The acquisition software looks at a "master schedule" to determine which images are to be acquired. When an image is acquired it is placed in a temporary network directory. A process in the background monitors the network directory for images and transfers them to the processing system. For more information on acquisition processing refer to Appendix A: Acquisition Dependancies of the System Managers Guide.
Receiving stations that can acquire HRPT, LAC, or GAC data require some type of mechanism to select and process desired images. ADAPS does this by generating a "master" schedule of images to be acquired. The "master" schedule is used to determine if a image is to be acquired. All AVHRR images acquired from DOMSAT are stored on disk until the data link is terminated. The "master" schedule is then checked to determine if the images should be processed or deleted.
ADAPS includes an interactive X-window system based scheduler (XSCEHD) that can be used to schedule both live (HRPT, LAC and GAC) acquisitions and data received via the DOMSAT. All images selected for acquisition are stored in the "master" schedule.
The "master" schedule contains the scene ID, satellite number, orbit number, start and end times of acquisition, station id, type of data (HRPT, LAC, or GAC), acquisition operation code (don't acquire, acquire the image to disk, acquire the image to High Density Tape (HDT), acquire to both, or pull the image from the HDT), status flag (pending, on disk, on tape, missed), tape file number (if the image was acquired to HDT), and a flag indicating if an image was acquired during the day or at night. The following is an example "master" schedule information:
Any additions, modifications, or deletions to the "master" schedule can be accomplished with the schedule editor.
For detailed information about scheduling refer to XSCHED, XPLOT, AUTOGEN, and SCHEDIT User's Guides.
To automate the reference aid processing and archiving the following directory structure was set up.
The TAE batch capability and procedure definition files (PDFs) are used to automate the ingesting, reference aid generation, and archiving of the acquired AVHRR data. Each batch procedure looks for images in a specific directory (using environment variables), processes them, and the processed output is written or moved to another directory for continued processing, until the image is archived. The batch procedures also generate log files for image process tracking. In this way the operators of ADAPS only need to check the error directory for images that had errors during processing and look at the log files for specific errors or tracking information based on an image scene ID.
The following is a graphical representation of the automated flow and interactions of the systems and peripherals for the data acquisition and archive process:
ADAPS also has a robust capability to ingest AVHRR data received over the network or on recorded media. The data received from other sources in this manner can be of varying formats and may contain ancillary files or records that may or may not be useful. A pre-processing step is used to extract the image data from the various media and formats. The extracted image data is ingested and reformatted into the standard EDC AVHRR archive. Refer to the Data Ingest section for a detailed description of this process.
AVHRR products generated with ADAPS and LAS applications can be radiometrically corrected, precision corrected, geo-registered to a specified projection, have associated solar zenith, satellite zenith and relative azimuth bands calculated, and atmospherically corrected for Rayleigh and ozone affects.
One of the standard products generated using ADAPS/LAS is a composited NDVI. The following is an example flow of an EDC AVHRR archive image in the composited NDVI product:
Problems similar to those which cause data gaps but only affect bits within the data are called bit drops. Bit drops can corrupt the image data or date and time values. When an unexpected date or time value is encountered due to a bit drop, the date is corrected with the previous line's date, and the time is corrected to be one sixth of a second from the previous line. When bit drops occur within the image data they can be detected and set to zero with the LNDETECT function. LNDETECT uses several statistical comparisons between adjacent lines and pixels of the archive image to identify corrupt data. When corrupt data is found it is set to zero so that it will not be used as part of the NDVI product.
To take advantage of the image processing capabilities of LAS an AVHRR archive image must be converted to the LAS image format. AVHRRIN converts an AVHRR archive image to a LAS image. Three subcommands are supplied to window the AVHRR image using line and sample, latitude and longitude, or a latitude range. The input image may be located on disk or on an AVHRR archive tape.
The HRPT minor frame data (".mnr"), dropped line data (;drpl), and the AVHRR data descriptor record (".addr") files are created and associated (same root name) with the AVHRR LAS image. For more information on the format and content of the minor frame file refer to the Calibration section of the ADAPS Programmer's Guide. The minor frame data can be displayed in LAS with DSPMNR and the ADDR can be displayed in LAS with DSPADDR.
AVHRRCAL radiometrically corrects the visible and thermal bands of an AVHRR LAS image. The visual and thermal sensors onboard the NOAA satellites experience a degradation of performance over time, thus making in-flight calibration necessary. A slope/intercept coefficient pair for each band is generated and applied linearly to the image data. The calibrated data for the visible bands (1, 2 and 3A) may be output as percent refectance or radiance. The thermal bands (3B, 4 and 5) are converted to temperature (Kelvin). The calibrated data is scaled to maintain desired accuracies. Refer to the AVHRRCAL User Guide for more detail on calibration of AVHRR data.
NOAA conducts prelaunch calibration of the visible bands. The visible band coefficients may be based on these prelaunch values, or may take into account a linear degradation of the sensor over time, based on how long the satellite has been in orbit. Also, corrections may be made for variability in solar irradiance and earth-sun distance. For more information on the prelaunch and degradation of the sensor refer to the Calibration section of the ADAPS Programmer's Guide.
To calibrate the thermal bands, the sensor takes, along with each image line, a reading of space and a part of its housing which is a constant temperature internal calibration target (ICT), or blackbody. The blackbody's true temperature is monitored by four onboard platinum resistance thermometers (PRT's). The constant zero radiance of space and the radiance from the PRT readings of the blackbody are compared to the sensor's readings of space and the blackbody to provide a two-point (linear) calibration. The PRT counts are retrieved from the HRPT minor frame (".mnr") associated with the AVHRR LAS image.
