1. Remote Sensing of the Sea
Lecture 3
Remote Sensing of the Sea
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

2. Remote sensing of the sea includes:
1. Sensor calibration
2. Atmospheric correction
3. Positional registration
4. Oceanographic sampling for "sea truth"
5. Image processing
6. Oceanographic applications of satellite remote sensing
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

3. Compare satellite remote sensing and the traditional sources of oceanographic information:
Remote sensing is better than traditional methods:
Synoptic view, because satellites collect huge amount of information much exceeding the data collected by contact oceanographic observations;
Satellite observations cover wide areas of the World Ocean hardly accessible for field observations.
Problems of remote sensing:
The parameters measured by the satellites cannot be directly attributed to traditionally measured oceanographic characteristics;
Some satellite observations (ocean color and infrared) are more sensitive to unfavorable meteorological conditions than traditional oceanographic methods.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

4. High vacuum of outer space;
1. Sensor calibration
Each oceanographic equipment should be calibrated both before and after deployment. In the case of satellites we need to take into account the following:
The stress of launch;
High vacuum of outer space;
3. The power limitations on board the satellite, often resulting in gradual deterioration in the power supply on the satellite;
4. No opportunity of retrieving the instrument for periodic recalibration in the laboratory.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

5. 1. Sensor calibration
On some scanners, part of the scan views a reference target, a lamp of known brightness for the visible wavelength scanners, or a black body of measured temperature for thermal IR sensors. In this way gradual drift of the sensor can be detected and corrections made in the data analysis.
Some sensors use the moon as a natural object with constant optical characteristics.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

6. 2. Atmospheric correction
The sensors look at the ocean surface through another medium, the atmosphere. The atmosphere is opaque to electromagnetic radiation at many wavelengths, and there are only certain wavelengths through which radiation may be fully or partly transmitted.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

7. 2. Atmospheric correction
The following compounds of the atmosphere change its transmittance:
Gas molecules themselves
Water vapor
Aerosols
Suspended particles of dust
Water droplets in the form of clouds
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

8. 2. Atmospheric correction
~40% of sunlight is reflected by clouds
~20% of sunlight is absorbed by the atmosphere
~40% of sunlight is absorbed by Earth’s surface
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

9. 2. Atmospheric correction
Ray 1 - the useful signal;
Ray 2 - the radiation leaving the sea which is absorbed by the atmosphere;
Ray 3 - the radiation, which is scattered by the atmosphere out of the sensor field of vision.
Ray 4 - the energy emitted by the constituents of the atmosphere;
Ray 5 - the energy reflected by scattering into the field of vision of the sensor;
Ray 6 - the energy which previously left the sea surface but from outside the field of view.
Atmospheric pathways of electromagnetic radiation between the sea and the satellite sensor.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

10. 2. Atmospheric correction
The ocean area within the IFOV emits rays 1+2+3
Rays 4, 5, and 6 reach the sensor without having left the sea surface in the field of view, and therefore constitute extraneous "noise" on top of the signal.
The sensor receives rays
The complete atmospheric correction should result in the sum of rays
Atmospheric pathways of electromagnetic radiation between the sea and the satellite sensor.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

11. 2. Atmospheric correction
In the case of optical sensors Ray 2 is absent.
On the contrast, in thermal IR sensors Ray 2 is important: the cool atmosphere absorbs radiation (Ray 2) and re-emits it with lower temperature characteristics (Ray 4).
Atmospheric pathways of electromagnetic radiation between the sea and the satellite sensor.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

12. 2. Atmospheric correction
Increased atmospheric pathlength resulting from oblique viewing.
The oblique view results in looking through longer path length of atmosphere than for nadir viewing. This feature is used in atmospheric correction. By viewing the same piece of sea twice, through different lengths of atmosphere, an objective estimate of atmospheric effect can be made.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

13. 2. Atmospheric correction
The main strategies of atmospheric correction:
No separate attempt of atmospheric correction, instead we calibrate each scene with ground data.
A universal atmospheric correction based on an average model of atmospheric effects.
Using different wavelengths, assuming that certain channels are unlikely to have any upwelling radiation from the sea. In this way we process each pixel of the image.
An atmospheric (microwave) sounding sensor can be mounted on the same satellite as an oceanographic sensor.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

