Nasa earth Observing Satellite Calibration and Validation



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NASA Earth Observing Satellite Calibration and Validation

CGMS-38 White Paper, November 2010

Introduction
An integrated pre- and post-launch program for satellite instrument calibration and characterization is essential for assessing instrument performance, including the quantification of error sources that can be used to establish instrument-related geophysical retrieval uncertainties. For long-term trend detection, time-series assessments of space-borne instruments are essential. Post-launch validation of direct instrument products (Level-1) takes many forms, and may rely on a combination of on-board systems and comparisons against ground and airborne observations (e.g., vicarious calibration) as well as other satellite instruments.
Geophysical products (Level-2 and higher) require validation across large spatial and temporal scales. This is extremely challenging, typically requiring a combination of ground networks and instrumented surface sites, airborne in situ and remote observations (including instruments with independent capabilities/methodologies), inter-satellite comparisons, and research and modeling efforts.
To the extent that a practical definition of validation is the establishment of uncertainties, all assets used to assess satellite products require knowledge of their own quantitative measurement and/or retrieval errors in order to be useful.

1. NASA Instrument Calibration Laboratories
NASA maintains several calibration laboratories in support of spaceflight, airborne, and ground-based instrument calibration. Collectively, the facilities maintain radiance sources and spectral measurement equipment from the UV through the thermal infrared, reflectometry measurement systems, and thermal and/or vacuum chambers. These labs maintain strong ties to the Optical Technology Division of the U.S. National Institute of Standards and Technology (NIST) that began in the mid-1990s during the Earth Observing System program. Details on these laboratories follow.

NASA’s center of expertise in the calibration and characterization of satellite, ground-based, and airborne remote sensing instruments resides within the NASA GSFC Sciences and Exploration Directorate (SED) instrument calibration facilities and the NASA Ames Earth Science Division’s Airborne Sensor Facility (ASF). The NASA GSFC facilities include the Radiance Calibration Laboratory (RCL), Diffuser Calibration Laboratory (DCaL), and the Calibration Development Laboratory (CDL). As shown in Fig. 1, these facilities collectively have performed state-of-the-art radiometric measurements of from the ultraviolet and visible through thermal infrared for almost 2 to 3 decades.


As shown in Fig. 2, these facilities (1) work closely with the National Institutes of Standards and Technology (NIST) in obtaining calibration standards and in establishing good measurement practices, (2) validating their common calibration scales through measurement intercomparisons, and (3) providing their services and expertise to an impressive number of instruments, projects, agencies, and missions on an international scale. In addition, NASA has also supported development of the Total Solar Irradiance Radiometer Facility (TRF) at the University of Colorado/LASP. These facilities are briefly described below in the contexts of their current and future measurement capabilities and customers.
1.1 Radiometric Calibration Laboratory (RCL)
The Radiometric Calibration Laboratory (RCL) has maintained instruments and NIST-traceable calibrated sources to calibrate, monitor, and assess the performance of satellite-, aircraft-, and ground-based remote sensing instrumentation for over two decades. The RCL is housed in a class 10,000 cleanroom at GSFC. The RCL currently offers extended, uniform sources of spectral radiance calibrated over the wavelength range of 0.36µm to 2.4µm with intensities ranging from dark scenes such as sunlit sea surface through bright scenes such as sunlit cloud tops to even higher intensities for calibrating sun photometers and spectrometers.  During operation, all laboratory sources are spectrally monitored at 11 wavelengths from 0.36µm to 2.4µm. In addition, the RCL has a sphere aperture mapping capability available to facility customers capable of characterizing the radiance uniformity of sources. In support of instrument characterization, the RCL maintains monochromator-based sources for the measurement of relative spectral responsivity and polarized uniform radiance sources for the measurement of instrument polarization responsivity.
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Fig. 1. The NASA Calibration Laboratories/Facility development timelines.


All RCL measurement capabilities have the unique design and advantage of being field-deployable over the globe. RCL instruments, equipment, sources, and expertise have been made available to US Government agencies, industry, the international remote sensing community, and academic institutions. As shown in Table 1, facility customers have included a large number of Earth and space science projects and programs, with many being repeat customers.
In addition to the class 10,000 cleanroom facility, the RCL maintains and operates an engineering development laboratory. This non-cleanroom laboratory is used to design, fabricate, and bench test calibration and characterization equipment in advance of cleanroom deployment. The engineering development laboratory houses irradiance and radiance sources, clean benches and an environmental test chamber.
The RCL conducts research into improved sources, detectors, and measurement approaches in an effort to achieve and maintain the lowest possible measurement uncertainties and highest measurement efficiencies. Coordination of the assets of the RCL and CDL has produced a cw laser-based calibration capability from the ultraviolet through the near infrared. With the current cw laser systems, the anticipated addition of quasi-cw laser sources in early 2010, and calibrated trap detectors, state-of-the-art instrument system and sub-system level measurements of absolute and relative radiance and irradiance responsivity, wavelength calibration, spectral out-of-band,
Fig. 2. The relationship between NIST, the NASA Calibration Facilities, and the international remote sensing community establishes a common calibration scale.

