|
MISR Level 1 Products |
This statement applies to MISR Level 1 Products with a production date of April 15, 2002, or later until such time as further improvements to MISR software or ancillary inputs are made. See the Versioning Page for an in-depth explanation of the differences between various MISR product versions.
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.
The MISR calibration team has been involved in an on-going effort to both validate and make incremental improvements to the radiometric accuracy of the Level 1B data products. Because the uncertainties in these products are well understood, those produced after April 15, 2002, are given the quality designation of "Validated." Thorough descriptions of calibration uncertainties for current and older products can be found on the Calibration Page. The uncertainties at this time are listed below.
Although MISR radiometric accuracy is predictable over uniform scenes, improvements are anticipated to remove scene-dependent effects. These have been attributed to point-spread-function (PSF), ghost-image, L1B2 resampling, illumination level, and detector non-uniformity of response effects. These effects typically reduce contrast by a few percent; however, in extreme cases radiometric error can increase to about 25% for dark ocean targets (< 10 W m-2 micron-1 sr-1) with bright areas in the camera field-of-view. Because the MISR absolute radiometric scale was established over Nevada desert test sites, and believed accurate to within 4%, radiometric error increases over targets that are more extensive in size. As an example, the radiometric error for Sahara desert sites, which fill MISR camera fields-of-view, increases to 7% uncertainty. Specific examples of scene-dependent effects are given in the table below. A Level 1B correction algorithm for ghost-image and PSF effects is planned for the next product upgrade in October of 2002.
| 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. |
| Ghost-images refer to the reduction in image contrast due to scene inhomogeneity on the scale of the camera field-of-view. Although in-focus ghost-images have been observed, they are radiometrically insignificant except where a bright primary image has a secondary ghost which falls on a dark target. (The secondary ghost has been measured to be 0.3% of the primary image). More significant is the reduction in contrast due to the broad-scale out-of-focus secondary images. These effects have been estimated to be 2% of the average scene brightness. As with PSF, this effect results in the reduction of the reported radiance of bright targets, and an increase in the reported radiance of dark targets. |
| 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 quadratic 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. |
MISR Level 1B2 products exhibit acceptable georectification and coregistration accuracy. In the nominal case, the expected mean geolocation error across all cameras is below 60 meters. Standard deviations range between 100 meters (Nadir camera) and 300 meters (D cameras). Exceptions exist, but the most evident problem in older data, the Da camera mis-registration, has been corrected. Degraded georectification accuracy should be expected of data acquired during the month of July, 2000, and of data acquired near the time of occasional spacecraft maneuvers. See the Georectification Page for more details, including a link to the list of degraded orbits.
Both georectification accuracy and the ability to automatically assess georectification accuracy will be improved somewhat in the future when MISR Level 1 software is enhanced to take advantage of navigation correction by means of image matching against a specially prepared ancillary dataset.
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 instrument out-of-sync condition can temporarily corrupt the times reported with MISR L1B1 lines.
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. The An camera L1B1 Local Mode product is entirely corrupted due to a software bug. This bug has not been scheduled for a high-priority fix because the An camera always acquires data at 275 m resolution in all four bands. Users may order the corresponding RP_GM product to view a local mode site at "Local Mode" resolution.
The two primary fields in the RCCM product are the Cloud Mask and the Glitter Mask.
The RCCM (Radiometric Camera-by-camera Cloud Mask) is one of three MISR clouds masks. The RCCM has been promoted to a Beta quality designation based upon visual inspection. Separate threshold data sets are used to determine the RCCM over ocean vs land. Over ocean, the RCCM algorithm makes use of static thresholds which have been tuned through case studies. Regular blunders in the RCCM over ocean are only apparent in the presence of thick aerosol, over ice-covered waters and over waters very near a coastline.
The RCCM over land is often useful but is in a less-reliable preliminary stage of maturity than is the RCCM over ocean.
The Glitter Mask indicates regions of the data which are likely to exhibit sun glint. As of February 5, 2002, the sun glint cone angle was increased from 30 degrees to 40 degrees in order to mask out sun-glint contaminated pixels.