14. 2. Atmospheric correction
Without atmospheric correction, each scene can be calibrated with ground data, but the slope of correlation for each scene is unique.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

15. 2. Atmospheric correction - Cloud detection
Cloud cover is a main obstacle for satellite imagery in visible and infrared spectral bands. Clouds are transient atmospheric features that consist of small ice and liquid water particles with dimensions from under a micrometer to a few millimeters, resulting from water condensation and freezing.
Cloud properties vary with height. In the visible and infrared part of spectrum, the liquid water and ice crystals contained in the clouds scatter and absorb radiation, so that thick clouds make it impossible to view the surface. At any time, clouds cover almost two-thirds of the globe.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

16. 2. Atmospheric correction - Cloud detection
In both ocean color and SST, the first step of the procedure of atmospheric correction is to determine if every oceanic pixel in the image under investigation is cloud-free.
The SeaWiFS (8 optical channels) cloud detection is most primitive.
It is assumed, that the water-leaving radiance of near-infrared wavelength is near zero. As such, the pixels with a reflectance greater than a preset threshold are classified as clouds.
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17. 2. Atmospheric correction - Cloud detection
In AVHRR and MODIS the cloud detection is based on two factors:
the clouds are colder and more reflective than the ocean surface;
for spatial scales of order 100 km, the ocean surface, in contrast to clouds, is nearly uniform in temperature and reflectance.
Three kinds of tests are used:
“Threshold” tests eliminate pixels that are more reflective or colder than the ocean surface.
2) “Uniformity” tests examine the variance of temperature or reflectance in a rectangular array of pixels.
3) The retrieved SSTs are compared with climatology and with SSTs retrieved using alternative algorithms; e.g., according to the “unreasonableness” test, SST must be within the range from -2ºC to +35ºC.
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18. 3. Positional registration
Positional registration means the identification on a map of the place to which a remote-sensed measurement refers. The problem of knowing where the satellite was when a measurement was made depends on type of sensor, first of all its spatial resolution.
An approximate estimation of the satellite position can be obtained from the time of observation. However, the precision of this estimation is within few kilometers.
AVHRR radiometer on NOAA satellite
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19. 3. Positional registration
Positional registration means the identification on a map of the place to which a remote-sensed measurement refers. The problem of knowing where the satellite was when a measurement was made depends on type of sensor, first of all its spatial resolution.
An approximate estimation of the satellite position can be obtained from the time of observation. However, the precision of this estimation is within few kilometers.
AVHRR radiometer on NOAA satellite
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20. 3. Positional registration
Often the "ground control points" are used. However, the ground control points can be used mostly in the coastal zones. The problem of distortion of the image results from oblique viewing of the spherical earth surface. During processing each pixel of the image should be attributed to geographical coordinates.
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21. 3. Positional registration
In recent satellites more precise estimation of the position is obtained using the signals of GPS (Global Positioning System) satellites.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

22. 3. Positional registration
Most sophisticated method of position registration is used in TOPEX/Poseidon radar-altimeter. It is based on Doppler effect.
When the sensor is moving from the signal the frequency it receives “decreases”.
When the sensor is moving toward the signal the frequency it receives “increases”.
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23. 3. Positional registration
DORIS system determines the position of TOPEX/Poseidon satellite orbit to within a few centimetres.
The technique used (known as orbit determination), consists of locating a satellite in relation to about fifty ground control points on the Earth's surface.
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24. 4. Oceanographic sampling for "sea truth"
The main problem is that in general the remote-sensed characteristics of the sea change on a much shorter time scale than those of the land. Using for this purpose the overpasses of the satellite should be done carefully. In some cases it is impossible (e. g., altimeter measuring swell waves).
In other case (SST or water color measured few hours one after another) we can compare overpasses of the satellites.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

25. 4. Oceanographic sampling for "sea truth"
The strategy of collecting of samples is very important. The samples must span as wide range of data values as possible. Typically, transects across the gradients are used.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

