The MISR Calibration Peer Review II was held at the Jet Propulsion Laboratory (JPL) on 27-28 March 1995. Skip Reber, Goddard Space Flight Center (GSFC), chaired the review. Board members were Jim Butler (GSFC); Stuart Biggar, University of Arizona (U of A); Peter Jarecke, TRW; Robert Lee, NASA/Langley; Carol Johnson and Joe Rice, National Institute of Standards and Technology (NIST); Hugh Kieffer, U.S. Geological Survey; and Frank Palluconi, JPL.

Fifteen speakers from the MISR engineering, science, and data teams presented the status of the instrument development and test activities, as well as the in-flight calibration and data validation plans. The following paragraphs describe presentation areas.

Detector Standards

The 3 percent (1 sigma) absolute radiometric calibration of MISR is to be established with use of detector standards. That is, for this program, detectors are used to define the radiometric scale rather than national-standards laboratory traceable lamps. Detectors have the advantage of greater accuracy and lifetime stability, as compared to source standards, e.g., lamps. During preflight calibration each of the nine MISR cameras views, in turn, the output of a 65" integrating sphere. The sphere is used as a stable, flat-field source. The detector standards view the source to define the sphere output prior to each camera calibration.

The detector standards used in the preflight program are termed Laboratory Standards. They are commercially available and consist of three photodiodes in a trapped configuration such that front-surface reflection losses are collected by the next diode in series. High-purity silicon, as well as this light trapping, allow the devices to obtain near 100 percent external quantum efficiency. The Laboratory Standards, as well as the MISR flight detector standards, are fitted with field-defining baffle-tubes which contain precision apertures (manufactured using photo-lithography techniques). They also use filters which have been manufactured to the same design as the camera flight filters. This allows the source output to be characterized in the same spectral passbands as the flight instrument.

Post launch, MISR will be calibrated with use of an On-Board Calibrator (OBC) employing detector standards and sun-illuminated diffuse panels. The OBC utilizes three types of detector standards, termed Red and Blue High Quantum Efficiency (HQE) and PIN detectors. These have been designed and assembled within JPL. PIN refers to the p-type intrinsic n-type diode architecture.

There is one HQE for each MISR spectral band, four in total. The blue HQE detectors have high quantum efficiency at blue wavelengths and are used to calibrate MISR bands 1-3 (443, 555, 670 nm). The red HQE detector has high quantum efficiency at red wavelengths and is used to calibrate MISR band 4 (865 nm). The HQE detectors also use high-purity silicon in a light trap and achieve near 100 percent external quantum efficiency. They provide commonality with the preflight calibration methodology.

In addition, the OBC has five PIN wavelength sets. These are neither light-trapped, nor of high internal quantum efficiency, but rather have been optimized by design to be extremely radiation resistant. Preflight testing of the PIN detectors has demonstrated stability to radiation dosages simulating the six-year mission.

Radiation testing of the blue High Quantum Efficiency (HQE) photodiode flight standards has determined that these devices should not degrade during the EOS mission life. The red HQE device degraded within an acceptable range. Prior to testing it was believed that neither the red nor blue HQE devices would be radiation stable; hence the radiation-resistant PIN photodiodes were included in the instrument design. The better-than-anticipated performance of the HQEs will provide long-term multi-calibration pathways, thus reducing calibration uncertainties.

For the Laboratory and HQE Standards, 100 percent external quantum efficiency is verified by comparing the measured quantum efficiency of several devices, all of different photodiode architectures. The largest sources of uncertainty are knowledge of the aperture areas and aperture displacements within the baffle-tubes, and effective filter spectral transmittance for the as-built standard configuration. PIN internal quantum efficiency must additionally be measured by comparison to the Laboratory Standards. Radiance uncertainties of all standards are believed to be within + 2 percent (1-sigma).

Round-Robin Results

MISR has emphasized the need for Round-Robin experiments to provide further verification of the detector-standard-established radiometric scale, and to provide a cross-comparison with other EOS instruments. In August 1994, MISR was the host for a Round-Robin experiment. In attendance were Stu Biggar, U of A, who brought a multi-channel filtered transfer radiometer; Sakuma Fumihiro, National Research Laboratory of Metrology (NRLM), Japan, with a 650 nm channel transfer radiometer; and John Cooper, GSFC, with a NIST-traceable source and spectroradiometer. In order to intercompare the sphere source output, as measured with one radiometer at one wavelength, to another radiometer at a different wavelength, a blackbody spectral distribution was assumed. The MISR and U of A sphere measurements agreed to better than 1.0 percent at 550 nm. The MISR and NRLM sphere measurements agreed to better than 0.9 percent at 650 nm. These results confirm MISR's claim of 2 percent (1 sigma) detector-standard accuracy.

Additionally, a filter transmittance intercomparison was conducted during the Round-Robin. It is recalled that accuracy of the detector standards is no greater than the accuracy of these filter transmittance measurements; therefore, results of this test were also of great interest. Transmittance measurements typically agreed to within 1 percent, except for one (non-MISR) instrument with known inconsistencies. Prior to this experiment MISR had assumed filter transmittance accuracy of 0.5 percent. Further spectrometer certification is recommended, although the added 0.5 percent uncertainty will not affect our calibration plans or schedule. It is noted that systematic biases can be removed at a later time.

Engineering Model Characterization

MISR completed the build of the Engineering Model (EM) A and D cameras (the largest and smallest focal lengths amongst the four lens designs) this past fall. Camera testing was then conducted from December to March of this year. The MISR team believes the Engineering Model has been invaluable in diagnosing and fixing problems that otherwise would have been undetected until manufacture of the flight hardware. These fixes include the identification and elimination of: 1) a white light leak due to bondlines between the respective filter bands; 2) a significant light leak due to illumination of silicon around the CCD bond pads; and 3) a focus error due to the interface ring-mating lens to camera head.

