Summary:

Table of Contents:

• 1 Dataset Overview
• 2 Investigator(s)
• 3 Theory of Measurements
• 4 Equipment
• 5 Data Acquisition Methods
• 6 Observations
• 7 Data Description
• 8 Data Organization
• 9 Data Manipulations
• 10 Errors
• 11 Notes
• 12 Application of the Dataset
• 13 Dataset Plans
• 14 Related Software
• 15 Data Access
• 16 Output Products and Availability
• 17 References
• 18 Glossary of Terms
• 19 List of Acronyms
• 20 Document Information

1. Dataset Overview:

    Dataset Identification:

This guide includes information for the following DAAC products, 102, 095, 091, 071, 051, under the product name AVHRR Oceans Pathfinder global equal-angle best SST (NOAA,NASA). In addition it includes info for products 101, 094, 090, 070, 050, under the product name AVHRR Oceans Pathfinder global equal-angle all SST (NOAA/NASA).  Information for Version 5.0 of the pathfinder SST algorithm, under the product number 216,  and name AVHRR Oceans Pathfinder global 4km equal-angle all SST V5 (NOAA/NODC).  is also included in the document.  Version 4.1 information has been maintained in this document to give users a historical context of the project.

Dataset Introduction:

Multichannel sea-surface temperatures (MCSST) have been computed from AVHRR radiances operational since 1981. As part of the Pathfinder SST task, a detailed re-analysis of the calibration procedures for the NOAA AVHRR based on thermal vacuum test data was performed by researchers at the University of Miami [Brown et al., 1993]. The current reprocessing effort has led to the development of a consistent set of calibration coefficients and non-linearilty corrections that will produce a long-term SST data set. In addition to improving the overall accuracy estimates, an improved processing scheme has led to a more-sensitive cloud-clearing technique. This has improved the number of estimates per global image by nearly a factor of two, thus substantially increasing the retrieval of valid SST values. Further advances made in version 5.0 of the algorithm include retrievals at a higher resolution of 4 km, along with an improved ice and land mask.

Objective/Purpose:

The mandate of the Pathfinder SST task is to produce, validate, and evaluate a long time-series of AVHRR-derived SST as a precursor to EOS data sets, and for use in climatologic investigations and modeling.

Summary of Parameters:

Sea Surface Temperature

Discussion:

In order to understand the processes involved in global climate change many different scientific measurements are needed. One of the parameters critical to understanding how the ocean affects climate on a global scale is sea surface temperature (SST). An example of the importance of this measurement for climate studies is their use in the study of the western boundary currents of the world's ocean. The western boundary currents play an important role in the Earth's heat balance. They carry a tremendous amount of heat poleward from low-latitude regions. Because the currents exhibit strong SST gradients, the SST measurements can be used to determine their displacements. Knowledge of the displacements, in turn, allows us to improve our understanding of ocean circulation and heat transport.

2. Investigator(s):

Edward Kearns, PHD
Research Assistant Professor, RSMAS/MPO
University of Miami, 4600 Rickenbacker Cswy
Miami, FL 33149
(305) 361 4837    fax: (305) 361 4622
ekearns@rsmas.miami.edu

Kenneth Casey, PHD
NOAA National Oceanographic Data Center
1315 East-West Highway
Silver Spring MD 20910 USA
301-713-3272 ext 133
http://www.nodc.noaa.gov/sog

Jorge Vazquez, PHD
Physical Oceanography Distributed Active Archive Center
4800 Oak Grove Dr. M/S 300/323
Pasadena, Ca. 91101
818 354 6980
jorge.vazquez@jpl.nasa.gov

3. Theory of Measurements:

The history of SST computation from AVHRR radiances is discussed at length by [McClain et al., 1985]. Briefly, radiative transfer theory is used to correct for the effects of the atmosphere on the observations by utilizing "windows" of the electromagnetic spectrum where little or no atmospheric absorption occurs. Channel radiances are transformed (through the use of the Planck function) to units of temperature, then compared to a-priori temperatures measured at the surface. This comparison yields coefficients which, when applied to the global AVHRR data, give estimates of surface temperature which have been nominally accurate to 0.3û C.

