| Organization and Implementation of Calibration in the EOS project -- Part 1 | James J. Butler (butler@highwire.gsfc.nasa.gov) EOS Calibration Scientist NASA Goddard Space Flight Center
B. Carol Johnson (cjohnson@enh.nist.gov) |
Introduction
The Earth Observing System (EOS) is an international multi-satellite program in global remote sensing of the Earth. The goal of the EOS mission is to advance the scientific understanding of the Earth as a system, i.e., land, oceans, atmosphere, and the influences of natural and anthropogenic processes on this system through the development of a deeper understanding of the components and their interactions within the system. In order to achieve this goal, EOS must produce accurate, precise, and consistent long-time series of radiometric measurement data from multiple instruments and multiple platforms. Understanding and correctly interpreting these data require the ability to determine what portion of the observed signal represents changes in the spectral-flux responsivity of the satellite sensor during the mission. The simultaneous goals of acquiring accurate data over many years and correctly identifying systematic effects depend crucially on: (1) calibrating all instruments against a set of recognized physical standards, (2) carefully characterizing the instruments' performances at the system level, (3) adhering to good measurement practices and established protocols, (4) intercomparing instrument measurements where possible, and (5) establishing traceability for all instruments to the common scale of physical quantities maintained at the national standards laboratories.
This two-part article outlines the overall organization and implementation of calibrations in the EOS project based on requirements that were established in 1989. Part 1 describes the organizational structure of the EOS Calibration Program and its position in the EOS Project Science Office's Panel for Data Quality. Part 1 also describes the implementation of the program with respect to planning, documentation, and peer reviews. Part 2, to be published in a future issue of The Earth Observer, describes the program's pre-flight and on-orbit calibration efforts and outlines their implementation through the National Institute of Standards and Technology (NIST)-supported measurement assurance programs, the United States Geological Survey (USGS)/Northern Arizona University (NAU) lunar radiometric measurement program, and other methods that will be used to ensure the accuracy of the Level 1B data. Where appropriate, examples of on-going calibration programs relevant to the EOS AM-1 instruments are provided.
EOS Calibration Requirements
The requirements for instrument calibration and Level 1B data, i.e., radiance data, validation were outlined early in the EOS project (EOS Level 1A Requirements Document 1989), and the calibration approach summarized in these articles is based on those requirements. The NASA/NIST calibration activities for the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) (see Johnson et al. 1996 a, b and references therein) and the multi-agency, ultraviolet intercomparison experiments (Thompson et al. 1996) have been used as baseline programs for evaluating and meeting the original EOS requirements and in formulating a key portion of the EOS Calibration Program. The NASA/NIST interagency collaboration in EOS calibration is described in part 2 of this article.
Organization of EOS Calibration Program
Figure 1 shows the organization of the EOS Calibration Program. The head of the EOS Project Science Office (EOS/PSO), the EOS Senior Project Scientist, established the position of EOS Calibration Scientist early in the program. In 1994 the EOS Panel for Data Quality (EOS/PDQ) was formed, and formal agreements between the EOS Project Science Office and NIST and the USGS were executed (King 1994). The EOS Calibration Scientist, a member of the EOS Panel for Data Quality, provides technical support and reports directly to the Chairman of this Panel and to the EOS Senior Project Scientist. The EOS Calibration Scientist is responsible for the following:
Also in 1994, the position of EOS Validation Scientist was established. As seen in Figure 1, the EOS Validation Scientist reports to the Chairman of the EOS Panel for Data Quality and to the EOS Senior Project Scientist and is responsible for the promotion of coordinated field experiments, algorithm development, review of data analysis procedures, and communication of validation issues between EOS and other related programs (King 1995).
Each EOS Instrument Team and/or Instrument Manufacturer has identified individuals to represent calibration, validation, and platform issues, so that responsibilities and lines of communication are clearly established and key calibration issues can be addressed by the appropriate calibration personnel. The EOS Instrument Calibration Scientists, representing the Instrument Teams, are the specific points of contact for the EOS Calibration Scientist, the EOS Panel for Data Quality, and ultimately the EOS Project Science Office on matters concerning instrument calibration and instrument Level 1 data validation.
