Data Set ID: 

SnowEx17 Cloud Absorption Radiometer BRDF, Version 1

This data set contains measurements of the bidirectional reflectance distribution function (BRDF) for two locations in Colorado, USA: Grand Mesa, a snow-covered, forested study site about 40 miles east of Grand Junction; and Senator Beck Basin approximately 80 miles to the SSE of Grand Mesa.

Measurements were acquired using the NASA Cloud Absorption Radiometer (CAR), an airborne multi-angular, multi-wavelength scanning radiometer. The CAR instrument measures scattered light in 14 spectral bands between 0.34 μm and 2.30 μm, which lie in the UV, visible, and near-infrared atmospheric windows.

Data were obtained for a variety of conditions including snow grain size (or age), snow liquid water content, solar zenith angle, cloud cover, and snowpack thickness. The data set can be used to assess the accuracy of satellite reflectance and albedo products in snow-covered, forested landscapes.

This is the most recent version of these data.

Version Summary: 

New data set.

STANDARD Level of Service

Data: Data integrity and usability verified

Documentation: Key metadata and user guide available

User Support: Assistance with data access and usage; guidance on use of data in tools

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Data Format(s):
  • NetCDF
Spatial Coverage:
N: 40.243618, 
S: 37.653967, 
E: -104.459534, 
W: -108.448729
Platform(s):NP-3C Orion
Spatial Resolution:
  • Varies x Varies
Temporal Coverage:
  • 16 February 2017 to 22 February 2017
Temporal ResolutionVariesMetadata XML:View Metadata Record
Data Contributor(s):Charles Gatebe, Rajesh Poudyal

Geographic Coverage

Other Access Options

Other Access Options


As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Gatebe, C. K. and R. Poudyal. 2018. SnowEx17 Cloud Absorption Radiometer BRDF, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: [Date Accessed].

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Detailed Data Description


The main parameters in this data set are CAR radiance measurements in the following 14 spectral bands:

Table 1. CAR Instrument Spectral Bands
Bands 1-7 Bands 8-14
339 nm 1266 nm
380 nm 1557 nm
474 nm 1638 nm
687 nm 1723 nm
870 nm 2094 nm
1030 nm 2188 nm
1229 nm 2323 nm

Each band's spectral range and spectral response function are also provided.

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Data files are formatted as NetCDF4/HDF5 (.nc). Each data file also has an associated XML file which includes additional metadata.

NetCDF is a set of software libraries and self-describing, machine-independent data formats that are specifically designed to help create, access, and share array-oriented scientific data sets. Note that NetCDF-4 is not a file format. It is a convention for storing data as HDF using the NetCDF data model. For more information, visit the HDF Group's HDF5 Home Page and Unidata's NetCDF Documentation website.

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File Contents

Five flights (one per day) were flown during February, 2017. Each .nc file contains the CAR measurements for the day indicated in the file name. All five flights obtained measurements at the Grand Mesa (GM) study site; two flights additionally acquired data for Senator Beck (SB) Basin (see Table 2 and Figure 1). 

Table 2. Sites Visited by Date 
Date Site
02/16/17 GM, SB
02/18/17 GM
02/20/17 GM
02/21/17​ GM
02/22/17 GM, SB

Figure 1 shows the flight line for 16 February, 2017. During this flight, CAR measurements were acquired at both the Grand Mesa and Senator Beck study sites.

Figure 1. CAR flight line for 02/16/17.


Data files contain radiance values, spectral ranges, and spectral response functions in separate variables for each of the 14 bands observed by the CAR instrument. Additional variables are provided that specify aircraft position and orientation, plus CAR viewing angles and solar angles and irradiance. Consult tables 3 and 4 for variable names and descriptions.

Table 3. CAR Spectral Variables
Variable Name Description Data Type
SRF_[λ]nm Spectral response function for central wavelength λ 32-bit floating point
SpectralRange_[λ]nm Spectral range for central wavelength λ 32-bit floating point
radiance_[λ]nm Radiance (W/m2·sr) for central wavelength λ 32-bit floating point
CentralWaveLength Array containing central wavelengths 32-bit floating point

Table 4. Aircraft/Instrument Variables
Variable Name Description Data Type
AircraftAltitude Aircraft altitude, meters above mean sea level 32-bit floating point
AircraftHeading Aircraft heading, degrees relative to local geodetic north 32-bit floating point
AircraftLatitude Aircraft latitude, degrees 32-bit floating point
AircraftLongitude Aircraft longitude, degrees 32-bit floating point
AircraftPitch Aircraft pitch, degrees relative to local sky-Earth normal 32-bit floating point
CAR_Viewing_Angles CAR viewing angles, 0 to 180° in half-degree increments  32-bit floating point
CoordinatedUniversalTime Time (UTC) for each aircraft position measurement 32-bit floating point
Date Data acquisition date and time (UTC), YYYYMMDDhhmmss.s String
Scan_Start_Angle Scan start angle, degrees relative to local sky-Earth normal 32-bit floating point
Scans Scan line count from start of data acquisition 32-bit floating point
SolarAzimuthAngle Solar azimuth angle, degrees relative to local geodetic north 32-bit floating point
SolarIrradiance Solar spectral irradiance (W/m2) 32-bit floating point
SolarZenithAngle Solar zenith angle, degrees relative to local sky-Earth normal 32-bit floating point
Time Seconds since 00:00:00.0, day of data acquisition 64-bit floating point
ViewingAzimuthAngle CAR pixel viewing azimuth angle, degrees relative to local geodetic north 32-bit floating point
ViewingZenithAngle CAR pixel viewing zenith angle, degrees relative to local sky-Earth normal 32-bit floating point
Data files also contain a NetCDF dimension scale corresponding to each spectral response function variable. When viewing the data files in software packages that use the HDF API (for example, HDFView), these arrays will be displayed as additional variables named "SRF_[λ]nm_dim," where λ represents the central wavelength. Software packages like Panoply that use the NetCDF API do not display NetCDF dimension scales.
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File Naming Convention

Data files utilize the following naming convention:

  • SnowEx17_car_p3c_[YYYYMMDD]_R1_[nnnn]_Level1C_[YYYYMMDD].[ext],


Table 5. File Naming Convention
Variable Description
SnowEx17 SnowEx 2017 field season
car Instrument code, car = Cloud Absorption Radiometer
p3c Aircraft code, p3c = Naval Research Lab NP-3C Orion research aircraft
YYYYMMDD Year, month, day of data acquisition
R1 Data revision number
nnnn Four-digit unique flight number based on historic CAR data records
Level1C NASA data processing level (1C)*.
YYYYMMDD Date data were processed or last updated
ext File format. Value = .nc (NetCDF4/HDF5) or .xml

*Level 1C modified for CAR data from EOSDIS data processing levels as described in NASA CAR | Data Overview (see Data subsection): "In Level-1C processing, georeferencing parameters are applied to Level 1B data for each scan line. Each scan line contains 361 pixels (representing a 180° field of regard). CAR data [are] expanded into 14, two-dimensional arrays and engineering data [are] excluded. Global attributes are kept to a minimum."

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File Size

Data files are typically between 0.5 GB - 1.0 GB. XML files are <10 KB. The complete set of NetCDF files (5) is approximately 3.2 GB.

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Spatial Coverage

Overall spatial coverage:

Northernmost Latitude: 40.243618° N
Southernmost Latitude 37.653967° N
Westernmost Longitude: 108.448729° W
Easternmost Longitude: 104.459534° W

Within the Grand Mesa and Senator Beck Basin study sites, flights primarily targeted the following meteorological station:

Table 6: Meteorological Stations Latitudes/Longitudes
Station Name Latitude, Longitude
Skyway 39.05074° N, 108.06143°W
LSOS 39.05225°N, 108.09792°W
West Tower 39.03388°N, 108.21399°W
Eastern Tower 39.10374°N, 107.88448°W
Middle tower 39.03954°N, 107.94174°W
Senator Beck Basin 37.90689°N, 107.72628°W

Spatial Resolution

Spatial resolution varies with altitude. At a height of 600 m above the surface, the resolution at nadir is about 10 m.

Projection and Grid Description

Aircraft latitudes and longitudes are provided for each flight. Latitude/longitude pairs correspond to nadir pixels in CAR scans.

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Temporal Coverage

Data are available for the following days:

  • 16 February, 2017
  • 18 February, 2017
  • 20 February, 2017
  • 21 February, 2017
  • 22 February, 2017

Temporal Resolution


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Software and Tools

NetCDF4/HDF5 data files can be accessed using software packages that read either NetCDF4 or HDF5, such as Panoply and HDFView.

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Data Acquisition and Processing

Data Acquisition Methods

During SnowEx 2017, the CAR instrument was flown aboard a Naval Research Lab (NRL) P-3 Orion research aircraft over the Grand Mesa and Senator Beck Basin study sites. To measure the BRDF of the surface–atmosphere system, the aircraft banked at a roll angle of about 20° and flew roughly 3 km diameter circles (assuming an altitude of 600 m above the ground) for approximately 2 minutes. Multiple circular orbits were acquired over selected surfaces and at different altitudes.

The CAR scan mirror rotates 360° in a plane perpendicular to the direction of flight and collects data (60 MB/hr) through a 190° aperture that allows the instrument to observe the earth-atmosphere scene around the starboard horizon from local zenith to nadir. Data are acquired at high angular (1°) and spatial resolution (>10 m at nadir, assuming 600 m altitude). During operations, CAR is stabilized by an independent system (the CAR Autonomous Navigation System) that corrects the sensor in real time with respect to aircraft roll, using inputs from a precision navigation sensor. This stabilization ensures that the resulting data and imagery are clear and require little or no post-processing to correct for aircraft motion. Radiometric calibration of the CAR is performed at NASA Goddard Space Flight Center.

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Derivation Techniques and Algorithms

CAR radiance data can be converted to reflectance following the van de Hulst formulation:

Rλ(θ, θ0, Φ) = πIλ(θ, θ0, Φ)/ μ0·Fλ


Iλ is measured reflected or scattered intensity (radiance);
θ, θ0 are the viewing and incident zenith angles, respectively;
Φ is the azimuthal angle between the viewing and incident light directions;
μ0 is the cosθ0;
Fλ is the solar flux density (irradiance) incident on the top of the atmosphere, assuming mean-earth distance;

The viewing directions range from 0–180°. The relative azimuth angles range from 0-360°, where 0° or 360° or 180° coincide with the solar principal plane.

To convert the reflectance data to BRDF, multiply Rλ (θ, θ0, Φ) by π, as defined by Nicodemus et al.

Processing Steps

  • In Level-1C processing, georeferencing parameters are applied to Level 1B data for each scan line.
  • Each scan line contains 361 pixels (representing a 180° field of regard).
  • CAR data are expanded into 14 two- dimensional arrays and engineering data is excluded.
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Sensor or Instrument Description

The NASA Goddard Space Flight Center (GSFC) Cloud Absorption Radiometer (CAR) instrument is an airborne multi-wavelength scanning radiometer that measures scattered light in fourteen spectral bands between 0.34 and 2.30 μm, located in the atmospheric window regions of the UV, visible, and near-infrared.

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References and Related Publications

Contacts and Acknowledgments

Charles K. Gatebe and Rajesh Poudyal
NASA Goddard Space Flight Center
Mail Code 613
Greenbelt, Maryland 20771

Document Information


March, 2018

No technical references available for this data set.

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