4. Equipment:

Sensor/Instrument Description:

Collection Environment:

NOAA-Series Satellites

Source/Platform:

NOAA-7, NOAA-9,NOAA-11, NOAA-12,  NOAA-14, NOAA-16 and NOAA-17  polar-orbiting satellites

Source/Platform Mission Objectives:

Each of the NOAA polar-orbiting satellites have carried an AVHRR as one of three sensors aboard the spacecraft. AVHRR was designed for multispectral investigations of meteorological, oceanographic, and hydrologic parameters, measuring emitted and reflected radiance in four or five spectral bands, spanning the visible portion of the spectrum to the thermal infrared.

Key Variables:

The sensor measures emitted and reflected radiation from Earth in two visible channels and three infrared channels.

Principles of Operation:

Each AVHRR scan views Earth for 51.282 milliseconds, during which time each channel of the analog data output is digitized. Scans occur at the rate of 6 per second, and the sampling rate of the AVHRR sensors is 39,936 samples per second per channel. During a scan, the detectors view an internal target, cold space, and the external scene. The temperature of the internal target is monitored, and space is assumed to have a black- body temperature of 3K. In this way, a simple two-point linear calibration is done internally (Schwalb, 1978). The nonlinear modification to this calibration is achieved at the time of postprocessing, and takes into account sensor nonlinearities, measurement of internal target temperature, calculation of target radiance, internal reflections and emissions, etc..

Sensor/Instrument Measurement Geometry:

The AVHRR has a cross-track scanning system which use an elliptical beryllium mirror rotating at 360 RPM about an axis parallel to the Earth. The 110.8° cross-track scan equates to a swath width of about 2700 km. This swath width is greater than the 25.3° separation between successive orbital tracks, and provides overlapping coverage. Coverage is global, twice daily, at an instantaneous field of view (IFOV) of ~1.4 milliradians, giving a ground field of view of ~1.1 km at nadir for a nominal altitude of 833 km.

Manufacturer of Sensor/Instrument:

ITT Aerospace

Calibration:

Specifications:

Channels 1 and 2 are calibrated to produce at-satellite radiances using a time dependent correction which accounts for sensor degradation and intercalibrates among the satellites. Channels 4 and 5 are calibrated using a non-linear function based on the internal calibration targets, baseplate temperatures, instrument dependent repsonse curves, and NOAA-provided gains and offsets. Channel 3 is calibrated using the gains and offsets in the GAC data record. The thermal channels are then converted to equivalent brightness temperatures using a lookup table based on the inverse Planck function convoloved with the instrument response.

Tolerance:

The instrument is designed to maintain a constant operating temperature for the IR detectors and provide a signal-to-noise ratio (SNR) of 3:1 at 0.5% albedo.

Frequency of Calibration:

The thermal infrared channels are calibrated in flight using a view of a stable blackbody and space as a reference. Channels 1 and 2 have no onboard calibration capabilities, however, they are calibrated before launch.

Other Calibration Information:

In an effort to develop a consistent set of in-flight calibration algorithms for channels 4 and 5, a radiance-based correction procedure was developed to account for the non-linear response characteristics of the detectors. This procedure resulted in a single correction algorithm applicable over the entire range of AVHRR operating temperatures, representing a significant improvement over the use of myriad tables to look up temperature corrections.

5. Data Acquisition Methods:

Full resolution AVHRR data are continuously transmitted and recorded in High Resolution Picture Transmission (HRPT) format. The Global Area Coverage (GAC) data are subsampled to approximately 4 km IFOV, recorded internally, and downlinked daily. These data are the starting point for the Pathfinder SST processing. The Level-1B data are defined as radiometrically-corrected and calibrated data in physical units at full instrument resolution as acquired. To produce the NOAA GAC Level-1B data, the Level-0 (unprocessed) instrument data are quality controlled, assembled into discrete data sets, and have calibration and Earth location information appended. Data are then stored as full orbits consisting of both ascending (daytime) and descending (nighttime) data.

