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Extension of MODIS Ocean Processing Capabilities to Include
the 250 & 500-meter Land/Cloud Bands

Bryan Franz
NASA Ocean Biology Processing Group
21 June 2006
Revised: 23 October 2006

See also: Franz, B.A., P.J. Werdell, G. Meister, E.J. Kwiatkowska, S.W. Bailey, Z. Ahmad, and C.R. McClain (2006). MODIS Land Bands for Ocean Remote Sensing Applications, Proc. Ocean Optics XVIII, Montreal, Canada, 9-13 October 2006.

Introduction

The 36 spectral channels of the MODIS instrument were selected to support observation of clouds, land, and oceans. The traditional channels used for ocean color observation are the 9 bands in the 412-869 nm spectral regime, with a spatial resolution of 1000-meters at nadir. These ocean bands were designed with high sensitivity over the dynamic range of reflectances typical over open oceans, including contributions from the surface and the atmosphere. Over highly turbid coastal and inland waters, it is possible for this dynamic range to be exceeded, such that the bands saturate and the true signal is unkown. Other bands on MODIS were specifically designed for land and cloud observations, with both increased spatial resolution and reduced sensitivity over a broader dynamic range. These land/cloud bands overlap the spectral range of the ocean bands and extend into the short-wave infrared (SWIR), from 469 nm to 2130 nm. The ocean processing code developed by the OBPG, the Multi-Sensor Level-1 to Level-2 code (msl12), has been extended to support these additional bands. The primary purpose of this effort is to provide a mechanism for exploring the potential value of the increased spectral information, as well as the higher spatial resolution and saturation limits of the land/cloud bands, for application to coastal and inland waters. The extended band suite is shown in the Table 1 below. For completeness, the additional thermal bands used for 4um and 11-12um SST retrievals are shown in Table 2.

It must be emphasized that our goal in this effort is to provide the capability to the research community for investigating potential applications of the additional spectral bands for ocean remote sensing. Within the OBPG, we will not be producing any standard geophysical products that require the 250 and 500-meter bands. Our primary effort has been to characterize the instrument response (e.g., polarization sensitivities, relative spectral response functions) in the additional channels, in a manner consistent with what we have done for the standard ocean bands, and to develop the appropriate software and tables to facilitate retrieval of oceanic optical properties at the additional spectral channels. We have also developed some mechanisms for utilizing the increased spatial resolution, and we have added additional options to our atmospheric correction algorithm for utilizing the SWIR information. We are providing these additional capabilities to the research community through SeaDAS, with the hope that it will encourage evaluation and application development.

Table 1: Extended MODIS Band Suite for Oceans

Band Number Wave Length (nm) Band Width (nm) Spatial Resolution (m) Signal-to-Noise SNR Radiance
(mw/cm^2/um/sr)		 Saturation Radiance
(mW/cm^2/um/sr)
8 412 15 1000 880 4.49
9 443 10 1000 838 4.19
3 469 20 500 243 3.53
10 488 10 1000 802 3.21
11 531 10 1000 754 2.79
12 551 10 1000 750 2.10
4 555 20 500 228 2.90
1 645 50 250 128 2.18
13 667 10 1000 910 0.95
14 678 10 1000 1087 0.87
15 748 10 1000 586 1.02
2 859 35 250 201 2.47
16 869 15 1000 516 0.62
5 1240 20 500 74 0.54
6 1640 * 35 500 275 0.73
7 2130 50 500 110 0.10
* the 1640 band on MODIS-Aqua is non-functional



Table 2: Thermal Bands Used for SST

Band Number Wave Length (nm) Spatial Resolution (m) NEdT (K) Radiance
(mw/cm^2/um/sr)
22 3.9 1000 0.07 0.067
23 4.0 1000 0.07 0.079
31 11 1000 0.07 0.955
32 12 1000 0.05 0.894

Input Level-1B Products

The standard MODIS Level-1B format divides the calibrated radiances fields into three separate files corresponding to the three distinct spatial resolutions of 250, 500, and 1000-meters, with filename identifiers QKM, HKM, and 1KM used to distinguish the quarter, half, and 1-kilometer variants, respectively. The 1KM file is also sometimes called LAC, based on historical conventions. The spectral bands associated with each file are provide in Table 3. Note that the HKM file also includes the two 250-meter bands (marked with * in Table 3). The radiances from the 250-meter bands are averaged to 500-meter spatial resolution, and written to special "aggregated" fields in the HKM file. Similary, the radiances from the 250 and 500-meter bands are averaged to 1000-meter resolution and stored in the 1KM file.