A raw AVHRR image is converted to a projected image by using a model that maps from line/sample location in satellite perspective to geographic latitude/longitude coordinates. An image that has been reprojected using this model, called a systematically corrected image, may have slight errors because the location and attitude of the satellite are not known exactly.
Navigation refers to the geo-registration of an AVHRR image by using ground control to correct the model. There are two methods for automatic navigation: one uses a vector source of reference, and the other uses a registered base image. The navigation process contains a series of modular steps that result in a registered, projected image (precision corrected). A set of coefficients for roll, pitch, yaw, and altitude corrections are generated to refine the satellite model. These coefficients are stored in the ADDR file (".addr") associated with the LAS AVHRR image. The corrections to the satellite model are applied when generating the geometric grid.
RECTIFY generates a geometric mapping grid and optionally generates a viewing angles grid. The geometric mapping grid defines a transformation from the image's satellite projection to another projection. This grid can then be used by GEOM to transform the image into the desired projection.
The angles grid contains the satellite zenith, the solar zenith, and the relative azimuth angles at grid locations in the output projection. ANGINTERP uses the angles grid to generate a three band angle image. A bilinear interpolation is used to generate the angles between the grid points.
GEOM performs the geometric rectification of an image as specified by a previously generated mapping grid using a one-pass, two-dimensional; a two-pass, one-dimensional; or a three-pass, one-dimensional algorithm. Resampling is accomplished using nearest neighbor interpolation, parametric cubic convolution interpolation, bilinear interpolation, or a user-entered table of resampling weights. See the RECTIFY, GEOM, and PROJPRM User's Guides for more detail on supported projections and rectification.
ANGINTERP creates the satellite zenith, solar zenith, and the relative azimuth angles for a projected AVHRR LAS image. These bands are created using angle values from selected grid points stored in an angles grid created by RECTIFY. The grid is used to generate an image containing the view angles for every pixel by interpolating the values at the surrounding grid points. The view angles are scaled to maintain desired accuracies.
NORMD calculates a normalized difference vegetation index (NDVI) image. The vegetation index can be used to monitor vegetation condition among other things. The NDVI is calculated from bands one and two of AVHRR data by subtracting band one from band two and dividing by band two plus band one. The NDVI values are scaled to maintain desired accuracies.
COMPOSITE generates a composite image using up to 52 different images. To generate a composite image, the input images are compared pixel by pixel with a specified band. The values for each pixel in the output image are taken from the image with the minimum or maximum value in that band. The last band of the output image identifies the image from which each pixel was selected.
ATMOCOR applies an ozone and Rayleigh scattering correction technique to an AVHRR LAS image containing calibrated radiance data from the AVHRR visible bands. The image data is atmospherically corrected using a series of Look-Up Tables (LUT), a LAS image containing satellite zenith, solar zenith, and relative azimuth angle information, a LAS image containing elevation data, and optionally a LAS image containing ozone data. All input images must be in the same projection space. The atmospherically corrected data is scaled to maintain desired accuracies.
The AVHRR Data Set products are processed to maintain the maximum precision of the data. The AVHRR channels 1-5 are scaled to 10-bit precision within a 16-bit (signed) integer data type. The NDVI, viewing geometry, and data pointer are stored as byte data. The data range, scale, offset, and scaling methods are available for US conterminous processing and 1KM global processing.
PRODSUM reads information from files associated with the AVHRR LAS image to produce an ASCII report containing a summary of information used to generate a product. The product summary contains information about the input scene, projection information, describes the output files, and describes the tape format. The product summary is the first file on tape. The following is an example of a product summary report:
AVHRR PRODUCT SUMMARY REPORT ---------------------------- Account: 000019 Date: 04/04/94 Order-Unit #: 9000-238 Scene Identification: ah11040194132245 Orbit: 1894 Acquisition Date: 04/01/94 Time: 13:12:45 Product Description: Global 1km Satellite: 11 Output Data Volume Description ------------------------------ Density: high Data Format: BSQ Data Type: I*2 Data Precision: 10 Number Lines: 100 Bands: 10 Number Samples: 100 WDB2 Layers: AF Production Information ---------------------- Pixel Size: 1000 Projection: goodes Resampling Technique: NN Center of (1,1) Pixel: X = 022220 Y = 00009 Area of Interest: Lat/Long Line/Samp Pixels From Nadir UL: 12345 12345 12345 12345 12345678 UR: 12345 12345 12345 12345 12345678 LL: 12345 12345 12345 12345 12345678 LR: 12345 12345 12345 12345 12345678 Source Data Information ----------------------- Source: orbit Precision/Resolution: .001 Tape Summary ------------ File 1: Product Summary File 2: Band 1: Channel One Band 2: Channel Two Band 3: Channel Three Band 4: Channel Four Band 5: Channel Five Band 6: I*2 Normalized Difference File 3: Band 1: Satellite Zenith Band 2: Solar Zenith Band 3: Relative Azimuth Band 4: Byte Normalized Difference Band 5: Date File 4: Processing History
The following is a description of a geo-registered product format:
Output Product Tape File 1 Product Summary 80 Bytes per line o ASCII text information about the output geo-registered product. File 2-X: Image Products Defined in the "Output o Geo-registered, Calibrated, Data Volume Description" Normalized Difference, and of the Product Summary. Satellite/Sun Angle images. File X+1: Processing History 80 Bytes per line o ASCII text information about the processing that occurred to generate the products.