6. Observations:

Data Notes:

  sst: 2001100.s04d3pfv50-sst-16b.hdf
         yyyyddd.brrkpaaaaa-ttt-bbb.hdf

YYYY = 4 digit year of observation (1985-200x)
DDD = 3 digit day of year (001-365, or 001-366 for leap years)
b = Indicates the bit length of the pixel values in the file. "s" is for 16 bit files, "m" is for
 8 bit files
rr = Approximate resolution in km. Set to "04" for 4 km files
k = Indicates the averaging period used to create the file.   For daily files the value has a character value of "d'.

p = pass which  indicates nighttime descending (1) or daytime ascending (3)
aaaaa=pfv50 = Shorthand for "Pathfinder Version 5.0"  or for version 5.0 interim;
aaaa= pfrt
ttt = Indicates the type of data stored in the file. TYPE may be one of the following:
sdev: Standard deviation
num: Number of observations
qual: Overall quality value
bbb = Number of bits in each pixel. Only present for 16-bit files (-16b)
hdf = Indicates HDF-SDS Version 4 file format

             Examples:

                 2001100.s04d3pfv50-sst-16b.hdf
                 A daily, daytime all-pixel SST file with 16-bit pixel values at 4 km resolution
                 from 2001, day 100.

NOTE: Version 5.0 interim refers to a year of data where coefficients from a previous year are used to calculate
              SST Values.  Such data will be replaced once new coefficients are calculated for the given year.

Additional fields, as well as those at the PO.DAAC and ncluding masks, standard deviations, and first guess fields maybe obtained through one of the following NODC interfaces:
 

ftp access:

ftp://data.nodc.noaa.gov/pub/data.nodc/pathfinder/Version5.0
 

web access:

http://data.nodc.noaa.gov/pathfinder/Version5.0

and the OPeNDAP server at:

http://data.nodc.noaa.gov/cgi-bin/nph-dods/pathfinder/Version5.0
 
 

More detailed information on the NODC interfaces can be obtained through
the userguide at:

http://www.nodc.noaa.gov/sog/pathfinder4km/

Field Notes:

A buoy match-up data set is used in computing the calibration coefficients used to calculate sea surface temperature.

7. Data Description:

Spatial Characteristics:

The Pathfinder SST data are distributed in a variety of resolutions, and temporal averages, to accomodate researchers with varying processing capabilities and needs. Each data product is produced as either an ascending (daytime) or descending (nighttime) image. These products are produced as daily composites, which are defined as spatial bins of all temperature retrievals at a maximum resolution of 9 km. Auxiliary information include quality and sampling data, as well as simple statistics. From the daily products, 8-day, monthly, and yearly composites are formed. Version 5.0 of the algorithm in daily fields at a spatial resolution of 4km.

Spatial Coverage:

Global

Spatial Coverage Map:

Spatial Resolution:

version 4.1 9 km, 18 km, 54 km

version 5.0 4 km

Projection:

Equal-Area, Equal-Angle

Grid Description:

The AVHRR Ocean Pathfinder data are processed in an equal-area grid based on one developed by the International Satellite Cloud Climatology Project (ISCCP). For version 4.1 and under the bin size is approximately 9.28 km on a side, which gives 5,940,422 bins over the globe. For version 5.0 of the algorithm the bin size is approximately 4.64. An advantage of this grid is that it can be easily combined into grids with different zonal resolutions because the number of bins per row is always an integer. Since the GAC data were originally sampled at approximately 4 km resolution, bin values are averages. The number of retrievals which were averaged into each bin is a standard data product, and can be obtained to correctly perform any special weighted averaging. After processing, the data are remapped into an equal-angle projection (9km resolution, 4096x2048 rectangular grid), in order to facilitate visualization and extraction of regional subsets. Data are also binned onto (2048,1024) (18km) and (720,360) (54km) grids. For version 5.0 of the algorithm the array size is (8192x4096).

 

 
 
 
 
 
 
 

          To calculate the geographic location of a specific pixel use the following equations for 9 km equal-angle imagery (other resolutions will be similar):

        longitude = (-180. + (x * dx)) + (dx/2)

        latitude = (90. - (y * dy)) - (dy/2)
          where for 9km resolution:
          x = grid point in x-direction (0-4095)
        dx = grid x-direction spacing (360 deg / 4096.)
          y = grid point in y-direction (0-2047)
          dy = grid y-direction spacing (180 deg / 2048.)

for the version 5.0 4 km resolution:

x = grid point in x-direction (0-8191)

        dx = grid x-direction spacing (360 deg / 8192.)
         y = grid point in y-direction (0-4095)
       dy = grid y-direction spacing (180 deg / 4096.)

Note that the upper left hand corner of first element of the data array is located at 180 West and 90 North, and that adding dx/2 and -dy/2 moves the geographic coordinates to the center of a grid box.