Other significant problems studied with the EM were insufficient out-of-band rejection and a low-level "halo" around the point-source image. Both problems were attributed to scattering within the filter and scattering between the filter and CCD. Improved quality of the flight filters has reduced the magnitude of these problems considerably. Our present plan is to compensate for any remaining deviations from specifications in the ground data processing software.

The filter scattering noted on the EM cameras has also affected the PIN photodiode performance. If uncorrected, out-of-band energy would lead to a violation of the calibration accuracy requirements. During the flight calibrations, the photodiode standards will measure sunlight reflected from diffuse panels which are spectrally flat. Because the spectral distribution of the input is known, the diode data can be adjusted for out-of-band response. This correction will be implemented in order to provide the required 3% calibration throughout the mission.

The primary objective of EM testing was to identify and correct weaknesses in the test and data analyses procedures (to dry run them), before the arrival of the flight cameras. Where possible the tests were additionally used to verify instrument design and recommend final changes. The unexpected halo, out-of-band response, and bond-pad light leak led to a more-extensive testing and characterization than originally planned. The flight procedures, therefore, have now been modified to include more extensive measurements.

Through EM radiometric calibration it was learned that the stability of the sphere lamps was increasing with time. Relying on vendor input, the original plan utilized each lamp set for 200 hours. Based upon experience with the EM, however, it appears there is a smaller window in which the lamps are stable to within our ± 1% requirement, thus a better plan is to replace the bulbs after 50 hours, in order to assure that stability is met. Our plans for the flight unit include changing the bulbs more frequently, as well as monitoring stability with a blue-filtered photodiode, the wavelength at which the lamp stability is greatest.

Electrically, the MISR cameras are extremely quiet. The camera signals are encoded to 14 bits and all signal-to-noise-ratio requirements are being met in EM testing. Requirements for local uniformity of response from pixel to pixel are also being met.

The spectral calibration data for MISR are collected from 400 to 1175 nm, in step sizes of 0.5 (1.0) nm for the in-band (out-of-band) characterization run. During EM testing it was discovered that if the monochromator output was spread sufficiently to illuminate the full-array (1504 pixels per array, ± 30 degrees for the A camera), the signal strength became insufficient to allow characterization of the out-of-band response. As out-of-band characterization is crucial, the output beam has been narrowed such that only 50 pixels are illuminated at a time. The beam will additionally be steered to characterize seven spatial positions across the array for the in-band test, and three positions for the out-of-band test.

Spectralon BRF Determination

In-flight the HQE and PIN detector standards view diffuse reflectance standards. These are flat panels made of Spectralon, a product of Labsphere, Inc. Spectralon is made of pure polytetrafluoroethylene (PTFE); is spectrally flat; and is highly reflective. The calibration panels are used to provide a flat-field source into the cameras. They are characterized preflight for relative reflectance versus view and illumination angle. In-flight degradation of the panel absolute reflectance, or illumination of the panels with a varying Sun/Earth atmosphere path length are accommodated in that the detector standards measure the radiance reflected from the panels throughout each calibration exercise.

Data from the EM Spectralon diffuse panel bidirectional reflectance factor (BRF) characterization were presented. MISR uses Spectralon panels as part of the in-flight calibration. The BRF set-up utilizes a HeCd (442 nm), HeNe (632.8 nm), and diode (860 nm) laser to characterize the panels over the MISR spectral observation range. BRFs were measured at 11 spatial positions, both in- and out-of the principal plane, at both s and p illumination polarizations. At 442 nm the panel spatial uniformity was poorer than the 0.5 percent requirement. It was determined that spatially distinct regions (top, center, and bottom) existed, probably resulting from the manufacturing processing. During the final stage of panel preparation a sanding disk is swept across the panel; a central hole in this disk may give rise to the observed spatial variations. The flight panels, although manufactured to the same procedures, have improved spatial uniformity, meeting the 0.5 percent manufacture requirement. Evidence of non-uniformity in BRF due to cleats in the Spectralon, which are used for gripping the Spectralon panels by the aluminum holding trays, was sought. No such effect was observed.

Image Registration and Geolocation

Achieving and verifying repeatable instrument pointing has been a focus of the preflight instrument phase. Testing verification has been provided by a tool which uses nine distinct collimators in a fixture which rests on the MISR optical bench. Imaging of a collimator target by each MISR camera provides camera displacement with respect to the calibrated collimator assembly. Repeatability of camera pointing through temperature excursions is thereby verified.

In-flight, pointing is to be determined to within tens of arcseconds. This is achieved by a three-step process involving: 1) using ground control points to provide an accurate camera pointing model; 2) establishing a set of reference imagery, one image per each MISR camera per unique orbit; and 3) the routine registration of follow-on MISR imagery to the reference data set. Geolocation and nine-camera image registration is provided with this processing, as the reference imagery is geolocated with respect to a Space-Oblique Mercator map.

Review Board Comments

The MISR team felt that the Calibration Peer Review was a constructive process, and several key recommendations will be implemented. It was felt that, in addition to the Round-Robin Experiments, calibration accuracy of our detector standards should be verified by a standards lab, such as NIST. The MISR team believes that this can be done after the camera calibration data have been collected. Any systematic biases in the calibration can be removed prior to final documentation of the preflight camera calibration. Concern was expressed about our plan to swap out the Spectralon diffuser panels after testing at the spacecraft integration facility. The MISR team still feels this is our best option, in view of the sensitivity of Spectralon to hydrocarbon contamination. A recommendation was made to have the Cary spectrometer certified for transmission accuracy. Another comment pointed to the need to coordinate field campaigns for validation among AM platform instruments. We concur with these observations.

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