TRUTHS Traceable Radiometry Underpinning Terrestrial- & Helio- Studies Nigel Fox NPL Nigel. Fox@npl. co. uk David Pollock U of Alab. James Aiken Plym Mar Lab Michael Schaepman U of Zur Xavier Briottet ONERA Werner Schmutz WRC/PMOD John Barnett Ox univ. Mike Sandford RAL Keith Shine Uni of Read Steve Groom Plym Mar Lab Phil Teillet Claus Frohlich WRC/PMOD Theocharous NPL Jo Haigh Imp Coll Kurtis Thome U of Ariz Olivier Hagolle CNES Terry Quinn BIPM Hugh Kieffer USGS Michel Verstraete JRC(Italy) Judith Lean NRL Emma Woolliams NPL John Martin NPL Ed Zalewski U of Ariz CCRS
Need for improved Quality Assurance Requirement - baseline for climate studies - global warming - Man or Nature? From: Hugh Keiffer - detection of change. USGS - improve models - prediction of weather systems - monitoring the treaties Difficulties - Bias between sensors MISR, MODIS, AVHRR …. . - Instruments change on launch and 0. 1 % degrade in-orbit (gain and spectral) - On-board calibration systems expensive! Reliable? Traceable? - Inter-team / manufacturer /agency - auditing carbon sinks - efficiency of carbon sinks debate - identify crops from weeds - automated farming - Need for International agreement 0. 1 % - QA of operational services (GMES) - No consistent statements of uncertainty & GEOSS or “degrees of confidence”. - instrument synergy - compatible data sets (interoperability) Solar constant – 25 yr record evidence of stability of the Sun
Reliable satellite data quality Ideally Requires: Pre-flight instrument design conformance Traceable sub-system characterisation/calibration “ End-to-end calibration Maintenance/life-test of witness samples/sub-systems Post-launch design/performance conformance Traceable calibration/validation of all key characteristics - on-board calibration system! - comparison with physical parameter - “ with reference data/instrument (comparison with existing similar instrument)
Infrastructure for innovation in measurement, validation and QA of EO data • Transfer standards • Comparisons Post-launch airborne • Measurement & test protocols • International link NPL ++ NPL+ Modelling & Data processing • Innovation on techniques • Independence NIST+ Calibration QA Traceability Pre-flight In-situ Advice Public sector Audit Validation Private Industry Academia
CCPR RECOMMENDATION P 1 (2005) ……. Post launch requirements include: Resolution adopted by CEOS Plenary 14 (Nov 2000) (Committee for Earth (a) Vicarious calibration of ground sites with temporally and spatially stable surface ON THE IMPORTANCE OF SI TRACEABLE MEASUREMENTS TO MONITOR CLIMATE CHANGE characteristics and generally clear skies, and where possible, observations of the Sun, Observation Satellites) Moon, and stars, are useful for characterizing calibration drifts of VIS and NIR Options The CCPR, recalling Resolution 4 of the 21 st General Conference on Weights and Measures (1999) instruments. If appropriately calibrated from benchmark instruments in space these can concerning the need to use SI units in studies of Earth resources, the environment, human well–being and • Provide link to SI via a sub-set of instruments flown to be used as reference standards. related issues, • 1/ All EO measurement systems should be verified simultaneously view same target as satellite (b) Space-based benchmark observations, with the required accuracy, spectral coverage Considering the increasing importance of optical radiation based measurements from ground, air and space traceable to SI units for all appropriate measurands. - high altitude aircraft, Balloon, Rocket, Shuttle, ISS and resolution and traceable to international standards as “gold” standards for validation which support research into the understanding of the causes and impacts of climate change; - degradation/outgassing, multiple targets, range of parameters and inter-calibration of other satellite sensors. ® 2/ Pre-launch calibration should be performed using Considering the cooperation between the World Meteorological Organisation, the BIPM and the CCPR, • Vicarious calibration via calibrated reference targets (c) Permanent reference sites and dedicated