Temporal Characteristics:

Temporal Coverage:

Data is avaliable from 1985 - ongoing. For details on what version numbers of the algorithm correspond to a specific algorithm see the users reference manual of the Pathfinder homepage.

Temporal Coverage Map:

Temporal Resolution:

Daily, 8day, Monthly

Data Characteristics:

Parameter/Variable:

Sea Surface Temperature

Variable Description/Definition:

SST - temperature of the sea surface.

Unit of Measurement:

for version 4.1 and below pixel value with slope of 0.15 and y-intercept of -3.0 to convert to °C.

for version 5.0 pixel value with slope of 0.075 and y-intercept of -3.0 to convert to °C.

Data Source:

AVHRR

Data Range:

The data range is greater than -3.0°C and less than 35°C.

Sample Data Record:

Not Available

Related Datasets:

• AVHRR monthly global MCSST coregistered with CZCS data (Miami, GSFC) CD-ROM
• AVHRR weekly global and regional 18km gridded daytime MCSST (Miami)
• AVHRR weekly global and regional 18km nighttime MCSST (Miami)
• NAVOCEANO mcsst 18km and 2.2km resolution data sets

8. Data Organization:

Data Granularity:

The basic granule is daily ascending data and daily descending data. The data volume varies according to the data product. However data are also averaged into 8day and monthly products. The daily files are simply averaged into 8day and monthly time periods.

A general description of data granularity as it applies to the EDG appears in the EOSDIS Glossary.

Data Format:

The daily data are stored in the Hierarchical Data Format (HDF). HDF was developed at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champain. The 8day averages are available in flat binary files.

Sample Data Record:

Information not available.

9. Data Manipulations:

Formulae:

Derivation Techniques and Algorithms:

The AVHRR Level-1B sensor counts in the visible channels (1 and 2) are first converted to Rayleigh-corrected radiances and then to optical depth for use in removing the effects of the atmosphere and viewing and illumination geometry. Channels 3-5 are transformed to units of "brightness temperature", using the Planck black body function and a newly- determined (Brown et al., 1993) correction for sensor calibration non-linearity in the longer-wavelength channels. The algorithm used is essentially the nonlinear SST (NLSST: Walton, 1988), with a modification for sensor calibration drift with time. In addition the algorithm was modified so that the coefficients were calculated for three different regimes of water vapor corresponding to three different regimes of T4-T5. The form of the algorithm is
 
 

SST = a + b*T4 + c*(T4-T5)*Tsurf + d*(sec(q)-1)*(T4-T5) + etime

where q is the zenith angle of the instrument, and T4 and T5 are the brightness temperatures from AVHRR channels 4 and 5, respectively. The empirical coefficients a, b, c, d, and e were determined through a multiple-regression of AVHRR radiances with a database of in-situ temperatures, measured using moored and drifting buoys. In order to be considered a match, the pixel location and in-situ measurement must differ by no more than 0.1 degree spatially, and temporally by no more than 30 minutes. Modifications to the NLSST came in the form of the T4-T5 coefficients being calculated for three different water vapor regimes and the addition of the temporal degradation coefficient etime. The algorithm,along with the calculation of the T4-T5 coefficients for three different water vapor regimes, was used to process version 1.0 of the Pathfinder SST data. In addition coefficients were calculated over a yearly period.

In version 4.0, 4.1 and 5.0 of the algorithm the coefficients are calculated for T4 - T5 <= 0.7 and T4 - T5 > 0.7. This form of the algorithm was approved for the reprocessing of the historical AVHRR derived SST data by the AVHRR Oceans Science Working group because it tended to lower the overall bias over the widest possible environmental conditions (Evans and Podesta., 1996). The coefficients for version 4.0 and 4.1 of the algorithm are calculated over 5 month periods centered on each month. Thus the etime term was no longer needed. Thus the form of the algorithm now becomes:

SST = a + b*T4 + c*(T4-T5)*Tsurf + d*(sec(q)-1)*(T4-T5)

where the etime term has been dropped and the SSTs are solved for two different water vapor regimes corresponding to T4 - T5 <= 0.7 and T4 - T5 > 0.7. The main difference between version 4.0 and version 4.1 of the algorithm is in the assignment of the quality flags. The improvement of the version 4.1 data set over previous algorithms such as version 1.0, 3.0 and 4.0 lies in the use of a tree algorithm to calculate the quality flags, thus making the procedure of quality flag assignment more objective. The tree algorithm leads to a quality flag between 0-7 being assigned to a plxel value, with 0 being the lowest quality and 7 being the highest quality. For version 4.1 pixels are defined as best that are assigned a quality flag greater than or equal to 4. For more details on the assignment of the quality flags see the users reference manual.