Table 3: Spectral bands as distributed in each Level-1B file
* aggregated to 500-meters, ** aggregated to 1000-meters

QKM HKM 1KM
645 nm 645 nm * 645 nm **
859 nm 859 nm * 859 nm **
469 nm 469 nm **
555 nm 555 nm **
1240 nm 1240 nm **
1640 nm 1640 nm **
2130 nm 2130 nm **
412 nm
443 nm
488 nm
531 nm
551 nm
667 nm
678 nm
748 nm
869 nm
3.9 um
4.0 um
11 um
12 um


The OBPG currently distributes Level-1A and Level-1B products via the Ocean Color Web (OCW). For MODIS, the primary purpose of the OBPG is to support global ocean color and SST production and distribution at 1km resolution. A document describing the OBPG processing flow is here.

Level-2 Processing

As discussed in the User's Guide, msl12 supports the processing of observed radiances from a variety of sensors, with the option to ouput a host of geophysical products. Two of the sensors supported by msl12 are MODIS on Aqua and MODIS on Terra, identified as MODISA and MODIST, respectively. To these we now add HiRes MODIS variants HMODISA and HMODIST. It is important to understand that these are treated as separate sensors within msl12, with separate atmospheric tables and default parameter sets. This was done to simplify maintenance of standard MODIS ocean color processing, during the development and evaluation of HiRes MODIS capablities. msl12 determines which sensor it is processing by examining the input file.

Spatial Resolution and Geolocation
If the HKM and QKM files are provided, msl12 now has the ability to process at 500 or 250-meter resolution. This is controlled through a new input parameter called resolution, which recognizes values of 250, 500, 1000. When processing at 250-meters, MSl12 bilinearly interpolates the 500-meter and 1000-meter band radiances to 250-meter resolution as well, so that the full band set is co-registered. It should be recognized, however, that only the 645 and 859-nm channels are truly at 250-meter resolution. Processing at 250-meter resolution requires the existance of all three Level-1B files: QKM, HKM, and 1KM (or LAC). When processing at 500-meter resolution, the QKM file is not required, as the code will use the aggregated fields within the HKM file for the 645 and 859-nm radiances, as well as the standard 500-meter channels, and it will interpolate the 1000-meter radiances from the 1KM file to 500-meter resolution. When processing at 1000-meter resoution, only the 1KM (or LAC) file is required, as this contains all spectral channels in native or aggregated form.

The processing of MODIS from Level-1B calibrated radiances to Level-2 geophysical products also requires a separate geolocation file. The geolocation file can be obtained from the Goddard DAAC or generated from the Level-1A files using SeaDAS. This file defines the centers of the 1km pixels, so msl12 will perform appropriate interpolations when processing at higher resolutions. There is no HKM or QKM equivalent geolocation file. The interpolation follows the methodology outlined by Liam Gumley, University of Wisconsin, as presented here (section 5). All path geometry (solar and sensor view zenith and azimuth) are similarly interpolated, as are the radiances when going from lower to higher spatial resolutions.

Input File Rules
The parameter "ifile" is used to specify the input Level-1 file to msl12. However, for MODIS processing at resolutions above 1000-meters, 2 or more input files are required. msl12 locates these additional files by assuming some standard naming conventions are being maintained. It is assumed that the three files are identical in filename, varying only in the existence the letters QKM, HKM, and 1KM (or LAC).

