This document provides a high-level quality assessment of the level 1B
lidar data products, as described in Section 2.1 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
Lidar Science Working Group (LSWG). 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 CALIOP Level 1B data product contains a half orbit (day or night) of
calibrated and geolocated single-shot (highest resolution) lidar profiles,
including 532 nm and 1064 nm attenuated backscatter and depolarization ratio at
532 nm. The product released contains data from nominal science mode
measurement.
The CALIOP Level 1B product also contains additional data not found in the
Level 0 lidar input file, including post processed ephemeris data, celestial
data, and converted payload status data. The major categories of lidar Level 1B
data are:
Lidar Profile Data
Lidar Footprint Position Data
Satellite Viewing Geometry
To make proper use of the CALIOP Level 1B products, all users must be aware
of the uncertainties inherent in the data products. The data quality of each
product is summarized briefly below:
This field reports the Coordinated Universal Time (UTC), formatted as
'yymmdd.ffffffff', where 'yy' represents the last two digits of year, 'mm'
and 'dd' represent month and day, respectively, and 'ffffffff' is the
fractional part of the day.
Profile ID
This is a 32-bit integer generated sequentially for each single-shot profile
record. Each profile ID is unique within each granule.
Latitude
Geodetic latitude, in degrees, of the laser footprint on the Earth's surface.
Longitude
Longitude, in degrees, of the laser footprint on the Earth's surface.
Land Water Mask
This is an 8-bit integer indicating the surface type at the laser footprint, with
International Geosphere/Biosphere Programme (IGBP) classification of the
surface type at the laser footprint. The IGBP surface types reported by
CALIPSO are the same as those used in the
CERES/SARB surface map.
This field indicates the lighting conditions at an altitude of ~24 km above
mean sea level; 0 = day, 1 = night.
Frame Number
This field reports the number of a frame within the sequence of 11 frames
making up a Payload Data Acquisition Cycle (PDAC). Each frame consists of
15 laser pulses. All 15 records in a frame have the same value of Frame Number.
Lidar Mode
This is a 16-bit integer representing the operating mode of the lidar. For
all Level 1B data, the lidar mode will have a value of 3, indicating that the
lidar is in autonomous data acquisition mode.
Lidar Submode
This is a 16-bit integer representing the operating submode of the lidar. For
all Level 1B data, the lidar submode will have a value of 4, indicating that
the lidar operating in its normal configuration.
Surface Elevation
This is the surface elevation at the laser footprint, in kilometers above
local mean sea level, obtained from the
GTOPO30 digital elevation map (DEM).
Laser Energy 532
This field reports the laser energy, in Joules, at 532 nm measured by the
laser energy monitor for each shot.
Laser Energy 1064
This field reports the laser energy, in Joules, at 1064 nm measured by the
laser energy monitor for each shot.
Perpendicular Amplifier Gain 532
This is the gain of the variable gain amplifier for the 532 nm perpendicular
channel, in volts per volt.
Parallel Amplifier Gain 532
This is the gain of the variable gain amplifier for the 532 nm parallel
channel, in volts per volt.
Amplifier Gain 1064
This is the gain of the variable gain amplifier for the 1064 nm channel, in
volts per volt.
The depolarization gain ratio is the ratio of the opto-electric gains between
the 532 perpendicular and the 532 parallel channels. This product is
determined from the Polarization Gain Ratio (PGR) mode measurement, in which
a pseudo-depolarizer is inserted into the optical path to generate equal
backscatter intensities in both the 532 parallel and 532 perpendicular
channels (see equation 5.8 in Section 5.1 of the CALIPSO
Lidar Level I ATBD (PDF)).
During the first several months of the mission, the depolarization gain
ratio has proved to be very stable, with values falling consistently between
1.02 and 1.05. The uncertainty in these measurements due to random noise is
estimated to be smaller than 1% (see the Depolarization Gain Ratio
Uncertainty 532, immediately below). Possible systematic errors have not yet
been quantified; however, these are estimated to be small, and thus the
measured depolarization gain ratio is considered highly reliable.
Depolarization Gain Ratio Uncertainty 532
This field reports the uncertainty in Depolarization Gain Ratio Uncertainty
532 due to random noise. Values are computed based on the 532 nm
noise scale factors (NSF) using equation 5.15
in Section 5.2 of the
Lidar Level I ATBD (PDF). The uncertainty due to
systematic errors is not included for this release, but is estimated to be
small.
This is the lidar calibration constant at 532 nm, as defined in section 3.1.2 of the
Lidar Level I ATBD (PDF).
