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"Recently Asked Questions" About SeaWiFS and Ocean Color

In the weeks following the August 1997 launch of SeaWiFS, several pertinent questions were received by the Ocean Color Data Support Team (OCDST) at the Goddard Distributed Active Archive Center (DAAC). Because the answers to these questions were of potential interest to the expanding community of ocean color data users, the following question-and-answer page was composed. New and interesting questions sent to the OCDST will occasionally be added here.

  1. What's ocean color?
  2. What's the difference between wave height, ocean color, and sea surface temperature images?
  3. What was CZCS?
  4. What is the difference between Level 1 GAC and Level 2 GAC?
  5. How can I get a GAC image showing a wider area?
  6. What software do I need to view images?
  7. What do I get when I order a file?
  8. Why is there an approximately one-week-long break in the data coverage in October 1997?
  9. Why does the SeaWiFS Project data browser have files that are not available from the DAAC?
  10. Can I get real-time data support from SeaWiFS?
  11. What does "reprocessing" mean?
  12. Can I get SeaWiFS data on a CD-ROM?
  13. What is a SeaWiFS data subscription?
  14. What information is necessary for a SeaWiFS data subscription?
  15. Why doesn't the DAAC have HRPT data that I can see on the SeaWiFS Data Browser?
  16. Why is EPTOMS ozone data missing between December 12, 1998, and January 3, 1999?
  17. Why do you have subsets of just the U.S. East Coast and the Mediterannean Sea region?
  18. I'm writing a paper. How should I give credit for the SeaWiFS data?
  19. How do I get data from two separate HRPT stations and put it in the same order?
  20. I don't like the new data browser. Can I use the old one?
  21. What's the "Ocean Color Spectrum", and how can I get into it?
  22. Why isn't there any SeaWiFS data on November 17 and 18, 1999?
  23. Why are there only two ancillary meteorological data files per day in October and part of November, 1999?
  24. How can the NDVI land data be complete over South America in the summer if the tilt segments always take place over the same region?

1. We don't know precisely what an ocean color image is, and what the colors represent. Could you explain this in simple terms?

The most familiar images from the Coastal Zone Color Scanner (CZCS), and now SeaWiFS, are false-color representations of phytoplankton pigment concentration, calculated from the radiances (light intensities) measured by the sensor. Sensors like CZCS and SeaWiFS measure radiance in specific bands of the visible light spectrum, and the data is used to calculate such variables as pigment concentration and light attenuation. False color is simply used to emphasize the concentration ranges. For the CZCS, violet and blue represent low concentrations (below 1.5 milligrams per cubic meter), green and yellow are intermediate, and orange and red are high concentrations (up to 30 milligrams per cubic meter). The SeaWiFS color scale is similar, but the improved data and better algorithms allow a better estimate of chlorophyll, rather than pigment, concentration.

Phytoplankton pigment is predominantly chlorophyll, but also contains degradation byproducts (phaeopigments). In addition, light-absorbing substances or reflective particles in the water can "confuse" the algorithms used to calculate the pigment concentration, particularly in coastal regions. Newer sensors, such as SeaWiFS and MODIS (the Moderate Resolution Imaging Spectroradiometer), have more spectral bands, i.e., they measure more "slices" of the visible light spectrum, allowing more sophisticated algorithms and better discrimination between chlorophyll, phytoplankton pigments, and other substances and particles in the water.

One other note: the algorithms also involve correcting for light scattering in the atmosphere. Atmospheric correction is quite important in the process, as about 90% of the light received by a satellite- borne sensor is from the atmosphere, and only about 10% from the ocean.

2. There appear to be three types of images from satellites used in oceanography: wave height images, ocean color images, and surface temperature images. What is the difference between these approaches, and what are their separate merits?