For example, a set of Level-1B files from the Goddard DAAC look like this:

MYD021KM.A2002312.0000.004.2004046053615.hdf
MYD02HKM.A2002312.0000.004.2004046053615.hdf
MYD02QKM.A2002312.0000.004.2004046053615.hdf

while a set of files generated by SeaDAS look like this:

A2005109180000.L1B_LAC
A2005109180000.L1B_HKM
A2005109180000.L1B_QKM

Either convention will work. Other possibilities are:

foo1KMbar
fooHKMbar
fooQKMbar

or even:

blah (assumed to be the 1KM file)
blah_HKM
blah_QKM

with the latter being a special case preferred by certain Canadians. Any one of the three files can be provided as the input to msl12, and it will locate the others as needed. The specific input file will determine the resolution at which the data will be processed, but this can be changed by the "resolution" parameter. The matrix is shown in Table 4.

Table 4: Effect of msl12 Input Parameters on Processing Behavior

Parameter
ifile resolution Required Files Effect
1KM 250 1KM, HKM, QKM Processing at 250-meter resolution
1KM 500 1KM, HKM Processing at 500-meter resolution
1KM 1000 1KM Processing at 1000-meter resolution
1KM -1 1KM This will revert to standard 9-band, 1000-meter MODIS ocean processing
HKM 250 1KM, HKM, QKM Processing at 250-meter resolution
HKM 500 1KM, HKM Processing at 500-meter resolution
HKM 1000 1KM Processing at 1000-meter resolution
HKM -1 1KM, HKM Processing at 500-meter resolution
QKM 250 1KM, HKM, QKM Processing at 250-meter resolution
QKM 500 1KM, HKM Processing at 500-meter resolution
QKM 1000 1KM Processing at 1000-meter resolution
QKM -1 1KM, HKM, QKM Processing at 250-meter resolution


Default Products
As stated, the purpose of providing access to the higher resolution bands is to encourage and support the development of new algorithms or applicatons by the research community. At present, the additional spectral bands in the visible can be processed to water-leaving radiances or remote sensing reflectances. For a given set of sensor wavelengths, the available wavelength-dependent products that can be produced with msl12 are described in the User's Guide. The default product set for HMODISA and HMODIST is currently set as shown in Table 5.

Table 5: Default Product Suite for HiRes MODIS

Product Description
chlor_a chlorophyll-a based on standard MODIS OC3 algorithm (443,488,551)
K_490 diffuse attenuation at 490nm
nLw_412 normalized water-leaving radiance at 412nm
nLw_443 normalized water-leaving radiance at 443nm
nLw_469 normalized water-leaving radiance at 469nm
nLw_488 normalized water-leaving radiance at 488nm
nLw_531 normalized water-leaving radiance at 531nm
nLw_551 normalized water-leaving radiance at 551nm
nLw_555 normalized water-leaving radiance at 555nm
nLw_645 normalized water-leaving radiance at 645nm
nLw_667 normalized water-leaving radiance at 667nm
nLw_678 normalized water-leaving radiance at 678nm
sst sea-surface temperature from 11-12um channels
tau_869 aerosol optical thickness at 869nm
angstrom_531 aerosol angstrom exponent (531,869)
eps_78 aerosol model epsilon (748,869)


New Products
The default product suite is effectively just the standard MODIS ocean product suite with additional water-leaving radiance products included. In addition, J. O'Reilly has provided a new chlorophyll algorithm based on the two 500-meter bands at 469 and 555nm. The product name is chl_oc2. Unlike the standard MODIS chlorophyll algorithm (OC4) that relies on the 1000-meter channels at 443, 488, and 551nm, the chl_oc2 product can provide true 500-meter chlorophyll-a retrievals. This is a good example of new products that can be developed for HiRes MODIS. It is also worth noting that the nLw_645 product can provide true 250-meter resolution of the water-leaving radiance in the red, which may be useful as a high-resolution, turbid-water indicator (e.g., Li 2003). In addition, there are algorithms already available in MSl12 that can make use of the additional spectral channels. For example, the 469, and 555 channels will be used within the spectral optimization of the GSM01 bio-optical model.