Technical support for the EOS Calibration Scientist is provided by the EOS Calibration Panel. Membership is decided by the EOS Calibration Scientist, but in general individuals or institutions are included because of their expertise, experience, and position in the EOS Calibration Program. The areas of interest of the EOS/CP include pre-flight and on-orbit instrument calibration and characterization, and all ancillary activities associated with estimating the accuracy of the results of the EOS-sponsored measurements. These latter activities are part of the general measurement assurance programs (MAPs) for EOS, described in part 2 of this article.
Implementation of the EOS Calibration Program
Implementation of the EOS Calibration Program occurs through a combination of detailed analyses, scientific reviews, measurement assurance programs, calibration meetings, Level 1B data validation field programs, and completion of necessary ancillary data bases such as the lunar radiometric data base. Close cooperation between the calibration and validation aspects of the program is essential, especially in matters concerning on-orbit validation of instrument Level 1B data.
The implementation of the EOS Calibration Program is accomplished through a number of important, parallel calibration tasks. These tasks are shown in Figure 2 and include the following:
The remainder of this article will examine the first four items. The last four items are described in part 2 of this article.
Planning and Documentation
EOS Calibration Panel
The EOS/CP provides technical support to the EOS Calibration Scientist and to the EOS Project Science Office in the areas of pre-flight and on-orbit calibration, instrument cross-calibration, Level 1B data validation, and long-term Level 1B data stability. Specific tasks of this panel include:
The EOS/CP is scheduled to meet once a year, with the next meeting planned for early 1996. Meetings will be coordinated and chaired by the EOS Calibration Scientist. Meeting results will be reported as articles to The Earth Observer and distributed as formal minutes. Current members of the EOS/CP include the EOS Calibration Scientist, Instrument Calibration Representatives, Instrument Principal Investigators, Level 1B data Validation Instrument Investigators, representatives from instrument manufacturers and NIST, and invited members from the remote sensing calibration community.
CEOS Cal/Val Dossier
Critical information necessary for planning is being electronically gathered in the form of a cal/val database for the CEOS. This effort continues and expands the previous effort of the CEOS Working Group on Calibration and Validation (WGCV) (CEOS Pilot Cal/Val Dossier 1993). Questionnaires on calibration laboratories; remote sensing field sites; and field instruments have been developed by a CEOS Dossier Development Team made up of the Deputy EOS Senior Project Scientist, the EOS Calibration Scientist and the EOS Validation Scientist. These questionnaires have been electronically distributed to key personnel performing calibration and validation in support of remote sensing programs. When completed, this on-line database will be available for use by the CEOS members, the EOS scientists, and the broader international Earth science community. The home page address to access the questionnaires is: http://spso.gsfc. nasa.gov/calval /calval_hpage.html.
Peer Reviews
The critical review of the calibration of EOS instruments is met using two complementary procedures. The first involves carefully evaluating the instrument calibration plans, and the second involves evaluating the theoretical algorithms used to convert instruments' raw digital output, i.e., Level 0 data, to calibrated, geolocated radiance data, i.e. Level 1B data.
Instrument Calibration Reviews
According to the EOS Background Information Package (BIP) Announcement of Opportunity No. OSSA-1-88 Part One: Guideline for Proposal Preparation (1988), a calibration plan is required from each EOS Instrument Investigation Team at the time of instrument proposal. This plan is updated and submitted by each team at the approximate time of the instrument engineering preliminary design review (PDR). At the time of the instrument engineering Critical Design Review (CDR), a final, mature version of the Instrument Calibration Plan is required. The Instrument Calibration Plan describes the approaches that the Instrument Investigation Team will use to produce the Level 1B data (geolocated and calibrated spectral radiances or band-averaged radiances). These approaches include a description of the test program used to calibrate and characterize the instrument before launch and the program(s) used to monitor the calibration and characterization of the instrument after launch. The instrument calibration plan must include an uncertainty budget that describes the performance of the instrument at the time of delivery, and the anticipated performance on orbit. The plan describes how calibration traceability in SI units (Taylor 1995) is established and maintained. In all cases, the method of traceability to the instrument's corresponding national standards laboratory is explained. It is the responsibility of the EOS Instrument Team Leader to deliver the Instrument Calibration Plans to the EOS Project Science Office.
The Instrument Calibration Plans generated at the time of the engineering PDR and the CDR are reviewed by a peer panel of calibration scientists and engineers, consisting of representatives of other instrument calibration teams, the EOS/CP, scientists from the team of the instrument under review, and representatives from NIST. Of the five individual instruments on EOS AM-1, MODIS, CERES, MISR, and ASTER have been reviewed (see, for example Bruegge [1995]). The final AM-1 calibration peer review, for MOPITT, is to be held in early 1996, and calibration peer reviews are currently being scheduled for the EOS PM-1 and EOS Chem instruments.
