Frequently Asked Questions and Information - MISR Information
MISR Frequently Asked Questions
See the MISR web site, and also Diner, D. J. et al. "Performance of the MISR Instrument During Its First 20 Months in Earth Orbit", IEEE. Trans. Geosci. Rem. Sens. 40 (7), 1449-1466 (July 2002).
|Camera View Zenith Angles at Earth's Surface||0.0 ° (nadir), 26.1, 45.6, 60.0 and 70.5 ° (both fore and aft of nadir)||
Local Mode (targeted):
|Swath Width||360 kilometers (224 miles) (9-day global coverage)|
|Cross-Track x Along-Track Pixel Sampling||
275 x 275 meters (902 x 902 feet)
550 x 550 meters (0.34 x 0.34 mile)
1.1 x 1.1 kilometers (0.68 x 0.68 mile)
275 meters x 1.1 kilometers (0.17 x 0.68 mile)
|Spectral Bands (Solar Spectrum Weighted)||446.4, 557.5, 671.7, 866.4 nanometers|
|Spectral Bandwidths||41.9, 28.6, 21.9, 39.7 nanometers|
Measuring the reflectance of a target from different directions is really useful because geophysical media (the atmosphere, including the clouds and aerosols, the ocean, and terrestrial surfaces such as soil, vegetation and snow or ice) reflect solar light differently in different directions. In fact, the variations between the reflectances acquired from a variety of observation angles can be interpreted (with appropriate models) to document the properties of the target, just as the more familiar spectral differences are exploited to document its chemical composition.
Most imaging space-borne instruments acquire measurements for each location on Earth from a single direction at a time, usually within the limited range of (across-track) observation zenith angles allowed by the scanner or push-broom design of the sensor. The accumulation of multiangular observations with such instruments necessitates revisiting the site over rather long periods of time, from multiple days to a few weeks or more. By incorporating nine separate cameras oriented at various angles along the track of the platform, the MISR instrument is capable of acquiring multiple observations of the same site from a wide variety of zenith angles in a matter of a few minutes. This greatly facilitates the interpretation of the measurements and significantly improves the accuracy of the retrieved information.
Specifically, using its multi-angular and multi-spectral capability, the MISR instrument
- better distinguishes between objects and surfaces than would be possible on the basis of spectral variations alone;
- exhibits an enhanced sensitivity to aerosols and thin cirrus clouds, both of which are ubiquitous and usually hard to detect, especially at large observation zenith angles and/or over bright backgrounds;
- provides a three-dimensional (stereoscopic) view of clouds that allows users to estimate their height;
- makes it possible to use clouds as tracers of winds aloft due to the time lapse (about 7 minutes) between the most forward and the most backward views;
- yields at once measurements that accentuate or minimize the effect of sun glint over the ocean and other water surfaces, thereby enabling observations even when traditional sensors are hampered by the very high reflectance of these surfaces, and
- permits users to much more accurately estimate the hemispherical albedo of the target, which is thus calculated on the basis of nine different values instead of only one.
More detailed information is provided on the MISR web site Science Goals and Objectives page.
The instantaneous swath of any imaging instrument, including MISR, is the width of the region that is actually observed across the track of the instrument at any time during any particular overflight. The instantaneous swathes of the cameras on the MISR instrument vary from 376 to 414 km, but the instantaneous swath of the overlap of all cameras is 360 km. This means that whenever MISR passes exactly overhead of a site, its nine cameras also observe the neighborhood of that site, up to 180 km on either side of the projection of the track of the Terra platform on the Earth's surface. The swath also refers to the entire region being observed by MISR in the course of one particular orbit: in this case, it refers to an area that is about 20,000 km long and 360 km wide.
The Terra platform that carries MISR and other scientific instruments flies at an altitude of 705 km above sea level on a sun-synchronous orbit. It revolves once around the planet in 98.88 minutes and thus completes about 14.5 revolutions per day. In the context of MISR data exploitation, each complete revolution is called an orbit, and orbits are consecutively numbered from launch. The number of the orbit is thus directly related to the time span since launch.
In practice, since MISR is an optical sensor that measures the reflectance of the Earth in the solar spectral range, it is acquiring useful data only while the Terra platform is over the illuminated (day) side of the planet, i.e., during one half of the complete orbit or a bit less. Of course, the Earth itself keeps turning around its own axis while Terra proceeds on its orbit. As a result, when Terra completes an orbit and initiates the next one, it actually flies over quite different regions. The orbit number thus also indicates the areas of the planet observed. For these reasons, the orbit number is explicitly included in the name of many MISR data and product files.
