Orbital stitching

Orbital stitching is the process of combining a group of AVHRR images to form an orbital image. Combining the images reduces the data volume in the archive by eliminating overlap areas between images and facilitates ease of use by having to access one image for product generation.

The Global Land 1-KM AVHRR data set consists of only the afternoon (ascending) images from the NOAA satellites, resulting in only one half of the images available world-wide. When these images are stitched, a half-orbital image is generated. The satellite number, acquisition date, start-time, and end-time of each image must be known to stitch each image into its appropriate half orbit. The images are sorted and grouped based on this information. There will be at least a 30 minute time gap (dark side of the earth) between each half-orbit. A half orbit image will consist of one to ten individual images. The following table is an example of three groups of images to be stitched:

The start and end times are used to group the images instead of the orbit number since the orbit number changes at the equator of the ascending pass. A stitched half-orbit starts at the last part of an orbit and ends in the first part of the next orbit.

The lines from each image in the stitch group are checked for bit drops. Several statistical comparisons are made between adjacent lines of an image to identify bad data. The bad data is set to zero. When 60% or more of a line is bad data the line is dropped (zero filled).

The stitching process begins by copying the first image (image in the group with the earliest start time) to the half-orbit image. The start time of the next image is then used to determine if an overlap area or gap between images exists. When the end time of the half-orbit image is after the start time of the next image an overlap area exists. The overlap area begins at the record in the half-orbit image that has the same time stamp as the start time of the next image. When there is an overlap between two images a station priority table, "priority.stations" located in $ADAPSTABLES, is checked to determine which image in the overlap area should have precedence in the stitched half-orbit. Receiving stations which have consistently fewer dropped lines and bit drops in overlap areas have higher priority. Station priorities were determined by monitoring images over a period of time. The priorities were assigned for each station in an attempt to generate a better half-orbit image in an automated process. It does not guarantee the best possible half-orbit because the priorities are based on qualitative judgments. The priority of a station can be easily changed in the "priority.stations" file if continued monitoring finds changes in the data quality from a receiving station. If two overlapping stations have an equal priority, the data from the beginning of the second image will replace data from the end of the previous image. The following is a graphical representation of the stitching process:

Each line from the priority image, in the overlap area, is read and checked to determine if it is a dropped line. When a dropped line is found, the record from the other image, with the same time stamp as the dropped line, is read to determine if it is also a dropped line. If it is not a dropped line, it will be used to replace the dropped line in the half-orbit image. When a replacement line is not found, the dropped line is left and the remainder of the overlap area is checked. The priority and dropped line checking is the same for two or more image overlaps of the same area. The remainder of the next image, past the overlap area, is copied to the half-orbit image. This process is repeated until all the images have been added to the orbital image.

A gap between images exists when the start time of the next image is later than the end time of the half-orbit image. This gap will be zero filled in the half-orbit image to maintain the proper along-track perspective. The number of lines to be filled is calculated by subtracting the end time of the previous image from the start time of the next image and dividing by 0.1666666 (1000 milliseconds divided by 6 scan lines per second). Data that doesn't exist at the beginning or end of the half-orbit image, such as missed images or ocean data, is not zero filled to conserve space. Thus, each half-orbit image may not stretch from pole to pole.

After all of the images in a group are stitched into the half-orbit image a browse image, meta data information, and an archive tape are created. The half-orbit image is copied to Storage Technology tape silo (EDC only), and the images within the half-orbit group are deleted from disk. The browse image is archived to tape and the meta data is transferred to the AVHRR database system.