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Up Topic Frequently Asked Questions / Data Access FAQ / Can I process data on a computer that's not connected to the internet? (locked)
- By mike Date 2008-07-07 13:54 Edited 2008-07-08 11:17
Can I process data on a computer that's not connected to the internet?

Currently only two types of data processing may require ancillary data files to be downloaded from the internet. These two types of processing are the Level-2 processing for all sensors, and the MODIS Level-1A geolocation file creation. Keep in mind that if you download the required ancillary data in advance and place it in the correct directories on your computer, then an internet connection is not required during the processing itself.

Level-2 Processing

The SeaDAS Level-2 processing program (l2gen) requires ancillary meteorological and possibly SST data as an input. These input files can come from either near-real time (NRT) data files available on the internet or from less accurate climatology files included with SeaDAS. Processing will complete successfully without downloading METOZ and OISST files since SeaDAS will fall-back on the climatologies. Using the climatologies is fine for learning how to use SeaDAS, but to get the best products the correct ancillary data shoud be used. (Using climatological data can result in data differences over 10%.)

These ancillary files can be easily downloaded in advance for your time period of interest and placed in the $SEADAS/var/metoz/ and $SEADAS/var/oisst/ directories. See this post for specific instructions.

Also note that OISST is only required for MODIS SST and a few certain product algorithms for all sensors (such as Carder products).


MODIS Level-1A Geolocation Processing

MODIS geolocation processing requires ancillary attitude and ephemeris files (with the exception noted below for Terra). SeaDAS automatically determines the files it needs and retrieves them from the internet during processing if they don't already exist on the user's hard disk. If your computer isn't connected to the internet, the attitude/ephemeris files can be downloaded in advance from our ftp server and placed in the $SEADAS/var/modis/atteph/ directory under their correct year and day subdirectories. The files are located here:


However, there is an exception to this requirement with MODIS Terra processing (not for Aqua). Since the Terra L1A file format includes onboard navigation information, the geolocation processing can be done without using ancillary attitude/ephemeris files. The geolocation accuracy will not be quite as good as when using the separate definitive attitude/ephemeris files, but should be adequate for most users. So to avoid the internet dependancy for Terra, the user must disable use of definitive attitude/ephemeris files in the GUI geolocation processing window or on the command-line.

Also, if performing geolocation processing on recent MODIS Level-1A data, the Earth motion file utcpole.dat and the leap seconds file leapsec.dat files must be kept up-to-date in the $SEADAS/var/modis/ directory. Though the SeaDAS processing programs will automatically download them if they're over 14 days old, it is still suggested that they be updated on a weekly basis by running the modis_update_utcpole_leapsec.csh script as a cron job.
Up Topic Frequently Asked Questions / Data Access FAQ / Can I process data on a computer that's not connected to the internet? (locked)

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