spatial out-of field, and polarization response will be possible from 0.27µm to 3.3µm. In addition, the RCL is currently examining the use of supercontinuum (i.e. white light) lasers as sources in radiometric applications. Recently, the RCL demonstrated and commissioned the use of commercial scanning field radiometers in the transfer of calibrations from facility to customer sources greatly reducing the time necessary for customer calibrations.


While calibration results from National Measurement Laboratories (NMLs) and other labs can take weeks to months to be made available to customers, the calibration and characterization results from the RCL are available to facility users as quickly as 1 hour from time of acquisition. This ensures no impact to established instrument deployment or flight schedules.
Additional information can be found on the laboratory website: spectral.gsfc.nasa.gov.

Table 1: RCL Customers and Projects.




NASA

Other U.S. Government Lab

Foreign Industry or Agency




GSFC/Aeronet

NIST/Triana/DISCVR

NEC/ASTER (Japan)




GSFC/Landsat

NIST/EOS Calibration

MELCO/ASTER (Japan)




GSFC/SIMBIOS




ESA/SPOT




GSFC/SSBUV




U. Lille/PHOTONS




GSFC/Safari-CAR










GSFC/LEISA

U.S. Industry

University




GSFC/SOLSE

Ball Aerospace/OMPS

Penn State U./DOE ARM




GSFC/MESSENGER

Ball Aerospace/SBUV-2

S. Dakota State U./ETM+




GSFC/ACE-Asia

Raytheon/MODIS

U. Idaho/HAMCAM




GSFC/THOR

Raytheon/VIIRS

U. Arizona/MODIS




GSFC/MODIS




U. Arizona/ETM+




ARC/MAS & MASTER










GSFC/MPL










JPL/MISR










LaRC/CERES










GSFC/SeaWiFS

GSFC/LOLA

GSFC/Cloud Scanner & Rainbow Camera

GSFC/2NFOV

GSFC/RSP


GSFC/ORCA











Note : Italics denotes space science project

1.2 Diffuser Calibration Laboratory (DCaL)
The DCaL has operated within the SED since May 1992. The laboratory is a class 10,000 cleanroom containing a unique, state-of-the-art, complete out-of-plane optical scatterometer capable of measuring the bi-directional scatter distribution function (BSDF) of transmissive or reflective, specular or diffuse optical elements and surfaces in addition to granular, powdered, or liquid samples. The current operating wavelength range of the DCaL is continuous from 0.23µm to 1µm using an incoherent, monochromator-based source and from 1.47µm to 1.63µm using tunable laser sources. The laboratory is a secondary standards calibration facility; the primary standard facility being the Spectral Tri-function Automated Reflection Radiometer (STARR) facility located at NIST in Gaithersburg, Maryland.
The DCaL scatterometer is also capable of making directional hemispherical measurements of optical scatter (i.e. reflectance and transmittance) over the complete scattering hemisphere of any sample and at any incident illumination geometry. The DCaF scatterometer can also raster scan reflective and transmissive samples to determine their BSDF uniformity.
A separate, new in-plane scatterometer offering continuous wavelength operation from 0.25 to 1.7µm has been assembled and is undergoing testing in the DCaL. Commissioning of this system in the UV through near-infrared (i.e. 0.25 to 1.0µm) is nearing completion; and commissioning in the shortwave infrared (i.e., 1.0 to 1.7µm) will take place following anticipated receipt of calibrated lab standard samples from NIST. Extension of measurements to 2.5µm is undergoing hardware design.
In accordance with the overall calibration facility policy of not impacting user-established flight and deployment schedules, quick-look BSDF and total hemispherical scatter data can be provided to DCaL users within minutes of data acquisition. Formal measurement reports can be generated and delivered to laboratory customers within 24 hours. The DCaL has recently instituted a materials solar exposure capability. This capability employs a combination small, oil-free vacuum system and solar simulator. The ability to introduce controlled, measurable amounts of contaminants onto solar irradiated samples is in the process of being implemented and tested. The optical impact of solarized contamination is determined by measuring the sample’s BSDF before and after exposure.
Since 1993, the DCaL has made on the order of one million optical scatter measurements for flight and non-flight projects from both the government and private sectors. Laboratory customers have included the domestic and international Earth and space remote sensing programs listed in Table 2. Of note in this table is the fact that the DCaL has provided hundreds of thousand BRDF measurements of laboratory and flight diffusers for the following heritage ozone measuring instruments: SSBUV, SBUV-2 on NOAA-14, 16,17, 18, N´, TOMS Earth Probes, QuikTOMS, Meteor TOMS, ADEOS TOMS, OMI, and OMPS. The laboratory has participated and will continue to participate in a number of international measurement comparisons.
Additional information on the DCaL can be found at spectral.gsfc.nasa.gov.


