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MISR Level 1 Products Quality Statement |
Radiometric Calibration: Carol Bruegge, Veljko Jovanovic
Georectifcation and Coregistration: Veljko Jovanovic
Exceptions/Anomalies: Veljko Jovanovic
Ellipsoid Color Browse: Veljko Jovanovic
Level 1A: Veljko Jovanovic
Level 1B1: Veljko Jovanovic
This statement applies to MISR Level 1 Products (Ellipsoid and Terrain) with a version number of 0024, and the Geometric Parameters with version number 0013. See the Versioning Page for an in-depth explanation of the differences between various MISR product versions. Quality statements covering earlier time periods may be accessed through links at the bottom of this page.
NOTE:The Radiometric Camera-by-Camera Cloud Mask is now considered to be part of Level 2, so the paragraph detailing the RCCM quality has been moved to the L2TC Quality Statement.
The MISR Level 1 software which generated these products is believed to be functioning quite well except where noted below. This statement lists known problems with Level 1 Products and clarifies issues which have confused some users.
There are no known problems with the current release of PGE7 software. Analysis of isolated case studies indicates that the software is meeting all of its requirements.
The Geometric Parameters exhibit one algorithmic quirk which has surprised some users. Solar zenith and azimuth angles near the swath edge occasionally appear to jump around. This inconsistency is the result of an intentional choice of algorithm whereby solar angles are computed at the mean time at which MISR cameras viewed the ground point in question. Adjacent points are not always visible to the same set of cameras. This can cause a bias in solar angle towards cameras which acquired that point.
This portion of the list is lengthy , so the sub-headings are listed for quick reference.
Update:The Ellipsoid and Terrain radiance products are now in their final state (version 0024), and the Level 1 software to produce these products will not be changed any further barring discovery of previously unknown problems. The MISR radiances are now Stage 3 validated. The DAAC will soon complete reprocessing the Level 1 data for the entire mission with this version of the software. Previous versions of the products should not be used.
The MISR band-relative radiometric calibration was last improved in version 0022 of the Level 1 product. Red band radiances have been reduced by 3% for all cameras, and NIR band radiances have been reduced by 1%. The need for a change was discovered during analysis of data from several MISR vicarious calibration experiments. Results from MISR vs MODIS, MISR vs MERIS, and Terra lunar calibration studies supported the decision to make this change.
Another significant improvement in Radiometric accuracy was made in versioned 0015. The improvment was achieved by discovering and correcting an error in the code used to derive the radiometric calibration coefficients. A-Nadir camera data produced before this version may have had absolute errors as large as 10% (too bright) at the left/western edge of the scene, and -5% (too dim) at the right/eastern edge. Radiometry at the swath center, and for other cameras has been in error to a lesser degree.
A history of the calibration changes can be found on the Calibration Page. The current uncertainties are listed below.
PSF effects refer to scene-contrast reductions due to local scene inhomogeneity. The radiometry of one pixel is affected in proportion to the contrast difference and proximity of another pixel. This is a camera optical effect, and is measurable for objects that are within 6 km crosstrack distance of each other. The downtrack PSF effect is believed to be negligible. A deconvolution algorithm is now exercised in the software to minimize PSF effects. |
Ghost-image refers to the presence of a secondary image, created as a reflection of a given target through the lens optical axis. This secondary ghost has been measured to be 0.3% of the primary image, and thus results in a negligible error except where the image of a bright target falls on a very dark scene. |
L1B2 resampling is implemented by bilinear interpolation, and thus errors in this process are a function only of a sample's radiance value as compared to that of its immediate neighbors. |
Illumination-level dependent errors are attributable to the goodness-of-fit of camera response data to a mathematical equation. The MISR cameras are described as having a linear relationship between incident radiance and camera output. For radiance levels less than 2% in equivalent reflectance, this assumption is valid to within 5% uncertainty. The error is considered negligible for larger input signals. |
Detector uniformity of response errors occur when a set of detector elements are non-uniform in response (10% non-uniformity or greater), are image inhomogeneous scenes, and are DN-averaged as part of the on-board data compression (Global Mode) algorithm. Only a dozen detector regions (out of 13,000 such pixel blocks) are non-uniform, and these are identified by data quality indicators in the products. For conditions where bright scenes are adjacent to dark scenes, an additional radiometric error of 6% may result in pixel regions where the Data Quality Indicator level is given a value of 2. |
Update: MISR Level 1B2 products exhibit as expected georectification and coregistration accuracy. In the nominal case, the expected mean geolocation error for all nine cameras is below 20 meters. Standard deviations range between 30 meters for A-Nadir camera, and 70 meters for most oblique D's cameras. The final operational version of the processing algorithm along with the globally validated georectification and coregistration performances is described by Jovanovic, V., Moroney, C., & Nelson, D. in "Multi-angle geometric processing for globally geo-located and co-registered MISR image data", Remote Sensing of Environment, Special Issue, Volume 107, 2007.
Also, all camera data acquired during occasional spacecraft maneuvers is of degraded accuracy. Data products generated during these times contain an Orbit Quality flag which indicates this problem. The nominal value of the Orbit Quality flag in the File Metadata (Global Attributes) is 0.0. Degraded data has an Orbit Quality flag value of -1.0. See the Georectification Page for more details, including a link to the list of degraded orbits.
The Nadir, single-band L1A browse product has been replaced with a new Ellipsoid-based color product. The new browse product is generated for all 9 cameras at 2.2 km resolution (sub-sampled). MISR Red, Green and Blue bands are used to create the color image, which is intentionally clipped and gamma-stretched in order to make cloud, ocean and land features visible. The jpeg compression is performed at 75% quality, which means that compression artifacts are occasionally visible.
The MISR Level 1A product is a reformatted version of the raw L0 data packet stream from the spacecraft. In this format, the CCD Data Numbers (DNs) may be viewed as an unregistered image with data quality indicators occupying the two low-order bits. By design, L1A does not differ greatly from the raw data except that gaps are filled in with appropriate fill values.
The MISR Level 1B1 product has been radiometrically corrected, but it has not been registered. The quality of L1B1 radiances is equivalent to that of L1B2 radiances discussed above, except that L1B1 pixels correspond directly to instrument CCD detector samples.
The L1B1 Local Mode product consists of the L1B1 output acquired when the MISR instrument is put into Local Mode in which all nine cameras view a scene at 275 m resolution in all four bands.