For the nighttime portion of an orbit, the 532 nm calibration constant is
determined for each 55 km averaged profile (11 frames) by comparing the
532-parallel signals in 30 km to 34 km altitude range to a scattering model
derived from molecular and ozone number densities provided by NASA's
Global Modeling and Assimilation
Office (GMAO). This calculation uses equation 4.7 in Section 4.1.2.1 of
the CALIPSO
Lidar Level I ATBD. The computed 532 nm calibration
constants are then smoothed over an interval of 1485 km using equation 4.8. A
constant value of the calibration constant is applied to all single-shot
profiles in each 55 km averaging region.
The calibration technique used during the nighttime cannot be used in the
daytime portions of the orbits, because the noise associated with solar
background signals (i.e., sunlight) degrades the backscatter signal-to-noise
ratio (SNR) in the calibration region below usable levels. Therefore, for the
daytime portion of the orbit, the calibration constants are derived by
interpolating between values derived in the adjacent nighttime portions of
the orbits.
Calibration Constant Uncertainty 532
The uncertainty due to random noise for 532 nm calibration constant is
computed based on the 532 nm noise scale
factors using equation 4.24 in Section 4.3.2 of the CALIPSO
Lidar Level 1 ATBD (PDF). Estimates of systematic
errors, if any, are not included in this release. An extensive assessment of
possible systematic errors is currently underway.
For nighttime calibrations, the uncertainty due to noise is estimated to
be typically smaller than 1%. Additional systematic errors may arise from
aerosol contamination of the calibration region (less than a few percent),
and from large signal spikes seen frequently in the
South Atlantic Anomaly (SAA) and occasionally outside
the SAA region.
A stratospheric aerosol model is currently being developed to correct for
the aerosol present in the calibration region. Upon completion, this model
will be applied to calibration processing for subsequent data releases.
Large noise spikes can be present both in the lidar return signals and in
the baseline signals. Baseline signals are determined on-board by
calculating the mean signal value over 15000 data points (1000 15 meter
samples in the 65 to 80 km altitude region from each of the 15 shots within a
frame). This calculation is performed for each frame, and the resulting value
is subtracted from each sample of all profiles in that frame. The presence of
large outliers -- i.e., "spikes" -- in the backscatter signals in
the calibration region tends to bias the calibration constant toward a larger
value. On the other hand, the spikes present in the baseline region can cause
and erroneous overestimate of the measured baseline signal, and the
subsequent subtraction of this baseline value will thus introduce a bias in
all data within the frame, causing it to be lower than it otherwise should
be. This in turn tends to bias the calibration constant toward a smaller
value. Threshold-based data filtering schemes are applied to 532 nm data to
remove large spikes in the lidar signal prior to performing the nighttime
calibration. Two threshold boundaries - a maximum and a minimum - are set.
By excluding values outside this range, large signal excursions are
effectively removed. Spikes with smaller magnitudes may remain, depending on
the selection of the maximum threshold value. Perturbations to the
calibration due to spikes in the baseline region can be only partly
eliminated by this kind of threshold-based filtering scheme. However, by
properly selecting the threshold limits, the impacts of spikes in the
calibration region and the baseline region will cancel each other out to some
degree. Preliminary comparisons of CALIOP's 532 nm attenuated backscatter
coefficients, which are critically dependent on the accuracy of the
calibration, with validation measurements acquired by the LaRC airborne
high-spectral-resolution lidar (HSRL) and Goddard's airborne
Cloud Physics Lidar
(CPL) show consistency to within a few percent.
Because the daytime calibration constants are interpolated from nighttime
values, the uncertainties contained in the nighttime calibration are
transferred to daytime. Additional error may arise from the selection of
interpolation scheme. In general, the uncertainty for daytime calibration
constants is somewhat higher than the uncertainty for the nighttime values.
The lidar calibration constant at 1064 nm, C1064, is determined by
comparing 1064 nm signals to 532 nm signals in properly selected high cirrus
clouds, using the procedure described in Section 7.1.2.2 of the CALIPSO
Lidar Level I ATBD (PDF). For the current data release,
the ratio of cirrus backscatter coefficients at 1064 nm and 532 nm is assumed
to be uniformly 1. This assumption is being extensively assessed in on-going
validation activities.
For each granule (day or night) a single, constant value (granule mean) for
C1064 is derived by averaging all individual calibration constant
estimates that were obtained. This granule mean serves as the calibration
constant that is subsequently applied to all 1064 nm profiles in the granule.
We note that the procedure used in the 532 nm calibration cannot be applied
for the 1064 nm measurements, because the molecular scattering at 1064 nm
is ~16 times weaker than at 532 nm, and because the avalanche photodiode (APD)
detector used in the 1064 nm channel has significantly higher dark noise than
photomultiplier tube (PMTs) used in the 532 nm channels.