1.3. Calibration Development Laboratory (CDL)
The CDL was the home of the SSBUV beginning in 1984. The laboratory provided test and maintenance and pre- and post-flight calibration for the eight successful SSBUV flights. The laboratory continues to maintain “flight hardware” handling capability to support ongoing flight missions. Because of its excellent record, the laboratory has become world renowned for ultraviolet calibrations and has provided calibration services to many ozone missions using backscatter ultraviolet techniques. This includes TOMS, SBUV/2, GOME and the SCanning Imaging SpectroMeter for Atmospheric ChartographY (SCIAMACHY) (ESA), Optical Spectrograph and InfraRed Imager System (OSIRIS) (Canada), GOME-2 (Eumetsat), Ozone Monitoring Instrument (OMI) (Aura), OMPS (NPP) and will continue with OMPS-JPSS. In addition, the laboratory has provided resources for and supported development of several other NASA and cooperative space flight missions. These include the Shuttle SOLSE/Limb Ozone Retrieval Experiment (LORE) mission for the Integrated Program Office (IPO) and the Mediterranean Israeli Dust Experiment (MEIDEX), an Israeli initiative to study aerosols, also from the Shuttle. Services have also been provided for the AERONET and the MODIS Airborne Table 2: DCaL Customers and Projects.


NASA

Other U.S. Government Lab

Foreign Industry or Agency

GSFC SSBUV

NIST/EOS Calibration

TPD-TNO/OMI

GSFC/ETM+

NRL/FAME

TPD-TNO/MERIS

GSFC/SIMBIOS

NOAA/SIMBIOS

DLR/GOME

GSFC/JWST







GSFC/Stereo-COR

U.S. Industry

University

GSFC/HST

OSC/TOMS

MIT/ALI

GSFC/SOLSE-LORE

Ball Aerospace/OMPS

U. Colorado/SORCE

GSFC/Tethered Satellites

Raytheon/MODIS

U. Arizona/MODIS

GSFC/GLAS

ITT/ABI

U. Arizona/ETM+

GSFC/CAR

NRAO/ALMA




GSFC/TRMM

Raytheon/ETM+




JPL/TES

Ball Aerospace/SBUV-2




JPL/MISR

Raytheon/VIIRS




SSC/Commercial Remote Sensing

GSFC/APS


GSFC/SBUV-2

GSFC/ORCA



GSFC/LRS

GSFC/JDEM/WFIRST

Swales/GLAS





Note: Italics denotes space science project

Simulator (MAS). The CDL has also been heavily involved with all phases of technology development programs within NASA’s Earth Science Technology Office (ESTO). The laboratory PI and support personnel have participated in the SBIR, ACT, and IIP programs.


Past and present CDL users are summarized in Table 3.
The CDL maintains an array of NIST standards and develops advanced procedures for calibrating backscatter instruments in the ultraviolet and visible. Some recent work has moved some activities into the near infrared. The CDL has also provided facilities for Director’s Discretionary Fund (DDF, now IRAD) and Instrument Incubator Program (IIP) developments.
The CDL also provides for the cross-calibration of other ground and airborne instruments including Dobson and Brewer spectrometers, the Airborne Compact Atmospheric Mapper (ACAM), and Skyrad (SSBUV reconfigured to make zenith sky observations). Laboratory work with Skyrad and the Dobson/Brewer instruments has led to improvements and new developments of trace gas retrieval algorithms within the branch.
The airborne work has recently been extended to the Global Hawk platform and we will be supporting the venture class project DISCOVER-AQ with ACAM on the B200 platform.

Table 3. CDL Customers and Projects.




