A54D-01 INVITED
Achieving Satellite Instrument Calibration for Monitoring Global Climate Change
For the most part, satellite observations of climate are not sufficiently accurate to establish a climate record that is indisputable and hence capable of determining at what rate the climate is changing. Furthermore, they are insufficient for establishing a baseline for testing long term trend predictions of climate models. The reasons for this state of affairs are many, but can be summed up in one all-encompassing statement: NOAA's operational satellite systems have been focused on short term weather observations, and NASA's research satellite systems have not had the long term continuity needed for monitoring climate change. As a result, highly accurate observations of decadal scale climate trends are generally lacking. The Workshop on Achieving Satellite Instrument Calibration for Climate Change (ASIC3) was organized to discuss the scientific issues involved and develop recommendations to improve the situation. The Workshop brought together experts in satellite instrument calibration, metrology scientists from the U.S. and U.K. national standards institutes, remote sensing specialists, and climate data analysts. The Workshop recommended a set of satellite benchmark missions to create irrefutable records traceable to International Standards (SI) and to calibrate other satellite sensors. This recommendation also includes a call to maintain continuous, overlapping missions for sea level, solar irradiance, and Earth radiation budget. The second overarching recommendation calls for establishing a U.S. National Center for Calibration (NCC). The NCC would bring together NOAA's expertise in operational missions and calibration/intercalibration of operational instruments, NIST's leadership in measurement science and standards, and NASA's capabilities in research missions and advanced calibration techniques.
A54D-02 INVITED
The role of climate benchmark records in climate-change attribution and projection
The detection, attribution, and projection of climate change present significant challenges to the climate sciences community. This presentation explores the potential utility of SI-traceable climate measurements from space in these three critical areas of research. The discussion is framed in terms of the current assessment reports by the Intergovernmental Panel on Climate Change (IPCC). Highly accurate decadal-length climate records of temperature, humidity, and the spatial gradients in these fields would represent a significant advance in detection and attribution activities. It is also increasingly urgent to quantify feedbacks from reductions in sea ice and snow and to detect feedbacks in the climate system from changes in cloud properties. Observations regarding the sign and magnitude of cloud feedbacks would help determine which climate models could produce the most realistic projections of future climate change. Finally, climate benchmark records would be particularly useful for detecting abrupt shifts in the climate regime despite inevitable discontinuities in satellite observing systems.
A54D-03
Advances in Data Reported SI
Recognizing that higher demands would be placed on remote sensing, the report on High Accuracy Space Based Calibration Requirements, 1998 (Ref.1) and the article on Traceable to SI Units (Ref. 2), laid the ground work for results to be traced to the International System of Units. Climate Data Records demand high accuracy measurements to detect small signal variations in long time series observations. To do so mandates calibration source and remote sensor data traceability to SI. Improved remote sensing nomenclature adopted by the National Institute of Standards and Technology (NIST) clarifies the meaning of physical units and uncertainty in measurement. The need to report results in the International System of Units is being accepted by the remote community as essential for the results to have meaning scientist to scientist and program to program. The statistical processes that support calibration data interpretation are evolving. Certain processes and procedures should be followed especially for remote sensor calibration and subsequent data reduction and analysis. As progress is being made in these areas then data items in principle are becoming better known within a stated uncertainty relative to the internationally adopted base units. We will discuss a complete package; nomenclature, processes and procedures, that gives full meaning to data reported as being SI. Remote sensing results will cease being misinterpreted frequently when the community has applied the complete package. 1. Murdock, T. L. & D. B. Pollock, High Accuracy Space-based Remote Sensing Calibration Requirements, National Institute of Standards and Technology, NIST GCR 98-748, March 1998. 2. Pollock, D. B., T. L. Murdock, R. U. Datla, A, Thompson, Traceable to SI Units, International Journal of Remote Sensing, Vol 24, Number 2, p225-236, 2003.