Version 5.0 of the algorithm uses the same formulation as above with the same quality flags applied.  However pixel values are calculated at a spatial resolution of 4km instead of 9km.

Data Processing Sequence:

Processing Steps:

Level-1B data are first ingested from optical disk, then converted from to a standard image format. Data are then navigated from line/pixel(image) coordinates to latitude/ longitude coordinates and subsetted. Next the non-linear correction algorithm adjusts for the calibrations of the AVHRR channels, and SST is computed from predetermined regression coefficients which are specific to 3 regimes of water vapor.
 

Once SST retrievals are determined, the data subsets are binned to produce the 9.28 km equal-area orbitals for both ascending and descending nodes and an initial quality flag is assigned to each retrieval. After the orbitals for an entire day have been completely processed, they are composited to a single daily file.

The next phase is declouding. This is effected through the creation of composite images over 3 weeks, from each of which a mean is computed. These are then used to fill a central weekly mean image which contains the day being declouded (if the central mean image is missing values). If the weekly means from week(n- 1) or week(n+1) cannot be used to fill empty values in the central (week n) mean, a spatial interpolation is done. The completely-filled weekly image is then compared to the central daily, and simple thresholding is used to indicate partial or complete cloudiness. This process generates a cloud mask for every day of data.

Processing Changes:

None

Calculations:

Special Corrections/Adjustments:

Adjustments and calibrations of the mapping from line/pixel(image) space to latitude/longitude are also performed. To determine the actual location of the line/pixel image, time and attitude parameters are corrected using navigation ephemeris files to get a comparable match-up of the coastlines that are visible from the actual image to a reference map outline. This generates a more refined set of parameters most importantly the earth location of the image in latitude and longitude values. Sectors are extracted data specified by a latitude/longitude center taken from the ingested files. The selection of the latitude and longitude center depends on the accuracy of the navigation process. The navigation file is updated for this piece or sector. It uses the same file but modifies the ingested file. For subsequent processing, the volume of data from a sector is far smaller compared to a typical ingested file. This is desirable in terms of effecient processing.

Calculated Variables:

Sea Surface Temperature is calculated from a non-linear correction algorithm (see above).

Graphs and Plots:

Information not available.

10. Errors:

Sources of Error:

One of the greatest limitations is the obstruction by clouds in the field of view. Other sources of error include atmospheric gases and emissions as well as water surface characteristics.

Quality Assessment:

A semi-automated quality-assurance (QA) scheme has been developed which examines AVHRR SST retrievals for temporal and spatial consistency. This is carried out in a two- part statistical post-processor, followed by a visual inspection. The automated portion of the analysis serves to guide an operator in the visual inspection phase, greatly reducing the time necessary to characterize spurious findings. The data quality information thus gained is passed to the end-user in the form of additions to the processing flags, as well as comments which are included in the metadata of each image. This combination forms a qualitative and quantitative description of anomalies found in the data.

11. Notes:

    Limitations of the Data:

No SST value under cloudy conditions

        Known Problems with the Data:

Cloud cover. Periods of high aerosols after major volcanic eruptions such as Mt. Pinatubo. Water Vapor attenuation

        Usage Guidance:

For more detailed information, see the NOAA/NASA Pathfinder AVHRR Oceans Pathfinder Sea Surface Temperature Data Set User's Reference Manual.

 

Additional information on access through the NODC interfaces may be found at:

                          ftp access:

             ftp://data.nodc.noaa.gov/pub/data.nodc/pathfinder/Version5.0

                web access:

              http://data.nodc.noaa.gov/pathfinder/Version5.0

                and the OPeNDAP server at:

              http://data.nodc.noaa.gov/cgi-bin/nph-dods/pathfinder/Version5.0

                More detailed information on the NODC interfaces can be obtained through the usersguide at:

             http://www.nodc.noaa.gov/sog/pathfinder4km/
 

    12. Application of the Dataset:
 

    Global climate studies, studies of ocean circulation and its interaction with the atmosphere, calculate heat transport in the ocean.