Calibration ATBD Reviews
In January 1994 the EOS Project Science Office requested that instrument teams produce a document describing in detail the algorithms used in the production of their data. This ATBD document describes the measurement equations and algorithms used in the transformation of raw Level 0 data to geolocated Level 1B radiances (or from Level 1B to Level 2 data) for a particular instrument. Calibration ATBDs are reviewed by a panel assembled by the EOS Project Science Office with the EOS Calibration Scientist acting in a support capacity. The review team evaluates the calibration ATBD in the areas of pre-flight and on-orbit calibration, general measurement approach, traceability of the measurements to SI units, extent and reliability of the sensor characterization measurements, estimation of uncertainties, plans for revising the radiometric calibration coefficients, maintenance of radiometric scales (for the field instruments as well as subsequent flight sensors), and methods used to communicate with other instrument teams (required for cross-calibration, validation, and all other calibration activities).
Summary
Achieving the goals of the EOS mission, namely to advance the scientific understanding of the Earth as a system and to distinguish between and determine the natural and anthropogenic influences on that system, depends on the production of accurate, precise, and consistent long-time series of measurements from multiple instruments on multiple platforms. The EOS Program Office and Project Science Office recognize the paramount importance of calibration in achieving those goals. Therefore, a state-of-the-art program in EOS calibration is being conducted under the auspices of the EOS Panel for Data Quality and is being headed by the EOS Calibration Scientist. Implementation of the program is multifaceted and involves planning and documentation, peer reviews, measurement comparison programs, measurement assurance programs, training, and workshops.
Part 1 of this article has examined the organizational structure of the EOS Calibration Program and the implementation of that program from the standpoint of planning and documentation and peer reviews. Part 2 of this article, to be published in a future issue of The Earth Observer, will examine the measurement comparison activities, measurement assurance programs, Level 1B data validation field programs, and calibration workshops.
References
Bruegge, C. 1995: Minutes of the Multi-Angle Imaging SpectroRadio-meter (MISR) Calibration Peer Review II, The Earth Observer, 7, (3), NASA Goddard Space Flight Center, 3-6.
Committee on Earth Observation Satellites (CEOS) Pilot Cal/Val Dossier, October 1993. Smith System Engineering, Ltd., Guildford.
Earth Observing System (EOS) Background Information Package (BIP), Announcement of Opportunity No. OSSA-1-88, Part One: Guidelines for Proposal Preparation, January 19, 1988.
Earth Observing System (EOS) Project Requirements Level 1A, pp. 15-16, May 10, 1989.
Johnson, B. C., S.S. Bruce, E.A. Early, J.M. Houston, T.R. O'Brian, A. Thompson, S.B. Hooker, and J.L. Mueller, 1996a: The Fourth SeaWiFS Intercalibration Round-Robin Experiment, SIRREX-4, May 1995. NASA Tech. Memo. 104566, Vol. 35, S.B. Hooker and E.R. Firestone Eds., NASA Goddard Space Flight Center, 61 pp.
Johnson, B. C., F. Sakuma, S.F. Biggar, J. Butler, J. Cooper, M. Hiramatsu, and K. Suzuki, 1996b: OCTS-SeaWiFS Preflight Cross-calibration Experiment at the OCTS Integrating Sphere Using Round Robin Radiometers (in preparation).
King, M., 1994: Editor's Corner, The Earth Observer, 6 (6), NASA Goddard Space Flight Center, 1-3.
King, M., 1995: Editor's Corner, The Earth Observer, 7 (1), NASA Goddard Space Flight Center, 1-3.
Taylor, B. N., 1995: Guide for the Use of International System Units (SI). NIST Special Publication 811, U.S. Department of Commerce, Gaithersburg, Maryland, 74 pp.
Thompson, A., E.A. Early, J. DeLuisi, P. Disterhoft, D. Wardle, J. Kerr, J. Rives, Y. Sun, T. Lucas, M. Duhig, and P. Neale 1996: The 1994 North American Inter Agency Intercom-parison of Ultraviolet Monitoring Spectroradiometers. In final review, to be submitted to J. Research NIST.