A54D-04
Experimental and Metrological Basis for SI-Traceable Infrared Radiance Measurements From Space
In order to establish a climate benchmark record and to be useful in interdecadal climate forecast testing, satellite measurements of high spectral resolution infrared radiance must have uncertainty estimates that can be proven beyond a doubt. An uncertainty in radiance of about 1 part in 1000 is required for climate applications. This can be accomplished by appealing to the best measurement practices of the metrology community. The International System of Units (SI) are linked to fundamental physical properties of matter, and can be realized anywhere in the world without bias. By doing so, one can make an accurate observation to within a specified uncertainty. Achieving SI-traceable radiance measurements from space is a novel requirement, and requires specialized sensor design and a disciplined experimental approach. Infrared remote sensing satellite instruments typically employ blackbody calibration targets, which are tied to the SI through Planck's law and the definition of the Kelvin. The blackbody temperature and emissivity, however, must be determined accurately on- orbit, in order for the blackbody emission scale to be SI-traceable. We outline a methodology of instrument design, pre-flight calibration and on-orbit diagnostics for realizing SI- traceable infrared radiance measurements. This instrument is intended as a component of the Climate Absolute Radiance and Refractivity Earth Observatory (CLARREO), a high priority recommendation of the National Research Council decadal survey. Calibration blackbodies for remote sensing differ from a perfect Planckian blackbody; thus the component uncertainties must be evaluated in order to confer traceability. We have performed traceability experiments in the laboratory to verify blackbody temperature, emissivity and the end-to-end radiance scale. We discuss the design of the Harvard standard blackbody and an intercomparison campaign that will be conducted with the GIFTS blackbody (University of Wisconsin, Madison) and radiometric calibration facilities at NIST. The GIFTS blackbody is a high-performance space-qualified design with a new generation of on-orbit thermometer calibration via miniaturized fixed point cells. NIST facilities allow the step-by-step measurement of blackbody surface properties, thermal properties, on-axis emissivity, and end-to-end radiometric performance. These activities will lay the experimental groundwork for achieving SI-traceable infrared radiance measurements on a satellite instrument.
A54D-05
A New Approach For Absolute Temperature Calibration: Application to the CLARREO Mission
A novel scheme to provide on-orbit absolute calibration of blackbody temperature sensors (on-demand) has been demonstrated using a copy of the engineering model version of a space flight hardware blackbody design (GIFTS). The scheme uses the phase change signature of reference materials to assign an absolute temperatures scale to the blackbody sensors over a large temperature range. Uncertainties of better than 0.020 K have been demonstrated over the temperature range from 234 to 303 K. Thermal modeling has been conducted to optimize the design, and to show that accuracies comparable to those measured in the laboratory should be obtainable in the less-controlled on-orbit temperature environment. The implementation if this scheme is very attractive due to its simplicity and relatively low mass. In addition, all aspects of the electronics (control and temperature readout) needed to support this scheme have been developed and demonstrated in the as-delivered GIFTS Engineering Model blackbody calibration system developed by the University of Wisconsin. NASA's anticipated plan for a mission dedicated to Climate (CLARREO) will hinge upon the ability to fly absolute standards that can provide the basis to meet stringent requirements on measurement accuracy. For example, instrumentation designed to measure spectrally resolved infrared radiances will require high-emissivity calibration blackbodies having absolute temperature uncertainties of better than 0.020 K (3 sigma). The novel blackbody temperature calibration scheme described here is very well suited for the CLARREO mission because if its low mass, high accuracy, and ease of implementation into a demonstrated flight blackbody design.
A54D-06
SI Traceable Stars
Astronomy, satellite, and defense programs require accurate SI traceable standards in space for calibrating telescope-based sensors, assessing their accuracy, and ensuring that their measurements can be compared with other sensors. For satellites, including space-based telescopes, the lack of such standards makes it difficult to ensure that the pre-launch calibration is maintained following exposure of the sensor to launch vibration, high-energy radiation, and out-gassed vapor condensation. Stars have been used for calibration; however star catalogs show discrepancies from 2~% to 5~%, even though approximately 50~% of stars have photometric stabilities greater than 0.5~%. Historically, absolute stellar calibration has been challenged by the lack of available optical standards that closely mimic stellar spectral and angular distributions, of measurement methods to use such standards, and of knowledge of the temporally variable transmittance of the atmosphere. In this talk, we will address these deficiencies with the goal of establishing a suite of SI traceable stars for optical sensor calibration with a radiometric uncertainty of 0.5~% over the spectral range from 380~nm to 2500~nm.
