefficients used in the data
calibration of the 1 Km data.
125m Radiance Calibration Coefficients
This is a 40 element array that contains the coefficients used in the data
calibration of the 125 m data.
Column Number of Center Image Pixel
This field reports the CCD array center column used for this granule.
Row Number of Center Image Pixel
This field reports the CCD array center row used for this granule.
Frame Time
This field reports the total amount of time for a frame of data.
Integration Time
This field reports the amount of time the CCD is exposed to light during a
single data acquisition.
Total Poss Day Packets
This field reports the possible number daytime packets that could be processed.
Total Proc Day Packets
This field reports the number of daytime packets processed.
Total Proc Night Packets
This field reports the number of nighttime packets processed.
Reflectance Bins Min
This is a 72 element array, which is the minimum value of each reflectance
bin reported.
Reflectance Bins Max
This is a 72 element array, which is the maximum value of each reflectance
bin reported.
Solar Zenith Bins Min
This is a 15 element array, which is the minimum value of each solar zenith
angle bin reported.
Solar Zenith Bins Max
This is a 15 element array, which is the maximum value of each solar zenith
angle bin reported.
The CALIPSO Team is releasing Version 3.02 which represents a transition of
the Lidar, IIR, and WFC processing and browse code to a new cluster computing
system. No algorithm changes were introduced and very minor changes were
observed between V 3.01 and V 3.02 as a result of the compiler and computer
architecture differences. Version 3.02 is being released in a forward
processing mode beginning November 1, 2011.
WFC Level 1B Scans Version 3.01 includes new metadata parameters and
corrections to several minor software bugs. Specifically, the Orbit Number and
Path Number metadata parameters are now included to facilitate improved
subsetting capabilities.
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. WFC geolocation accuracy 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. On-orbit performance assessments of the WFC
radiometric products are refularly performed-based on analysis of coincident
WFC and MODIS data. Using deep convective clouds as vicarious calibration
targets, direct comparisons of WFC and MODIS radiance measurements from the
first four years of the CALIPSO mission indicate that the WFC radiance tracks
the MODIS data very closely with daily absolute mean differences never
exceeding 1.5% and typically less than 1%. Analysis of WFC and MODIS deep
convective cloud reflectance distributions also indicate that the WFC has
exhibited very good radiometric stability during the four years of operation
with at most a <1% drift relative to MODIS. For more details, please see
Pitts et al. (2007) (PDF).