NASA

Other U.S. Government Lab

Foreign Industry or Agency

GSFC/SSBUV

NIST/EOS Calibration

KNMI/OMI

GSFC/ACAM




ESA ESTEC/SCIAMACHY

GSFC/Aeronet

U.S. Industry

DLR/GOME

GSFC/Gotz, Brewer, Dobson

ESTO/SBIR/IIP/ACT

ESPO/AVE/GloPac

LARC/Discover-AQ



OSC/TOMS

Ball Aerospace/GeoSpec/OMPS

SBUV-2


CSA/OSIRIS

National Institute of Environmental Research (NIER), Korea/GEMS










University







Washington State Univ./MAXDOAS

UMBC/Pandora














1.4 Airborne Sensor Facility (ASF)
Instrument calibration and characterization in the ASF began in 1992 being established jointly by NASA HQ’s Ecosystems and Airborne Science programs. In 1995, measurement capabilities were expanded to support the calibration and characterization of the Earth Observing System (EOS) airborne simulators.
The calibration laboratory within the ASF is designed to spectrally and radiometrically calibrate nadir-viewing airborne imagers and radiometers operating in the 0.4 to 15µm range.  Radiance calibrations in the visible through shortwave infrared are performed using integrating spheres or lamp-illuminated panels calibrated by transfer spectroradiometers. Two 30" diameter integrating spheres and two 20" hemispheres are employed in laboratory and field calibrations. Radiance calibrations in the thermal infrared are performed using two blackbodies. Spectral calibrations in the visible through thermal infrared are performed using a monochromatic source from 0.4µm to 15µm and an interferometer from 2.5 to 25 µm. The spectral bench is particularly set up to accommodate line-scanning sensors, and has recently been adapted for pushbroom devices operating in the visible to shortwave infrared. Radiometric and spectral calibrations are performed in the ASF laboratory using the output of a collimated source fed into a thermal chamber simulating instrument operation at flight temperatures. The lab is made available to outside users, with prior approval, and a list of facility users is provided in Table 4.
1.5 TSI Radiometer Facility (TRF)
The total solar irradiance (TSI) climate data record includes overlapping measurements from 10 spaceborne radiometers. The continuity of this climate data record is essential for detecting potential long-term solar fluctuations, as offsets between different instruments generally exceed the stated instrument uncertainties. The risk of loss of continuity in this nearly 30-year record drives the need for future instruments with <0.01% uncertainty on an absolute scale. The new NASA Glory-funded TSI Radiometer Facility (TRF) is the first facility in the world to provide Table 4: ASF Customers and Projects.


NASA


Other U.S. Government Lab

University

Ames & JPL/MASTER

DOE/TMS

U. North Dakota/Ag Cam

GSFC/CAR

Ames/AATS-14



NIST/EOS Calibration

Various Universities/Field Spectrometers

Ames/Field Spectrometers

U.S. Industry




Ames/UAS AMS

JPL/TIMS


Headwall/VNIR Imaging Spectrometer




Ames/TMS

GeoEye/IKONOS




Ames/SSFR







Ames/MAS

Ames/L-CROSS









Ames/AOCI







Ames/MAMS

Ames/AIRDAS









instrument end-to-end irradiance calibrations at solar power levels to the required accuracy. Based on a cryogenic radiometer with a uniform input light source of solar irradiance power levels, the TRF allows direct comparisons between a TSI instrument and a NIST-calibrated reference cryogenic radiometer viewing the same light beam in a common vacuum system.


This facility improves the calibration accuracy of future TSI instruments, establishes a new ground-based radiometric irradiance standard, and provides a means of comparing existing ground-based TSI instruments against this absolute standard under flight-like operating conditions.


The TRF has validated NASA's Glory/TIM flight TSI instrument, the European PICARD/PREMOS flight instrument, ground-based SORCE/and SoHO/VIRGO instruments, and been used to diagnose instrumental causes of differences in the TSI record. A ground-based ACRIM instrument will be tested on the TRF later this year.

2. Ground Networks and Ground-Based Calibration Sites
2.1 AERONET
The AERONET (AErosol RObotic NETwork) program is a federation of ground-based remote sensing aerosol networks established by NASA and PHOTONS (University of Lille 1, CNES, and CNRS-INSU) and is greatly expanded by collaborators from national agencies, institutes, universities, individual scientists, and partners. The program provides a long-term, continuous and readily accessible public domain database of aerosol optical, microphysical and radiative properties for aerosol research and characterization, validation of satellite retrievals, and synergism with other databases. The network imposes standardization of instruments, calibration, processing and distribution. AERONET site locations are shown in Fig. 3.
AERONET collaboration provides globally distributed observations of spectral aerosol optical depth (AOD), inversion products, and precipitable water in diverse aerosol regimes. Aerosol optical depth data are computed for three data quality levels: Level 1.0 (unscreened), Level 1.5 (cloud-screened), and Level 2.0 (cloud-screened and quality-assured). Inversions, precipitable water, and other AOD-dependent products are derived from these levels and may implement additional quality checks.
There are approximately 450 instruments registered in the network. An on-line “AERONET Data Synergy Tool” has been developed. It has recently evolved with the incorporation of solar flux from the Solar Radiation Network (SolRad-Net) and ocean color from AERONET-Ocean Color ground-based networks. Furthermore, AQUA-MODIS, TERRA-MODIS, SeaWiFS and other satellite products available through disc.sci.gsfc.nasa.gov/Giovanni have been linked to the Data Synergy Tool to compliment the modeled (GOCART, back-trajectories) and observational data (Rapid response, MPLNET and NOGAPS) data. Further information is available at
aeronet.gsfc.nasa.gov.
::bamgomas_maps (1993-2010).gif

Fig. 3. Location of AERONET sites, cumulative from1993-2010.


2.2 MPLNET
The NASA Micro-Pulse Lidar Network (MPLNET) is a federated network of Micro-Pulse Lidar (MPL) systems designed to measure aerosol and cloud vertical structure continuously, day and night, over long time periods required to contribute to climate change studies and provide ground validation for satellite sensors in the Earth Observing System (EOS) and related aerosol modeling efforts. Most MPLNET sites are co-located with sites in the NASA Aerosol Robotic Network (AERONET). These joint super sites provide both column and vertically resolved aerosol and cloud data, such as: optical depth, single scatter albedo, size distribution, aerosol and cloud heights, planetary boundary layer (PBL) structure and evolution, and profiles of extinction and backscatter.

MPLNET results have contributed to studies of dust, biomass, marine, and continental aerosol properties, the effects of soot on cloud formation, aerosol transport processes, and polar clouds and snow. MPLNET also serves as a ground calibration network for space-based lidars such as the Geoscience Laser Altimeter System (GLAS) on the ICESat spacecraft. (launched in 2003) and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) (launched in 2006). MPLNET data is also used to validate and help interpret results from NASA satellite sensors (e.g., GLAS, CALIOP, MISR, and TOMS).

MPLNET is a federated network composed of NASA sites, and others run by, or with help from, partner research groups from around the world. Principal investigators for individual sites may be from NASA, other U.S. government agencies, universities, or foreign research groups.

All sites in MPLNET currently use the Micro Pulse Lidar (MPL) that was developed at NASA Goddard Space Flight Center (GSFC) in the early 1990s. Further information is available at mplnet.gsfc.nasa.gov. MPLNET site locations are shown in Fig. 4.



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Fig. 4. Location of historic and active MPL sites, cumulative from 2000.



2.3 Landsat Ground and Targets Sites
The Land Cover Project Science Office (LPSO) at NASA GSFC provides support for monitoring the radiometric and geometric calibration of the current Landsat missions (Landsat-5 and Landat-7), and establishing a long-term calibrated radiometric record going back to Landsat-1 in 1972. Principal radiometric calibration sites and site leads include:

  • Brookings, South Dakota (Dennis Helder, South Dakota State University): An agricultural site used for characterizing radiometric response over vegetation.

  • Railroad Valley, Nevada and Ivanpah, California (Stewart Bigger, University of Arizona): Desert playa sites used for characterizing bright target radiometric response.

  • Lake Tahoe and Salton Sea, California (Simon Hook, JPL): Uses an automated buoy network (Tahoe) and platform (Salton Sea) to record near-surface water and air temperatures for thermal calibration. The cold (Tahoe) and warm (Salton Sea) examples provides a dynamic range of ~30K.

  • Lake Ontario (John Schott, U. Rochester): Vicarious lake cruises measuring water and air temperatures for thermal calibration.

The LPSO uses a network of ground targets for geometric characterization and calibration. Imagery from Lake Ponchatrain Bridge (Louisiana) is routinely acquired to characterize the Landsat-7 Modulation Transfer Function (MTF). In addition, there are 102 global sites used for band-band registration studies, and 46 sites with ground control used for assessing geodetic accuracy.


NASA and the U.S. Geological Survey (USGS) have funded long-term trending studies of the radiometry of the entire Landsat satellite series. This work was carried out by Dennis Helder (SDSU), and has recently resulted in the first definitive absolute radiometric calibration extending back to 1972. These studies relied on sensor cross-calibration using near-simultaneous acquisitions from a global distribution of hyper-arid test sites (also see section 4). Validation of the long-term calibration was provided by analyzing the radiometric stability of a calibrated Landsat-1 through 7 data series of the Sonoran desert.
2.4 Radiometric Vicarious Validation Activities
The primary satellite intercomparison and ground validation activities have been through NASA-supported work at universities and other government agencies.  Much of the work by the USGS to develop the lunar model needed for trending temporal changes in sensors and for intercomparison has come from NASA sources including ROSES (competitive funding) as well as direct funding through the Earth Observing System (EOS) Project Science Office. Ground calibration and validation funds through Landsat (section 2.3), EOS, and ROSES have permitted the characterization of numerous test sites ranging from large dry lake beds in the remote western U.S., small grass and asphalt-based sites, to large bodies of water (e.g., Lake Tahoe). Automation of these sites has already been shown to be near-operational in the thermal infrared and feasible in the reflected solar.
Intercomparisons between international groups making ground measurements have taken place at these sites, and most recently at the Tuz Golu test site in central Turkey as part of a CEOS (Committee on Earth Observation Satellites) directed activity.
2.5 Tropical Rainfall Measurement Mission (TRMM) Ground Validation
Because of the highly variable nature of precipitation it is important to have high quality validation data from a diverse variety of cases and climate regimes, and over extended time periods. To achieve this objective, a key component of the TRMM project is the Ground Validation (GV) effort (http://trmm-fc.gsfc.nasa.gov/trmm_gv). This is primarily a data collection and product generation program. Ground-based radar and rain gauge data are collected and quality-controlled, and validation products are produced for comparison with TRMM satellite products. Detailed information and product analysis is available from the TRMM GV web site.
The four primary GV sites are Darwin, Australia; Houston, Texas; Kwajalein, Republic of the Marshall Islands; and, Melbourne, Florida (Wolff et al 2005). There is also a significant effort being supported at NASA Wallops Flight Facility (WFF) and vicinity to provide high quality, long-term measurements of rain rates (via a network of rain gauges collocated with National Weather Service gauges), as well as drop size distributions (DSD) using a variety of instruments, including impact-type Joss-Waldvogel, laser-optical Parsivel, and other disdrometers. DSD measurements are also being collected at Melbourne and Kwajalein using Joss-Waldvogel disdrometers. The largest part of the validation effort involves the routine collection, processing and product generation of ground-based radar, rain gauge and disdrometer data in order to produce standard validation products. Products are produced using techniques developed to carefully quality control ground radar data sets and estimate surface rainfall rates, adjusted by quality-controlled rain gauge data. The procedures for performing these tasks are optimized to take advantage of each site’s strengths.
2.6 NDACC
The international Network for the Detection of Atmospheric Composition Change, (NDACC) formerly the Network for the Detection of Stratospheric Change (NDSC) was formed to provide a consistent standardized set of long-term measurements of atmospheric trace gases, particles, and physical parameters via a suite of globally distributed research stations. Official operations began in 1991. U.S. participation is led by NASA and NOAA.
The NDACC currently consists of more than 70 high-quality, remote-sensing research stations. Site instruments include Dobson/Brewers, lidars, FTIR spectrometers, sondes, and UV and microwave radiometers. Site locations are shown in Fig. 5.
While the NDACC remains committed to its initial objective of monitoring changes in the stratosphere, with an emphasis on the long-term evolution of the ozone layer, it’s measurement and analysis priorities have broadened to encompass both the stratosphere and free troposphere as well as to explore the interface between changing atmospheric composition and climate. Examples NDACC measurement and analysis accomplishments and network details (its implementation, structure and operation, data archiving, and related protocols and publications) can be found at www.NDACC.org.

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Fig. 5. Location of NDACC sites, currently involving more than 20 countries.



2.7 SHADOZ
The Tropical Ozonesonde Dataset for Satellite Validation (SHADOZ) is a NASA project to augment and archive balloon-borne ozonesonde launches and to archive data from tropical and sub-tropical operational sites. The project was initiated in 1998 by NASA GSFC with other U.S. and international co-investigators. There are currently twelve stations launching ozonesondes in the SHADOZ network (see Fig. 6). The collective data set provides the first climatology of tropical ozone in the equatorial region, enhances validation studies aimed at improving satellite remote sensing techniques for tropical ozone estimations, and serves as an educational tool to students, especially in participating countries. SHADOZ has been designated as a “Cooperating Network” under NDACC (Section 2.6).
The SHADOZ homepage provides technical information for each station and contact information. Annual summary newsletters that report recent satellite validation efforts and other activities are available at SHADOZ newletters. The SHADOZ PI is Dr. Anne Thompson (Pennsylvania State University).
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Fig. 6. Location of current SHADOZ sites.




2.8 COVE
The Clouds and the Earth's Radiant Energy System (CERES) Ocean Validation Experiment (COVE) provides continuous radiation measurements at the Chesapeake Lighthouse for validation of CERES and other satellite products. No other ocean platform provides continuous atmospheric radiation measurements of this kind and quality. COVE is located on the Chesapeake Lighthouse (a U.S. Coast Guard platform located 25 km East of Virginia near the mouth of the Chesapeake Bay). The site provides an excellent validation site for space-borne retrievals of cloud and aerosol microphysics. The aerosol climatology at this location is consistent with polluted urban aerosols, which is not surprising given its close proximity to Virginia Beach and Norfolk, Virginia. These urban aerosols will also have an impact on the marine clouds at the site. However, easterly winds from occasional synoptic systems and frequent sea breezes provide a marine aerosol source.
Instrumentation includes the GEWEX Radiation Panel Baseline Surface Radiation Network (BSRN) instrument suite (uplooking shaded and unshaded broadband pyranometers, shaded pyrgeometers; downlooking pyranometers and pyrgeometers; normal incidence pyrheliometers), an AERONET sunphotometer, uplooking and downlooking multifilter rotating shadowband radiometers (MFRSRs), a micropulse lidar (MPL), pressure, temperature, relative humidity, National  Oceanic and Atmospheric Administration (NOAA) global positioning system integrated precipitable water vapor (GPS-IPW), National Data Buoy Center (NDBC) wave height and period. Much of the current instrumentation has been providing continuous data since March, 2000. A local area network (LAN) at the facility and a microwave link to shore provides robust digital communications. Further information is available at cove.larc.nasa.gov
2.9 MOBY
The Marine Optical Buoy (MOBY) is an autonomous optical buoy moored off the island of Lanai in Hawaii. It was developed by NASA and NOAA to support the validation of satellite ocean color imagery data collected by the Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS). The project in now continued with support from NOAA. MOBY is managed by the Moss Landing Marine Laboratory (MLML).
MOBY consists of a 14-meter long buoy system instrumented to measure upwelling radiance and downwelling irradiance at the sea surface and at three depths.  Submarine light is transmitted by fiber optics to the MOBY spectrograph for continuous energy measurements at subnanometer resolution from 340 (ultraviolet) to 950 (near-infrared) nanometers.  Standard meteorological observations are collected concurrent with the submarine light measurements, and supplemental oceanographic measurements, such as natural phytoplankton fluorescence, are also collected. Data is available on-line from the MLML web site (moby.mlml.calstate.edu).
2.10 Mobile Instrument Suites
NASA supports several mobile instrument suites used in field campaigns and monitoring studies. These include:


  • Surface-sensing Measurements for Atmospheric Radiative Transfer 
(SMART): ground-based remote sensing instruments for atmospheric solar and terrestrial radiation studies.

  • Chemical, Optical, and Microphysical Measurements of In-situ Troposphere 
(COMMIT): In-situ observatory for studying basic chemical, optical and microphysical properties of atmospheric aerosols and trace gases.

  • Aerosol, Cloud, Humidity, Interactions Exploring and Validating Enterprise 
(ACHIEVE): Under development. To be equipped with active instruments (95 GHz and 24 GHz radar, and UV-wavelength lidar for aerosol extinction).

Each instrument suite is integrated into a twenty-foot weatherized and thermally controlled trailer to facilitate the shipping to and operation in the field. More information is available at smartlabs.gsfc.nasa.gov.

 

3. Airborne Sensors, Platforms, and Validation Efforts
NASA funds and maintains facility sensors in addition to supporting development and flight of a broad range of PI sensors for Earth Science investigations. A large number of aircraft are available for remote sensing and in situ observations through the NASA Airborne Science Office (within the Earth Science Division). The instrument and platform assets are used for satellite validation, core science research investigations, and for application and natural hazard purposes.
3.1 Airborne Instruments
Facility instruments are generally funded directly by NASA for use by investigators or campaign organizers upon request. These instruments include:


  • Airborne Visible Infrared Imaging Spectrometer (AVIRIS). Hyperspectral imager, visible through shortwave-infrared (224 contiguous spectral channels). [aircraft: ER-2, WB- 57, Twin Otter; see section 3.3; aviris.jpl.nasa.gov]




  • MODIS Airborne Simulator (MAS): Airborne scanning grating spectrometer (50 spectral channels), 50 m nadir spatial resolution imagery, visible through the 14µm spectral coverage. Used for cloud, aerosol, fire detection, and surface features. [aircraft: ER-2; mas.arc.nasa.gov]




  • MODIS-ASTER Simulator (MASTER). Similar to the MODIS Airborne Simulator (MAS), with changes in the spectral band positions in order to better simulate both ASTER and MODIS. [aircraft: WB-57, B-200, ER-2; masterweb.jpl.nasa.gov]




  • National Polar-orbiting Operational Environmental Satellite System Airborne Sounder Testbed - Interferometer (NAST-I): scanning interferometer measures emitted thermal radiation at high spectral resolution between 3.3 and 18 microns, 2 km nadir spatial resolution from a nominal altitude of 20 kilometers. [aircraft: ER-2, PROTEUS; cimss.ssec.wisc.edu/nasti/].