New Atmospheric Correction Capabilities
The additional spectral channels in the NIR and SWIR provide an opportunity to explore a variety of new atmospheric correction options that may be of particular value in highly reflective, turbid waters. The standard atmospheric correction approach used of global ocean processing makes use of the NIR bands at 748 and 869nm to determine aerosol type and concentration. The method requires a priori knowledge of the water-leaving radiance in these longer wavelengths. In Morel case 1 waters, where the spectral distribution of water-leaving radiances can be assumed to vary with chlorophyll concentration alone, a simple iteration scheme (i.e., Stumpf 2003) can be used to model and predict the water-leaving radiances in the NIR. However, in turbid waters this case 1 assumption does not hold, so water-leaving radiances in the NIR are difficult or impossible to estimate, and the resulting aerosol determination based on the NIR will be corrupted and erroneous. In contrast, water is so strongly absorbing in the SWIR spectral regime that even highly reflective turbid waters appear black at these longer wavelengths. Following the recent work by Wang & Shi (2005), the SWIR bands on MODIS can be used to determine aerosol type and concentration. This information can then be used either to predict the water-leaving radiance in the NIR (thereby allowing the standard algorithm to procede), or the aerosol determination from the SWIR can be extrapolated all the way to the visible. These capabilities have been incorporated into msl12 to facilitate evaluation. It should be noted, however, that the signal-to-noise in the SWIR bands (Table 1) is quite low, and this may be a limiting factor in any advantage gained by using the SWIR. The aerosol correction options for msl12 are described in Table 6.

Table 6: Relevant Aerosol Correction Options in msl12

aer_wave_short lower wavelength used for aerosol model selection 748
aer_wave_long upper wavelength used for aerosol model selection and aerosol concentration 869
aer_opt Option for aerosol calculation mode.
1 - 12 extrapolation to visible with fixed model (aer_opt) with concentration from aer_wave_long
-1 Model selection using aer_wave_short and concentration from aer_wave_long
-3 Model selection using aer_wave_short and concentration from aer_wave_long,
with iteration for non-zero water-leaving radiance
-3


With the above options, it is possible to test a multitude of processing permutations. For example, the aer_wave_short and aer_wave_long can be set to 1240 and 1640, respectively, with aer_opt set to -1, and the aersol correction will be determined using the two SWIR bands rather than the standard NIR bands. Setting aer_wave_long to 2130 and aer_opt to 4 will yield aerosol concentration determined at 2130nm (where water is very dark), with the resulting aerosol reflectance extrapolated to the visible via the fixed model (maritime 90%, see User's Guide). Setting aer_wave_long to 859 and aer_opt to 4 will yield aerosol concentration determined with the 250-meter channel at 859nm, with the result extrapolated to the visible via the same fixed model (yielding true 250-meter aerosol retrievals). The aer_wave_short and aer_wave_long can be set to any sensor wavelength (though only wavelengths longward of 600nm make any sense).

Gaseous Absorption
One issue with the SWIR bands is high sensitivity to water-vapor and CO2. New corrections were developed to account for these effects. In msl12, these corrections can be enable via the gas_opt parameter.

gas_opt=gaseous transmittance bitmask selector (default: 1)
1: Ozone
2: CO2
4: NO2
8: H2O
0: no correction

Note that gas_opt is a bit mask. For example, to enable Ozone and H2O (water-vapor) correction, set gas_opt=9 (i.e., 1+8). The water-vapor and ozone concentrations are determined from ancillary files. A method to determine water-vapor directly from the MODIS bands is in development. The correction for NO2 is also still in development (currently not active).