Ocean color is primarily used for research into phytoplankton, the base of the oceanic food web and the main biological entity in the marine carbon cycle. Phytoplankton are the plants of the ocean, producing organic carbon by photosynthesis. Ocean color is therefore valuable in research into biological systems and patterns in the ocean, but the data is quite dependent on the algorithms used to derive pigment concentration and other geophysical variables from the measured radiances. Considerable research effort with SeaWiFS data will concentrate on algorithm development. For this reason, the data is somewhat dependent on research conducted at sea to refine the algorithms. As this research progresses, the reliability of the calculated geophysical variables will improve. Use of the data also requires statistical combination of several views of the same region to eliminate the pervasive presence of clouds and haze.

Sea surface temperature (SST) measures the infrared radiation emitted by the ocean surface. It is simply a measure of the intensity of this radiation and relies less on in-situ verification. However, atmospheric correction and comparison to in-situ sensors for calibration is still quite important. Because oceanic currents and water masses can vary considerably in temperature, SST data is particularly useful in observing currents and circulation in the oceans. The data is quite sensitive to atmospheric effects, and is also obscured by clouds. SST data is most commonly obtained from polar-orbiting satellites operated by the National Oceanic and Atmospheric Administration.

Wave height and sea surface height (SSH) are obtained from altimetry, which measures a radar signal broadcast from the satellite and reflected back. Precise timing of the signal indicates the distance between the satellite and the surface of the ocean. Use of orbit and gravity models provides the actual "height of the ocean", which indicates highs and lows in the ocean surface that are somewhat analogous to meteorological highs and lows. The data can be used to indicate large scale patterns of circulation and variability over large scale. The current El Nino -- Southern Oscillation event is quite visible in SSH data, as the high "dome" of heated water normally present in the western Pacific essentially migrates eastward in the absence of the normal wind pattern. Waves of warm water can be observed moving eastward. Wave height, which can indicate both storm influence and regions of high variability (such as the Agulhas retroflection) is derived from the spectral waveform of the radar signal reflected from the ocean surface. The radar signal is somewhat influenced by water vapor in the atmosphere, but this can be corrected for with a separate instrument that measures specific water absorption bands in the microwave region. The NASA and Centre National Etudes Spatiales (CNES) satellite TOPEX/Poseidon has such a microwave instrument, and the current accuracy of the SSH measurement is better than 5 cm.

 

3. Was the CZCS a NASA satellite, or an international joint venture?

The Coastal Zone Color Scanner (CZCS) was a NASA sensor, flown on the NIMBUS-7 satellite. It collected ocean color data from 1978 - 1986. For a good overview of the sensor, go to the Coastal Zone Color Scanner (CZCS) Instrument Guide

Though CZCS was a NASA instrument, calibration of the data relied on an international team of scientists. Considerable work with the CZCS data was also performed by research groups in France and Italy.

4. I am interested in the images from the SeaWIFS Browser. However, I do not understand the following: What is the difference between Level 1 GAC and Level 2 GAC? The level 1 shows the landmass as very red. I am interested in the chlorophyll concentration levels.

The GAC (Global Area Coverage) images that you are seeing are browse images. The browse image for Level 1A data is made from the raw radiance received at the satellite in band 8. This essentially allows land and clouds to be distinguished from open ocean. Level 1 data can be processed to Level 2 using the free software package called SeaDAS, which can be accessed and obtained from at the URL http://shark.gsfc.nasa.gov. Note that SeaDAS only works on UNIX platforms at present and it requires an IDL license.

NOTE: On October 16, 1997, the SeaWiFS Project changed the Level 1A browse product to a true-color image that uses bands 1, 5, and 6. The browse image should be much more pleasing to the eye, as well as more informative.

The SeaWiFS chlorophyll product is a Level 2 geophysical data product. Remember that the current data uses an at-launch algorithm which will be analyzed by the Calibration and Validation element of the SeaWiFS Project, and improved algorithms will be used in the future. The entire database of SeaWiFS data will be periodically reprocessed using new algorithms during the course of the mission.

