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Multi-Spectral Remote Sensing Systems and Design Lecture 4

Multi-Spectral Remote Sensing Systems and Design Lecture 4 . Summer Session 21 July 2011. Radiometers and Spectrometers. Radiometer – An instrument that measures radiance in a specified wavelength region

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Multi-Spectral Remote Sensing Systems and Design Lecture 4

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  1. Multi-Spectral Remote SensingSystems and Design Lecture 4 Summer Session21 July 2011

  2. Radiometers and Spectrometers • Radiometer – An instrument that measures radiance in a specified wavelength region • Spectroradiometer or spectrometer – An instrument that measures radiance continuously across a region of the EM spectrum or in multiple-bands across a region of the EM spectrum

  3. Near IR

  4. Questions to ask when designing a multi-channel spaceborne radiometer • What reflectance characteristics are you trying to measure? Spectral resolution • How precisely do you have to measure these characteristics? Radiometric resolution • How large are the features of interest? Spatial resolution • What is the size of the patch being detected by the satellite? • How frequently and when do you have to measure the features of interest? Temporal resolution • How much data are being recorded by the radiometer and how do we retrieve these data? • How do variations in surface and atmospheric conditions affect the data quality?

  5. Elements of a Remote Sensing System 4. Data Recorder 3. Sensing Device 5. Information Production System 6. Information Delivery System 2. Area or scene of interest 1. Information User

  6. Instantaneous Field of View- IFOV • All radiometers have an instantaneous field of view • i.e. the angular dimension over which radiation is detected • Essentially, the footprint that a satellite is detecting at any one instant. • It’s circular.

  7. Sensor IFOV (in degrees, ) H r Radius of circle within IFOV, r = H tan (/2) For very small IFOV, e.g., <<< 0.01º, r = H /2, where  is in radians

  8. Controls on Frequency of Coverage by a Satellite • The orbital time of the satellite • The width of the area being imaged by a satellite when it passes over the earth

  9. Types of satellites (by orbit) • Polar orbiting • Geo-stationary

  10. Polar Orbiting Satellites/Instruments

  11. A satellite in low earth orbit (~800 km) takes about 90 minutes to complete a single circling of the planet (16 orbits/day) inclination Inclination: how far off of the poles they circulate - Many satellites have 99oinclination Earth Circumference = 39,350 km 39,350/16 = 2460 km Why is it designed to cross the equator early in the morning? - solar illumination - cloud cover

  12. Satellite What would the viewing angle have to be in order to image the entire earth daily?? H = 800 km Swath width = 2460 km

  13. Tan() = 0.5*width height Tan() = (0.5*2460)/800 Tan() = 1.65  = arctan(1.65) = 57 degrees H = 800 km Swath width = 2460 km

  14. Satellite Viewing angle of 57° off nadir to image swath H = 800 km Swath width = 2460 km

  15. Satellite How long to cover the Earth with this system? Viewing angle of 6.1° off nadir to image swath 16 orbits/day * 172 km = 2752 km/day imaged (width) H = 800 km Earth’s circumference = 39,350 km 39, 350/2752 = 14.3 days Swath width = 172 km

  16. So, why doesn’t every imaging system have a wide swath and high resolution?? • There are trade-off’s! • Data volume  storage limits • Perceived vs. actual footprint • BRDF • Atmospheric effects

  17. Wide Swath / Low Resolution Narrow Swath/ High Resolution Image Size 2460 by 2460 km 172 by 172 km Ground area size (resolution or pixel size) 1 by 1 km 0.05 by 0.05 km (50 by 50 m) Number of radiometer channels 4 4 Images per orbit 16 228.8 Pixels per image per channel 6 million 11.8 million Pixels per orbit per channel 96 million 2.7 billion Pixels per orbit for all channels 384 million 10.8 billion If a sensor were to be both high resolution AND wide swath: Pixels per orbit for 4 channels: 155 billion Daily Pixels for Earth: 2.5 trillion Monthly Pixels for Earth: 743 trillion

  18. Transferring data to the ground • Transmit the information directly to the receiving station as the satellite passes over the site • On-board recorder – record data, and then transmit to a ground receiving station • Use a satellite relay system to transmit data to a receiving station • TDRSS Tracking and Data Relay Satellite System Data volume is a major consideration in developing satellites radiometer systems

  19. Problems with imaging over wide swaths • The size of your (perceived) ground footprint gets bigger as the angle off nadir gets large • Atmospheric effects increase • The bidirectional reflectance at the surface often changes • The emittance from the surface for the same surface cover type changes

