Ancillary Data Descriptions
Ancillary Meteorological Data
This is the NCEP Meteorological (MET) ancillary data used for
L1-L2 processing. The files are updated every 4 hours on
http://oceandata.sci.gsfc.nasa.gov/Ancillary/Meterological
and are organized by YYYY/DDD.
The SeaDAS Level-2 processing program (l2gen) requires meteorological
data as an input. The input file(s) can come from either near-real time
(NRT) or climatology sources. All ancillary inputs into l2gen are in the same
SeaWiFS-specified HDF file format.
The default for the Ocean Data Production System is to use NRT
ancillary data, whereas the current default for SeaDAS is to use the
climatology ancillary data which are distributed with SeaDAS
in the directory $SEADAS/run/data/common/. It is possible to instead
specify NRT ancillary inputs when running l2gen within SeaDAS.
The NRT MET data available from the Ocean Biology Processing Group
have already been quality-controlled and can be input directly into
l2gen. The current source for MET data is NCEP. The ancqc program
can be useful in displaying and optionally editing the NRT HDF files.
It is also possible to view the qc arrays in the NRT files with the
ancqc program. The qc arrays track if any points had been modified
from the initial source values.
Ancillary Ozone Data
The Level 2 data processing within l2gen also requires ozone data as an
input. The files are updated every 4 hours on
http://oceandata.sci.gsfc.nasa.gov/Ancillary/Meterological
and are organized by YYYY/DDD.
The ozone component of the atmosphere at a given time affects the
atmospheric signal measured by a satellite and its concentration needs to
be taken into account and removed from the atmospheric signal.
The Ocean Biology Processing Group makes use of several sources of
ancillary ozone in the processing of both SeaWiFS and MODIS data.
See http://oceancolor.gsfc.nasa.gov/VALIDATION/ozone.html
for more information.
NOAA OISST
This is the NOAA Optimum Interpolation (OI) Sea Surface Temperature
analysis archive, available at
http://oceandata.sci.gsfc.nasa.gov/Ancillary/OISST.
The MODIS SST algorithm requires a reference field for initial guess and
quality assessment. The Reynolds optimally interpolated 1-deg grids on 8-day
time-scales was the source for this reference SST field in past reprocessings.
All processing will now use the new Reynolds 0.25-deg daily optimally
interpolated SST fields. In the OBPG environment, these netCDF files are simply
renamed for consistency with standard conventions. As in previous
reprocessings, there is no sea surface temperature (SST) requirement for ocean
color processing; however, it is expected that SST knowledge will be required
for future ocean color product suites such as inherent optical properties.
The Reynolds 0.25-deg daily optimallyinterpolated SST fields
are available at:
http://oceandata.sci.gsfc.nasa.gov/Ancillary/Meterological.
These files can be used as input for the MDOIS L1-L2 SST processing instead
of the climatological SST input data ($SEADAS/run/data/common/sst_climatology.hdf).
SST is also required for certain derived product algorithms. For sensors
without SST capability, the OISST file can be provided as input for use in
derived product algorithms. Otherwise, the climatology will be used.
Aqua and Terra Attitude and Ephemeris
These files are either the definitive attitude and ephemeris data
for MODIS Aqua and Terra, or are the near-real-time (NRT) attitude and
ephemeris for Aqua. (Terra has NRT attitude and ephemeris embedded
in its Level-0 data files.) Attitude and ephemeris data are required for
producing geolocation files from Level 0 or Level 1A files.
SeaDAS includes a feature to automatically download both definitive
and real-time attitude and ephemeris files during processing. It is
suggested to simply use this feature to ensure you have the correct files.
These files can be manually downloaded from:
http://oceandata.sci.gsfc.nasa.gov/Ancillary/Attitude-Ephemeris/
These files must be placed under the $SEADAS/run/var/modis/atteph/
directory in their corresponding subdirectories divided by year and day.
There are different rules for what attitude and ephemeris files are
required depending on the start time of the input file, and these
rules are handled by the processing scripts. To ensure you have the correct
files you can simply download all the attitude and ephemeris files for the
day before, of, and after the day of the file you are processing. Remember
to place the attitude and ephemeris files in your corresponding
$SEADAS/run/var/modis/atteph/YYYY/DDD/ directories.
Again, is highly recommended to simply use SeaDAS's auto-download feature.
utcpole.dat and leapsec.dat
The Earth motion file "utcpole.dat" and the leap seconds file
"leapsec.dat"
are required for MODIS geolocation processing and are included with SeaDAS.
These files must be kept up-to-date. 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
update_luts.py script
as a cron job. To quantify the errors caused by not updating these files
(called "latency") Peter Noerdlinger has prepared an excellent
document.
elements.dat
The SeaWiFS program l1agen_seawifs needs a file called elements.dat to
calculate the navigation information that is included in the L1A output product.
The data in elements.dat are orbit position and velocity elements produced by
l1agen_seawifs, using onboard GPS data and a high order orbit propagation model,
and are used to meet navigation accuracy (goal is 1 pixel). The elements.dat
file is needed as input to navigation algorithms in order to initialize the
GPS filtering methods in l1agen_seawifs.
To obtain the best navigation, users should download the most recent version
of elements.dat from the oceans.gsfc.nasa.gov ftp site every 2 or 3 days if
l1agen_seawifs is run regularly. If l1agen_seawifs is run on an occasional basis,
the elements.dat file should be downloaded before running the program. The
most recent elements.dat file may be used for any previously collected L0 data.
Running l1agen_seawifs will automatically update the elements.dat file.
If l1agen_seawifs is run on a daily basis, a fairly accurate elements.dat file
can be maintained. However, it is still recommended to download elements.dat
every 2 or 3 days since it is derived using a larger number of GPS data and
may incorporate occasional navigation corrections. (Users will be notified of
any navigation corrections.)
The elements.dat file is available from:
http://oceandata.sci.gsfc.nasa.gov/Ancillary/LUTs/seawifs/elements.dat
