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CALIPSO: Data User's Guide - Data Product Descriptions - Wide Field Camera (WFC) Level 1 Products


CALIPSO HOMECALIPSO User’s Guide HOMEData Product Summaries → Wide Field Camera Level 1 Products

Introduction

The primary Wide Field Camera (WFC) 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:

Don’t forget to check out the detailed data quality summary for this data product. (At end of page).

The main data quality summaries page may be found in this link: https://asdc.larc.nasa.gov/documents/calipso/quality_summaries/WFC_L1BScansProducts_3-01_v02.pdf

Data Descriptions

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.

In the text below we provide brief descriptions of individual data fields reported in the CALIPSO WFC data products. Where appropriate, we also provide an assessment of the quality and accuracy of the data in the current release. The data descriptions are grouped into several major categories, as follows:

Additionally all the science data sets (SDSs) are listed in the table to the right, click on the SDS name to go directly to the description.


1 km x 1 km Native Science Data

Science Data Set (SDS) Data Maturity
Scan_Time NA
Scan_UTC_Time NA
Latitude NA
Longitude NA
Radiance Validated Stage1
Reflectance Validated Stage1
1km_Homogeneity Validated Stage1
Solar_Zenith NA
Solar_Azimuth NA
Viewing_Zenith NA
Viewing_Azimuth NA
CCD_Temperature NA
BasePlate_Temperature NA
Reflectance_Bins NA
Pixel_QC_Flag NA

125 m x 125 m Native Science Data

Science Data Set (SDS) Data Maturity
Scan_Time NA
Scan_UTC_Time NA
Latitude NA
Longitude NA
Radiance Validated Stage1
Reflectance Validated Stage1
125m_Homogeneity Validated Stage1
Reflectance_Bins_125 NA
Pixel_QC_Flag NA


1 km x 1 km Registered Science Data

Science Data Set (SDS) Data Maturity
Lidar_Shot_Time NA
Lidar_Shot_UTC_Time NA
Latitude NA
Longitude NA
Radiance Validated Stage1
Reflectance Validated Stage1
1km_Homogeneity Validated Stage1
Solar_Zenith NA
Solar_Azimuth NA
Viewing_Zenith NA
Viewing_Azimuth NA
Pixel_QC_Flag NA


WFC 1 km Native Science Data

(see Table 20 in CALIPSO Data Products Catalog (Version 3.3))

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 simply the standard deviation 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, pixels with no geolocation will be identified with fill values in the position fields.

Bit Definition:

  1. Center pixel not defined in level 0 data
  2. Radiance exceeds max count...saturated pixel
  3. Cannot geolocate
  4. Negative radiance
  5. Negative reflectance

WFC 125 m Native Science Data

(see Table 21 in CALIPSO Data Products Catalog (Version 3.3))

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 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, pixels with no geolocation will be identified with fill values in the position fields.

Bit Definition:

  1. Center pixel not defined in level 0 data
  2. Radiance exceeds max count...saturated pixel
  3. Cannot geolocate
  4. Negative radiance
  5. Negative reflectance

WFC 1 km Registered Science Data

(see Table 18 in CALIPSO Data Products Catalog (Version 3.3) (PDF))

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 simply the standard deviation 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, pixels with no geolocation will be identified with fill values in the position fields.

Bit Definition:

  1. Center pixel not defined in level 0 data
  2. Radiance exceeds max count...saturated pixel
  3. Cannot geolocate
  4. Negative radiance
  5. Negative reflectance


File Metadata Parameters

Product ID
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.

Initial Subsatellite Latitude
This field reports the first subsatellite latitude of the granule.

Initial Subsatellite Longitude
This field reports the first subsatellite longitude of the granule.

Final Subsatellite Latitude
This field reports the last subsatellite latitude of the granule.

Final Subsatellite Longitude
This field reports the last subsatellite longitude of the granule.

Orbit Number at Granule Start
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.


Data Quality Statements

Wide Field Camera (WFC) Level 1B Scans Information
Half orbit (Day) geolocated data radiances
Release Date Version Data Date Range Maturity Level
December 2011 3.02 November 1, 2011 to present Validated Stage 1
November 2010 3.01 June 13, 2006 to October 31, 2011 Validated Stage 1
December 8, 2006 1.10 June 13, 2006 to September 21, 2010 Provisional

Summary Statement for the release of the CALIPSO Product WFC Level 1B Scans Version 3.02, December 2011

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.

Summary Statement for the release of the CALIPSO Product WFC Level 1B Scans Version 3.01, November 2010

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.

Summary Statement for the release of the CALIPSO Product WFC Level 1B Scans Version 1.10, December 8, 2006

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. The initial assessment of the WFC radiometric performance was based on comparisons with the well-calibrated Aqua MODIS Channel 1 data. Preliminary comparisons 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. The WFC V1.10 radiometric measurements now exhibit excellent agreement with MODIS, with differences typically less than 2% over bright targets.

Data Quality Statement for the release of the CALIPSO Product WFC Level 1B Scans Version 1.10, December 8, 2006

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).



NASA
Last Updated: November 22, 2021
Curator: Charles R. Trepte
NASA Official: Charles R. Trepte

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