campaigns to collect in situ measurements relating to the metrological needs of the WMO; equipment and techniques that can be demonstrably - Desserts, Moon, Stars, Snow fields of the state of the surface and atmosphere. All instruments used for in-situ Considering the difficulty of demonstrating and maintaining traceability to the SI in the space environment measurements should be calibrated and traceable to SI standards. - Maintenance/establishment of radiometric accuracy traceable to and consistent with the SI system of units, and because the levels of accuracy needed are often more demanding than those needed to satisfy current • Designation of one or group of instruments as “reference (d ) Satellite inter-calibration from simultaneous and collocated observations: and traceability should be maintained throughout the industrial requirements; - Rely on mission overlap for traceability continuity - Simultaneous observations from collocations between a LEO and all GEO sensors lifetime of the mission. Considering the particular need for space-based experiments to be traceable to SI units and the difficulty of - Which one ? Improved calibration/reliability ? Degradation have also been demonstrated and can be used as a means to inter-calibrate GEO obtaining a calibration during the operational phase of a mission ; satellites. Conversely, an instrument with high accuracy, precision and stability in GEO • Dedicated International Calibration mission – “Std Lab in-orbit” Traceability – Property of the result of a measurement or the value of a Strongly recommends relevant bodies to take steps to ensure that all measurements used to make observations orbit can be used as a means to inter-calibrate all LEO sensors; which may be used for climate studies are made fully traceable to SI units; standard whereby it can be related to stated references, usually through - Optimised calibration system, international agreement, long-term an unbroken chain of comparisons all having stated uncertainties reliability, mimic terrestrial systems, (could additionally do science!!) And further recommends appropriate funding bodies to support the development of techniques which can vicariously calibrating broader band radiometers. make possible a set of SI-traceable radiometric standards and instruments to allow such traceability to be - Cost, Degradation? Traceability? Accuracy? “Vocabulary for International Metrology (VIM) ISO” established in space. -From CEOS strategy document for GEOSS (2005) - Transference to other missions Collocated high spectral resolutions observations are important for validating and -
Radiometric traceability Cryogenic Radiometry SI ~0. 01 % Spectral Responsivity ~0. 5 % Spectral radiometry Appearance Solar ~0. 1 % Pyrometry Photometry Remote Sensing Lighting Transport Aerospace Medicine Industry Environment
Traceability for Optical radiation measurements Fundamental constants (SI) Primary standard cryogenic radiometer Spectral Radiance/Irradiance calibrations LAND OCEAN ATMOSPHERE
Electrical Substitution Radiometry a 100 yr old technology When thermometer temperature T=To=TE then Po=PE Optical power =Po Absorbing black coating Thermal shroud Optical power =Po Electrical Heater Power = PE Copper disk When T =To=TE then Po=PE Mechanical cryogenic cooler “Fridge” (T = 20 K) Shutter Cooling improves sensitivity by 1000 X Absorbing cavity (~ 0. 99999) Electrical Heater Power = PE Principle of Cryogenic radiometry
Cryogenic Radiometry – international agreement and consistency • High diffusivity - potential of large cavity, (high absorbtance) - rapid isothermal conditions - controlled heat flow paths • Superconductive leads - no joule heating loss High sensitivity thermometry • Stable thermal environment - low external load (background) - low cavity radiative loss Accuracy to SI <~0. 01 %