The sun-synchronous orbit of Terra was selected in such a way that after 233 revolutions around the planet, or some 16 days, the platform returns to exactly the same locations and observes them under nominally identical angular conditions. Because of the 360 km instantaneous swath width of the MISR instrument, it is possible to gather multiple sets of observations (each with 9 cameras and 4 spectral bands) of a particular site in 2 days (Poles) to 9 days (Equator), depending on its latitude, but of course under a variety of angular conditions.
An Orbit/Date Conversion tool is available from the ASDC web site.
The generic data file for MISR is a swath, i.e., a set of measurements for the entire area observed during the day part of the orbit. This is a very large amount of data. To simplify the storing and processing of these data, swathes are broken into a series of predefined, uniformly-sized boxes along the ground track called blocks. Each path is divided into 180 blocks measuring 563.2 km (cross-track) x 140.8 km (along-track). For a given path, a numbered block always contains the same geographic locations. Hence, areas that are close to each other in latitude will belong to blocks with similar numbers. Because of seasonal variations in the portion of the Earth that is in daylight, only up to about 142 blocks will contain valid data at any particular time.
The MISR instrument continuously acquires data everywhere on Earth, but with a frequency that is dependent on the latitude of the location of interest. Indeed, due to the overlap of the swathes near the poles and their wide separations at the equator, repeat coverage varies from 2 to 9 days, respectively.
The nine MISR cameras acquire measurements every 275 m across track (250 m for the An camera). However, the data rate that would be required to transmit all the measurements from the nine cameras in the four spectral bands at that full resolution would exceed the capacity allocated to the MISR instrument on the Terra platform. To retain some of the high spatial resolution of the sensors without exceeding the data transfer quotas, MISR can be operated in two different data acquisition modes:
- In global (default) mode, a special averaging scheme is conducted on-board such that the data from all cameras other than An and all spectral bands other than red are averaged to 1.1 km before transmission to the receiving station. The spectral data (all 4 bands) from the An camera and the directional data in the red spectral band from all 9 cameras are therefore always available at the full spatial resolution.
- In local mode, it is possible to transmit all the data at the full resolution, but only for limited periods of time and therefore for limited regions, typically about 300 km in length (along track). Please refer to the MISR technical documentation for a detailed description of the various programming modes. Users may contact the ASDC User and Data Services office to request local mode observations.
Bands 1-4 are blue, green, red, and near-infrared, respectively. Variations in spectral response from one camera to another, for the same band, are minor and a standardized, downloadable spectral response curve in text format for each of the bands is available. Summary spectral response information (e.g., bandcenter, bandwidth) is also available in the Ancillary Radiometric Product ARP_INFLTCAL file, and the mathematical definitions of the product contents are provided in MISR Level 1 In-flight Radiometric Calibration and Characterization Algorithm Theoretical Basis Document (ATBD-MISR-02). A moments analysis is used to define the "in-band" spectral region. Because the filters are not perfect, there is a small amount of "out-of-band" light (less than 2% of the integrated energy), and the ARP_INFLTCAL file provides spectral response information for both the in-band region of each filter as well as for the total band.
Zenith angles are positive values measured from the local vertical at each point. Azimuth angles are measured clockwise from the direction of travel to local north. For both the Sun and cameras, azimuth describes the direction in which the photons are traveling. According to this convention the solar azimuth angle is therefore near 270 degrees when Terra is close to the equator, because of its morning equator crossing time. Additionally, the difference in view and solar azimuth angle will be near 0 degrees for forward scattered light, and near 180 degrees for backward scattered light. Sun and View angles are available in the MISR Geometric Parameters (MIB2GEOP) data product. Sun angles are also available in the L1B2 data files.
The format Version is used to distinguish between software deliveries to ASDC that result in a product format change. The format version is given in the MISR data file name using the designator _Fnn_ where nn is the version number.
The product version is given in the MISR data file name using the designator _nnnn_ where nnnn is the product version number. Early on in the mission, several significant changes to ancillary datasets were required, and these changes triggered Product Version increments. At this point in time, most of the static ancillary data sets have stabilized and major changes to them are not predicted. Therefore, product version numbers are not necessarily incremented because of ancillary data set deliveries.
As MISR continues to reprocess the data products, Collections of scientifically consistent sets of data product versions become available. Collections may contain multiple format, production, and ESDT Versions.
An example of MISR's file naming convention is: MISR_AM1_GRP_TERRAIN_GM_Pxxx_Oxxxxxx_XX_Fnn_nnnn.hdf, where
MISR = Instrument Name
AM1 = Satellite Name
GRP_Terrain_GM = Georectified Radiance Product Terrain (Land Surface) Global Mode
Pxxx = Path Number
Oxxxxxx = Orbit Number
XX = Camera
Fnn = Format version of the product
nnnn = Product Version number
Individual MISR data set file naming conventions are found in the Project Guide.
Intensive assessment of MISR product data quality is an ongoing activity. Focused studies of some parameters are completed. Quality designators indicate the maturity of products and individual parameters. A product may contain parameters at varying quality levels. Below are the Maturity Level definitions:
|Beta:||Early release products for users to gain familiarity with data formats and parameters.|
|Provisional:||Limited comparisons with independent sources have been made and obvious artifacts fixed.|
|Validated Stage 1:||Uncertainties are estimated from independent measurements at selected locations and times.|
|Validated Stage 2:||Uncertainties are estimated from more widely distributed independent measurements.|
|Validated Stage 3:||Uncertainties are estimated from independent measurements representing global conditions.|
There are three reasons that the ground coverage of two MISR images within the same path and block may differ.
- The position and pointing of the Terra satellite can vary by a small amount from orbit to orbit. For instance, the position is allowed to vary by no more than 20 km from one overpass to the next. If two images from the same camera for different overpasses are compared, a slight horizontal offset in image location can often be seen. This offset is usually the result of a shift in position of the satellite.
- The nine MISR camera footprints do not all cover the same exact extent on the ground. This variance occurs because the different camera views are acquired from devices which have different optical designs. In addition, the Earth rotates underneath the satellite in the time that elapses between acquisitions of a particular ground target by two different MISR cameras. Crosstrack boresight offsets are included in the MISR camera pointing angles to compensate, but this does not result in perfect overlap at every latitude. Therefore, two images from the same orbit but from different cameras almost never display precise overlap. A slight horizontal offset between any two camera images is usually visible.
- The MISR cameras are only commanded to acquire imagery over the sunlit portion of the Earth. Therefore a significant seasonal variation of the start block and stop block occurs for orbits within the same path. For example, some blocks acquired in Antarctica in December will not be acquired in June because they are dark during the later month.
The nadir camera Level 1 images have a swath width of about 376 km, compared to 414 km for the off-nadir cameras. The widths are different because the focal lengths of the MISR cameras change in relationship to the varying distance to the Earth for the different cameras. When viewing the same Earth area with a more oblique-pointing camera the focal length must be greater in order to preserve resolution. All of the camera images are 1504 pixels wide, and the focal lengths of the D, C, B, and off-nadir A cameras are chosen so that each pixel is 275 m wide. However, the nadir A camera uses the same focal length as the off-nadir A cameras, so each of its 1504 pixels is only 250 m wide. Another factor affecting overlap is Earth rotation during the time interval between when each camera acquires its image of a given area. The area of overlap among the cameras depends on latitude. Even though the forward and backward D camera (for example) have the same swath width, it is possible for the common area viewed by both to be less than 414 km.
SOM is a space-based map projection in which the reference meridian nominally follows the spacecraft ground track. It provides a mapping from latitude and longitude to a coordinate system that is approximately aligned with the MISR swath. SOM was used because it minimizes distortion and resampling effects, and permits the greatest flexibility in the choice of Earth-based projections. SOM also allows direct cross-comparison with data from other instruments and simplifies global mapping, since the data have already been geolocated. For more information see the MISR Level 1 Georectification and Registration Algorithm Theoretical Basis Document (ATBD-MISR-03), and also Snyder, J.P., Map Projection - A Working Manual, United States Geological Survey Professional Paper 1395, U.S. Government Printing Office, Washington, DC (1987).
SOM vs Lat/Lon
MISR, the Multi-angle Imaging SpectroRadiometer instrument, was successfully launched into sun-synchronous polar orbit aboard Terra, NASA's first Earth Observing System (EOS) spacecraft, on December 18, 1999. MISR measurements are designed to improve our understanding of the Earth's environment and climate. Viewing the sunlit Earth simultaneously at nine widely-spaced angles, MISR provides radiometrically and geometrically calibrated images in four spectral bands at each of the angles.
MISR provides new types of information for scientists studying Earth's climate, such as the partitioning of energy and carbon between the land surface and the atmosphere, and the regional and global impacts of different types of atmospheric particles and clouds on climate. The change in reflection at different view angles affords the means to distinguish different types of atmospheric particles (aerosols), cloud forms, and land surface covers. Combined with stereoscopic techniques, this enables construction of 3-D models and estimation of the total amount of sunlight reflected by Earth's diverse environments.
MISR acquires systematic multi-angle imagery for global monitoring of top-of-atmosphere and surface albedos and to measure the shortwave radiative properties of aerosols, clouds, and surface scenes in order to characterize their impact on the Earth's climate.
Illustrations of MISR instrument viewing the Earth:
MISR data products are grouped into three processing Levels. Level 1 processing provides corrected (or calibrated) instrument data. These data are processed and calibrated to remove many of the instrument effects. The resulting products thus contain minimal instrument or spacecraft artifacts and are most suitable for subsequent scientific derivations. Level 2 processing provides retrieval of derived scientific quantities, such as atmospheric aerosol and cloud measurements. Level 3 processing produces global grids of various parameter elements from the Level 2 products. These global grids are produced daily, monthly, quarterly, and yearly. More detailed descriptions about each of these processing levels can be found in the MISR Project Guide. The MISR data products by processing level are available.
Level 3 globally gridded products became available in July 2002. Level 2 cloud, aerosol, and land surface products became available in February 2001. Level 1 products are available from February 24, 2000 to present. These include Level 1 raw instrument data (Level 1A); radiometrically calibrated radiances (Level 1B1); geolocated, co-registered, map-projected radiances (Level 1B2); browse data; and geometric parameters on a swath-by-swath basis. Additionally, engineering, navigation, and on-board calibrator files are also available, though these are not generally required in order to make use of Level 1 image data. Static data sets also available include the Ancillary Radiometric Product (latitudes and longitudes and surface elevations) and the Ancillary Geographic Product. All currently available data products are listed on MISR Data and Information.
|The MISR Browse Tool allows quick viewing of images from the MISR instrument. The color images are produced from the L1B2 Ellipsoid product for each camera reduced to 2.2km resolution. The MISR red, green and blue bands are used to create the color image, which are intentionally clipped and gamma-stretched to make cloud, ocean and land features visible. The browse images are JPEG format.||Browse Tool Instructions|
MISR data are processed and archived at the NASA Langley Research Center Atmospheric Science Data Center (ASDC). A MISR Order and Customization Tool is available that enables users to order and customize data in a single interface. Some features are non-consecutive path and orbit search; and sorting search results by date, camera, path, orbit, and file version. Customization options are subsetting by parameter, block, and spatial coordinates; additional latitude and longitude layers, and unpacking and unscaling applicable fields; and output data in HDF-EOS stacked-block grid or conventional grid. The reformatting option, referred to as "Conventional HDF-EOS", is designed to be an easier-to-use alternative to MISR's stacked-block format. The interface currently supports two output formats:
- Stacked-block HDF-EOS (original file format)
- Conventional HDF-EOS grid
See FAQ on Customizing and Subsetting MISR products for more information.
MISR data are also available through the ASDC Data Pool (an on-line, short-term data cache that provides a Web interface and FTP access). Specially subsetted and/or reformatted MISR data products supporting field campaigns are also available. Passive FTP mode is required to access the Data Pool from the command line as "ftp -p l4ftl01.larc.nasa.gov", where the system translates to "lower case L four ft lower case L zero one .larc.nasa.gov". A UNIX C shell script method may be used to assist in downloading files from the Data Pool (Readme | Sample script). Data files are stored in directories based on data product and acquisition date. Corresponding metadata files are available in Extensible Markup Language (XML) format.
Depending on the MISR data product, the files can be quite large (for example, a Level 2 Aerosol data product file is approximately 35 Megabytes and a Level 1B2 Radiance file is approximately 600 Megabytes).
MISR observes the entire day-lit side of each orbit. The MISR Production Report allows you to determine what data are available for an orbit range, a date range, or a path range. To obtain an inventory of MISR archive holdings for a specific location or time, submit a query through the MISR Order and Customization Tool which contains the required spatial and temporal constraints. When the query is complete, a list of MISR data files which satisfy the constraints will be displayed in the query results form.
Data for particular geographic locations can be searched via the MISR Order and Customization Tool by selecting products, region of interest, and date. The MISR Browse Tool provides information to help determine paths, orbits and block numbers for specific geographic regions. There is also an Orbit/Date Tool and a Lat/Lon to MISR Path/Block Tool available to determine MISR paths, orbits and blocks. The MISR Production Report, allows you to determine what data are currently available for an orbit range, a date range, or a path range.
The MISR Browse Tool allows quick viewing of images from the MISR instrument. The color images are produced from the L1B2 Ellipsoid product for each camera reduced to 2.2km resolution. The MISR red, green and blue bands are used to create the color image, which are intentionally clipped and gamma-stretched to make cloud, ocean and land features visible. The browse images are JPEG format. After viewing the browse images for your region, note the orbit numbers of interest. The MISR Production Report lists the most currently available data for the selected orbits. Include those orbit numbers when submitting the search for data through the MISR Order and Customization Tool.
Customization allows users to specify the contents and the output format of their MISR products. Currently available features include:
- Spatial subsetting
- Parameter subsetting
- Additional latitude and longitude fields
- unpack and unscale data values
- HDF-EOS conventional grid output format
Subsetting / reformatting is available for MISR Level 1 and Level 2 Standard Data Products. The product maturity designations and quality statements of the original products are applicable to the subsetted / reformatted products. However, some of the transformations executed by the MISR Order and Customization Tool may affect data content. The following products may be customized:
- MIANCAGP - MISR Ancillary Geographic Product
- MIB2GEOP - MISR Level-1B2 Geometric Parameters
- MIRCCM - MISR Level-1B2 Radiometric Camera-by-camera Cloud Mask
- MI1B2E - MISR Level-1B2 Global Mode Ellipsoid-projected Radiance
- MI1B2T - MISR Level-1B2 Global Mode Terrain-projected Radiance
- MB2LME - MISR Level-1B2 Local Mode Ellipsoid-projected Radiance
- MB2LMT - MISR Level-1B2 Local Mode Terrain-projected Radiance
- MIL2TCCL - MISR Level-2 TOA/Cloud Classifiers Parameters
- MIL2TCAL - MISR Level-2 TOA/Cloud Albedo Parameters
- MIL2TCST - MISR Level-2 TOA/Cloud Stereo Parameters
- MIL2ASAE - MISR Level-2 Aerosol Parameters
- MIL2ASLS - MISR Level-2 Land Surface Parameters
One MISR data file is the full swath of data (from terminator to terminator), and the MISR swath-based products are very large. Many users will be interested only in those parts of the swath that cover their region of interest. The MISR Order and Customization Tool enables users to order and customize data in a single interface. Customization options are subsetting by parameter, block, and spatial coordinates; additional latitude and longitude layers, and unpacking and unscaling applicable fields; and outputting data in HDF-EOS stacked-block grid or conventional grid.
Stacked-block HDF-EOS format is a MISR specific extension to the HDF-EOS library that is only used by MISR and is not widely supported by third party data analysis tools. Tools which support HDF-EOS or HDF can usually import stacked-block data to a limited extent. Two problems commonly encountered are: 1) piecing together the MISR blocks so they are aligned correctly with respect to each other; and 2) geolocating the MISR data in a map context. Many tools do not support geolocation of the stacked-block HDF-EOS format. A detailed discussion of the stacked-block HDF-EOS format including instructions on how to write software to geolocate MISR data are discussed in Appendix A of the Data Product Specifications Document.
vs Aligned File
The conventional HDF-EOS format is an alternative format that uses only common HDF-EOS elements, specifically excluding use of the stacked-block extensions. As a result, the conventional HDF-EOS format should be easily imported into any tools that support generic HDF-EOS. In practice however, there are still many tools which have only limited support for HDF-EOS. Conversion to conventional HDF-EOS format attempts to reproduce the data content of the original products, only changing the data format. However, there are some notable differences in the data content for some fields which are discussed below. Geolocation is based on the standard parameters used by the General Cartographic Transformation Package (GCTP) library which is distributed with the HDF-EOS library. Data are map projected using the same Space Oblique Mercator (SOM) projection as the original products.
vs Aligned File
The choice of format largely depends on the tools you are using. Some MISR specific tools, such as misr_view, only support stacked-block format. Tools with only generic support for HDF or HDF-EOS, will be able to display stacked-block data a block-at-time, but will not automatically stitch blocks together with the proper alignment, and will not provide geolocation information.
- If you are using MISR-specific tools such as misr_view or the MISR ENVI Tools, you should use the stacked-block format.
- If you are using generic HDF or HDF-EOS tools, you should use the conventional format.
Most tools that generically support HDF or HDF-EOS should be able to import conventional format. However, tools that only support generic HDF format will not automatically import the HDF-EOS geolocation information.
ENVI version 4.0 (a product of ITT Visual Information Solutions), can successfully import and display most data fields in a conventional HDF-EOS format file. ENVI does not automatically import HDF-EOS geolocation parameters, but this information can be entered manually, using provided ENVI instructions. ENVI does not support import of HDF fields with more than three dimensions, including the X, Y, spatial dimensions. Therefore, MISR parameters including both camera and band dimensions cannot be displayed in ENVI.
In the conversion from stacked-block to conventional format, data is reprojected onto a flat 2-dimensional grid, so that the MISR blocks can be aligned appropriately with respect to each other. For most fields this reprojection is trivial, requiring no resampling; i.e. the reprojected data is identical in content to the original stacked-block format.
The exception occurs for data at resolutions of 35.2 km and 70.4 km. The root of this problem lay within the stacked-block format, which allows the MISR block locations to be offset with respect to each other by multiples of 17.6 km. At pixel resolutions larger than 17.6 km, the block offset is a fraction of a pixel. Therefore, the MISR data is actually computed over an irregular grid in which the pixels are not lined up in regular columns, but instead are shifted by fractions of their size as shown in the illustration of geolocation error (PDF).
This problem affects the following parameters:
|Level 2 TOA/Cloud Albedo (MIL2TCAL)||AlbedoParameters_35.2_km||35.2 km|
|Level 2 TOA/Cloud Stereo (MIL2TCST)||DomainParams||70.4 km|
|Level 2 TOA/Cloud Classifiers (MIL2TCCL)||CloudClassifiers_35.2_km||35.2 km|
|Level 2 Land Surface (MIL2ASLS)||DomParamsAer||70.4 km|
Data for all parameters listed above are shifted by up to one-half a pixel, to align with the nearest pixel in the conventional HDF-EOS grid. All the data numbers, however, remain the same as those in the original products. The effect is a geolocation error of up to 17.6 km for 35.2 km resolution parameters; and up to 35.2 km for 70.4 km resolution parameters. To account for this error, "Latitude" and "Longitude" fields are provided which give the original location of each pixel prior to shifting.
This means there will be two sources of geolocation information for the above parameters. First is the implicit geolocation of each pixel as defined by the GCTP map projection information. This implicit geolocation information will have an error of up to one-half a pixel. Second is the "Latitude" and "Longitude" fields which explicitly specify the lat/lon location for the center of each pixel. The "Latitude" and "Longitude" fields will always have the correct geolocation information. By comparing the implicit lat/lon against the explicit lat/lon, one can determine which pixels have been shifted, and by how much.
See MISR Data and Information for a complete list of data products and the Data Product Specifications for detailed descriptions of the parameters within those products. The main products are the ellipsoid (MI1B2E) and terrain (MI1B2T) projected radiances for each of the cameras, the browse images (MISBR), geometric (MIB2GEOP) parameters (sun and view angles), and the cloud and sun glint mask (MIRCCM).
Level 1A data (the Reformatted Annotated Product, file names containing "FM_SCI") contain raw digital counts from the MISR camera charge-coupled device (CCD) focal planes. At Level 1B1 (the Radiometric Product, file names containing "RP"), the values correspond to calibrated radiances. At Level 1B2 (the Georectified Radiance Product, file names containing "GRP_ELLIPSOID" or "GRP_TERRAIN") the radiance data are geolocated, and geometric co-registration of the data from the different bands and camera angles is performed by resampling to a Space Oblique Mercator (SOM) grid. L1A and L1B1 use the HDF-EOS "swath" interface, and L1B2 uses the HDF-EOS "grid" interface. Most users do not need the L1A and L1B1 data.
The MISR instrument can acquire measurement in Global Mode (GM), Local Mode (LM), and Calibration. Global Mode is the normal acquisition with pole to pole coverage on the day-lit side or the orbit, full resolution (275m) in all 4 nadir bands and the red band of all other angles and 1.1km sampling in all other off nadir bands.
MISR can be configured to disable the on-board data averaging and provide high resolution images in all 36 channels for selected targets and observation times. This capability is referred to as Local Mode (LM). The result is a scene with a crosstrack pixel spacing of 275 meters, with downtrack sampling also at 275 meters, over a spatial area of approximately 300 kilometers downtrack by 360 kilometers crosstrack. LM acquisition can be requested by providing latitude and longitude coordinates, dates, and explanation of data need to User and Data Services.
Most of the time that MISR is acquiring Earth imagery it operates in a configuration called Global Mode, which allows the spatial resolution to be set for each individual channel (there are 36 channels on MISR: 4 bands at each of 9 angles). Currently Global Mode is defined to provide 275-m resolution data in all bands of the An (nadir) camera and in the red band of all the off-nadir cameras (this is the highest resolution available and is called 1x1 data since it involves no on-board pixel averaging). In the remaining channels 4 samples x 4 lines are averaged within the instrument to create a 4x4, or 1.1-km resolution sample. In the L1B2 product, 1x1 data map to a 2048x512 array, whereas 4x4 data map to a 512x128 array. Thus, samples 1-4 in lines 1-4, for example, in 2048x512 data correspond on the SOM grid to the same location as sample 1, line 1, in a 512x128 array.
In order to resample the data from the nine MISR cameras to the SOM grid, it is necessary to assign an altitude to each location on the grid. In the Ellipsoid product, this altitude is represented by the WGS84 ellipsoid. In the Terrain product, it is the altitude of Earth's terrain. Both products are broken arranged into 180 blocks measuring 563.2 km (cross-track) x 140.8 km (along-track). The Terrain product does not contain valid data for blocks that are considered to be completely over water.
|Illustration of Ellipsoid vs Terrain Projection|
Any MISR target which is not at the altitude at which the L1B2 product is registered will show a residual misregistration, or parallax, from one angle to another. This parallax is a function of the target's height relative to the projection altitude. For example, to visualize both topographic relief and cloud altitudes stereoscopically, the Ellipsoid is the appropriate product. Applications which require the data to be co-registered at the land surface altitude should use the Terrain product.
For blocks which contain no land, the 14 most significant bits (MSB) of the Terrain product are assigned the fill value 16379, and the actual image data must be obtained from the Ellipsoid product.
MISR Level 1B2 data products use various high data values to signify fill, and one of the fill values (16377) in the 14 MSB's of the scaled radiances signifies that this location on the SOM grid was obscured from the camera's view by intervening topography. Because greater amounts of topographic obscuration occur at the more oblique angles, the prevalence of this particular fill value increases as the view angle increases. This particular fill value does not apply to the Ellipsoid product, because for each sample, whatever radiance was observed along the line of sight is projected to the ellipsoid grid location.
The Aerosol data (MIL2ASAE) contains aerosol optical depth, aerosol compositional model, ancillary meteorological data, and related parameters on a 17.6 km grid. The Land Surface data (MIL2ASLS) includes bihemispherical and directional-hemispherical reflectance (albedo), hemispherical directional and bidirectional reflectance factor (BRF), BRF model parameters, leaf-area index (LAI) and fraction of photosynthetically active radiation (FPAR), and normalized difference vegetation index on a 1.1 km grid. MISR Top of Atmospheric/Cloud Stereo (MIL2TCST) data parameters include a stereoscopically-derived cloud mask and cloud height on a 1.1 km grid, and reflecting level reference altitude on a 2.2 km grid. Cloud motion parameters are calculated on a 70.4 km grid. The TOA/Cloud Albedo (MIL2TCAL) data contain local albedo values, and the TOA/Cloud Classifiers (MIL2TCCL) contain altitude-binned cloud classifications and angular cloud fractions. Further information is available at MISR Data and Information.
To obtain the complete information about the aerosol models, you will need to order the MISR Aerosol Climatology Product (MIANACP) which contains the Aerosol Physical and Optical Properties (APOP) and the Mixture files. The Mixture file lists the pure particles in each model identifier. The APOP then gives the detailed information for the pure particles. More information on the MISR aerosol model is available from the MISR publications page and the Algorithm Theoretical Basis Document (ATBD).
The Level 3 Component Global Georectified Radiation Product (CGGRP), Component Global Land Surface Product (CGLS), Component Global Aerosol Product (CGAS), and Component Global Albedo Product (CGAL) are generated for daily, monthly, quarterly, and yearly time periods. These data are globally gridded on several grids including 0.5°x0.5°, 1°x1°, or 10°x10°. Selected parameters from these products are available for viewing from MISR Level 3 Imagery. Further information is available at MISR Data and Information.
The Level 3 products are globally gridded statistical summaries of a range of parameters from select Level 1 and Level 2 products. The Levels 1 and 2 products are in swaths, each derived from a single MISR orbit, where the imagery is 360 km wide and approximately 20,000 km long. At Level 3 the product parameters from multiple swaths are combined to make complete, global maps. The Level 3 products are averages of select Level 2 parameters on daily, monthly, quarterly and annual time scales. The MISR Level 3 Imagery provides easy access to images of select parameters within these products.
The "Clim-Likely" aerosol climatology data set was developed as an initial step in identifying a range of components and mixtures for the MISR Standard Aerosol Retrieval Algorithm climatology, and as one standard against which to compare MISR aerosol air mass type retrieval results. Six component aerosols included in the model were medium and coarse mode mineral dust, sulfate, sea salt, black carbon, and carbonaceous aerosols. Five aerosol air mass "Mixing Groups" and thirteen sub-groups were identified from a cluster analysis of the entire set of data. Each Mixing Group contains the four most abundant component particles in the column for climatologically common aerosol air masses. Each sub-group identifies the dominant particles within the Mixing Group. The data are derived from 'typical-year' aerosol transport model results and are available for individual 1° x 1° grid boxes or as global monthly files. The "Clim-Likely" aerosol climatology data set is available from the ASDC. "Clim-Likely" documentation and examples are available from this reference: Kahn, Ralph, Pranab Banerjee, and Duncan McDonald (2001). The sensitivity of multi-angle imaging to natural mixtures of aerosols over ocean. J. Geophysical Res., 106 (D16), 18219-18238 (PDF).
MISR data and imagery are available for many field campaigns. Select data products are subset for the region and dates of interest. Special gridded regional products may be available as well as Local Mode data for select targets within the field campaign region. Specific details on MISR-supported field campaigns are available.
At Level 1A, the 14 most significant bits (MSB) directly represent the raw digital count from the camera's Charge-Coupled Device (CCD). The 2 least significant bits (LSB) of the 16-bit data values are data quality indicators (DQI). A DQI of 0 means the data meet all specifications; 1 means some degradation in quality has occurred but the data are still valid; 2 means the data should not be used for scientific analysis; and 3 means the data are not usable. At Level 1B1, the image data and the data quality indicators are stored as separate fields. The image data values represent "scaled radiances", which means that they must be multiplied by a scale factor to obtain radiance in units of Watts/square meters/steradian/micrometer. These band-by-band scale factors are stored in the swath metadata of L1B1 files, and are found within the attributes of the swath structures. The data quality indicators follow the same convention described above.
For Level 1B2 data, the image data and the data quality indicators are once again packed into 16-bit words. The 14 MSB's represent "scaled radiances", and the 2 LSB's are DQI values that follow the same convention described above. The conversion from scaled radiance to radiance (Watts/square meters/steradian/micrometer) requires multiplication by a scale factor. These band-by-band scale factors are stored in the grid metadata of L1B2 files, and are found within the attributes of the grid structures. Note that certain high values of the scaled radiances at L1B1 and L1B2 are reserved for fill, as described in the Data Product Specification Document.
Since November 2002 (product version 0016 and later), the MISR L1B2 products contain parameters within the file that make it easier to convert to BRF. The BRF conversion factor for each band is derived from the equation:
(pi * SunDistanceAU2) / (std_solar_wgted_height * cos(SolarZenith))
This factor can be used to calculate the BRF:
BRF = BRF conversion factor * Radiance
The Radiance may be obtained from the Radiance/RDQI by right-shifting 2 bits, then multiplying the result by the Scale factor (radiometric) contained in the Radiance Grid Metadata.
The value is stored in the MISR Ancillary Geographic Product (AGP), of which there are 233 unique files, one for each of the World Reference System-2 (WRS-2) paths. Like the L1B2 products, these are divided into blocks, and the fields within the "Standard" grid of the AGP are provided at 1.1-km sampling. The average elevation over a 1.1-km grid sample are provided in the "AveSceneElev" field. AGP files can be obtained from the MISR Order and Customization Tool and through the ASDC Data Pool.
This process is described in the MISR Data Product Specifications, Appendix A. There are two methods for doing this:
Look up the Lat/Lon of the corresponding block, line, sample in the Ancillary Geographic Product (AGP) data sets.
MISR geolocation information is located in the MISR Ancillary Geographic Product (AGP) files. The fields "GeoLatitude" and "GeoLongitude" contain the desired information. These values are stored on the "Standard" (1.1 km) grid of the AGP; you may need to interpolate or extrapolate if the parameter you are working with is on a different grid. Latitudes are geodetic, and longitudes are relative to the Greenwich meridian, positive to the east and negative to the west.
You will need to obtain the AGP file that corresponds to the path of the data you are trying to geolocate. The path is given in the MISR data file name using the format _Pxxx_ where xxx is the path number (1-233).
Mathematically convert the SOM block, line, and sample (pixel) to latitude and longitude. This is a two-step process. Algorithms are provided in the DPS Appendix A.
Convert(block, line, sample) <=> SOM(x,y)
Requires several metadata values from the data file and some arithmetic
Convert SOM(x,y) <=> Lat/Lon
Requires use of GCTP (General Coordinate Transformation Package) map projection coordinate conversion library in HDF-EOS distribution or some other software that incorporates GCTP, such as IDL.
- Convert(block, line, sample) <=> SOM(x,y)
The Geometric Parameters files (file names containing "GP_GMP") contain, within each SOM block, solar zenith angle, solar azimuth angle, and view zenith angle and view azimuth angle for each of the 9 cameras. The geometry is established by the MISR camera views, the Sun, and the Terra orbit. To conserve data volume, these parameters are provided on a 17.6-km grid, so you will find the data blocks to be dimensioned 32x8. To get values at 1.1 km or 275 m, you can either interpolate these arrays, or, since the geometric parameters vary slowly spatially, the nearest neighbor should provide sufficient accuracy for most applications. Solar zenith and azimuth are also included in L1B2 file version 16 and later.