26. 4. Oceanographic sampling for "sea truth"
Spatial resolution of the sensor is important as compared with spatial variability of the measured parameter, because the value measured within a point may not be representative of the average parameter within the whole pixel measured by the satellite.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

27. 4. Oceanographic sampling for "sea truth"
Comparing satellite observations and “sea truth” data we should keep in mind that the data collected by contact methods can be not more precise than remotely-sensed data.
In practice, the satellite data and “sea truth” data are nothing but two data arrays collected by different methods.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

28. Level of data processing
5. Image processing
Level of data processing
Level 0 -
Level 1 -
Level 2 -
Level 3 -
Level 4 -
Raw data received from satellite, in standard binary form;
Image data in sensor coordinates, containing individual calibrated channels;
Derived oceanic variable, atmospherically corrected and geolocated, but presented in sensor coordinates;
Composite images of derived ocean variable resampled onto standard map base and averaged over a certain time period (may contain gaps);
Image representing an ocean variable averaged within each grid cell as a result of data analysis, e.g., modeling.
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29. These data are called “Level 0” data.
5. Image processing
The “raw” information measured by the sensor onboard the satellite (raw radiance counts from all bands as well as spacecraft and instrument telemetry) is transmitted by radio-signal and received by the ground station.
These data are called “Level 0” data.
Level-1 Data Products
Level-1 products contain all the Level-0 data, appended calibration and navigation data, and instrument and selected spacecraft telemetry that are reformatted and also appended.
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30. The radiances are measured at different wavebands, called “channels”.
5. Image processing
The radiances are measured at different wavebands, called “channels”.
Different channels provides information on different properties of the Earth’ surface.
One method of analysis is when the images observed at different wavebands can be combined to result in a “true color image”.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

31. The radiances are measured at different wavebands, called “channels”.
5. Image processing
The radiances are measured at different wavebands, called “channels”.
Different channels provides information on different properties of the Earth’ surface.
One method of analysis is when the images observed at different wavebands can be combined to result in a “true color image”.
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32. 5. Image processing
At this MODIS image of the Mississippi River delta you can see clouds, coastline, river, the zones of phytoplankton bloom and pollution in the coastal ocean, etc.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

33. 5. Image processing
True color images are an important source of information about natural disasters like these wildfires in California in autumn 2003.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

34. 5. Image processing Level-2 Data Products
Each pixel of Level-2 data contains geophysical values (e.g., sea surface temperature, surface chlorophyll concentration, etc.) estimated from the radiances measured by the satellite
Each Level-2 product is generated from a corresponding Level-1 product.
Level-2 data are derived from the Level-1 raw radiance counts by applying the sensor calibration, atmospheric corrections, and the algorithms specific for each kind of geophysical value (e.g., bio-optical algorithms for water color data, etc.).
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35. MODIS Sea Surface Temperature, 2000 December 6, 17:05
5. Image processing
Example of Level 2 data:
MODIS Sea Surface Temperature, 2000 December 6, 17:05
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36. MODIS Surface Chlorophyll Concentration, 2000 December 6, 17:05
5. Image processing
Example of Level 2 data:
MODIS Surface Chlorophyll Concentration, 2000 December 6, 17:05
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37. MODIS Total Suspended Solids , 2000 December 6, 17:05
5. Image processing
Example of Level 2 data:
MODIS Total Suspended Solids , 2000 December 6, 17:05
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38. For other satellites these periods can be different.
5. Image processing
Level-3 Data Products
Level-3 means geophysical parameters observed during a certain period and interpolated on a global grid. For The SeaWiFS the periods of Level 3 data are:
one day,
8 days,
a calendar month, or
a calendar year.
For other satellites these periods can be different.
The spatial resolution of the global grid can be:
1 degree (360 x 180 grid);
18 km (2048 x 1024 grid) - MC SST;
9 km (4096 x 2048 grid) - SeaWiFS, Pathfinder SST v.1-4;
4.5 km (8192 x 4086 grid) - MODIS, Pathfinder SST v.5.
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39. SeaWiFS Level 3 chlorophyll image, 1997 December 8 (daily)
5. Image processing
SeaWiFS Level 3 chlorophyll image, 1997 December 8 (daily)
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40. SeaWiFS Level 3 chlorophyll image, 1997 December 11 – 18 (8-day)
5. Image processing
SeaWiFS Level 3 chlorophyll image, 1997 December 11 – 18 (8-day)
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41. SeaWiFS Level 3 chlorophyll image, 1997 December 1 – 31 (monthly)
5. Image processing
SeaWiFS Level 3 chlorophyll image, 1997 December 1 – 31 (monthly)
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42. The satellite data are disseminated via internet.
5. Image processing
The satellite data are disseminated via internet.
Users select the images in online databases and either download or order data files. Typical satellite images are very big (e.g., one MODIS image is about Mb).
To enable the users to have a brief look at each image before selecting it low-resolution “browse” images are often produced. If the area of interest is free from clouds, the user orders the data file, downloads it, and works with it using appropriate software.
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43. 5. Image processing Data format.
Many types of satellite information are stores in Hierarchical Data Format (HDF).
HDF is a cross-platform file format for storing a wide variety of scientific data. This public-domain open standard was created by the National Center for Supercomputing Applications at the University of Illinois Urbana-Champaign (NCSA).
A typical HDF file might contain a dataset, data table, descriptions of data, images produced from the data and other related information. It can be processed using special software, such as Noesys, MATLAB, IDL, etc.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