 

5. I wish to obtain a GAC image of a large area, not just strips. How can I get an image showing a wider area?

The only wide-area images are from the HRPT stations, which are LAC (Local Area Coverage) data. LAC data is only available for level 1, and will require SeaDAS (or software developed by the user) for processing. The GAC data is in 1500 km wide strips for a purpose - the pixel sizes are more uniform in the center of the SeaWiFS scan. The width of a LAC scan is 2800 km, but the pixels are elongated near the edge due to the curvature of the Earth.

If you prefer a very wide area, you can also look at the level 3 product, which is a map of the whole Earth. The resolution is considerably reduced, of course.

 

6. When I download an image I can not open it as a Windows95 file. Do I need additional software?

Yes, additional software is required. The image itself is a GIF file, so you will need software capable of displaying GIF images. On a UNIX system, a package like XV will work. There should be software packages capable of displaying GIF images on a PC, and either Netscape or Internet Explorer (properly configured) will also display the images.

If you download the browse image data, you are receiving a Hierarchical Data Format (HDF) file that includes the image. 2008 update: The following site has a Windows HDF browser that allows viewing of HDF files: http://hdf.ncsa.uiuc.edu/plugins/UsersGuide/index.html.

 

7. I don't understand the "order" system. What information will I be receiving, and how will it differ from the files I select and view?

If you order data, you will receive the digital HDF files that contain all the data obtained by SeaWiFS, and additional data that allows the data to be processed (to add coastlines to an image, for example). The browse files are smaller files with less information, so they cannot be used as effectively for research.

8. Why is there an approximately one-week-long break in the data coverage in October 1997?

The satellite went into "safe" mode during this period, because it received erroneous navigation information. Virtually all satellites have this contingency in their control software, which helps to prevent a catastrophic failure. The instrument essentially goes to "sleep" until the ground controllers can diagnose the problem and wake the instrument up again in the proper fashion.

9. Why does the SeaWiFS Project data browser have files that are not available from the DAAC? And why do two files from the same day and the same area look different?

The SeaWiFS Project at Goddard Space Flight Center receives twice-daily transmissions of data from the satellite. Once they have received the raw data, they can immediately produce images and data files, and make them visible on their data browser. In some cases a browse image will be produced, but it will take longer to make the actual digital data file. The initial processed data are then examined by the Calibration/Validation element of the SeaWiFS Project to insure data integrity. Occasionally the data will be processed again. The data files have to "pass" calibration and validation before they are sent to the DAAC. It is common to have a1-3 day delay from the time an image can be seen on the SeaWiFS Project data browser to the time the data is ingested at the DAAC and it is visible on the DAAC Web browser.

If the browse image is obtained very soon after it is first produced, it may not have been processed with the best meteorological and ozone data available. (These data types are called "ancillary" data, because they are not data from the sensor but they are necessary for data processing.) Climatological data, data which is the average value over year or longer time span, may be used to initially process the data. When the ancillary data corresponding to the time the data was obtained is substituted, the appearance of the new browse image may be slightly different.

10. My research group is going on a cruise in two months. Can we get SeaWiFS data during the cruise so that we won't miss interesting ocean color features?

Your research group can't get SeaWiFS data in real time from the DAAC, but there is a group to contact for real-time data support. As part of the SIMBIOS (Sensor Intercomparison and Merger for Biological and Interdisciplinary Ocean Studies) Project, real-time images can be provided. It is necessary to be a SeaWiFS Authorized Research User and to receive approval from SeaWiFS Project Scientist Charles R. McClain. Researchers who receive this support are expected to provide optical and/or pigment data to the SeaWiFS/SIMBIOS Projects within six months of the cruise. For more detailed information and online data request forms, go to the SIMBIOS/SeaWiFS Support Services and Schedules page.

It is also possible to request recorded LAC data for a specific region through the URL given above, because there is a limited amount of storage (about 10 minutes of data daily) available on board the satellite for one-kilometer resolution LAC data. If a location and date are provided, LAC recording can be scheduled for that area. This option might be important if a study area is out of the receiving range of any HRPT station.

