The Space Metrology Program David B. Pollock*a , Alexander Panfilov and Inessa Glazkova b, Thomas Humpherys and Victor Privalsky c, Victor Sapritsky, Svetlana Morozova, and Boris Khlevnoy d, Raju U. Datla e, Victor Misnik and Valery Sinelschikov f a
The University of Alabama in Huntsville, 301 Sparkman Drive, OB 422, Huntsville, Alabama 35899, U.S.A. The M.V. Khrunichev State Space Research and Production Center, Novozavodskaya 18, Moscow 121087, Russia c Space Dynamics Laboratory / Utah State University, 1695 N Research Pkwy, No. Logan, Utah 84341, U.S.A. d The All-Russian Institute for Optophysical Measurements, Ozernaya 40, Moscow 119361, Russia e National Institute of Standards and Technology, 100 Bureau Drive, MS 8441, 20899-8441Gaithersburg, MD, U.S.A. f NPO Kometa, 5 Velozavodskaya ul., Moscow, Moscow, 115280, Russian Federation b
ABSTRACT The full potential of current remote sensor technology is limited by the inability to correct biases once an exoatmospheric remote sensor becomes operational. Even when the calibration is traced to the International System of Units, SI, and the instrument is performing within the operational envelope wherein it is calibrated, the problem exists and a Space Metrology Program is a potential solution to the problem. This paper discusses such a program, suggests a feasibility study to address the issues and recommends a plan of action. Any operational instrument has a bias and reducing the magnitude of the bias can only be accomplished when an adequately accurate standard is accessible by the instrument while the instrument is in its operational environment. Currently the radiometric flux from the sun, the moon and the stars is inadequately accurate SI to provide a standard that is consistent with the remote sensor state-of-the-art technology. The result is data that is less accurate than it could be often leading to confusing and conflicting conclusions drawn from that data. Planned remote sensors such as those required to meet future program needs (e.g. the United States National Polar-Orbiting Operational Environmental Satellite System (NPOESS) and the proposed international Global Earth Observation Program) are going to need the higher accuracy radiometric standards to maintain their accuracy once they become operational. To resolve the problem, a set of standard radiometers on the International Space Station is suggested against which other exo-atmospheric radiometric instruments can be calibrated. A feasibility study for this program is planned. Keywords: spaceborne earth observations, radiometry, calibration
*
[email protected]; telephone (256) 824-2514; facsimile (256) 824-6618; http://www.eb.uah.edu/ece/faculty/Pollock/index.htm
Accuracy SI C 1.0 0.9 0.8 0.7
Frequency
1. THE PROBLEM Simply stated there is a broadly based problem with remote sensor datai,ii. Correlated errors begin to appear as operational remote sensors age introducing a bias into their output data. The problem exists because the total uncertainty traced to the International System of Units, abbreviated SI, of existing remote sensor radiometric standards, σT, is greater than the relative measurement uncertainty of the sensors, σM, see Figure 1, making it difficult to identify and correct for the correlated errors. The total uncertainty SI, i.e. the probability with which the data is true, is obtained from the root-sum-square of σT, σM, and C. The measurement uncertainty shown in Figure 1 is four times greater than the uncertainty of a calibration source resulting in the probability the data is true being dominated by calibration source uncertainty. One would reasonably expect a calibration source to have an uncertainty that is less than that of the instrument being calibrated. That is simply not the case for many modern sensors.
σΜ
σΤ
0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.6
0.7
0.8
0.9
1.0
1.1
1.2
S. I. Truth
Figure 1 The uncertainty of the calibration truth is four times the measurement uncertainty.
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This problem is particularly acute for those operational sensors in-orbit where recovery and re-calibration on the ground is either impractical or impossible. Reducing the total uncertainty of the respective solar, lunar and stellar spectral flux traced to the International System of Units is required. Among the sensors affected are those used for the U. S. Global Climate Change Research Program (U.S. GCRP), the sensors of National Aeronautics and Space Administration, National Ocean and Atmospheric Administration, Tennessee Valley Authority, Department of. Defense, Department of Energy, Health and Human Services, National Science Foundation, United States Department of Agriculture, Department of the Interior and Environmental Pollution Agency. The scale of the problem can be appreciated by reference to Table 1 where the calibration sources are seen to be 2 to 15 times more uncertain than current sensor technology. Physical Characteristic Solar Total Irradiance
Current Technology
Calibration Sources
Objective
0.05 - 0.1 %
0.15 - 0.2 %
0.01%
Solar Spectral Irradiance
0.5 - 1 %
1-5%
0.01%
Lunar spectral irradiance
0.5 - 1%
6 – 15%
0.10%
Lunar Spectral Radiance / phase model
0.5 - 1 %
2-5%
0.12%
Earth Reflected Radiance
0.5 - 2 %
2-5%
0.20%
Stellar Spectral Irradiance
0.5 - 2 %
2 - 10 %
2%
Table 1 The scope of the uncertainty problem.