Calibration Constant Uncertainty 1064
This field reports the uncertainty in the 1064 nm calibration constant due
solely to random noise contained in 1064 nm data. Systematic errors are not
estimated in this release.
If a sufficient number of cirrus clouds are present in any granule, the
uncertainty due to noise in the granule mean of the 1064 nm calibration
constant can be very small. Larger systematic errors may arise from the
assumption that the cirrus color ratio (the ratio of backscatter coefficients
at 1064 nm and 532 nm) has a constant value of 1.0. A very preliminary study
on the ratio of gain and energy-normalized, range-corrected signals (i.e.,
the quantity X defined in equations 3.7 and 3.8 in the CALIPSO
Lidar Level I ATBD (PDF)) at 1064 nm and 532 nm in
selected dense cirrus clouds shows a distribution having a width of exceeding
10% of the mean value.
The total attenuated backscatter at 532 nm, β′532
in Section 6.2.2 of the
Lidar Level I ATBD (PDF), is one of the primary lidar
Level 1 data products. β′532 is the product of the 532
nm volume backscatter coefficient and the two-way optical transmission at 532
nm from the lidar to the sample volume. The construction of the 532 nm total
attenuated backscatter from the two constituent polarization components is
described in detail in Section 6 of the
Lidar Level I ATBD (PDF). The attenuated backscatter
profiles are derived from the calibrated (divided by calibration constant),
range-corrected, laser energy normalized, baseline subtracted lidar return
signal.
The 532 nm attenuated backscatter coefficients are reported for each laser
pulse as an array of 583 elements that have been registered to a constant
altitude grid defined by the Lidar Data
Altitude field.
Note that to reduce the downlink data volume, an on-board averaging scheme
is applied using different horizontal and vertical resolutions for different
altitude regimes, as shown in the following table.
This field reports the perpendicular component of the 532 nm total attenuated
backscatter, as described in section 6 of the CALIPSO
Lidar Level I ATBD (PDF). Profiles of the perpendicular
channel 532 nm attenuated backscatter are reported in the same manner as are
profiles of the 532 nm total backscatter.
Profiles of the parallel component of the backscatter can be obtained by
simple subtraction of the perpendicular component from the total.
The attenuated backscatter at 1064 nm, β′1064, is
computed according to equation 7.23 in section 7.2 of the
Lidar Level I ATBD (PDF). Like
β′532, β′1064 is one of the
primary lidar Level 1 data products. β′1064 is the
product of the 1064 nm volume backscatter coefficient and the two-way optical
transmission at 1064 nm from the lidar to the sample volume. Profiles of the
1064 nm attenuated backscatter are reported in the same manner as are
profiles of the 532 nm total backscatter.
However, the first 34 bins of each profile contain fill values (-9999),
because no 1064 nm data is downlinked from the 30.1 - 40 km altitude range.
This field reports the background signal, in digitizer counts, for the 532 nm
perpendicular channel. Background signals are measured at very high
latitudes, where no backscattering signal will be returned from the
atmosphere. Background signals include such things as detector dark current
and background radiation signals (e.g., from daytime sunlight). In general,
any lidar sample will include both an atmospheric scattering signal and the
background signal. The latter is subtracted from lidar samples during data
processing. For CALIOP, the background signal is computed on board and
subtracted from the lidar data prior to downlink.
Parallel Background Monitor 532
This field reports the background signal, in digitizer counts, for the 532 nm
parallel channel.
This field reports the root mean square (RMS) noise, in digitizer counts, of
the background signals from the 532 nm perpendicular channel The RMS noise is
determined on-board for each laser pulse by computing the standard deviation
of 1000 15 m samples acquired in the 65-80 km altitude range.
The random error contained in lidar measurements consists of two parts.
One is due to the variation in the received laser scattering signal from the
atmosphere. The other is due to the variation in the background signal. Both
parts have to be taken into account when estimating the random error. The
random error arose from the scattering signal can be estimated using the
NSF. The random error due to the background
signal is the measured RMS noise.
Parallel RMS Baseline 532
This field reports the RMS noise, in digitizer counts, of background signal
in the 532 nm parallel channel.
RMS Baseline 1064
This field reports the RMS noise, in digitizer counts, of the background
signal in the 1064 nm parallel channel. We note that the magnitude of
the background signal at 1064 nm is not measured by CALIOP, because this
quantity is dominated by the detector dark noise.
This field reports the noise scale factor (NSF) for each shot for the 532 nm
perpendicular channel. This product is computed from daytime measurements of
the Perpendicular RMS Baseline 532 and the
Perpendicular Background Monitor
532. The theoretical basis for the calculation relies on the fact that
the photons from solar background radiation follow a Poisson stochastic
process
(Liu et al., 2006 (PDF)). The procedure to compute the NSF
is described in Section 8 of the
Lidar Level I ATBD (PDF).
This field reports the NSF for the 1064 nm channel. CALIOP does not measure
the background signal level at 1064 nm, because the APD detector dark noise
is dominant during both nighttime and daytime measurement. For this reason,
the procedure to estimate the NSF for the 532 nm channels cannot be used for
the 1064 nm channel. The 1064 nm NSF is therefore set to 0 for Version 1.10
of the CALIPSO lidar Level 1 product, which causes negligible error because,
as above, the APD detector dark noise is the dominant error source.
Perpendicular Column Reflectance 532
Perpendicular column reflectance for 532 nm is reported for each lidar Level
1 profile.
Perpendicular Column Reflectance Uncertainty 532
Perpendicular column reflectance for 532 nm is reported for each lidar Level
1 profile.
Parallel Column Reflectance 532
Parallel column reflectance for 532 nm is reported for each lidar Level 1
profile.
Parallel Column Reflectance Uncertainty 532
Parallel column reflectance for 532 nm is reported for each lidar Level 1
profile.
Molecular number density, in units of molecules per cubic meter, reported for
each lidar Level 1 profile at the 33 standard altitudes recorded in the
Met Data Altitudes field. Molecular number
density values are obtained from the ancillary meteorological data provided
by the GMAO.
Ozone number density, in units of molecules per cubic meter, reported
for each lidar Level 1 profile at the 33 standard altitudes recorded in the
Met Data Altitudes field. Ozone number
density values are obtained from the ancillary meteorological data provided
by the GMAO.
Temperature, in degrees C, reported for each lidar Level 1 profile at the 33
standard altitudes recorded in the Met Data
Altitudes field. Temperature values are obtained from the ancillary
meteorological data provided by the
GMAO.
Pressure, in millibars, reported for each lidar Level 1 profile at the 33
standard altitudes recorded in the Met Data
Altitudes field. Pressure values are obtained from the ancillary
meteorological data provided by the
GMAO.
Relative Humidity
Relative humidity reported for each lidar Level 1 profile at the 33 standard
altitudes recorded in the Met Data Altitudes field. Relative humidity values
are obtained from the ancillary meteorological data provided by the GMAO.
Surface Wind Speeds
Surface wind speeds, in meters per second, are reported for each lidar Level
1 profile as eastward (zonal) and northward (meridional) surface wind stress.
Surface wind speed values are obtained from the ancillary meteorological data
provided by the GMAO.
Tropopause Height
Tropopause height, in kilometers, reported for each lidar Level 1 profile.
Tropopause height values are obtained from the ancillary meteorological data
provided by the GMAO.
Tropopause Temperature
Tropopause temperature, in degrees C, reported for each lidar Level 1 profile.
Tropopause temperature values are obtained from the ancillary meteorological
data provided by the GMAO.
QC Flag #1
This is an unsigned 32-bit integer with each bit indicating a specific error
condition, as defined by Table 2.
Suspicious mean signal value, 532 nm parallel channel (any/all regions)
20
Suspicious mean signal value, 532 nm perpendicular parallel channel
(any/all regions)
21
Suspicious mean signal value, 1064 nm channel (any/all regions)
22
Suspicious signal range, 532 nm parallel channel
23
Suspicious signal range, 532 nm perpendicular parallel channel
24
Suspicious signal range, 1064 nm channel
25
Laser energy low, 532 nm
26
Laser energy low, 1064 nm
Off Nadir Angle
This is the angle of the viewing vector of the lidar off the nadir, in
degrees. Since the beginning of operations in June 2006, CALIPSO has been
operating with the lidar pointed at 0.3 degrees off-nadir (along track in
the forward direction) with the exception of November 7-17, 2006 and August
21 to September 7, 2007. During these periods, CALIPSO operated with the
lidar pointed at 3.0 degrees off nadir. Beginning November 28, 2007, the
off-nadir angle was permanently changed to 3.0 degrees.
Viewing Zenith Angle
This is the angle, in degrees, between the lidar viewing vector and the
zenith at the lidar footprint on the surface. This angle is close to Off
Nadir Angle in value.
Viewing Azimuth Angle
This field reports the azimuth angle from north of the lidar viewing vector,
in degrees.