44. All good software packages are commercial.
5. Image processing
All good software packages are commercial.
To understand the basic features of HDF files you can use free program HDFExplorer from the Internet site
1. Select <<Download>>
2. Fill out the form with your name, etc.
3. Click <<Submit>>
4. Store the HDFExplorerSetup.exe file on the hard drive of your computer
5. Double-click HDFExplorerSetup.exe to install HDFExplorer.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

45. From the section Lecture 3 “Remote Sensing of the Sea”
5. Image processing
Download from the site
From the section Lecture 3 “Remote Sensing of the Sea”
two example files:
MO36MWN2.sst4.zip and
C L2_BRS.hdf.zip
Uncompress these files using WinZip and open them in HDFExplorer. On the left you see the content of the file. Clicking “+” expand the structures.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

46. 5. Image processing
MO36MWN2.sst4.zip contains data on sea surface temperature (SST4) collected by MODIS Terra satellite.
The dataset sst4_mean contains the data array. Double-click it to see the content.
Let us analyze the content of the dataset at the example of one grid node (x=1; y=150).
The grid node with column = 1 and row = 150 contains the value To understand its meaning double-click on the records Scale_type, Slope and Intercept.
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47. Y=Slope * x + Intercept; Slope = 0.01 Intercept = -300
5. Image processing
You see the following:
Scale_type =
Y=Slope * x + Intercept;
Slope = 0.01
Intercept = -300
31706 * 0.01 – 300 = 17.06
Double-click Units and Name.
Now you see that it is the temperature of the ocean surface measured in Degrees C.
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48. Double-click Start Year, Start Day, End Year, End Day.
5. Image processing
Double-click Start Year, Start Day, End Year, End Day.
You see that the data were collected from 2000, Julian Day 336 (December 1) to 2000 Julian Day 344 (December 9).
Double-click
Northernmost Latitude
Southernmost Latitude
Westernmost Longitude
Easternmost Longitude.
You can see that the data array covers the entire Earth surface from 90S (i.e., -90) to 90N and from 180W (i.e., -180) to 180E.
Number of Columns and Number of Lines indicate the grid size:
1024 x 512.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

49. 5. Image processing
HDFExplorer cannot transform 2-byte arrays into images, but other software can help you to make a graphical representation of the HDF file content.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

50. 5. Image processing
Open the file C L2_BRS.hdf and expand the structures clicking “+”.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

51. 8-bit Raster Image 2 indicates the snapshot location.
5. Image processing
This file contains CZCS chlorophyll image; the snapshot was obtained in the western Pacific.
8-bit Raster Image 1 contains the image of surface chlorophyll concentration.
8-bit Raster Image 2 indicates the snapshot location.
Point the mouse cursor at the Raster Image 1, right-click and select “View”. You see the values from 0 to 255, i.e. bytes.
Point the mouse cursor at the Raster Image 1, right-click and select “Image View”. You see the graphic representation of the data array made using the Image Palette.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

