Data Set ID: 
AFLVIS1B

AfriSAR LVIS L1B Geolocated Return Energy Waveforms, Version 1

This data set contains return energy waveform data over Gabon, Africa. The measurements were taken by the NASA Land, Vegetation, and Ice Sensor (LVIS), an airborne lidar scanning laser altimeter. The data were collected as part of a NASA campaign, in collaboration with the European Space Agency (ESA) mission AfriSAR.

This is the most recent version of these data.

Version Summary: 

Initial release

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

See All Level of Service Details

Parameter(s):
  • INFRARED WAVELENGTHS > SENSOR COUNTS
Data Format(s):
  • HDF5
Spatial Coverage:
N: 1, 
S: -2, 
E: 12, 
W: 8
Platform(s):AIRCRAFT, B-200, C-130, DC-8, G-V, HU-25C, P-3B, RQ-4
Spatial Resolution:
  • Varies x Varies
Sensor(s):ALTIMETERS, LASERS, LVIS
Temporal Coverage:
  • 20 February 2016 to 8 March 2016
Version(s):V1
Temporal ResolutionVariesMetadata XML:View Metadata Record
Data Contributor(s):J. Blair, Michelle Hofton

Geographic Coverage

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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.

Blair, J. B. and M. Hofton. 2018. AfriSAR LVIS L1B Geolocated Return Energy Waveforms, Version 1. [Indicate subset used]. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: https://doi.org/10.5067/ED5IYGVTB50Z. [Date Accessed].
Created: 
15 October 2018
Last modified: 
14 November 2018

Data Description

The data in this Level-1B product were collected as part of the AfriSAR mission by the NASA Land, Vegetation, and Ice Sensor (LVIS) in collaboration with the European Space Agency (ESA) and the Gabonese Space Agency. The AfriSAR mission was an airborne campaign that collected radar and field measurements of tropical forests in Gabon, West Africa. The AfriSAR data is a precursor to upcoming spaceborne missions that examine the role of forests in Earth's carbon cycle. The data are also distributed in the Level-2 format, through the AfriSAR LVIS L2 Geolocated Surface Elevation Product data set. The Level-2 data files contain canopy top and ground elevations, as well as relative heights derived from the Level-1B data. Other related LVIS data sets include Level-0, Level-1B, and Level-2 products collected as part of the Operation IceBridge campaigns. See the Related Data Collections section for links to these data sets.

Parameters

The data files include geolocated return energy waveforms (RXWAVE), transmitted waveforms (TXWAVE), and ancillary data.

Parameter Description

Parameters contained in the HDF5 files are described in Table 1.

Table 1. HDF5 File Parameters

Group

Parameter

Description

Units

/(root) LFID LVIS file identification. The format is XXYYYYYZZZ, where XX identifies instrument version, YYYYY is the Modified Julian Date of the flight departure day, and ZZZ represents the file number. N/A
SHOTNUMBER LVIS shot number assigned during collection. Together with LFID, it provides a unique identifier to every LVIS laser shot. N/A
AZIMUTH Azimuth angle of laser beam Degrees
INCIDENTANGLE Off-nadir incident angle of laser beam Degrees
RANGE Distance along laser path from the instrument to the ground Meters
TIME UTC decimal seconds of the day Seconds
LON0 Longitude of the highest sample of the return waveform Degrees East
LAT0 Latitude of the highest sample of the return waveform Degrees North
Z0 Elevation of the highest sample of the waveform with respect to the reference ellipsoid Meters
LON1023 Longitude of the lowest sample of the return waveform Degrees East
LAT1023 Latitude of the lowest sample of the return waveform Degrees North
Z1023 Elevation of the lowest sample of the waveform with respect to the reference ellipsoid Meters
SIGMEAN Signal mean noise level, calculated in-flight Counts
TXWAVE Transmitted waveform, 128 bins long, 16 bits at 1GHz Counts
RXWAVE Return waveform, 1024 bins long, 16 bits at 1GHz Counts
/ancillary_data/ HDF5Version HDF5 version number based on IDL version Number
Maximum Latitude Maximum value of latitude to be found in this file Degrees North
Maximum Longitude Maximum value of longitude to be found in this file

Degrees East

Minimum Latitude

Minimum value of latitude to be found in this file

Degrees North

Minimum Longitude

Minimum value of longitude to be found in this file

Degrees East

ancillary_text Ancillary information relevant to data collection and processing N/A
reference_frame Reference frame for LVIS data products, derived from 
reference frame for global navigation satellite system (GNSS) orbits.
Using International Terrestrial Reference Frame (ITRF 2005).
N/A

File Information

Format

The data files are in HDF5 format (.h5). Each data file is paired with an associated XML file (.xml), which contains additional metadata.

File Contents

Figure 1 shows an illustration of LAT1023 and RANGE values from a sample of the LVIS1B_Gabon2016_0220_R1808_038024.h5 data file as displayed in the HDFView tool.

Figure 1. A sample of values for the parameters LAT1023 (left) and RANGE (right), visualized using the HDFView software.

Naming Convention

Example file names:

LVIS1B_Gabon2016_0220_R1808_038024.h5
LVIS1B_Gabon2016_0220_R1808_038024.h5.xml

Files are named according to the following convention, which is described in Table 2:

LVIS1B_GabonYYYY_MMDD_RYYMM_nnnnnn.NN

Table 2. File Naming Convention
Variable Description
LVIS1B Short name for LVIS L1B Geolocated Return Energy Waveforms
GabonYYYY Campaign identifier. Gabon = location of AfriSAR mission; YYYY= four-digit year of campaign
MMDD Two digit month, two-digit day of campaign
RYYMM Date (YY year / MM month) of the data release
nnnnnn Number of seconds since UTC midnight of the day the data collection started
NN Indicates file type: .h5 (HDF5 file) or .h5.xml (XML metadata file)

File Size

The total HDF5 file volume is approximately 103 GB.

Spatial Information

Coverage

The data set covers rainforests in Gabon, Africa, as noted by the coverage below.

Northernmost Latitude: 1° N
Southernmost Latitude: 2° S
Westernmost Longitude: 8° E
Easternmost Longitude: 12° E

Resolution

The spatial resolution is on average 20 m, but varies with aircraft altitude. Laser spot size is a function of beam divergence and altitude. Nominal spot spacing is a function of scan rate and pulse repetition rate.

Geolocation

International Terrestrial Reference Frame 2008 (ITRF08), WGS-84 ellipsoid

Temporal Information

Coverage

20 February 2016 to 08 March 2016

Resolution

The AfriSAR campaign was conducted on nine days between 20 February and 08 March 2016. Table 3 lists all the flight dates and locations for those days. For more detailed information, visit NASA's AfriSAR ORNL DAAC web page.

Table 3. Flight Dates and Locations
Date Location
20 Feb 2016 Mabounie site
22 Feb 2016 TanDEM-X and GEDI lines
23 Feb 2016 Biomass transect 1
25 Feb 2016 Mondah site
02 Mar 2016 Lope site
03 Mar 2016 Mondah site -2
04 Mar 2016 Pongara site
07 Mar 2016 RABI site
08 Mar 2016 Fill in: Biomass, Mondah, Pongara sites

Data Acquisition and Processing

Background

As described on the NASA LVIS website, a laser altimeter is an instrument that measures the range from the instrument to a target object or surface. The device sends a laser beam toward the target and measures the time it takes for the signal to reflect back from the surface. Knowing the precise round-trip time for the reflection to return yields the range to the target.

Figure 2 shows two example return waveforms. A simple waveform (left) occurs when the surface is relatively smooth within the laser footprint, which generates a laser return waveform that consists of a single mode. The detection threshold is computed relative to the mean noise level and is used to detect the return signals that are geolocated for Level-2 data products. Multilayered surfaces, such as forests or vegetated landcover, produce complex waveforms (right) containing more than one mode. Different modes represent the various surfaces within the footprint, such as the canopy top or the ground, and are distributed according to their relative elevations within the footprint.

Figure 2. Sample Level-1B product waveforms illustrating some possible distributions of reflected light.

Acquisition

LVIS employs a signal digitizer, disciplined with a very precise oscillator, to measure both the transmitted and reflected laser pulse energies versus time. These digitized and captured photon histories are known as waveforms. For the outgoing pulse, it represents the profile of the individual laser shot, and for the return pulse it records the interaction of that transmitted pulse with the target surface.

Processing of these waveforms yields many products, but the primary product is range from the instrument to the Earth's surface and the distribution of reflecting surfaces within the area of the laser footprint. For vegetated terrain these surfaces are tree canopies, branches, other forms of vegetation, and open ground.

LVIS uses a waveform-based measurement technique to collect data instead of just timing detected returns of the laser pulse. The return signal is sampled rapidly and stored completely for each laser shot. Retaining all waveform information allows post-processing of the data to extract many different products. With the entire vertical extent of surface features recorded, metrics can be extracted about the sampled area. An advantage of saving all of the waveform data is that new techniques can be applied to these data long after collection to extract even more information. See the NASA LVIS website for more information.

Processing

This data set is generated from the raw instrument data as described under Processing Steps. More details can be found in Hofton et al. (2000).

Processing Steps

The following processing steps are performed by the data provider to produce the Level-1B data.

  1. The differential kinematic GPS data are post-processed to generate the airplane trajectory. The trajectory is merged with the laser data to produce the latitude, longitude, and altitude of the airplane for each laser shot.
  2. An atmospheric correction is applied to each laser measurement. This adjustment is necessary because temperature and pressure affect the speed of light through the atmosphere. It is computed using a model, and data extrapolated from the nearest meteorological station.
  3. Laser pulse timing errors, due to the internal system response time and further affected by the amplitude of the return, are determined by calibration experiments. These are performed at the beginning and end of each flight. Each range measurement is corrected accordingly.
  4. The attitude (roll, pitch, and yaw) of the airplane is recorded by the Inertial Navigation System (INS), and is interpolated for the time of each laser shot to know the precise pointing.
  5. Several instrument biases are determined next. Timing biases are due to the delay between the actual observation of aircraft attitude and the recording of those data in the computer following the calculations. Laser mounting biases come from slight angular differences between the orientations of the three axes of the INS and those of the airplane. The timing and angle biases are determined after flying the airplane through controlled roll and pitch maneuvers over a known, preferably flat, surface.
  6. The offset between the GPS antenna and the laser scan mirror must be known in order to relate the airplane trajectory and the range measurement. The offset vector is found by performing a static GPS survey between several system components inside and outside the grounded airplane.
  7. The laser range measurement is transformed from a local reference system within the airplane to a global reference frame and ellipsoid. This creates the geolocated data product.

Quality, Errors, and Limitations

Currently, there are no known errors or limitations in this data set.

Instrumentation

As described on the NASA LVIS website, LVIS is an airborne lidar scanning laser altimeter used by NASA to collect surface topography and vegetation coverage data. LVIS uses a signal digitizer with oscillator to measure transmitted and reflected laser pulse energies versus time capturing photon histories as waveforms. The laser beam and telescope field of view scan a raster pattern along the surface perpendicular to aircraft heading as the aircraft travels over a target area. LVIS has a scan angle of approximately 12 degrees, and can cover 2 km swaths from an altitude of 10 km. Typical collection size is 10 m to 25 m spots. In addition to waveform data, GPS satellite data is recorded at ground tie locations and on the airborne platform to precisely reference aircraft position. An IMU is attached directly to the LVIS instrument and provides information required for coordinate determination.

Software and Tools

The following external links provide access to software for reading and viewing HDF5 data files. Please be sure to review instructions on installing and running the programs.

Related Data Sets

Related Websites

Contacts and Acknowledgments

Bryan Blair
Laser Remote Sensing Laboratory, Code 694
NASA Goddard Space Flight Center
Greenbelt, MD 20771

Michelle Hofton
Department of Geography
2181 LeFrak Hall
University of Maryland
College Park, MD 20742

Acknowledgments:

This work was supported through funding from Hank Margolis (NASA - SMD - ESD Terrestrial Ecology).

References

Blair, J. B., D. L. Rabine., and M. A. Hofton. 1999. The Laser Vegetation Imaging Sensor: A Medium-Altitude, Digitisation-Only, Airborne Laser Altimeter for Mapping Vegetation and Topography, ISPRS Journal of Photogrammetry and Remote Sensing, 54: 115-122.

Hofton, M. A., J. B. Blair, J. B. Minster., J. R. Ridgway, N. P. Williams, J. L Bufton, and D. L. Rabine. 2000. An Airborne Scanning Laser Altimetry Survey of Long Valley, California, International Journal of Remote Sensing, 21(12): 2413-2437.

Hofton, M. A., J. B. Blair, S. B. Luthcke, and D. L. Rabine. 2008. Assessing the Performance of 20-25 m Footprint Waveform Lidar Data Collected in ICESat Data Corridors in Greenland, Geophysical Research Letters, 35: L24501, doi:10.1029/2008GL035774.

No technical references available for this data set.

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