  • UAV-Synthetic Aperture Radar (UAVSAR). High resolution, fully polarimetric, L-band SAR designed for repeat pass InSAR applications. [aircraft: Gulfstream III; uavsar.jpl.nasa.gov].




  • Other: UAS-Autonomous Modular Sensor (UAS-AMS), Cirrus Digital Camera and Digital Mapping System (DCS/DMS), Precision Attitude/position equipment (POS-AV) [aircraft: various]

NASA has also assisted in the development of many PI remote sensing (radar, lidar, imagers, radiometers, sounders) and in situ instruments (chemistry, aerosol, cloud, meteorological parameters) via the Research Office, Earth Science Technology Office (ESTO), and the Instrument Incubator Program (IIP). The breadth of the instruments that have flow on NASA-funded field campaigns is beyond the scope of this white paper but can be surveyed on any number of field campaign web sites (e.g., http://www.espo.nasa.gov).


As an example, the High Spectral Resolution Lidar (HSRL) provides aerosol backscatter and depolarization at 532 and 1064 nm, and aerosol extinction at 532 nm (science.larc.nasa.gov/hsrl). HSRL takes advantage of the spectral distribution of the lidar return signal to discriminate aerosol and molecular signals and thereby measure aerosol extinction and backscatter independently. HSRL was designed to fly on small aircraft (NASA B200, LearJet) and is primarily used to validate measurements made by the CALIPSO spaceborne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) and aerosol retrievals from satellite-based passive sensors. A summary of one validation underflight is shown in Fig. 7. HSRL is also used in campaigns focused on regional process studies and the validation of chemical transport models. Further information is available at the above web site.
3.2 Airborne Platforms
The Airborne Science Program within the Earth Science Division is responsible for providing aircraft systems that further science and advance the use of satellite data (airbornescience.nasa.gov).
The Airborne Science Program maintains a catalog of available aircraft, with access to both NASA (e.g., Global Hawk, ER-2, WB-57, DC-8, P-3, G-III, B-200, etc.) and contracted commercial assets. Other U.S. agency assets have also been used to acquire observations.

Fig. 7. Top panels: CALIOP and HSRL attenuated backscatter profiles from 10 August 2006 indicate CALIOP calibration to within 2% for the case shown. To date, 100 coincident HSRL-CALIPSO underflights have been made on the LaRC King Air B200. Bottom panels: Summary of all CALIPSO underflights (daytime observations are coded red, nighttime in blue). The calibration differences described above are shown in the bottom panel for all 100 underflights.




3.3 Satellite Validation Efforts
Validation and correlative measurements in support of satellite sensors and retrievals is of critical importance. NASA has a long history of validation field campaigns and programs, heavily leveraging off the NASA aircraft fleet and infrastructure, facility and PI instruments, calibration laboratories and NIST partnerships, and other U.S. agency and international efforts (see Section 2.4 regarding radiometric vicarious validation). In many cases, combined observations from spaceborne, airborne, and in situ instrument is required to obtain all physical quantities needed for scientific studies.
A brief summary of Earth Observing System (EOS) and EOS-related field campaigns can be found at this link. Other field campaign information can be found at the NASA Ames organization that provides project management for many NASA's Science Mission Directorate field research efforts (www.espo.nasa.gov).

4. Inter-Satellite Calibration
Satellite intercalibration with operational Geosynchronous imagers and/or AVHRR sensors has been supported for many years for generation of consistent multiple-instrument time series observations for ISCCP and ERBE/CERES data processing. More recently, NASA has supported NOAA activities to cross-compare AVHRR to multiple NASA assets including Terra and Aqua MODIS (J. Xiong, GSFC). A number of NASA-funded science teams have used inter-satellite and/or -instrument comparisons to evaluate long-term total solar irradiance data records (SORCE TIM, ACRIMSAT, ERB, etc.) and comparison of high spectral resolution and filter radiometer infrared spectra (AIRS, IASI, MODIS). These efforts are supported through a combination of support from the NASA Research Office and Flight Office.
NASA supports international efforts at establishing satellite intercalibration through participation in the Global Space-based Intercalibration System (GSICS, a space component to WWW GOS). NASA is represented on the executive panel (J. Butler, GSFC) and the Research Working Groups (Dave Doelling, LaRC and J. Xiong, GSFC). K. Thome (GSFC) and Xiong participate regularly in CEOS-related activities including the development of best practices and protocols for sensor intercomparisons.  J. Butler (GSFC) is active within the GEOSS community to ensure that calibration standards are consistent across sensor development as well as in supporting preflight calibration intercomparison activities.

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