True-Color Images

The msl12 software is actually a suite of programs that take advantage of the generalized approach to reading and processing the data from multiple spaceborne, earth-viewing radiometers. One of the programs, msl1brsgen, is used to generate browse imagery in a variety of formats. This program is also distributed with SeaDAS. It can produce true-color images, with or without atmospheric correction, at varying resolutions, for any sensor and level-1 format that msl12 supports. Full usage is:

msl1brsgen [Optional arguments] l1_filename [geo_filename] output_file_name

Optional arguments:
[-h 8|24] write output in hdf format, 8-bit or 24-bit (default=8)
[-p] write output in 24-bit ppm format
[-b] write output in 24-bit flat binary format
[-a] remove atmosphere, use surface reflectance
[-r subsample-factor ] (default=10)
[-f first-scan-number ] (default=0)
[-l last-scan-number] (default=nscan-1)
[-s start-pixel-number] (default=0)
[-e end-pixel-number ] (default=npix-1)

If given a QKM, HKM, or 1KM file and a specified a subsample-factor of 1, it can generate 250, 500, or 1000-meter images at full resolution. The images can be output as ppm files (-p), a common image format that most image conversion utilities can read.

A Processing Example

The SeaDAS development has put together a number of scripts to simplify MODIS processing. Starting from a Level-0 MODIS-Aqua file for a five-minute granule over the north east coast of Australia, MOD00.P2006115.0325_1.PDS, I will use the utilities distributed with SeaDAS to convert to Level-1B, make a 250-meter true-color image, select a region of that image, and process to level 2. Note that all commands are issued from the UNIX commandline (NOT THE SEADAS COMMANDLINE). No IDL is involved in the making of Level-2 products. Assume my unix commandline prompt is "unix%"

Generate Level-1A and geolocation file

unix% $SEADAS/etc/modis_L0_to_L1A_GEO.csh MOD00.P2006115.0325_1.PDS

This produces two files: A2006115032511.L1A_LAC and A2006115032511.GEO

Generate Level-1B

unix% $SEADAS/etc/modis_L1A_to_L1B.csh A2006115032511.L1A_LAC A2006115032511.GEO This produces three files: A2006115032511.L1B_LAC, A2006115032511.L1B_HKM, and A2006115032511.L1B_QKM

Generate True-Color image, full 5-minute granule at 250-meter resolution

unix% $SEADAS/bin/msl1brsgen -p -a -r 1 A2006115032511.L1B_QKM A2006115032511.GEO A2006115032511.RGB_QKM.ppm

This may take 30-minutes or more, depending on the system. The -a option says to do some atmospheric correction, which generally makes the image look less hazy. If you provide the HKM or 1KM file, the apparent resolution of the image will be reduced accordingly. In reality, the true resolution is 500-meters at best, since msl1brsgen uses a combination of the two 500-meter bands at 469 and 555-nm and the 250-meter band at 645 nm. I use the cjpeg utility to convert to jpeg (e.g., cat A2006115032511.RGB_QKM.ppm | cjpeg > A2006115032511.RGB_QKM.jpg). Click on the image to see the full resolution, 5416 x 7560 image.



Locate a subregion

There is another program in SeaDAS that will give the start and end pixel and line for a given longitude/latitude box, for any Level-1 file msl12 supports or any Level-2 file that msl12 produces. Here it is run for the box bounded by 20 to 23 south latitude and 149 to 152 east longitude; a region encompassing a portion of the Great Barrier Reef, Broad Sound, and Shoalwater Bay.

unix% $SEADAS/bin/msscanpixlimits A2006115032511.L1B_QKM A2006115032511.GEO 149.0 -23.0 152.0 -20.0

The program returns:
sline=6450
eline=7560
spixl=3461
epixl=4529

Now we can make an image of just the subregion via:

unix% $SEADAS/bin/msl1brsgen -p -r 1 -a -f 6450 -l 7560 -s 3461 -e 4529 A2006115032511.L1B_QKM A2006115032511.GEO A2006115032511.RGB_QKM_sub.ppm
unix% cat A2006115032511.RGB_QKM_sub.ppm | cjpeg > A2006115032511.RGB_QKM_sub.jpg

Click on the image to see the full resolution, 1069 x 1111 image.