11. What does "reprocessing" mean? Is reprocessed data better than the data I obtained in October?

When the data comes from the satellite, it's just digital data -- strings of numbers. The data has to be processed to derive meaningful remote sensing data. The first step in SeaWiFS data processing is to produce "raw" radiance data, the intensity of light for each band detected at the satellite. When navigation information is added, this becomes Level 1A data. Algorithms are then used to remove the effect of atmospheric light scattering and to account for the angle of the sun, producing normalized water-leaving radiances. This data can be combined in algorithms to produce other parameters, such as chlorophyll concentration. This level of processing is called Level 2 data. Level 3 data means that the data has been combined statistically into "bins" which contain all the data from a certain region collected daily, weekly, monthly, or annually. The SeaWiFS bins are square areas 9x9 kilometers in dimension.

"Reprocessing" means that the SeaWiFS data has been processed again, using new and improved algorithms, or by applying calibration data from the instrument itself. When SeaWiFS was launched, the algorithms used to produce the initial images were "at-launch" algorithms. In the early days of the mission, the operating characteristics of the sensor were adjusted, and the algorithms used to produce the data products were tested and refined by comparing in situ data to data from the sensor. The definitions of data flags (for conditions in which the data may be inaccurate) or masks (which screen out clouds or land) were examined and improved as well.

The SeaWiFS Project began reprocessing the data in early January of 1998. The reprocessed data is considered "science-quality" data, more accurate and reliable than the data which was initially distributed during the last quarter of 1997. The reprocessed data can now be obtained from the DAAC SeaWiFS data browser. Of course, all of the data now being received is processed with the new algorithms and calibration data. For a detailed description of the changes involved, see SeaWiFS Data Reprocessing.

The current algorithms aren't the final word, however. The SeaWiFS Project plans to reprocess the whole data set about once a year, as more research in optical oceanography takes place. The goal is to make the current algorithms even better.

12. Can I get SeaWiFS data on a CD-ROM?

CD-ROMs are not one of the current data distribution options for SeaWiFS data. CD-ROMs hold a large volume of data, and although SeaWiFS is currently producing a lot of data, it isn't enough (yet) to merit distribution on CD-ROM. NASA customarily puts a comprehensive data set, such as a year of data or a decade of data, on CD-ROMs. Very large amounts of data, such as from the TOPEX/Poseidon altimetry mission, are also distributed this way. Because of the costs of producing CD-ROMS, this option is usually used for data with a great deal of potential interest and a large community of users. Because SeaWiFS data is restricted to Authorized Research Users, it can't be distributed as freely as other kinds of data -- at this time.

13. What is a SeaWiFS data subscription?

A data subscription means that an Authorized Research user has requested that certain types of data be acquired for their use, and that the data should be sent to them at regular intervals. The data subscription works this way: when data is sent to the DAAC from the SeaWiFS Project, the subscription software selects data corresponding to a subscription request. The data is placed in a file corresponding to an individual researcher. When the time interval is completed, the data is either written on magnetic tape and sent by mail, or placed in an FTP directory. In the latter case, an email notification is sent to the user that the data is ready to be transferred electronically.

14. What information do I need to provide for a SeaWiFS data subscription?

First of all, data subscriptions are only available for Authorized Research Users, so you must be registered with the SeaWiFS Project. Registration also serves to put mailing address and email address information in our database. Once that is accomplished, we need to know the following: the data products desired in the subscription, how you would like to get the data (by FTP or on magnetic tape), and what interval for data delivery is desired (daily, weekly, or monthly). Many subscribers currently get data from individual HRPT stations.

15. I found data from an HRPT station on the SeaWiFS Data Browser, but it's not available on the DAAC SeaWiFS Data Browser. Why not?

There are two possible reasons for this discrepancy. The most likely reason is the two-week data embargo. The SeaWiFS Global Browse Utility, which is intended to be used primarily by SeaWiFS Authorized Research Users for planning purposes, displays GIF images from real-time HRPT stations, but no actual HDF data file scan be downloaded from there. The Goddard DAAC does not receive any SeaWiFS data until the end of the two-week data embargo period. I.e., for data received on May 1, the first day the DAAC could receive it would be May 15.