An informal program to solve this fundamental problem began about seven years ago as the result of a Workshop at the National Institute of Standards and Technology in the fall of 1997iii that was held to identify the requirements for High Accuracy Space Based Remote Sensors. Since then there has been an expanding recognition, discussion and realization that the magnitude of the uncertainty for the physical characteristics listed by Table 1 is greater than it was initially thought to be. Presentations and discussions at Nationaliv and International Conferencesv and Workshopsvi,vii have contributed to understanding the scope of the problem and to the initiation of efforts to find a solution. A recent Workshopviii held in November 2002 addressed remote sensor calibration requirements to measure global climate change. A radiometer, SORCEix, launched January 25, 2003 is providing precise measurements of the total solar flux radiation. Independent efforts such as these contribute to a comprehensive plan that should have adequate standing in the science community to get the recognition essential to fund the various plan activities that will show progress in reducing remote sensor calibration source uncertainty SI. Study shows that remote sensor data relative standard uncertainty is one-quarter to one-fifth the total uncertainty SI for either the spectral or the total radiant flux from the moon, the stars and the suni. The consequence is data uncertainty continues to be large because there are inadequately uncertain calibration sources available to remove the remote sensor correlated errors that arise during operations. Even beyond that the data available to analysts to reduce the uncertainty of these calibration sources is bound by the uncertainty induced from the limited source calibration data. The stellar calibrations are traced to SI units through measurements of a single spectral line at 5556 ∆ from a ground based observatoryx supported by theory, models and analyses. The traceable path for the lunar calibrationsxi is through the reflectance of lunar rocks, ground based observations of Vega, an FEL calibration lamp, atmospheric corrections, and analyses. The total solar flux path is traced through one time calibrations prior to launch. Vicarious terrestrial calibrations are traced through on-site observations, atmospheric corrections, the solar flux, and analyses. An example of a glaring deficiency with the Lunar Flux as a radiometric standard is that a recognized, accepted Lunar Flux model created by the Remote Optical Lunar Observatory, ROLO, Program of the United States Geological Survey over a number of years although stable to better than 0.1% shows a significant wavelength dependent uncertainty on an absolute scale thought to be on the order of 5 – 15% SI. Differences up to 6% are found when the model predictions are compared to satellite instrument measurementsxii,xiii. An effort to reduce this uncertainty SI over the ROLO 300 to 2,300 nm spectral region is in progressxiv,xv,xvi.
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2. A SPACE METROLOGY PLAN The informal program described by the efforts in the preceding paragraphs serve to illustrate the scope of the activities necessary for a more formal Space Metrology Program to address the total task, an update of the flux uncertainty SI at the top of the atmosphere for stars, the sun, the moon as well as terrestrial sources. Seven years have passed since the initial realization that there is a problemiii. It will take additional years to design, build and calibrate – re-calibrate the transfer standards, gather the field data and analyze the data to reduce each source uncertainty. A coordinated, organized Space Metrology Program under the direction of a central leadership is suggested for the following reasons: 1st, the current remote sensor state of the art is seriously deficient in producing the quality of data the technology permits; 2nd, programs such as NPOESS that will address global geo-physical environmental issues cannot realize their performance requirements because of inadequate exo-atmospheric radiometric standards; 3rd, the results realized from such a program affects the data quality produced by the International Remote Sensing effort, governmental and commercial; 4th, the calendar time and the costs to achieve the program goals are reduced. The goal of such a program should be an uncertainty for each calibration source and the maintenance of that uncertainty traced to the internationally accepted flux scale. It is the radiation uncertainty SI of the existing solar, lunar, stellar and terrestrial calibration sources that needs to be reduced. A means to maintain the reduced uncertainty of each scale is a challenge. The issue being how to correct correlated errors that will arise with a Transfer Radiometer just as they do for any operational remote sensor. This issue can be resolved by requiring that a Transfer Radiometer be retrievable. Central to begin A Space Metrology Program should be a feasibility study with a Program Plan as the output. The study can identify and bound the scope of the effort necessary to improve and maintain the existing scales for each source. Among the feasibility study inputs are the current and future remote sensor spectral, spatial and temporal uncertainty requirements, work of which a significant part has already been doneviii. Those who should participate in the feasibility study are key engineers and scientists chosen from those who design and produce the sensors, use the data and are responsible to maintain National Standards such as VNIIOFI, NPL, NIST. The Space Metrology Program has a single goal, reduce the solar, lunar, stellar, terrestrial respective calibration source uncertainty SI to acceptable levels that will support near term data requirements supported by a carefully thought through means of maintaining and where possible improving those levels in the future. As is evident from an examination of Figure 2xvii, multiple instruments will be required to span the spectral and the irradiance range of these calibration sources.
