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 3.0) (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:
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:
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.
Uncertainties for the attenuated backscatter are not explicitly reported
in the CALIOP Level 1 data products to save data volume, which would
otherwise approximately double the Level 1 data volume. If needed, users can
compute random errors for the attenuated backscatter products as described in
Uncertainties for Attenuated Backscatter (PDF). IDL code
for computing the attenuated backscatter uncertainties is contained in
IDL Code for Uncertainty Calculations (PDF).
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 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 (PDF). 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.
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 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.
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.
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.
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).
Orbit Number
Orbit Number consists of three HDF
metadata fields that define the number of revolutions by the CALIPSO
spacecraft around the Earth and is incremented as the spacecraft passes the
equator at the ascending node. To maintain consistency between the CALIPSO
and CloudSat orbit parameters, the Orbit Number is keyed to the Cloudsat
orbit 2121 at 23:00:47 on 2006/09/20. Because the CALIPSO data granules are
organized according to day and night conditions, day/night boundaries do not
coincide with transition points for defining orbit number. As such, three
parameters are needed to describe the orbit number for each granule as:
Orbit Number at Granule Start: orbit number at the granule start time
Orbit Number at Granule End: orbit number at the granule stop time
Orbit Number Change Time: time at which the orbit number changes in the granule
Path Number
Orbit Number Path Number consists of
three HDF metadata fields that define an index ranging from 1-233 that
references orbits to the Worldwide Reference System (WRS). This global grid
system was developed to support scene identification for LandSat imagery.
Since the A-Train is maintained to the WRS grid within +/- 10 km, the Path
Number provides a convenient index to support data searches, instead of
having to define complex latitude and longitude regions along the orbit
track. The Path Number is incremented after the maximum latitude in the orbit
is realized and changes by a value of 16 between successive orbits. Because
the CALIPSO data granules are organized according to day and night
conditions, day/night boundaries do not coincide with transition points for
defining path number. As such, three parameters are needed to describe the
path number for each granule as:
Path Number at Granule Start: path number at the granule start time
Path Number at Granule End: path number at the granule stop time
Path Number Change Time: time at which the path number changes in the granule
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.
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.
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.
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.
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.
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.
Profile ID
This is a 32-bit integer generated sequentially for each single-shot profile
record. Each profile ID is unique within each granule.
This field indicates the lighting conditions at an altitude of ~24 km above
mean sea level; 0 = day, 1 = night.
IGBP Surface Type
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.
Land Water Mask
This is an 8-bit integer indicating the surface type at the laser footprint, with
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.
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 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).
Noise Scale Factor 1064
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.
Parallel Background Monitor 532
This field reports the background signal, in digitizer counts, for the 532 nm
parallel channel.
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 RMS Baseline 532
This field reports the RMS noise, in digitizer counts, of background signal
in 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.
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 is an unsigned 32-bit integer with each bit indicating a specific error
condition, as defined by Table 2.
The CALIPSO lidar data are averaged on-board the satellite, prior to being
downlinked, using the variable averaging scheme shown in Table 1. Regions 1
and 2 contain single shot data (albeit at different vertical resolutions). In
regions 3, 4, and 5, the downlinked data have been averaged to horizontal
resolutions of, respectively, 3 shots, 5 shots, and 15 shots. The level 1
processing constructs Pseudo Single Shot Profiles (PSSP) by replicating the
data from regions 3, 4, and 5, and then stacking data arrays from the
different averaging regions. Two sets of QC flags, as shown in Tables 2 and
3, are computed for each one of these pseudo single shot profiles.
The laser energy assessments reported in QC Flag #1 are computed as follows:
the laser energies associated with bits 5, 6, 13, and 14 correspond to
the laser energies for the genuine single shot portion (i.e., from regions
1 and 2) of each PSSP;
bits 15-19 are toggled on if any single shot within the horizontally
averaged regions of a PSSP falls below the 'near zero energy' threshold; and
bits 20-24 are toggled on if any single shot within the horizontally
averaged regions of a PSSP falls below the 'data quality' threshold.
For example, suppose that (a) the energies for shot #5 in a 15 shot frame
fail the data quality threshold tests but are above the 'near zero'
threshold; and (b) the energies for all other shots in the frame are normal.
In this case, bits 5, 6, 13, and 14 in profiles 1-4 and 6-15 are all set to
zero, to indicate acceptable laser energy. In profile #5, bits 5 and 6 are
zero (because the energies were above the 'near zero' threshold) and bits 13
and 14 are one (because the energies were below the data quality threshold).
Because the averaged data in region 3 of PSSP #4, #5, and #6 was constructed
using the low energy data recorded for shot #5, bits 15 and 16 in these three
profiles are set to one, whereas in the remaining profiles bits 15 and 16 are
set to zero. Similarly, bits 17 and 18 are set to one for PSSP #1 - #5, and
bit 19 is set to one for all profiles in the 15 shot frame.
Table 2: Bit assignments for the first QC Flag
Bits
Interpretation
1
532 nm parallel channel missing
2
532 nm perpendicular channel missing
3
1064 nm channel missing
4
Not geolocated
5
Single shot 532 laser energy below calibration threshold (near zero energy)
6
Single shot 1064 laser energy below calibration threshold (near zero energy)
7
Historical value used for the depolarization gain ratio
Single shot 532 laser energy below data quality threshold (low energy)
14
Single shot 1064 laser energy below data quality threshold (low energy)
15
Near zero 532 nm laser energy profile included in region 3 average
16
Near zero 1064 nm laser energy profile included in region 3 average
17
Near zero 532 nm laser energy profile included in region 4 average
18
Near zero 1064 nm laser energy profile included in region 4 average
19
Near zero 532 nm laser energy profile included in region 5 average
20
Low 532 nm laser energy profile included in region 3 average
21
Low 1064 nm laser energy profile included in region 3 average
22
Low 532 nm laser energy profile included in region 4 average
23
Low 1064 nm laser energy profile included in region 4 average
24
Low 532 nm laser energy profile included in region 5 average
25-32
Spare
QC Flag #2
This is an unsigned 32-bit integer with each bit indicating a specific error
condition, as defined by Table 3. QC Flag #1 contains an
explanation of how bits are set for each Pseudo Single Shot Profile (PSSP).