Process the subregion to Level-2

Here, I prefer to create a parameter file first. I will call it A2006115032511.L2_QKM.par. It will tell msl12 to process at 250-meter resolution, using the SWIR bands at 1240 and 2130 to fully determine the aerosol contribution. The parameter file looks like this:

ifile=A2006115032511.L1B_QKM
geofile=A2006115032511.GEO
ofile=A2006115032511.L2_QKM_sub
resolution=250
l2prod=chl_oc2,default
ctl_pt_incr=1
aer_wave_short=1240
aer_wave_long=2130
aer_opt=-1
sline=6450
eline=7560
spixl=3461
epixl=4529
MET1=/ancillary/2006/115/S200611500_NCEP.MET
MET2=/ancillary/2006/115/S200611506_NCEP.MET
MET3=/ancillary/2006/115/S200611506_NCEP.MET
OZONE1=/ancillary/2006/115/S20061150011523_TOAST.OZONE
OZONE2=/ancillary/2006/116/S20061160011623_TOAST.OZONE
OZONE3=/ancillary/2006/116/S20061160011623_TOAST.OZONE

Now, run it through msl12 to produce A2006115032511.L2_QKM_sub, a Level-2 file of the sub-region containing the default products from Table 5 plus the OC2 chlorophyll product developed by O'Reilly.

unix% $SEADAS/bin/msl12 par=A2006115032511.L2_QKM.par

This processed in about 5-minutes on a 3 Ghz Intel Xeon running Fedora Core 1. The resulting file can be loaded into SeaDAS for analysis and display. The following images show standard MODIS chlorophyll (based in the OC3 algorithm using 1KM bands at 443, 488, and 551 nm), the OC2 chlorophyll based on the HKM bands at 469 and 555 nm, and the normalized water-leaving radiance at 645 nm (indicating high reflectance in the red, indicative of high turbidity or bottom reflectance).

Chlorophyll (OC3)


Chlorophyll (OC2)


nLw(645)


Extraction at Level-1A

If you know in advance what region you want to process, or you plan to archive data for future processing. a better approach is to produce an extracted Level-1A and geolocation file and process that to Level-1B and Level-2. Processing time will be reduced (about 20% in this test), and the extracted Level-1A requires less storage space. Archiving of Level-1A also provides the option to reprocess to Level-1B when the instrument calibration (Level-1B LUT) is updated.

unix% $SEADAS/etc/modis_L1A_extract.csh A2006115032511.L1A_LAC A2006115032511.GEO 149.0 -23.0 152.0 -20.0 A2006115032511.L1A_LAC_sub A2006115032511.GEO_sub
unix% $SEADAS/etc/modis_L1A_to_L1B.csh A2006115032511.L1A_LAC_sub A2006115032511.GEO_sub -o A2006115032511.L1B_LAC_sub -h A2006115032511.L1B_HKM_sub -q A2006115032511.L1B_QKM_sub

You can then use the subscened geolocation file A2006115032511.GEO_sub and L1B files to produce the same Level-2 results. The parameter file would look like this:

ifile=A2006115032511.L1B_QKM_sub
geofile=A2006115032511.GEO_sub
ofile=A2006115032511.L2_QKM_sub
resolution=250
l2prod=chl_oc2,default
ctl_pt_incr=1
aer_wave_short=1240
aer_wave_long=2130
aer_opt=-1
MET1=/ancillary/2006/115/S200611500_NCEP.MET
MET2=/ancillary/2006/115/S200611506_NCEP.MET
MET3=/ancillary/2006/115/S200611506_NCEP.MET
OZONE1=/ancillary/2006/115/S20061150011523_TOAST.OZONE
OZONE2=/ancillary/2006/116/S20061160011623_TOAST.OZONE
OZONE3=/ancillary/2006/116/S20061160011623_TOAST.OZONE

It is also useful to know that you need not store the geolocation file, as it can always be regenerated via the SeaDAS script, e.g.:

unix% $SEADAS/etc/modis_L1A_to_GEO.csh A2006115032511.L1A_LAC_sub -o A2006115032511.GEO_sub