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Figure 2 Calibration source irradiances at the top of the atmosphere span a significant dynamic range.
3. RECOMMENDED PLAN OF ACTION At risk of stepping on existing metrological organization toes a plan of action is suggested to initiate a more formal Space Metrology Program under the control and guidance of an internationally selected committee. The elements of such a program could be the establishment of; 1. A Central Committee charged with the responsibility to produce a Program Plan that will set priorities and make recommendations to cover the spectral flux range shown by Figure 2; 2. A Feasibility Study team to address the specific technical issues associated with the recommended order of calibrations as set forth by the Program Plan; and, 3. Science teams to execute the Program Plan elements. As with all matters such as this the issue of who is going to pay for the effort to be expended will need to be resolved before any action can occur. 3.1. SPACE METROLOGY COMMITTEE A committee supported by the National Standards organizations can provide the technical leadership needed to establish a Space Metrology Committee and produce a formal Program Plan. The committee members should be well steeped in how National Standards are established and maintained as well has have had hardware related experience with the design, fabrication and operation of remote sensing radiometers in the laboratory and in the field. To establish the committee will take some cooperation between the National Standards organizations that would be called upon to support the work necessary to bring the committee into existence. The committee challenge will be to select and to prioritize spectral regions of Figure 2 and document those selections in a Program Plan. 3.2. FEASIBILITY STUDY Each spectral, irradiance region can be addressed by a feasibility study team to assess the technical issues associated with individual transfer radiometer performance requirements, deployment, operation, and data analysis. The issues include a time frame to reduce the uncertainty for a spectral – flux region, the frequency with which a transfer radiometer should be retrieved and recalibrated, signal dynamic range, quantity of data, spectral range, platform, ambient operating conditions, non-retrievable versus retrievable transfer standard, costs, who should build and operate
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an instrument for each region, data reduction, error analysis, how the results can be validated and finally made available to the broad remote sensing community. 3.2.1. Study inputs Suggested feasibility study team inputs are the current and future remote sensor spectral, spatial and temporal uncertainty requirements, work for which a significant part has already been doneviii. Participants in the study should be key engineers and scientists chosen from those who design and produce the sensors, use the data and those who are responsible to maintain National Standards such as VNIIOFI, NPL, NIST. 3.2.2. Study outputs Feasibility study outputs should address how to establish with data the spectral, spatial and temporal characteristics of each transfer radiometer and how to remove those characteristics from the respective source data. Technical issues include signal dynamic range, quantity of data, spectral range, platform, ambient operating conditions, non-retrievable versus retrievable transfer standard, costs, qualifications to build and to operate which instruments, data reduction, error analysis, time scale of the activities, responsibilities to maintain the uncertainty, how often to retrieve and re-calibrate a transfer radiometer. In summary, how A Space Metrology Program Plan can be implemented in both the near and long term to reduce the uncertainty and to maintain or reduce that level of uncertainty for the sun, the moon, the stars and terrestrial sources.
3.3. Concept Demonstration A demonstration of the technical feasibility to reduce the uncertainty for a limited spectral, spatial and temporal dynamic flux range with a retrievable transfer standard radiometer that would precede an expansion of the feasible concept to include the balance of the spectral irradiance space illustrated by Figure 2 seems to make sense. The deployment of a transfer standard radiometer on the International Space Station where retrieval, re-calibration and redeployment of a transfer standard can be demonstrated is considered to be crucial to the success of the plan. The transfer standard re-calibration could be against a standard such as the High Accuracy Cryogenic Radiometer, HACR, at the National Institute of Standards and Technology. The calibration and use of such a transfer standard to reduce the uncertainty of an exo-atmospheric source and then the retrieval and the re-calibration of the transfer standard supports both of the major elements for A Space Metrology Program, reducing the uncertainty of an existing scale and maintaining that uncertainty. Which source and the spectral range to use for such a demonstration would be a subject for the feasibility study and would be a study output. 4. Conclusions Unless the uncertainty of exo-atmospheric radiometric standards are of adequate quality to justify “… reliance on and need for more powerful sensor systems, with increased performance and functionality”† the utility of their data will remain limited by the uncertainty of their calibration, regardless the sensor stability, and data quality will remain at the current level unless and until adequate radiometric standards are established. Conclusions drawn from inadequate data quality can lead to all sorts of faulty political and economic decisions. The ongoing debate over global warming or cooling is a case in point.
