gac [ parameter=value ... ] [ outputfile ... ] gac [ parameter=value ... ] [ directory ]
Parameters are: first_file, num_files, tape_device, use_lat_range, min_lat, max_lat, use_master, master_file, start_time, num_lines, start_sample, num_samples, channels, calibrate, temp_units.
gac creates datasets from Environmental Data Information Service (EDIS) Global Area Coverage (GAC) tapes. gac reads tapes containing 1, 2, 3, or 5 channel data; 5 channel packed GAC tapes with missing headers are also supported. Multiple passes on a single tape can be processed in succession with this function. The user can specify one or more channels to extract.
A spatial subset can be specified in one of three ways:
1) the smallest rectangular area required to cover a master dataset.
2) a rectangular area of specific size beginning at a given time and scan sample.
3) A rectangular area of specific width, beginning at a user-specified scan sample, and intersecting intersecting a user-specified latitude range.
Data can be left as raw counts or converted to radiances or brightness temperatures. If the output is raw counts, the hrpt header records are stored in the dataset along with the counts. If the data are converted to radiances or temperatures, the header information is not retained.
There are no input datasets. The source for input data were described above. Multiple output TeraScan datasets are supported. Output is either to a user supplied directory or a user supplied list of dataset names. If a directory name is entered, output dataset names will automatically be generated and the files written to the output directory. The format of the output file names is n#.yyddd.hhmm, where # is the NOAA satellite number, yyddd is the julian date, and hhmm is the start hour and minute.
This is number of the pass on tape at which processing is to begin. This number is relative to the current position on the tape. For example, if the tape is positioned at the beginning of the tape, first_file=2 will process the second pass on the tape. If the tape is positioned at the third pass on the tape, first_file=1 will process the third pass and first_file=2 will process the fourth pass.
Valid range [>=1]. The default is 1.
This is number of consecutive passes to process when the outputfile is specified to be a directory.
Valid range is [>=1]. The default is 1.
This is the name of the input tape device. Tape device names are machine specific.
Valid responses for gac on the Sun SPARC Station:
/dev/nwd0 - DAT tape, no rewind
The default is extracted from the UNIX environment variable TAPE.
Indicates that only data corresponding to AVHRR lines that intersect a user specified latitude range will be ingested.
Valid reponses are [yes or no]. The default is yes.
Minimum and maximum latitudes, used only when use_lat_range=yes. If min_lat is greater than max_lat, they are interchanged invisibly to the user.
Latitudes must be specified in standard format, e.g., 34.5 n, 34 30 n, or 34 30 00 n. See formats. There is not default for either parameter.
Used only if use_lat_range=no. Answer yes if the data is to be selected from the intersection of a master dataset (number 1 in the description above, also see master). Answer no if subsets are to be specified by start time and sample (number 2 in description above).
Valid responses are [yes or no]. The default is yes.
If use_master=yes, this is the name of the master dataset used to specify a region for data extraction (see master).
Valid responses are any TeraScan dataset that contains an earth transform. The default is Master.
If use_master=no and use_lat_range=no, this is the start time for data extraction. The first line with a time greater than start_time is the first output line. A response of 00:00:00 instructs gac to begin processing at the first scan line.
Any valid time is allowed (see formats). The default is 00:00:00.
If use_master=no and use_lat_range=no, this is the number of lines written to the output dataset. If the end of pass is detected before the output dataset is complete, the output dataset is truncated.
The valid range is [>=1]. The default is 700.
If use_master=no, this is the first sample to extract from the pass. Samples are numbered starting with 1.
The valid range is [1 to 409]. The default is 1.
If use_master=no, this is the number of samples written to the output dataset. If the edge of the pass is detected before the output dataset is complete, the output dataset is truncated.
The valid range is [1 to 409]. The default is 409.
This is a list of the AVHRR channels to extract from the HRPT pass. The five channels correspond to 0.6-0.7, 0.7-1.1, 3.5-3.9, 10.3-11.3, and 11.5-12.5 micrometers.
Valid responses are [1,2,3,4,5]. The default is 1,2,3,4,5.
Answer yes to convert raw sensor counts to radiance or temperature using EDIS provided transforms. Answer no if raw count data is desired. Raw data can subsequently be calibrated and converted using the avcal function.
Channels 1 and 2 are converted to radiance, with units 100*albedo. Channels 3, 4, and 5 are converted to brightness temperatures with units of Celsius, Kelvin or Fahrenheit.
Valid responses are [yes or no]. The default is no.
If calibrate=yes, it is necessary to specify the temperature units. Temperature units can be degrees celsius, kelvin, and fahrenheit.
Valid values are [ celsius, kelvin, fahrenheit]. The default is celsius.
Extract two TeraScan AVHRR datasets named dataset1 and dataset2 from two consecutive GAC files archived on 1/2" computer compatible GAC tape. The area for data selection is specified by the master dataset.
[1] % gac dataset1 dataset2 first_file : int ? [1] tape_device : char(16) ? [/dev/nwd0] use_lat_range : (char 3) ? [yes] no use_master : char( 3) ? [yes] master_file : char(75) ? [Master] channels : int( 5) ? [1 2 3 4 5] calibrate : char( 3) ? [no] yes temp_units : char(10) ? [celsius] Is tape up ? y dataset1 starts at time 13:38:29.679, sample 1. Actual size is 500 lines by 400 samples. Processing tape file 2 dataset2 starts at time 02:35:23.679, sample 1. Actual size is 502 lines by 403 samples.
Extract one TeraScan AVHRR dataset from a GAC file archived on 1/2" computer compatible tape. Area is selected from the beginning of the file by specifying the time and start sample.
[2] % gac dataset1 first_file : int ? [1] tape_device : char(16) ? [/dev/nwd0] use_lat_range : (char 3) ? [yes] no use_master : char( 3) ? [yes] no start_time : char(15) ? [00:00:00] num_lines : int ? [700] start_sample : int ? [1] num_samples : int ? [409] channels : int( 5) ? [1 2 3 4 5] calibrate : char( 3) ? [no] Is tape up ? yes Processing tape file 1 dataset1 starts at time 13:38:29.679, sample 1. Actual size is 700 lines by 409 samples.
lac, avin, avcal, master, etx, avhrr, nitpix, formats
Only AVHRR data is available on the EDIS LAC or GAC tapes. Due to an unfortunate decision by EDIS, the TIP header data has been removed, thus negating the possibility of ingesting TOVS or DCS data from LAC or GAC tapes.
Last Update: $Date: 1998/05/29 17:17:18 $