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IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

Lecture 3 Remote Sensing of the Sea. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea. Remote sensing of the sea includes: . 1. Sensor calibration. 2. Atmospheric correction. 3. Positional registration. 4. Oceanographic sampling for "sea truth".

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IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

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  1. Lecture 3 Remote Sensing of the Sea IoE 184 - 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 184 - 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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  4. 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 184 - 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 184 - 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 184 - 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 184 - 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 184 - 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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  10. 2. Atmospheric correction The ocean area within the IFOV emitsrays1+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 rays1+4+5+6 The complete atmospheric correction should result in the sum of rays1+2+3. Atmospheric pathways of electromagnetic radiation between the sea and the satellite sensor. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  11. 2. Atmospheric correction In the case of optical sensorsRay 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 184 - 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 184 - 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 184 - 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 184 - 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 184 - 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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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 IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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 IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  22. 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”. 3. Positional registration Most sophisticated method of position registration is used in TOPEX/Poseidon radar-altimeter. It is based on Doppler effect. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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 184 - 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 184 - 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 184 - 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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  28. 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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  29. 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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  30. 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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  31. 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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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 184 - 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 184 - 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.). IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  35. 5. Image processing Example of Level 2 data: MODIS Sea Surface Temperature, 2000 December 6, 17:05 IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  36. 5. Image processing Example of Level 2 data: MODIS Surface Chlorophyll Concentration, 2000 December 6, 17:05 IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  37. 5. Image processing Example of Level 2 data: MODIS Total Suspended Solids , 2000 December 6, 17:05 IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  38. 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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  39. 5. Image processing SeaWiFS Level 3 chlorophyll image, 1997 December 8 (daily) IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  40. 5. Image processing SeaWiFS Level 3 chlorophyll image, 1997 December 11 – 18 (8-day) IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  41. 5. Image processing SeaWiFS Level 3 chlorophyll image, 1997 December 1 – 31 (monthly) IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  42. 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 250-300 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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  44. 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 http://www.space-research.org/ 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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  45. 5. Image processing Download from the site www.obee.ucla.edu/test/faculty/nezlin From the section Lecture 3 “Remote Sensing of the Sea” two example files: MO36MWN2.sst4.zip and C1978341012416.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 184 - 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 31706. To understand its meaning double-click on the records Scale_type, Slope and Intercept. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  47. 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. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  48. 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 184 - 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 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

  50. 5. Image processing Open the file C1978341012416.L2_BRS.hdf and expand the structures clicking “+”. IoE 184 - The Basics of Satellite Oceanography. 3. Remote Sensing of the Sea

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