i
Pollock D. B. and Thompson A., 1999, Remote Sensing Accuracy Current State of The Art, Briefing to National Physical Laboratory, 11 p. ii Pollock, D. B., T. L. Murdock, R. U. Datla, A. Thompson, Radiometric Standards in Space, The Next Step, presented at NEWRAD99, 7th International Conference on New Developments and Applications in Optical Radiometry, Departmento de Metrologia, Instituto de Fisica Aplicada (IFA), Consejo Superior de Investigancines CientΡficas, †
National Science Foundation solicitation NSF 04-522, 26 February 2004.
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Madrid, Spain, October 25-27, 1999, and published, International journal of pure and applied metrology, metrologia, Vol 37, Number 5, p403-406, 2000. iii Murdock, T. L., Pollock, D. B., High Accuracy Space Based Remote Sensing Requirements, United States Department of Commerce, National Institute of Standards and Technology, NIST GCR 98-748, March 1998. iv The annual remote sensor radiometric calibration conferences, CALCON, held at Utah State University, Logan, Utah. v NEWRAD99, the 7th International Conference On New Developments And Applications In Optical Radiometry, October 25-27, 1999, Madrid, Spain vi ISPRS, International Society for Photogrammetry and Remote Sensing and American Society for Photogrammetry and Remote Sensing, Dec 2 -5, 2003, Gulfport, MS vii CALCON 2003, 12TH Conference on Characterization and Radiometric Calibration for Remote Sensing, Utah State University, September 15-18, 2003. viii Ohring, G. et al, Satellite Instrument Calibration for Measuring Global Climate Change, Report of a Workshop at the University of Maryland Inn and conference Center, College Park, MD, November 12-14, 2002, United States Department of Commerce, NISTIR 7047, March 2004. ix http://lasp.colorado.edu/sorce/ x Cohen, M., R. G. Walker, M. J. Barlow, J. R. Deacon, Spectral Irradiance Calibration in the infrared. I. Groundbased and IRAS Broadband Calibrations, The Astronomical Journal, Vol. 104, Num. 4, October 1992. This is the first in a comprehensive series of papers that led to S. D. Price, M. Egan, M Cohen, R. G. Walker, D. B. Pollock, A Network of Infrared Calibration Stars, Proceedings of the Space Situational Awareness Symposium, 2001, that contains extensive references for star calibrations. xi Stone, T.C., H. H. Kieffer and J. M. Anderson, Status of Use of Lunar Irradiance for On-orbit Calibration, Earth Observing Systems VI, Proc. SPIE 4483, 165-175, 2002. xii Kieffer, H.H., et al, On-orbit Radiometric Calibration Over Time and Between Spacecraft Using the Moon, Proc. SPIE 4881, 287-298, 2003 xiii Stone, T. C., H. H. Kieffer, Assessment of Uncertainty in ROLO Lunar Irradiance for On-orbit Calibration, Proc. SPIE 5542, paper 5542-41, 2004. xiv Pollock, D. B., T. C. Stone, H. H. Kiefer, J. P. Rice, Reducing the RObotic Lunar Observatory (ROLO) Irradiance Model Uncertainty SI, CALCON 2004, 13TH Conference on Characterization and Radiometric Calibration for Remote Sensing, Utah State University, August 23 - 26, 2004. xv Stone, T. C., H. H. Kieffer, ROLO Capabilities for On-orbit Calibration Using the Moon, CALCON 2004, 13TH Conference on Characterization and Radiometric Calibration for Remote Sensing, Utah State University, August 23 26, 2004. xvi Kieffer, H. H., More than you want to know, CALCON 2004, 13TH Conference on Characterization and Radiometric Calibration for Remote Sensing, Utah State University, August 23 - 26, 2004. xvii Murdock, T. L., private communication, 1998.
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