A54D-07 INVITED
TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies): A Mission to Achieve "Climate Quality" Data
Over recent years the debate as to whether climate change is real has largely subsided, however there is still significant controversy over its cause and most importantly the scale of its impact and means of mitigation. Much of the latter relies upon the predictive capabilities of sophisticated, but highly complex models. Such models need globally sampled measurements of a variety of key indicative physical parameters, and in some cases proxies of others, as input data and whilst generally predicting similar things the detail of their outputs in the decadal time scales can be highly variable. Clearly the quality of the input data is crucial to such models. However, since the key indicators of climate change may only vary by a few percent per decade, the absolute accuracy of such data also needs to be very small to allow detection and provide some means of validating/discriminating and improving the models. At the present time, the accuracy of currently measured data from space is rarely, if ever, adequate to meet this requirement. Instead, high risk strategies are developed which rely upon overlapping and renormalizing data sets from consecutive flights of similar instruments to establish a long-term trend. Such a strategy is doomed to failure!. The only means of achieving robust data sets of sufficient quality and accuracy with a guarantee of long term reproducibility sufficient to detect the subtle indicators of climate change and its cause (anthropogenic from natural) is through traceability to SI units. Such traceability needs to be regularly re-established and guaranteed throughout the lifetime of a mission. However, given that most sensors degrade in performance during launch, and most importantly whilst in orbit, this is difficult to achieve with sufficient accuracy, since such sensors cannot easily be retrieved and taken back to a national standards laboratory for recalibration. TRUTHS (Traceable Radiometry Underpinning Terrestrial- and Helio- Studies) is a mission proposal which allows for the first time, direct regular traceability to SI of all optical radiometric quantities: incident solar irradiance (total and spectrally resolved) and reflected solar spectral radiance from the Earth (or Moon) in-orbit with uncertainties small enough to meet the needs of climate. The TRUTHS satellite is in effect a "standards laboratory in orbit" providing regular calibration of its on-board instrumentation using a methodology mimicking that used on the ground directly against an on board primary standard cryogenic radiometer. In addition to making its own measurements of the Earth and Sun its primary objective is to transfer its high accuracy and tracebility to all other in-flight optical EO sensors, through near simultaneous observation of the same targets, e.g. moon, Earth deserts, snow fields etc. In this way providing a means to establish truly global benchmark measurements from all sensors directly traceable to SI both now and in the future. The use of a flexible moving platform facilitates the matching of view angle of different sensors and the use of high spectral and spatial resolution the ability to simulate footprints and spectral bands.
A54D-08
Ionospheric Error Contribution To GPS Radio Occultation Temperature Retrievals
The NRC Decadal Survey calls for long-term climate records. GPS radio occultation (GPS RO) is viewed as a promising measurement technique that fulfills the need for high accuracy measurements of upper atmospheric temperature and pressure with robust systematic error bounds known on orbit. Geophysical observables are derived by measuring propagation delay induced by the atmosphere, a measurement whose fundamental unit—the second—is absolutely determined by calibration against atomic clocks. Agreement between collocated pairs of observations obtained from the COSMIC GPS RO constellation confirm the method is very precise and inter-satellite retrieval biases are less than 0.1 K. Close agreement between nearly collocated measurements does not directly establish an upper bound for all sources of systematic error that may affect individual retrievals. An important example arises from the ionospheric contribution to signal delay that is largely common to pairs of collocated soundings. Ionospheric impact on GPS RO temperature retrievals must be thoroughly understood before on-orbit SI-traceability can be established at the desired accuracy level (approximately 0.1 K or better). We present results of a realistic simulation study to determine the impact of large-scale ionospheric structure on geophysical retrieval error. GPS transmits signals at two frequencies to compensate for ionospheric error. The two signals do not follow identical paths within the ionosphere possibly leading to incomplete calibration of ionospheric delays and a biased atmospheric retrieval. We use a three-dimensional ray-tracing code that computes the paths of GPS signals through the ionosphere to estimate this impact of the ionosphere. Resultant GPS phase and amplitude data are input to the GPS Occultation Analysis System (GOAS) at JPL to retrieve the geophysical quantities of interest. A new feature of this approach is using JPL's Global Assimilative Ionosphere Model (GAIM) to provide realistic electron density profiles for the ray-tracing calculation. GAIM is a new generation of ionospheric "weather model" that uses data from an extensive measurement network of ground-based GPS receivers to produce realistic electron density maps over a wide range of ionospheric conditions, from quiescent to disturbed. Simulation results and estimates of temperature error are presented for a variety of geophysical conditions such as quiet, geomagnetically disturbed, and relatively quiet periods of enhanced density levels. We conclude with a discussion of implications for on-orbit SI-traceability.