Table 3: Bit assignments for the second QC Flag
Bits
Interpretation
1
Reserve
2
Excessive underflows, 532 nm parallel channel in region 6*
3
Excessive underflows, 532 nm perpendicular parallel channel, region 6*
4
Excessive underflows, 1064 nm channel, region 6*
5
Excessive overflows, 532 nm parallel channel, region 6*
6
Excessive overflows, 532 nm perpendicular parallel channel, region 6*
7
Excessive overflows, 1064 nm channel, region 6*
8
Excessive overflows, 532 nm parallel channel, region 2
9
Excessive overflows, 532 nm perpendicular parallel channel, region 2
10
Excessive overflows, 1064 nm channel, region 2
11
LRE Flags in SAD packet indicate bad data,
532 nm parallel channel
12
LRE Flags in SAD packet indicate bad data, 532 nm perpendicular channel
13
LRE Flags in SAD packet indicate bad data,
1064 nm channel
14
Quality Flags in SAD packet indicate bad data, 532 nm parallel channel
15
Quality Flags in SAD packet indicate bad data, 532 nm perpendicular channel
16
Quality Flags in SAD packet indicate bad data, 1064 nm channel
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.
Scattering Angle
This is the angle, in degrees, between the lidar viewing vector 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.
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.
Viewing Azimuth Angle
This field reports the azimuth angle from north of the lidar viewing vector,
in 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.
This field reports the attitude data, expressed as a set of Euler angles in
degrees, of the CALIPSO satellite. The Euler angles represent the rotation
between orbital and spacecraft coordinates and expresses as roll, pitch,
and yaw angles.
This field reports the position, in kilometers, of the CALIPSO satellite.
The position is expressed in Earth Centred Rotating (ECR) coordinate system
as X-axis in the equatorial plane through the Greenwich meridian, the Y-axis
lies in the equatorial plane 90 degrees to the east of the X-axis, and the
Z-axis is toward the North Pole.
This field reports the velocity, in kilometers per second, of the CALIPSO
satellite. The velocity is expressed in Earth Centred Rotating (ECR)
coordinate system.
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.
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.
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.
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.
Lidar Level 1B Version 3.01 includes corrections to the 532 nm and 1064 nm
extinction, backscatter, and ozone cross-sections that were applied in the
release of Lidar Level 1B Version 3.00. The data were reprocessed using the
corrected values and are being released as Version 3.01.
The following six parameters were added in the file metadata:
Version 3.00 includes algorithm improvements, modifications to existing data
parameters, and new data parameters. The maturity level of Version 3.00 Lidar
Level 1B data product is assigned Validated Stage 1. In this stage, all obvious
errors have been identified and corrected, and intercomparisons of attenuated
backscatter products have been performed at selected locations and times.
Algorithm improvements were implemented for:
532-nm daytime calibration
The revised 532-nm daytime calibration algorithm produces improved
corrections to the thermally-induced drift in signal level that occurs
over the course of the daytime orbit segment. In Version 3.00, the
empirically determined correction factors are applied using a 34-point
linear approximation as compared to the 5-point linear approximation
implemented in Versions 2.01 and 2.02. This allows for better
characterization of the small scale changes in signal level that take
place over the daytime orbit segment. Comparisons of nighttime and newly
calibrated daytime clear-air, attenuated scattering ratios over 8-12 km in
altitude were made for multiple seasons of LOM 2 (first laser) operation
and for the first three months of LOM 1 (backup laser) operation. In all
cases the agreement between night and day was within 5% for the entire
orbit segment.
laser energy calculations and signal normalization by laser
energy
Two updates to the 532-nm and 1064-nm laser energy calculation algorithm
were implemented in Version 3.00 in order to reduce errors in both
calibration and the processing of signal profiles for low energy laser
shots. The first update uses new laser energy conversion coefficients to
improve the accuracy of the laser energy calculation. In the second
update, the signal normalization by laser energy is changed to normalize
by averaging region instead of by shot. That is, for each averaging
region, normalize all averaged shots by the corresponding average energy
for that region. Data are averaged on-board over 15, 5, 3, or 1 shot(s)
before downlink, with the amount of averaging depending upon the altitude.
Application of the new normalization scheme improves the signal
normalization for frames with low energy laser shots and has little effect
on frames with nominal laser energies.
interpolation of GMAO gridded data products to the CALIPSO orbit
tracks
Corrections were made to the code used to interpolate the GMAO gridded
data products to the CALIPSO orbit tracks. In Versions 2.01 and 2.02, two
bracketing GMAO files were used to derive meteorological parameters. In
some cases, the CALIPSO measurement times fell outside of the bracketing
file times causing parameters to be extrapolated. In Version 3.00, this
problem was rectified by selecting three GMAO files for each orbit track
segment. This assures the orbit track times are completely contained
within the GMAO data file times.
The modified parameters are:
Met_Data_Altitude
The altitudes reported in the parameter Met_Data_Altitude were modified so
they are now coincident with an altitude reported in the Lidar_Data_Altitude
array.
QC_Flag and QC_Flag_2
The quality flags QC_Flag and QC_Flag_2 were updated to identify the
profiles that were normalized using low energy.
New data parameters describe the orbit and path number and are included in
the file metadata. The following six parameters were added:
