The AVIRIS Data Facility
The first step in processing a new tape is scanning the tape to make sure it matches the hardcopy list of runs provided by the AVIRIS Experiment Coordinator. Sometimes problems result in short throwaway runs or missing runs. These must be noticed so that the proper site names can be matched with the good data.
AVIRIS processing is done on a per run or per scene (512 lines) basis. However, as the AVIRIS archive media have limited storage space, no more than six scenes is processed at one time.
Normal processing begins with downloading and decommutating the data, known as the download process. AVIRIS data is stored as 10-bit (12-bit for '95 and later) fields, but computers do not have any standard 10-bit data structures. Thus, the first processing reads the data from the tape, pads it with leading zeroes for storage in 16-bit integers, and writes it to magnetic disk.
All data is stored in 16-bit integers, but some data (ex: navigation) is actually encoded 32-bit floating point data. The ADF archiving process "expands" these fields to their proper size for easier understanding. In addition, the image data is reversed (within each scan line), since the data coming off AVIRIS, if displayed directly, is actually reversed from how the data would look from the aircraft. Each line of data is expanded and reversed, with any bad data marked as such, and then written to ADF archive media (currently 4mm tapes).
The AVIRIS archiving process also compiles information about the image, navigation, and engineering data and stores it in the ADF database. This stored data is extracted at will with the Performance Evaulation Programs, or PEPs, which also plot the data to model instrument behavior graphically.
The AVIRIS quicklook images are also created during the archive process. These are initially stored as 2048x1536 Sun rasterfiles, and show band 36 of each of the scenes in the run (up to the maximum of six scenes, longer runs will have more than one quicklook). These are printed and stored in folders in the ADF. Then they are processed to reduce them to 307x1536 strips with no header or separating data. These reduced quicklooks are then stored on the AVIRIS Anonymous FTP Site.
When an investigator wishes AVIRIS data, the data must go through the Product Generation (PG) software. If the investigator wishes raw data, then PG only copies the data from archive tape to the desired distribution medium. However, most of the time the investigator wishes radiometrically corrected data.
Radiometrically corrected means converted into units of radiance, as opposed to the unitless AVIRIS digital numbers. Radiance is measured in units of microwatts per square centimeter per nanometer per steradian, or uW / (cm^2 * nm * sr). AVIRIS radiometric calibration factors are calculated by measuring the response of AVIRIS to an integrating sphere (a known target illuminated by a known light source). This calibration is accurate to within 7%, absolute, over time. Intra-flight accuracy is within 2%.
In addition to radiometric correction, the PG process performs detector readout delay correction when necessary. The data from the instrument's detectors is not read truly simultaneously. For each of the four spectrometers, the bands are read in order, so that the last band is read somewhat later than the first. As the instrument is scanning, the later bands are looking over a slightly different ground position than the first bands. Therefore, PG does a weighted average to "slide" the data back to it's proper position, insuring that for each pixel each of its 224 bands contains data from the same area on the ground.
However, for the 1995 flight season the instrument hardware was improved so that there is no delay in 1995 data.
The ADF also has software for general image processing which is used for image display and creation of pictures for the JPL Public Information Office (such as the image cube at the top of this page) and any technical conferences or presentations that ADF members are involved in. This software is also used by the PEPs and for detailed anomaly analysis.
The ADF hardware are as follows:
1 Sun Microsystems Sun 4/490
1 Solbourne 5E-90
1 IBM RS6000 workstation
2 Sun Microsystems SPARC IPX workstations
2 Sun Microsystems SPARC 1 workstations
2 NCD X-terminals
2 Metrum VLDS (Very Large Data Storage) tape drives
Several HP 4mm tape drives from Artecon
Several Wangdat 4mm tape drives from Apunix
Several EXABYTE 8mm tape drives from Artecon and ANDATACO
1 1/4" tape cartridge drive
1 9-track (1/2") tape drive
1 Kodak XL7700 image printer
1 Sun Microsystems SPARCprinter
IDL (Interactive Display Language) from Research Systems Inc. (RSI)
ENVI (ENvironment for the Visualization of Images), also from RSI
SQL Server and Open Client from Sybase
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