A54D-09
Review of NIST Capabilities for Improving Calibration and Validation of Climate Sensing Systems
During the past two decades, a few key developments have emerged at NIST and other national measurement institutes that are destined to have a positive fundamental impact on the ability to improve the future SI traceable climate record. These began with the establishment of absolute cryogenic electrical substitution radiometry to provide highly accurate scales, followed by the development of practical methods to propagate the scales to the types of spectral radiance and irradiance measurements that pervade the remote sensing of climate change. This presentation will review these developments and consider their impact on calibration and validation of future climate sensor systems. We discuss how the radiant power scale is established at NIST through cryogenic radiometers such as the Primary Optical Watt Radiometer (POWR), and how NIST propagates this scale to the other radiometric scales at the Spectral Irradiance and Radiance Calibrations with Uniform Sources (SIRCUS) facility. Some examples will be discussed where calibration issues on specific remote sensing instruments were resolved by comparison with these scales. Also, the importance of independently arriving at radiometric scales through completely different physical means will be highlighted, for the case of spectral radiance, by discussing the comparison of electrical substitution-based scales to blackbody-based scales. This is particularly important to the problem of verifying absolute uncertainty claims of 0.1 K, 3 sigma, in the spectrally- resolved thermal-infrared radiance. Finally, we discuss the recent development at NIST of a Hyperspectral Image Projector (HIP), which propagates electrical-substitution-based scales to complex, realistic spectra and even to realistic hyperspectral spatial scenes, and its proposed application to climate sensor calibration and validation.
A54D-10
Sampling Errors of Climate Monitoring Constellations
Satellites need to observe the earth with great accuracy to capture the long-term trends in climate. As part of an effort to design a constellation of low earth orbiting satellites to benchmark climate observations, we explore the impact of imperfect sampling of satellites on the effort to monitor mean radiance on a variety of spatial and temporal scales. Our aims are (1) to find those orbits which provide an accuracy of at least 0.1 K in brightness temperatures at different temporal and spatial resolutions, and (2) to look for alternate ways of calibrating existing and future satellites like NPOESS. The 0.1 K level of accuracy in brightness temperature was chosen to agree with the expected magnitude of decadal trends in temperature forced by changes in greenhouse gas concentration. Sampling studies carried out at different frequencies representative of the lower, middle and upper troposphere will be presented here. Model generated radiances rather than observed radiances are used for this study, since a good representation of diurnal variability in the original data is essential; polar orbiters don't have enough observation times, and geostationary orbiters don't cover enough of the world, and have problems because of high observing angles. The brightness temperatures are obtained by using MODTRAN on the archived simulations from the GFDL coupled model. These are then sub-sampled along the paths traversed by the satellite footprint for various potential orbits at different inclinations. Maps of retrieval accuracy for monthly mean, seasonal mean and annual mean radiance will be presented for single satellites and for a constellation of satellites. Also, the temporal and spatial distribution of simultaneous nadir overpasses for various satellite configurations will be examined to inspect the expected effectiveness for cross calibrating other existing and future satellites. Results for the annual mean indicate that: A single satellite in a precessing orbit can achieve sampling errors in 15 degree grid boxes less than 0.1 K for brightness temperatures in the spectral regions that mostly sample the upper troposphere and lower stratosphere; In the mid-troposphere channels and in the window channel, a single precessing orbiter requires zonal averaging to reliably attain errors of less than 0.1 K; Since the primary source of sampling bias arises due to inadequate sampling of the diurnal and semidiurnal cycle, a constellation of satellites surely reduces the errors considerably and combinations with precessing polar orbits normally fare better than their sun-synchronous counterparts.