However, if the data embargo time period has expired, the other possible reason is that the DAAC is not storing duplicate HRPT station data. If the data collected by one HRPT station is entirely contained in the data from another station, the DAAC will only receive the larger data file.

This situation might be a little difficult to visualize. As SeaWiFS observes the Earth, it broadcasts to any station that can receive the data. For example, as it passes over California, HRPT stations in Monterey, Los Angeles, and even Texas can receive the satellite telemetry. (It's the same data!) Because the station in Texas is farther from the satellite, and may only receive a portion of the data that the Monterey station receives. And if the antenna at the Monterey station is larger than the antenna at the station in Los Angeles, the Monterey station may get all the data received at the Los Angeles station, and more on either "end" of the data transmission. Thus, all the data received at the Los Angeles station is included in the data received by the Monterey station, and the DAAC will only get the data file from the Monterey station.

16. Why is EPTOMS ozone data missing between December 12, 1998, and January 3, 1999? What should I use in place of this data?

The Earth Probe - Total Ozone Mapping Spectrometer (EPTOMS) satellite suffered a serious navigational malfunction on December 12, 1998. Basically, the navigational system indicated that the instrument was not properly oriented. The satellite tried to re-orient itself before going into "safe" mode, losing almost all of its maneuvering propellant. The ground controllers were able to devise a new method of orienting the satellite (using magnetic torque control) and the instrument was restored to normal science mode on January 3, 1999. The TOMS Project cautions that the January 3rd data may contain small errors. For more information about the TOMS Project, visit their Web site, http://toms.gsfc.nasa.gov.

The backup ozone data source is the TIROS Operational Vertical Sounder (TOVS). Ancillary ozone data from TOVS is also available as SeaWiFS Ancillary Data at the DAAC.

17. Why do you have subsets of just the U.S. East Coast and the Mediterranean Sea region? Can you subset all of the HRPT station data files this way?

Our regional subsets were created to reduce both processing time and memory storage requirements for areas (primarily coastal regions) where there is considerable localized oceanographic and estuarine research activity. In order to create them to be compatible with SeaDAS processing, we actually create them using the subsetting function within SeaDAS. Therefore, it takes considerable processing time to create them, and we chose areas where interest in this type of research is particularly high. Remember that one of the features of our regional subsets is that they can be directly downloaded by FTP (they don't have to be ordered through the DAAC system). For more information about the subsets, contact the OCDST.

18. I'm writing a paper. How should I give the correct attribution for the SeaWiFS data?

You should use the following template as a guide:

"The authors would like to thank the SeaWiFS Project (Code 970.2) and the Goddard Earth Sciences Data and Information Services Center/Distributed Active Archive Center (Code 610.2) at the Goddard Space Flight Center, Greenbelt, MD 20771, for the production and distribution of these data, respectively. These activities are sponsored by NASA's Earth Science Enterprise."

In addition, If you have used data from an HRPT station, please give an attribution to that station.

19. I want to search for data from two different HRPT stations for the same time period and put the data in the same order. How do I do that with the new browsing system?

The new browsing system offers several different ways to search for, and order, data from a specific time period. The method to use primarily depends on whether or not you wish to examine browse images of the data files. Examining browse images and selecting specific data files reduces the volume of data that will be sent to you, but it takes longer.

The first method is to select your specific time period. There are two ways to do this -- one way is to choose data by yearor by month by "navigating" the calendars and checking the year(s) or month(s) you prefer. If you want one specific time range in one year or one month, you can provide the beginning and end dates below the calendar table. If the time range spans the New Year (January 1), you'll have to order data from individual years separately using this method. (You can even select data from the same range of dates from different months using this method, perhaps for comparison to data from a buoy that is collected at the same time each month.) The main drawback to this method of ordering is that it orders the files in "blocks"; browse images can't be examined, and files can't be deleted from a "block".