52. 5. Image processing
The records Start Year, Start Day, End Year, End Day indicate that the image was obtained in 1978, Julian Day 341 (December 7).
Northernmost Latitude, Southernmost Latitude, Westernmost Longitude and Easternmost Longitude indicate the approximate location of the image.
Latitude and Longitude structures indicate the exact geographic locations of selected pixels of the image. These arrays can be used by the software like MATLAB Mapping Toolbox to produce the image in any geographical projection you wish.
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53. Double-click the records Scaling Equation, Base, Slope, and Intercept.
5. Image processing
Double-click the records
Scaling Equation,
Base,
Slope, and
Intercept.
You see:
Base = 10;
Slope = 0.012;
Intercept = -1.4;
Scaling Equation =
= “Base**((Slope*brs_data) + Intercept) = chlorophyll a”
We can check some values within the range and see that
the brs_data = 100 results in …,
the brs_data = 200 results in …, etc.
Double-click Parameter and Units. You see the description of the data
“chlorophyll a concentration” and “mgm^-3”.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

54. Oceanographic applications of satellite remote sensing include:
1. Visible wavelength "ocean color" sensors
2. Sea surface temperature from infrared scanning radiometers
3. Passive microwave radiometers
4. Satellite altimetry of sea surface topography
5. Active microwave sensing of sea-surface roughness
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

55. 1. Visible wavelength "ocean color" sensors
These sensors operate in the visible part of the electromagnetic spectrum, measuring electromagnetic radiation emitted by the sun and reflected by land and ocean surface.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

56. 1. Visible wavelength "ocean color" sensors
The color of the Earth’ surface, especially the color of the ocean, results primarily from biological processes.
Measuring the absorption and backscattering characteristics of ocean surface, we can estimate the concentrations of different kinds of matter suspended in seawater, including phytoplankton cells.
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57. 2. Sea surface temperature from infrared radiometers
Infrared sensors measure electromagnetic radiation within the band 1-30 µm, emitted by the ocean surface and resulting from the temperature of the upper sea layer.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

58. 2. Sea surface temperature from infrared radiometers
The near-infrared and infrared radiation is processed to sea surface temperature (SST). The most important SST sensors are Advanced Very High Resolution Radiometer (AVHRR) on NOAA satellites, MODIS, GOES geostationary satellites, and some others.
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

59. 3. Passive microwave radiometers
Passive microwave radiometers operate at electromagnetic wavelengths 1.5–300 mm (i. e., the frequency 1–200 GHz).
Their advantage is the comparatively long wavelength, which is not sensitive to scattering by the atmosphere or aerosols, haze, dust, or small water particles in clouds. So, the microwave sensors are all-weather devices. This principle advantage is countered by the fact that thermal emission is very weak at these longer wavelengths. To overcome noise levels a large field of view must be received; that results in low spatial resolution (25–150 km). So, these observations are used for studies of heat balance of the ocean.
The emissivity of the sea at microwave frequencies varies with the dielectric properties of sea water (including salinity) and the surface roughness. Hence, the development of this technique in future can enable the measurements of surface salinity.
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60. 4. Satellite altimetry of sea surface topography
Satellite altimeters are radars, which transmit short pulses toward the earth beneath them. The return time of the pulse after reflection at the earth's surface is measured, and this yields the height of the satellite. The most important are ERS-1, ERS-2, TOPEX/Poseidon, and Jason-1 satellites.
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61. Synthetic Aperture Radar (SAR)
5. Active microwave sensing of sea-surface roughness
Synthetic Aperture Radar (SAR)
Synthetic aperture radar (SAR) is based on the comprehensive analysis of contribution from individual points to the signal received when the sensor is at a particular point. The result is very high resolution.
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62. Synthetic Aperture Radar (SAR)
5. Active microwave sensing of sea-surface roughness
Synthetic Aperture Radar (SAR)
SAR images enable the analysis of small-scale and mesoscale eddies, river plumes,
oil slicks,
ice packs, etc.
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63. IoE 184 - The Basics of Satellite Oceanography. 3
IoE The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea