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SAR Subsets for Selected Field Sites, 2007-2010
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Revision Date:  February 23, 2011

Summary

This data set provides Synthetic Aperture Radar (SAR) images for 42 selected sites from various terrestrial ecology and meteorological monitoring networks including FLUXNET, Ameriflux, Long Term Ecological Research (LTER), and the Greenland Climate Network (GC-Net).

The data set contains at least one image for all 42 sites, and six sites have multiple images. See Table 1 for the sites and the temporal range of the available images. The scenes are in GeoTIFF format in Universal Transverse Mercator (UTM), WGS-84 projection, and 15-meter resolution.

The SAR images are subset scenes of approximately 60 km x 70 km that include an established site in one of the monitoring networks. The spatial resolution of all scenes is 15 meters. These scenes are distributed as GeoTIFF files with appropriate projection information defined within the file. The acquisition mode for all data is the Fine Beam Double Polarization or FBD with the HH/HV polarization. The HH and HV channels are distributed as 3 channels to allow for an intuitive image display. The HH band is displayed in the red and blue channels and the HV band is displayed in the green channel. For some images only single polarization is available; these images are distributed as grayscale images.

The source of the data is the PALSAR (Phased Array type L-band Synthetic Aperture Radar) sensor flying on the Advanced Land Observing Satellite (ALOS). The PALSAR data are in dual Polarization, HH+HV, mode. Bands HH (red and blue) and Band-HV (green) can be used to visualize land use patterns. The resulting images show vegetation in shades of green and barren land in shades of pink or purple.

niwot ridgewalker branch


Figure 1. SAR images for (a) Niwot Ridge, Colorado and (b) Walker Branch Watershed, Tennessee sites. Visualizations for land use are generally set such that green is tree canopy, pink is crop or barren soil, black is water, and grays are low vegetation. The star icon indicates the location of the field site.


The data can be used for a number of purposes (1) to validate the SAR measurements using FLUX tower site characterization data; (2) to examine the impacts of vegetation dynamics on climate; (3) to understand human impacts on vegetation at a local scale; (3) to detect deforestation and forest degradation; (4) to map and differentiate growth stages and change; (5) to retrieve woody biomass and structural attributes; and (6) to characterize, map and monitor ecoregions such as mangroves and wetlands.

Support Acknowledgment

The National Aeronautics and Space Administration (NASA) funded this EOSDIS Tech Infusion project (2010) as a collaboration between the Alaska Satellite Facility (ASF), the National Snow and Ice Data Center (NSIDC), and the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC) in 2010. The purpose of the project was to provide subsets of PALSAR data for selected field sites, such as flux tower locations, to increase terrestrial ecology users’ understanding of SAR, to make the community more aware of SAR data, and to promote its overall use.

This project produced many scenes of SAR satellite imagery for use by terrestrial ecologists in their geo-spatial investigations. The PALSAR subsets are provided in non-proprietary, unrestricted, and user friendly GeoTIFF file format and are now publicly available at the ORNL DAAC Web site.

Data and Documentation Access:

Description and Links to Companion Files and Supplemental Information:

SAR Documentation:

Alaska Satellite Facility, SAR Data Center (http://www.asf.alaska.edu/program/sdc)

Alaska Satellite Facility, SAR Data Center, PALSAR Summary (http://www.asf.alaska.edu/program/sdc/sensors#palsar)

Japan Aerospace Exploration Agency (JAXA), Advanced Land Observing Satellite "DAICHI" (ALOS) Site
(http://www.jaxa.jp/projects/sat/alos/index_e.html)

ORNL DAAC Spatial Data Access Tool (SDAT) link: http://webmap.ornl.gov/wcsdown/dataset.jsp?ds_id=993

Get Data: http://daac.ornl.gov/cgi-bin/dsviewer.pl?ds_id=993

Citation:

Cite this data set as follows:

Oak Ridge National Laboratory Distributed Active Archive Center, Alaska Satellite Facility Distributed Active Archive Center, and Japan Aerospace Exploration Agency. 2011. SAR Subsets for Selected Field Sites, 2007-2010. Data set. Available on-line [http://daac.ornl.gov] from ORNL DAAC, Oak Ridge, Tennessee, U.S.A. doi:10.3334/ORNLDAAC/993

Copyright 2010 Alaska Satellite Facility derived from data copyright JAXA/METI


Table of Contents

1 Data Set Overview
2 Data Description
3 Applications and Derivation
4 Quality Assessment
5 Acquisition Materials and Methods
6 Data Archive Access
7 References

1. Data Set Overview:

This data set provides Synthetic Aperture Radar (SAR) images for 42 selected sites from various terrestrial ecology and meteorological monitoring networks including FLUXNET, Ameriflux, Long Term Ecological Research (LTER), and the Greenland Climate Network (GC-Net).

The data set contains at least one image for all 42 sites, and six sites have multiple images. See Table 1 for the sites and the temporal range of the available images. The scenes are in GeoTIFF format in Universal Transverse Mercator (UTM), WGS-84 projection, and 15-meter resolution.

The SAR images are subset scenes of approximately 60 km x 70 km that include an established site in one of the monitoring networks. The spatial resolution of all scenes is 15 meters. These scenes are distributed as GeoTIFF files with appropriate projection information defined within the file. The acquisition mode for all data is the Fine Beam Double Polarization or FBD with the HH/HV polarization. The HH and HV channels are distributed as 3 channels to allow for an intuitive image display. The HH band is displayed in the red and blue channels and the HV band is displayed in the green channel. For some images only single polarization is available. These images are distributed as grayscale images.

The source of the data is the PALSAR (Phased Array type L-band Synthetic Aperture Radar) sensor flying on the Advanced Land Observing Satellite (ALOS). The PALSAR data are in dual Polarization, HH+HV, mode. Bands HH (red and blue) and Band-HV (green) can be used to visualize land use patterns. The resulting images show vegetation in shades of green and barren land in shades of pink or purple.

The data can be used for a number of purposes (1) to validate the SAR measurements using FLUX tower site characterization data; (2) to examine the impacts of vegetation dynamics on climate; (3) to understand human impacts on vegetation at a local scale; (3) to detect deforestation and forest degradation; (4) to map and differentiate growth stages and change; (5) to retrieve woody biomass and structural attributes; and (6) to characterize, map and monitor ecoregions such as mangroves and wetlands.

2. Data Description

This data set provides Synthetic Aperture Radar (SAR) images for 42 selected sites from various monitoring networks including FLUXNET, Ameriflux, LTER, and the Greenland Climate Network (GC-Net).

There is at least one image for all 42 sites, and six sites have multiple images. See Table 2 for the sites and the temporal range of the available images. The scenes are in GeoTIFF format in Universal Transverse Mercator (UTM) projection and 15-meter resolution.

Spatial Coverage

Sites: Selected field sites, such as carbon and heat flux tower locations, to increase terrestrial ecologist's understanding of SAR and to promote its use.

SAR SITES MAP

Figure 2. Locations of sites with SAR images.


Table 1.  Selected environmental network sites and physical attributes.

Site Name Land_unit Latitude Longitude Site Type Vegtype_igbpclss
Arctic LTER (ARC1)North America 68.62833-149.593331LTER Site Open shrublands
Baltimore Ecosystem Study (BES1) North America39.400281-76.7702806 LTER SiteUrban and built-up
Bartlett Experimental ForestNorth America 44.06464-71.2880769Flux Tower Deciduous broad-leaf forest
BOREAS NSA - Old Black SpruceNorth America55.87962-98.48081 Flux Tower Evergreen needle-leaf forest
BOREAS SSA Young AspenNorth America 53.65601-105.32314Flux Tower Mixed forest
British Columbia- Campbell River - Clearcut Site North America49.87048-125.29087Flux Tower Evergreen needle-leaf forest
British Columbia- Campbell River - Mature Forest Site North America49.86725-125.3336Flux Tower Evergreen needle-leaf forest
Buffalo13 ESE- SDSU Antelope Research Station (Calving Pasture Site)North America45.516 -103.3017Climate reference network station Grasslands
Cascades/H.J. Andrews LTER- Oregon North America44.24885-122.180347  Evergreen needle-leaf forest
Chamela Biological StationNorth America19.509281-105.040175 Flux TowerDeciduous broad-leaf forest
CP1Europe69.8819-46.9736  GC-Net** 
Duke Forest HardwoodsNorth America 35.973582-79.1004304Flux Tower Mixed forest
Harvard Forest EMS Tower(HFR1) North America42.537756-72.1714778 Flux TowerMixed forest
HJ Andrews Aeronet Sunphotometer (AND1) North America44.238889-122.223889 LTER SiteEvergreen needle- leaf forest
Howland Forest (Main Tower) North America45.20407-68.7402778 Flux TowerMixed forest
Humboldt Gl.Europe78.5266-56.8305  GC-Net 
Juniper Woodland Site North America40.259322-112.478125 Flux TowerGrassland
KULU Europe65.7584-39.6018  GC-Net 
Lost Creek North America46.08268-89.97919 Flux TowerDeciduous broad-leaf forest
Luquillo LTER (LUQ1)North America18.323889-65.8175 LTER SiteEvergreen broadleaf forest
Metolius Eyerly BurnNorth America44.579442-121.500076 Flux TowerEvergreen needle-leaf forest
Metolius Intermediate PineNorth America44.452432-121.557166 Flux TowerEvergreen needle-leaf forest
Missouri Ozark SiteNorth America38.74411 -92.200009Flux Tower Deciduous broad-leaf forest
NASA-E Europe 75 -29.9997  GC-Net  
NGRIP Europe 75.0998 -42.3326  GC-Net  
Niwot Ridge (LTER NWT1) North America 40.032878 -105.546403 Flux Tower Evergreen needle-leaf forest
Park Falls North America 45.945878 -90.2723042 Flux Tower Deciduous broad-leaf forest
Phillips Creek Marsh (PHCK) North America 37.46069 -75.8347115 LTER Site Woody Savannas
Rond.- Faz. Nossa Senhora-Ji Parana-pasture South America -10.76181 -62.3572222 Flux Tower Savannas
Rond.- Rebio Jaru Ji Parana- Tower B South America -10.07806 -61.9330972 Flux Tower Evergreen broad-leaf forest
Santarem-Km77-Pasture South America -3.011896 -54.53652 Flux Tower Cropland/natural vegetation mosaic
Sask - SSA Old Aspen North America 53.62889 -106.19779 Flux Tower Mixed forest
Sask- SSA Old Jack Pine North America 53.91634 -104.69203 Flux Tower Evergreen needle-leaf forest
Sioux Falls Portable North America 43.2408 -96.902   Croplands
Sky Oaks North America 33.384444 -116.640261 Flux Tower Closed shrublands
Summit Europe 72.5794 -38.5042  GC-Net  
Swiss Camp Europe 69.5732 -49.5952  GC-Net  
Tablelands Juniper Savanna North America 34.425489 -105.861545   Open shrublands
Tonzi Ranch North America 38.4316 -120.965983 Flux Tower Woody savannas
Valles Caldera Mixed Conifer North America 35.888447 -106.532114   Woody savannas
Walker Branch Watershed North America 35.958767 -84.2874333 Flux Tower Deciduous broad-leaf forest
Western Peatland- LaBiche-Black Spruce/Larch Fen North America 54.95384 -112.46698 Flux Tower Mixed forest

** Greenland Climate Network (GC-Net) (http://cires.colorado.edu/science/groups/steffen/gcnet/)

Collection boundaries: (All latitude and longitude given in decimal degrees)

Westernmost Longitude Easternmost Longitude Northernmost Latitude Southernmost Latitude
-156.6650 -29.9997 78.5266 -10.7618

Site Information

The purpose of the project was to provide subsets of PALSAR data for selected field sites from established monitoring networks for carbon and heat flux (AmeriFlux Tower locations), long-term ecological monitoring sites (LTER), Greenland Climate Network (GC-Net), and EOS Land Validation Sites to increase terrestrial ecology users’ understanding of SAR data and promote its use. The PALSAR subsets are provided in non-proprietary, unrestricted, and user friendly GeoTIFF file format.

Spatial Resolution

The SAR images are subset scenes of approximately 60 km x 70 km that includes an established site in one of the monitoring networks.  The spatial resolution of all scenes is 15 meters.

Temporal Coverage

Temporal Resolution

SAR subsets were extracted for the dates shown in Table 2.  The exact time of the image is included in the documentation.

Table 2.  The selected sites, the name of their respective compressed image files, the date(s), and projections of the SAR subset image(s) provided.

Site Name Image and Documentation Filename (compressed) Image Date(s) Image Projection
Arctic LTER (ARC1)Arctic_LTER.zip2010/07/11 WGS84, UTM, Zone 6N
Baltimore Ecosystem Study (BES1)Baltimore_Ecosystem_Study.zip 2009/07/28WGS84, UTM, Zone 18N
Bartlett Experimental Forest Bartlett_Experimental_Forest.zip 2007/07/20
2007/09/04
2007/10/20
2008/06/06
2009/07/25
2009/10/25
2010/07/28
2010/09/12
2010/10/28
WGS84, UTM, Zone 19N
BOREAS NSA - Old Black SpruceBOREAS_NSA_Old_Black_Spruce.zip 2010/09/24WGS84, UTM, Zone 14N
BOREAS SSA Young AspenBOREAS_SSA_Young_Aspen.zip 2010/07/19WGS84, UTM, Zone 13N
British Columbia-Campbell River - Clearcut Site British_Columbia_Campbell_River_Clearcut_Site.zip 2010/06/05WGS84, UTM, Zone 10N
British Columbia- Campbell River - Mature Forest Site British_Columbia_Campbell_River_Mature_Forest_Site.zip 2010/06/05WGS84, UTM, Zone 10N
Buffalo 13 ESE- SDSU Antelope Research Station (Calving Pasture Site) Buffalo_13_ESE_SDSU_Antelope_Research_Station.zip 2010/10/02WGS84, UTM, Zone 13N
Cascades/H.J. Andrews LTER- OregonCascades_H_J__Andrews_LTER_Oregon.zip 2008/10/25WGS84, UTM, Zone 10N
Chamela Biological StationChamela_Biological_Station.zip 2010/10/10WGS84, UTM, Zone 13N
CP1CP1.zip 2009/11/10WGS84, UTM, Zone 23N
Duke Forest Hardwoods Duke_Forest_Hardwoods.zip2010/09/25 WGS84, UTM, Zone 17N
Harvard Forest EMS Tower(HFR1)Harvard_Forest_EMS_Tower.zip2007/08/23
2010/08/31
2010/10/16
WGS84, UTM, Zone 18N
HJ Andrews Aeronet Sunphotometer (AND1) HJ_Andrews_Aeronet_Sunphotometer.zip 2010/10/25 WGS84, UTM, Zone 10N
Howland Forest (Main Tower) Howland_Forest.zip 2010/10/18WGS84, UTM, Zone 19N
Humboldt Gl. Humboldt_Gl.zip 2010/06/29WGS84, UTM, Zone 21N
Juniper Woodland Site Juniper_Woodland_Site.zip 2010/07/20 WGS84, UTM, Zone 12N
KULUKULU.zip2008/05/21
2008/07/06
2008/08/21
WGS84, UTM, Zone 24N
Lost CreekLost_Creek.zip2010/10/08 WGS84, UTM, Zone 15N
Luquillo LTER (LUQ1)Luquillo_LTER.zip2010/10/11 WGS84, UTM, Zone 20N
Metolius Eyerly BurnMetolius_Eyerly_Burn.zip 2010/06/29WGS84, UTM, Zone 10N
Metolius Intermediate PineMetolius_Intermediate_Pine.zip 2010/06/29WGS84, UTM, Zone 10N
Missouri Ozark SiteMissouri_Ozark_Site.zip 2010/08/04WGS84, UTM, Zone 15N
NASA-ENASA_E.zip2009/12/23 WGS84, UTM, Zone 25N
NGRIP NGRIP.zip 2010/06/29WGS84, UTM, Zone 23N
Niwot Ridge (LTER NWT1)Niwot_Ridge.zip 2007/06/05
2007/07/21
2007/10/21
2008/06/07
2009/07/26
2010/06/13
2010/07/29
2010/10/29
2010/12/14
WGS84, UTM, Zone 13N
Park FallsPark_Falls.zip2010/10/08 WGS84, UTM, Zone 15N
Phillips Creek Marsh (PHCK)Phillips_Creek_Marsh.zip 2010/10/31WGS84, UTM, Zone 18N
Rond.- Faz. Nossa Senhora-Ji Parana-pastureRond_Faz_Nossa_Senhora_Ji_Parana_pasture.zip 2010/10/21WGS84, UTM, Zone 20S
Rond.- Rebio Jaru Ji Parana-  TowerBRond_Rebio_Jaru_Ji_Parana_Tower .zip2010/07/17WGS84, UTM, Zone 20S
Santarem-Km77-PastureSantarem_Km77-Pasture.zip2007/06/12
2008/05/30
2009/06/18
2009/08/03
2010/06/21
2010/07/21
2010/08/06
2010/11/06
WGS84, UTM, Zone 21S
Sask- SSA Old AspenSask_SSA_Old_Aspen.zip 2010/11/05WGS84, UTM, Zone 13N
Sask- SSA Old Jack PineSask_SSA_Old_Jack_Pine.zip 2010/10/31WGS84, UTM, Zone 13N
Sioux Falls PortableSioux_Falls_Portable.zip2010/03/20
2010/06/29
2010/07/16
2010/08/31
2010/10/16
2010/12/01
WGS84, UTM, Zone 14N
Sky OaksSky_Oaks.zip2009/11/18 WGS84, UTM, Zone 11N
SummitSummit.zip 2010/01/23WGS84, UTM, Zone 24N
Swiss CampSwiss_Camp.zip 2010/01/05WGS84, UTM, Zone 22N
Tablelands Juniper Savanna Tablelands_Juniper_Savanna.zip2010/09/18 WGS84, UTM, Zone 13N
Tonzi RanchTonzi_Ranch.zip2010/09/17 WGS84, UTM, Zone 10N
Valles Caldera Mixed ConiferValles_Caldera_Mixed_Conifer.zip 2010/10/05WGS84, UTM, Zone 13N
Walker Branch WatershedWalker_Branch_Watershed.zip 2010/10/27WGS84, UTM, Zone 16N
Western Peatland- LaBiche-Black Spruce/Larch FenWestern_Peatland_LaBiche_Black_Spruce.zip 2010/09/23WGS84, UTM, Zone 12N

Data File Information

As show in Table 2, each site has a single compressed data file. Within the *.zip file is the GeoTIFF image file(s) with the image date appended to the file name. A *.pdf documentation file is also included and contains image-specific metadata, image analysts notes about channel assignments and colors, and a thumbnail of the SAR image as show in Figure 1. For sites with multiple images, only one thumbnail is included as images are quite similar in appearance.

Example Documentation File:

Site Name:Baltimore Ecosystem Study (BES1)
GeoTIFF: Baltimore_Ecosystem_Study_20090728.tif
Imagery Date: 28-Jul-2009, 03:32:51
Location: Maryland
The SAR image has greens, pinks, blacks and some gray colors. By assigning the HH to the red and blue channels
and the HV polarization to the green, we get these slightly more intuitive colors. Green is tree canopy. Black is water.
Polarization: FBD 34.3 HH+HV
Bands: HH (red & blue)
Bands: HV (green)

3. Data Application and Derivation:

PALSAR Terrestrial Biophysical Applications

The data can be used for a number of purposes (1) to validate the SAR measurements using FLUX tower site characterization data; (2) to examine the impacts of vegetation dynamics on climate; (3) to understand human impacts on vegetation at a local scale; (3) to detect deforestation and forest degradation; (4) to map and differentiate growth stages and change; (5) to retrieve woody biomass and structural attributes; and (6) to characterize, map and monitor ecoregions such as mangroves and wetlands.

Satellite radar can be important to Earth system monitoring because the properties of the signal return are better suited for certain vegetative biophysical estimates and are more accurate or not otherwise obtainable by passive remote sensing systems. A number of studies have shown a significant relationship between L-Band SAR backscatter coefficients and forest structure parameters including above ground biomass and vegetative structural attributes. Other examples of terrestrial applications include wetland characterization, mapping, and monitoring and forest change analysis

The PALSAR subsets provided in this data set might be useful for visual interest and preliminary analysis of the field area.  For in-depth analyses, such as biomass estimation, vegetation characterization, etc., users might have to download the lower level products from ASF.

Quantitatively Comparing Multi-Temporal Data

The data values in the image are Digital Numbers (DN) that can be used in the following equation to extract the Normalized Radar Cross Section (NRCS).

NRCS (dB) = 10*log10(<DN^2>) + CF

Where the Calibration Factor (CF) is a constant -83.

The cross section parameter is useful to quantitatively compare multi-temporal data.

4. Quality Assessment:

Because Radar emits its own signal, imaging can occur anytime of the day or night independent of sun angle. This is in contrast to passive imaging systems that require the Sun’s illumination. Due to its longer wavelength than visible light, the microwaves used in Radar also have the advantage of not being impeded by cloud cover or other atmospheric contamination.

5. Data Acquisition Materials and Methods:

Imagery from synthetic aperture radar (SAR) satellites is not a familiar data set for most users of geographic information systems (GIS). There are several reasons why radar imagery is not commonly used, primarily because of the nature of the technology and its specialized applications. Another is that radar imagery is not optical, requiring more technical processing and specialized image interpretation skills.

Overview

The SAR satellite used to create these images is the Advanced Land Observing Satellite (ALOS). ASF is a downlink and archive for ALOS in the Americas. The SAR sensor is an L-Band phased array radar capable of imaging in several resolutions and polarizations. Because radar data is ranging data that measures the strength and scatter of the radar pulse, it is not like optical imagery which is visually intuitive. To make the SAR scenes more user friendly, the polarization data was classified as reds, greens, and blues in the image. Another aspect of radar remote sensing is that the ranging data must be terrain-corrected by a process that assigns the ranging values to geographic coordinates by utilizing a digital surface model (DSM). High resolution DSM data are not available for the entire planet and existing data at high latitudes is problematic, especially in areas of very little terrain relief, such as sheet glaciers. To use a consistent DSM for this project and all of the sites being investigated, the DSM data from the ASTER satellite was used for the terrain correction.

ALOS PALSAR

Launched on January 24th, 2006 aboard the Advanced Land Observing Satellite (ALOS), the Phased Array Type L-band Synthetic Aperture Radar (PALSAR) instrument has promising applications for natural resource and land applications including parameters applicable to terrestrial nutrient cycle estimates. Research has shown that SAR data by itself, or combined with other optical or active systems, can enhance land characterization with information not otherwise available from passive remote systems. Because Radar emits its own signal, imaging can occur anytime of the day or night independent of sun angle. This is in contrast to passive imaging systems that require the Sun’s illumination. Due to its longer wavelength than visible light, the microwaves used in Radar also have the advantage of not being impeded by cloud cover or other atmospheric contamination. Some examples of PALSAR land applications include estimates and mapping of vegetation above ground biomass, deforestation mapping, wetland (including high latitude) characterization, and cropland monitoring.

Radar Imaging Basics

The PALSAR instrument is a type of Synthetic Aperture Radar (SAR) that emits energy in the long wavelength L-Band (1270 MHz) frequency. SAR radar systems are able to generate high-resolution imagery with a synthetic aperture (or virtual long antenna) by combining signals received by the physically short (real) antenna as it moves along its flight track.

As an imaging radar system moves along a flight path it emits and receives pulses in a single particular microwave wavelength and orientation (waves polarized in a single vertical (V) or horizontal (H) plane). The radar pulse interacts with the Earth’s surface and is scattered in all directions, with some energy reflected back toward the radar’s antenna. Known as backscatter, the returned signal is received by the antenna a fraction of a second later and in a specific polarization (H or V). The brightness, or amplitude, of the backscatter is measured and recorded and the data are used to derive an image. Radar waves interact differently with soil, vegetation, water, ice, and man-made objects such as buildings and roads because the backscatter is affected by the surface properties of objects. For a smooth surface such as water or a road, most of the incident energy is reflected away from the radar system resulting in a very low return signal. In contrast, rough surfaces will scatter the emitted energy in all directions and return a significant portion back to the antenna. In general, vegetation is usually moderately rough with most radar wavelengths.

6. Data Archive Access:

This data set is available through the Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).

Data Archive:

Web Site: http://daac.ornl.gov

Contact for Data Center

E-mail: uso@daac.ornl.gov
Telephone: +1 (865) 241-3952

7. References:

Shimada, M.; Isoguchi, O.; Tadono, T.; Isono, K.; , "PALSAR Radiometric and Geometric Calibration," Geoscience and Remote Sensing, IEEE Transactions on , vol.47, no.12, pp.3915-3932, Dec. 2009 doi: 10.1109/TGRS.2009.2023909