Fundamental constants (SI) GERB Detector Satellite Pre-flight Primary standard cryogenic radiometer Calibration Traceability ? ? Laser Cal interval ~100 nm Photodiode (spectral responsivity or SIRCUS Laser Cal interval ~0. 1 nm Filter Radiometer Radiance Temperature Ultra High Temperature Black Body (3500 K) Radiance continuum (Planck) Spectroradiometer (multi-band filter radiometer Spectral Radiance/Irradiance calibrations Satellite In-flight Calibration Lamp Solar illuminated Diffuser Geostationary Earth Radiation Budget (GERB) Vicarious Spectral radiance using filter radiometer with plancks law allows Meteosat SG) of (in-flight on-board determination T of Ultra high temp blackbody (~3200 K) Atmosphere/ Spectral Responsivity calibrations - Plancks law then predicts spectral Spectral response of filter radiometer. Model determined - Transferenceall radiance for of reference detector calibration over full bandwidth using tuneable lasers to each GERB Pixel Comparing response of reference detector to that - Detector array (256 elements ~ 50 m Sq) each of filter radiometer pixel cal from 300 nm to 20 m at NPLData products – uncertainty in spectral radiance ~ 0. 02% LAND OCEAN ATMOSPHERE
TRUTHS: Traceable Radiometry Underpinning Terrestrial- and Helio- Studies Satellite based mission to: • make SI traceable high accuracy measurements of solar radiation incident on, and reflected from, the Earth • transfer its unprecedented calibration accuracy to other satellite-based EO instruments through the calibration of reference targets such as the Sun, Moon and the Earth’s deserts • Supporting measurements of land processes, ocean colour, Earth radiation budget, atmospheric chemistry and aerosol distribution - Wide spectrum (380 to 2500 nm) baseline - Spatial resolution ~ 25 m (multi-angle) - Spectral radiance uncertainty <0. 5% (using novel in-flight calibration system)
Geophysical parameters measured by TRUTHS (baseline) Measurand Spectral resolution Spatial resolution Accuracy nm Total Solar Irradiance Total m - % 0. 01 Solar Spectral Irradiance 200 – 2500 (0. 5 - 1) Lunar Spectral Irradiance and Radiance 380 – 2500 (10) - 0. 1 - <0. 5 Earth Spectral Radiance 380 – 2500 (Polarised and Non-pol) (10) multi-angle ~ 25 (20 x 20 km) <0. 5 via filter rads TBD 20 km (TBD) <0. 5 for Aerosols / E Rad Bud Optional orbit for consideration Observing conditions (near polar 700 km) Oblique angle away from pole: Solar viewing - ~ 10 mins per orbit - fewer repeats - more satellite coincidences Earth viewing ~ 10 sites (20 * 20 km) at 5 angles per orbit
TRUTHS Traceability 0. 001 % 0. 005 % Cryogenic Solar Absolute Radiometer 0. 05 % 0. 02 % CSAR High sensitivity cavity Provides: 0. 03 % 0. 1 % Polarised Using monochromator dispersed solar Use calibrated • Measure of TSIrads filterradn ~ 10 nm bandwidth beam power • Primary standard for maintenance Earth / as input to 0. 2 % 0. 05 % of SI traceability atmosphere To calibrate photodiode (working std) solar spec As on. Imager ~0. 3 % ground irradiance monitor correction 0. 1 photodiode never exposed to Calibration drift, spectral and gain, n. b. % removed by performing calibrations in Sun/Earth space directly against a primary Option of broad band Filter radiometers standard using terrestrial • Earth Radiation budget UV to IR methodologies adapted for space. • TIR channels ~0. 3 %
Instrument integration on Truths satellite (baseline, other than calibration system all are TBD) SSIM RPs TASS CSAR FRTW CSAR SCM Payload: Instrument mounting plate PMO SCM SSIM Mass = 130 kg Power = 185 W FRTW 1 m PMO Cooler EI Diffuser EI 0. 8 m Solar: Cryogenic Solar Absolute Radiometer CSAR - TSI , Primary standard WRC PMO ambient temperature radiometers PMO - TSI Solar Spectral Irradiance Monitor SSIM - SSI Earth: Earth Imager (spectrometer) EI - Spectral radiance Polarised Filter Radiometers PFR - Polarised spectral radiance
TRUTHS Earth Imager Hyperspectral and high spatial to simplify matching to other sensors VNIR Primary mirror SWIR Artist’s impression of TRUTHS EI * Prism based spectrometer * 212 channels nominal 10 nm bandwidth (1 to 8 nm) * 200 mm diameter primary mirror * 380 to 2400 nm * 20 m ground resolution * Data rate ~ 1 Gbyte/second Design based on upgrade of planned ESA / APEX aircraft spectrometer 4 independent filter radiometers measure s and p polarisation for atmospheric correction and to monitor TRUTHS EI.