Solar Zenith Angle
This is the angle, in degrees, between the zenith at the lidar footprint on
the surface and the line of sight to the sun.
Solar Azimuth Angle
This field reports the azimuth angle from north of the line of sight to the
sun, in degrees.
Scattering Angle
This is the angle, in degrees, between the lidar viewing vector and the line
of sight to the sun.
Surface Altitude Shift
Surface altitude shift contains the altitude difference between the profile
specific 30 meter altitude array and the fixed 30 meter altitude array at the
array element that includes mean sea level. Profile specific altitude arrays
are computed as a function of the actual spacecraft off-nadir angle, which
varies slightly from the commanded spacecraft off-nadir angle. The fixed
altitude array is computed using the commanded spacecraft off-nadir angle
(0.3 or 3.0 degrees). The units are in kilometers and the values may be
positive or negative. The difference is calculated as:
Surface_Altitude_Shift = altitude (profile specific 30 meter mean sea level
bin) - altitude (fixed 30 meter mean sea level bin).
Number Bins Shift
Number bins shift contains the number of 30 meter bins the profile specific
30 meter array elements are shifted to match the lowest altitude bin of the
fixed 30 meter altitude array. Profile specific altitude arrays are computed
as a function of the actual spacecraft off-nadir angle, which varies slightly
from the commanded spacecraft off-nadir angle. The fixed altitude array is
computed using the commanded spacecraft off-nadir angle (0.3 or 3.0 degrees).
The profile specific array elements may be shifted up or down.
Spacecraft Altitude
This field reports the altitude, in kilometers above mean sea level, of the
CALIPSO satellite.
Spacecraft Position
Spacecraft Velocity
Spacecraft Attitude
Spacecraft Attitude Rate
Subsatellite Latitude
This field reports the latitude of the geodetic subsatellite point which is a
point on the surface where the geodetic zenith vector (perpendicular to the
surface tangent) points toward the satellite.
Subsatellite Longitude
This field reports the longitude of the geodetic subsatellite point which is
a point on the surface where the geodetic zenith vector (perpendicular to the
surface tangent) points toward the satellite.
Earth-Sun Distance
This field reports the distance from the Earth's surface to the Sun, in AU.
Subsolar Latitude
This field reports the latitude of the geodetic subsolar point which is a
point on the surface where the geodetic zenith vector (perpendicular to the
surface tangent) points toward the sun.
Subsolar Longitude
This field reports the longitude of the geodetic subsolar point which is a
point on the surface where the geodetic zenith vector (perpendicular to the
surface tangent) points toward the sun.
Latest Data Release Date: December 2007 Version: 2.01 Data Release Date: August 12, 2007 Version: 1.22 Data Release Data: March 1, 2007 Version: 1.20 Data Release Date: January 6, 2007 Version: 1.11 Initial Data Release Date: December 8, 2006 Version: 1.10
Lidar Level 1B Profiles, Version 2.01, June 13, 2006 to present.
Lidar Level 1B Profiles, Version 1.22, August 12, 2007 to November 11, 2007.
Lidar Level 1B Profiles, Version 1.20, March 1, 2007 to August 11, 2007.
Lidar Level 1B Profiles, Version 1.11, January 6, 2007 to February 28, 2007.
Lidar Level 1B Profiles, Version 1.10, June 13, 2006 to January 5, 2007.
Data Quality Statement for the latest release of the CALIPSO Lidar
Level 1B Profile Product (Version 2.01, December 2007).
Version 2.01 includes revised algorithms for the 532 nm daytime calibration
and the 1064 nm daytime and nighttime calibration. The 532 nm daytime
calibration coefficients are now scaled relative to systematic variations in the
measured backscatter signal that occur over the course of the daytime orbit
segments. The Version 2.01 532 nm daytime calibration corrections produce
significant improvements to the overall quality of both the Lidar Level 1 and 2
daytime data products, particularly in the northern hemisphere where the Level 1
data contain significant daytime calibration biases. It is recommended to use
the Version 2.01 for all analyses of the 532 nm daytime data.
The Version 2.01 1064 nm calibration coefficients also vary as a function of
orbit elapsed time, in the same manner and for the same reasons as the 532 nm
calibration constants. In all previous versions, a single value for the 1064 nm
calibration coefficient was computed and applied for each daytime and nighttime
orbit granule. The revised calibration procedures produce substantial
improvements in the quality of the 1064 nm measurements. These changes are most
noticeable in the daytime granules. Use of the Version 2.01 data products is
recommended for all analyses that rely on the 1064 nm measurements.
A Portable Document Format (PDF) reader (such as
Adobe Acrobat Reader) is required to open and view PDF documents.