The other way to select a specific time period is to use the "Temporal Search" feature. Click on the words "Temporal Search" or the spinning globe. Using temporal search allows the specification of ANY time range; just make sure that the start date is earlier than the end date! The output is a list of files from the HRPT station. The browse images can be examined so that only the desired files are placed in the order, using the "Add to Order" button.Skip the file without ordering by using "Next File", and "Continue" when you're done examining the files. If you reach the last file on the list, you will "Continue" automatically to the next step in the ordering process.

The information above covers how to search for data from one HRPT station. To get data from two (or more) stations in the same order, you first have to create a data order. Once you click on the "Create Order" button and you see the phrase "The following has been added to your order:" and a descriptive table below, the order has been created. "Continue Ordering" takes you back to the first page for the HRPT station supplying the data you just ordered. To get to a different station, we recommend clicking "Back to TOP", and navigating back to the HRPT station selection page. Select the next station, and begin ordering data by your favorite method.

20. I don't like the new data browser. The old one seemed to be much simpler to use; this one has too many options! Can you make the old browser available again?

Sorry, we can't put the old browser back "on-line" again. You might not think so, but the new browsing system is better than the old one. One of the main reasons is that now the Goddard DAAC has the same system for ordering data from any data set (ocean color data is only one of several Earth Science data sets at the DAAC). Previously, there were several different kinds ofdata browsers for the different data sets.

The new browser allows a lot more flexibility in the creation of data orders, and it helps us save on resources and the "manual" steps needed to get the data to you. Plus, it's modular, so we can make changes faster.

However, we understand that using the new browser might be confusing. If you have questions about how to use it, you should email us (or call us) to describe your specific requirements, and we'll suggest the best strategy to use.

21. I saw the "Ocean Color Spectrum" in Backscatter magazine. I have a research cruise next year with two open berths for scientists. How do I get mentioned in the magazine?

Just ask!

"Ocean Color Spectrum" is a collaborative effort between the Goddard DAAC and the Alliance for Marine Remote Sensing (AMRS), which publishes Backscatter . Besides being published in the magazine, "Ocean Color Spectrum" also goes out to every email address on the Goddard DAAC's distribution list, composed of marine scientists associated with ocean color-related research.

You can submit any related information to James Acker at the Goddard DAAC, acker@disc.gsfc.nasa.gov.

22. Why is SeaWiFS data not available on November 17-18, 1999?

The Orbview-2 satellite was commanded to enter "safe" mode and aligned to present the smallest possible impact profile, in order to safeguard against possible damage from meteoritic particles associated with the Leonid meteor shower. There is a similar data gap for the period November 17-20, 1998.

23. For most of the mission, there are four daily ancillary meteorological files from the National Center for Environmental Prediction (NCEP). But from October 1 to November 14, 1999, there are only two ancillary meteorological files, then there are four per day again. Will the missing files ever be replaced?

No. A fire damaged the main computer that NCEP used to generate the ancillary data files. The two files per day were generated by a slower backup system while the main computer system was repaired.

24. In the SeaWiFS Level 3 (Normalized Differential Vegetation Index) NDVI images that are found on the SeaWiFS Project Web site, they appear to have full coverage in the over South America in the summer. But the Level 2 GAC data always has the tilt segment in the about the same region. Why is the tilt segment in the same place, and how do the data gaps get filled in anyway?

The tilt segment (the period of time when the instrument is tilting, which is done to minimize sun glint) is generally performed over the same region of South America in the Northern Hemisphere summer so that critical sites for in situ research in the Caribbean Sea will be imaged.

The NDVI images posted to the SeaWiFS Web site use max-NDVI compositing (i.e., the highest NDVI value observed within each bin is selected). No masking of any kind is done. Clouds are minimized by the fact that they have low NDVI. The tilt is slow enough that we probably get some data in every 9-km bin of the mapped data over a 1-week period.

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