13. Dataset Plans:
 

Description of Future Plans:

Reprocessing efforts are planned for version 5.1.

 
14.  Related Software:

           Software Description:

The PO.DAAC is supplying IDL (interactive data languate), and C drivers, to read and write HDF data.  F C drivers must download the HDF library to use.

Software Access:

The HDF subroutine library is available via anonymous ftp at ftp.ncsa.uiuc.edu (141.142.3.135). PO.DAAC also supplies example IDL code for reading and writing HDF data; if you use IDL the HDF library is already included, however IDL version 3.5 or better must be installed at your site.

15. Data Access:

Contact(s) Name, Address, Telephone and E-mail:

User Services Office

Physical Oceanography Distributed Active Archive Center (PO.DAAC)
Jet Propulsion Laboratory (JPL)

Phone: (626) 744-5508
Fax: (626) 744-5506
Email: podaac@podaac.jpl.nasa.gov
URL: http://podaac.jpl.nasa.gov

Procedures for Obtaining Data:

Through an order form format a user specified temporal and spatial subsetting routine is available. The subset is produced and staged for pick-up via ftp. The requester is notified of the completion of the subset and is given the file name(s) and their location via an e-mail message.

 

Newly processed data can be accessed via ftp or the web:

FTP site: ftp://podaac.jpl.nasa.gov/pub/sea_surface_temperature/avhrr/pathfinder/
 

16. Output Products and Availability:

Tape Products:

    The following global products are available on 8mm

    Equal-angle Best SST (9km and 18km)
    Equal-angle All SST (9km and 18km)
    0.5-degree spatial resolution SST (54km)

Film Products:

Not Applicable

    Other Products:

Subsets of Best SST products are available via wide world web (http://podaac.jpl.nasa.gov/DATA_PRODUCT/SST/index.html) in GIF, raw binary, and HDF formats.

17.  References:

Brown J. W., O. B. Brown, and R. H. Evans, 1993. Calibration of AVHRR Infrared channels: a new approach to non-linear correction, Journal of Geophysical Research, 98 (NC10), 18257-18268.

JPL Physical Oceanography Distributed Active Archive Center (PO.DAAC) Data Availability, Version 1-94, JPL Publication 90-49, rev. 5.

Kidwell, K., 1991. NOAA Polar Orbiter User's Guide. NCDC/NESDIS, National Climatic Data Center, Washington, D.C..

McClain E. P., W. G. Pichel, and C. C. Walton, 1985. Comparative performance of AVHRR based multichannel sea surface termperatures, Journal of Geophysical Research 90, 11587-11601.

McMillin, L. M., and D. S. Crosby, 1984. Theory and validation of the multiple window sea surface temperature technique. Journal of Geophysical Research, 89(C3), 3655- 3661.

Stowe, L. L., E. P. McClain, R. Carey, P. Pellegrino, G. G. Gutman, P. Davis, C. Long, and S. Hart, 1991. Global distribution of cloud cover derived from NOAA/AVHRR operational satellite data, Adv. Space Research, 3, 51-54.

18.  Glossary of Terms:
 

Sea Surface Temperature
The temperature of the layer of sea water nearest the atmosphere.

19.  List of Acronyms:

AVHRR....Advanced Very High-Resolution Radiometer

EOS....Earth Observing System
FTP....File Transfer Protocol
GAC....Global Area Coverage
HDF....Hierarchical Data Format
JPL....Jet Propulsion Laboratory
MCSST....Multichannel Sea Surface Temperatures
NASA....National Aeronautics and Space Administration
NOAA....National Oceanic and Atmospheric Administration
NODC...National Oceanographic Data Center
PO.DAAC....Physical Oceanography Distributed Active Archive Center
SST....Sea Surface Temperature

20. Document Information:
 

Document Revision Date:

27 April 1999

07 May 2004

10 June 2004

Document Review Date:

This document is under review.

Document ID:

Citation:

Document Curator:

Jorge Vazquez

           jv@pacific.jpl.nasa.gov

Document URL:

ftp://podaac.jpl.nasa.gov/pub/documents/dataset_docs/avhrr_pathfinder_sst.html