  20. Bowtie effect

  21. Maximum look off-Nadir most scientists use Still usable, but starting to be un representative Relatively 1:1

  22. Atmospheric effects • Effects of atmosphere on incoming/outgoing EM energy is proportional to distance traveled through the atmosphere • As incidence angle increases, atmospheric effects (scattering, absorption, attenuation) increase • Using wide swath width increases the requirements for precise atmospheric adjustmentof the data

  23. Further information on this slide can be viewed at http://snrs.unl.edu/agmet/908/brdf_definition.htm

  24. Narrow-Swath, Higher Resolution Wide-Swath, Lower Resolution -Coverage only every 15 to 20 days (less if cloud cover exists) +Daily coverage of area +High resolution imagery -Low resolution imagery -Higher data volumes requires on-board recording or direct transmission +Lower data volumes result in less stringent recording/direct transmission requirements +Narrow viewing angle results in lower atmospheric / bi-directional scattering effects, and consistent across-swath resolution -Wider viewing angle results in greater atmospheric / bi-directional scattering effects, and variable across-swath resolution Summary of System Tradeoffs Temporal resolution Spatial resolution Data volume Atmospheric effects

  25. Categories of satellite radiometers • Wide swath, low resolution • 1000-2600 km swath, 500 to 1100 m • Moderate swath, moderate resolution • 100 to 200 km swath, 10 to 50 m resolution • Narrow swath, fine resolution • 5 to 15 km swath, 1 to 4 m resolution

  26. Digitization of signals Recording device Radiance is detected by the sensor and converted to a number based on the intensity of the signal by an analog to digital converter Different sensors have different levels of sensitivity, which depends on radiometric resolution and determines: - Minimum and maximum signature levels - Number of divisions signal is divided into - e.g. Landsat = 8-bit radiometric resolution  0-255 (28)

  27. The Pixel • A two-dimensional picture element that is the smallest non-divisible element of a digital image

  28. Key components of a pixel • Size of the pixel on the ground - spatial resolution • Number of gray levels (brightness values aka digital numbers) recorded by a pixel - radiometric resolution

  29. 8 bits = 28 = 256 levels 10 bits = 210 = 1024 levels 16 bits = 216 = 65536 levels

  30. Airborne MSS system in the NIR range, (0.76 to 0.90µm)

  31. Types of Scanning systems • Multispectral scanners – scanning systems that detect and record reflected, emitted or backscattered energy from an area of interest in multiple, broad bands of the electromagnetic spectrum • Hyperspectral scanners – scanning systems identical to multispectral scanners, except they detect and record energy from hundreds of narrow bands of the electromagnetic spectrum

  32. First Spaceborne MSS SatelliteEarth Resources Technology Satellite – ERTS-1 (Changed later to Landsat 1) • Launched 23 July 1972 • Contained two systems • 3 Channel Return Beam Vidicom (RBV) (channels 1 to 3) • 4 Channel Multispectral Scanner (channels 4 to 7)

  33. ERTS-1 RBV System Return Beam Vidicon cameras Essentially 3 TV cameras operating in separate channels • 0.48-0.57 µm (green) • 0.58-0.68 µm (red) • 0.69-0.83 µm (near IR)

  34. Landsat 1 MSS Channels or bands Band 4 is 0.5 to 0.6 um (green) Band 5 is 0.6 to 0.7 um (red) Band 6 is 0.7 to 0.8 um (near IR) Band 7 is 0.8 to 1.1 um (near IR)

  35. Key Features of a Spaceborne MSS System • Detectors in the wavelength bands of operation • Optics to provide a fine instantaneous field of view to define the pixel size • Method to scan the earth’s surface • Data recording system • Data transmission system

  36. Design Considerations in Land Surface MSS Systems • Number of bands sufficient to discriminate key land surface features • Trade-offs between swath width, pixel size, and frequency of coverage • Coarse resolution, wide swath, high frequency • Fine resolution, narrow swath, low frequency • Recording and down-loading data

  37. Landsat • The first Landsat satellite was launched in July 1972 (what we just went over) • Initially called Earth Resources Technology Satellite (ERTS) and renamed Landsat at a later date • The Landsat MSS system evolved into the Thematic Mapper (TM) in 1982 • Thematic Mapper evolved into Enhanced Thematic Mapper (ETM+) in 1999 • Next: Landsat 8 – Landsat Data Continuity Mission…Operational Land Imager (OLI) • December 2011 (cross your fingers)

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