Artifacts and Caveats

Image Striping
User's of standard Level-2 MODIS ocean data are familiar with the cross-track striping artifacts that are sometimes visible in the imagery. This effect is due to imperfect relative corrections between the 10 along-track detectors associated with each 1KM band. For the HKM and QKM bands, 20 and 40 detectors are distributed along-track to provide the higher along-track resolution. As such, cross-track striping artifacts at 20 and 40 line intervals can occur for the HKM and QKM bands, respectively. This problem is potentially exacerbated by the reduced sensitivity (higher digitization error) of each detector over the dynamic range of ocean observations. In addition, within each physical scan of MODIS, the HKM detector sets are sampled at double the rate of the 1KM, and the QKM detectors are sampled at four times the rate. This temporal sub-sampling is what provides the cross-track pixel resolution. Slight variation in this temporal subsampling rate, and imperfect reset of the sampling registers, can give rise to vertical (along-track) striping in the higher resolution bands. Corrections have been applied to correct for this effect, but the corrections are never perfect. In conclusion, both along-track and cross-track striping can be seen in the HKM and QKM bands, when viewed at native resolution.

File Size & Processing Time
When processing at higher resolutions, or when including a large number of products in the Level-2 output, file size can exceed 2GB. This will halt processing on most, if not all, systems. This is particularly true when working with larger data granules (i.e., Direct Broadcast files in excess of the usual 5-minute duration). It is strongly recommended that users limit the processing to the minimum region of interest. The best way to do this is to use the MODIS subscene tool, available in SeaDAS, to extract a subscene from a Level-1A file. The Level-1A extract can than be processed to Level-1B and Level-2. Alternatively, the full Level-1B can be processed to Level-2, but the output can be limited to a subregion of the granule (via msl12 parameters sline, eline, spixl, epixl). The former method is more efficient, as the latter requires some level of processing on the full file before it can be reduced to the subscene. In either case, for the same region, processing at 250-meter resolution will generally increase processing time by a factor of 16 relative to processing at 1000-meter resolution.

Differences with Standard Processing
In principal, running with aer_opt=-3, aer_wave_short=748, and aer_wave_long=869 would yield identical results to standard MODIS ocean processing, for the standard ocean bands. However, the OBPG has rederived all aerosol and Rayleigh tables, including those for standard ocean bands, so slight differences can be expected.

Questions and Support

SeaDAS-specific questions (e.g., interface, program errors, portability) can be posted to the SeaDAS Support Forum. General questions about the algorithms and implementation details can be posted to the Satellite Data Products and Algorithms Forum. Appropriate members of the OBPG monitor these forums.

References

Gao, B.-C., M. J. Montes, Z. Ahmad, and C. O. Davis (2000). Atmospheric correction algorithm for hyperspectral remote sensing of ocean color from space, Appl. Opt., 39, 887-896. Gordon, H.R., and Wang, M. (1994). Retrieval of water-leaving radiance and aerosol optical-thickness over the oceans with SeaWiFS - a preliminary algorithm. Appl. Opt., 33, 443-452.

Li, Rong-Rong, Yoram J. Kaufman, Bo-Cai Gao, and Curtiss O. Davis (2003). Remote Sensing of Suspended Sediments and Shallow Coastal Waters, IEEE Transaction on Geoscience and Remote sensing, Vol. 41, No. 3 PP. 559.

O'Reilly, J.E., Maritorena, S., Mitchell, B.G., Siegel, D.A., Carder, K.L., Garver, S.A., Kahru, M., and McClain, C. (1998). Ocean color chlorophyll algorithms for SeaWiFS. J. Geophys. Res., 103, 24937-24953.

Stumpf, R.P., R.A. Arnone, R.W. Gould Jr., P.M. Martinolich, and V. Ransibrahmanakul (2003). A partially coupled ocean-atmosphere model for retrieval of water-leaving radiance from SeaWiFS in coastal waters. In: Algorithm Updates for the Fourth SeaWiFS Reprocessing, NASA/TM-2003-206892, Vol. 22, NASA Goddard Space Flight Center, Greenbelt, Maryland, 51-59.

Wang, M. and W. Shi (2005). Estimation of ocean contribution at the MODIS near-infrared wavelengths along the east coast of the U.S.: Two case studies, Geophys. Res. Lett., 32, L13606, doi:10.1029/2005GL022917.

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