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MISR Level 1 Products |
This statement applies to MISR Level 1 Products with a production date of March 30, 2001, or later until such a 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.
An extensive review of product quality has not yet been performed. Please read the summary words of caution if you have not done so already.
In spite of all the warnings, 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. Preliminary analysis indicates that the software is meeting all of its requirements. Ongoing quality analysis is planned.
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.
MISR Level 1 products generated with the current ARP release are generally accurate to the 4 percent level at 1.0 equivalent reflectance level (one sigma confidence level). Exceptions do exist for particular channels and at particular response levels. See the Calibration Page for more details.
Users should consider that the radiance reported for an individual pixel in one of the L1B2 products is obtained by resampling to the SOM map projection. Therefore an individual L1B2 pixel does not necessarily correspond directly to an observation made by a single camera-CCD pixel.
MISR Level 1B2 products exhibit acceptable georectification and coregistration accuracy. In the nominal case, the expected mean geolocation error across all cameras is below 80 meters with the standard deviations ranging between 150 meters for the nadir view angle, up to 350 meters for the most oblique angles (i.e., the D cameras). Exceptions definitely exist, including those caused by on-orbit maneuvers and those caused by instrument out-of-sync, which is described below. See the Registration Page for more details.
The overall registration accuracy and ability to assess registration accuracy will be improved somewhat in the future when MISR Level 1 software is enhanced to take advantage of navigation correction by image matching.
The raw MISR data contains occasional gaps. These gaps usually consist of a few lost lines. Straight lines of raw data are resampled to gentle curves in the SOM map projection. Radiances in the gap regions are filled in with pre-defined fill values. Gaps then usually look like narrow, curved, bright, horizontal stripes in the L1B2 image. There is at least one small gap in almost every swath. In rare cases, data gaps of many lines have been observed.
The MISR instrument tends to go out-of-sync momentarily if the data rate from the hardware exceeds the real-time flight computer's capacity to write data out. This condition can occur whenever a change of camera state is commanded. The resultant error is seen most often in the forward and nadir cameras near the beginning of the swath.
Image lines acquired while the MISR instrument was out-of-sync may contain sporadic fill and/or repeats of previous lines. The resulting image contains a brief vertical smear across the swath. Normally, this phenomenon only lasts for a handful of lines. In order to avoid geolocation errors, fill values are inserted in the line time fields in these regions.
The line-of-sight between an off-nadir camera and a ground point is sometimes blocked by a topographic feature, such as a mountain. In such cases, fill values are reported instead of radiances in the terrain product. Large patches of obscuration fill can be seen in the D cameras over mountainous regions.
Blocks which encompass no land at all get entirely filled with ocean-fill values in the terrain product. Terrain algorithms are wasteful over ocean since height variation is negligible there. The Ellipsoid product already contains radiances for these blocks. If ocean blocks are required, blocks from the Ellipsoid product may be substituted.
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 L1A Browse product is a compression of the red band L1A data for low-volume viewing purposes. L1A Browse is provided in two formats, jpg and hdf. The Version 7 browse products no longer exhibit the misnaming of the jpg file.
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.