Spectral Calibration Monochromator (SCM) • Three separate double grating monochromators stacked and driven by a common drive shaft. • Wavelength calibration via laser diode at input - Higher power for irradiance calibration * Use of 3 separate fibre delivery systems allows throughput to be maximised. * Transmitted power calculated using realistic commercial fibre, mirror and grating specifications, now lab tested.
Solar Spectral Irradiance Monitor (SSIM) (could be SIM of SORCE) Spectral range: 200 to 2500 nm Spectral resolution: 0. 5 nm 200 – 1000 nm 1. 0 nm 1000 – 2500 nm Dynamic range: 0. 001 – 5 Wm-2 nm – 1 Temporal resolution: Variable Accuracy: 0. 1 % Two single grating spectrometers - Each utilising two “orders” via a beam splitter and two linear arrays Solar input via a common integrating sphere diffuser and precision aperture
Transfer of calibration to global EO missions • Establishment of reference data for Sun and Moon. ® In-orbit Comparison of solar viewing instruments e. g SORCE. -Link to VIRGO of SOHO. ® Establishment of network of Earth based Reference test sites. -E. g Railroad Valley, Libyan Desert, Antarctica etc -Sites to be characterised by field studies -Instrumented with remotely controllable/readable monitors -Calibration coefficients updated regularly by TRUTHS satellite -Data accessible over WWW to allow reprocessing to suit individual satellite footprints and spectral characteristics - Improve accuracy of all sensors but particularly those with no on-board e. g. MSG, DCM … and a reference for NPOESS etc DATA GAPS! ® Archived data reprocessable to improve historical reference. -Many in-flight sensors have the resolution, dynamic range and stability to allow update of calibration and viewed same desert targets. Targeted Science: Surface BRDF, Carbon cycle, atmosphere, coastal zones ….
Summary TRUTHS “in-flight calibration laboratory” removes uncertainty due to storage, launch and degradation and its mission provides this benefit, together with SI traceability, to all other EO optical sensors. • Set of SI traceable reference targets: Sun, Moon, network of ground sites • Utilises terrestrially implemented techniques and technology - In-flight calibration concept applicable to other missions • Order of magnitude improvement in measurement accuracy • Baseline for detection of climate change – reduce need for overlapping data sets • Quality Assure data used by ‘decision makers’ and improve synergy between sensors • Tools to underpin GMES and GEOSS initiative • Identify the polluters • Improved algorithms to allow quantitative measurement of bio-physical products • Provide data to improve understanding of natural solar induced variation on climate and compare with anthropogenic effects. The step change reduction in uncertainty and spin-off benefits is analogous to that obtained in NMIs when cryogenic radiometers were introduced in
TRUTHS : - Status • Proposed to EEOP (2002) - Not selected although received significant interest – reviewers conclusion: – “If the aim is merely to provide accurate, calibrated measurements of Earths spectral radiances and of solar and lunar irradiance, the mission can be classified as a solution. To the best of the reviewers’ knowledge, there is presently no strong need for absolutely accurate Earth spectral radiances since other errors dominate the radiometric error budgets of planned missions. ” ? ? ? – Models and atmospheric correction can only improve with better data to constrain them and test improvements - Mission seeks to provide solutions to wide range of EO communities Atmosphere Solar Land Ocean Should be benefit – seen as a weakness Media interest Broad support from UK govt departments (looking at funding mechanisms!) International collaborations seen as essential
Status 2 Costs: Estimates by EU industrial team 5 yr mission operations, Satellite, Launch - ~$40 M (SSTL) In-flight calibration system including CSAR and TSI measurements - ~$12 M Hyperspectral imager + solar spec irradiance - ~$12 M - Plan to start designing and building operational engineering model of CSAR from Apr 2007 in collaboration with WRC PMOD (Swiss) -Awaiting Decision by EU on New Metrology funding programme - potential to fund flight Calibration system Need study: - to optimise observation / mission requirements - identify operational instruments/suppliers/partners FOR SUCCESS MISSION SHOULD BE INTERNATIONAL Perhaps developed under CEOS? GEO?
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