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Lecture 10 Multi-Spectral Remote Sensing Systems 14 October 2008

Lecture 10 Multi-Spectral Remote Sensing Systems 14 October 2008. Reading Assignment. Campbell, Chapters 6, 21 Note: Most of the figures and tables from today’s lecture are from Jensen’s Introduction to Remote Sensing. Lecture Topics. Measuring radiance

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Lecture 10 Multi-Spectral Remote Sensing Systems 14 October 2008

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  1. Lecture 10Multi-Spectral Remote SensingSystems14 October 2008

  2. Reading Assignment • Campbell, Chapters 6, 21 Note: Most of the figures and tables from today’s lecture are from Jensen’s Introduction to Remote Sensing

  3. Lecture Topics • Measuring radiance • Key questions for designing spaceborne radiometers • Considerations for deploying a spaceborne radiometer • Problems in imaging over wide swaths • Summary of system tradeoffs  categories of satellite radiometers • Types and key features of multi-spectral scanning systems • Key Spaceborne Remote Sensing Systems • Land Surface Remote Sensing • Water Surface Remote Sensing

  4. 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

  5. Fig. 1 Radiance (W/sq m/sr)

  6. Fig. 2 Radiance (W/sqm/sr)

  7. Lecture Topics • Measuring radiance • Key questions for designing spaceborne radiometers • Considerations for deploying a spaceborne radiometer • Problems in imaging over wide swaths • Summary of system tradeoffs  categories of satellite radiometers • Types and key features of multi-spectral scanning systems • Key Spaceborne Remote Sensing Systems • Land Surface Remote Sensing • Water Surface Remote Sensing

  8. 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 radiance in order to be able to discriminate or measure the features of interest? Radiometric resolution • How large are the features of interest? Spatial resolution and swath width • How frequently and when do you have to measure the features of interest? Temporal resolution and swath width

  9. Resolution • Spectral Resolution – the wavelength regions of and bandwidths for a radiometer • Radiometric Resolution - the sensitivity of a remote sensing detector to variations in the emitted, reflected or scattered EM energy that is being detected • Spatial Resolution - The measure of the smallest distance (linear or angular separation) between objects that can be resolved by the sensor • Temporal Resolution – the timing and frequency for collection of data by a satellite system

  10. Vegetation Geology/Soils Spectral Resolution Sensors are designed to contain spectral bands specific to the type of surface feature being monitored or mapped Fig. 3

  11. Fig. 4 Radiometric Resolution If our system was designed to detect radiance values between 0 and 0.0015 watts/sq m, and we had 8 bit recording (0 to 255), then the radiometric resolution would be 0.0000006 watts/sq m

  12. Spatial Resolution is determined by the Instantaneous Field of View- IFOV • All scanning radiometers have an instantaneous field of view over which the sensor detects EM energy for a specific pixel

  13. Fig. 5 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

  14. Changes in IFOV • If you are imaging over a very wide swath, then H will increase as you scan away from nadir, meaning the size of the IFOV on the ground will increase

  15. Temporal Resolution Temporal resolution has three important components – • How frequently you have to observe a specific area on the earth’s surface to capture variations over time of the phenomena being observed • When during the year do the phenomena you are interested in take place • The diurnal (e.g., 24 hour) variations in the signature being observed • Variations in solar illumination • Variations in the occurrence of the phenomena • Variations in characteristics of the atmosphere

  16. Swath Width • How large of an area does the remote sensing system have to capture in order to collect data about the features or processes of interest? • Swath widths of satellite remote sensing systems range between 7 and 2,500 kilometers • Swath width + the orbital path of the satellite determine temporal resolution

  17. Lecture Topics • Measuring radiance • Key questions for designing spaceborne radiometers • Considerations for deploying a spaceborne radiometer • Problems in imaging over wide swaths • Summary of system tradeoffs  categories of satellite radiometers • Types and key features of multi-spectral scanning systems • Key Spaceborne Remote Sensing Systems • Land Surface Remote Sensing • Water Surface Remote Sensing

  18. CONSIDERATIONS FOR SATELLITE DEPLOYMENT OF A RADIOMETER • What is the size of the area or patch being detected by the satellite radiometer? • How frequently can a satellite view the same piece of ground on the earth’s surface? • How large an area is imaged by the sensor? • 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?

  19. CONSIDERATIONS FOR SATELLITE DEPLOYMENT OF A RADIOMETER • What is the size of the area or patch being detected by the satellite radiometer? Determined by • The IFOV of the sensor • The height of the satellite platform • The scanning angles of the radiometer

  20. CONSIDERATIONS FOR SATELLITE DEPLOYMENT OF A RADIOMETER • What is the size of the patch being detected by the satellite? • How frequently can a satellite view the same piece of ground on the earth’s surface? • How large of an area is imaged by the sensor? • 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? • How do you record/transmit the data being detected/recorded by the systems

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

  22. A satellite in low earth orbit (~800 km) takes about 90 minutes to complete a single passage from equator to equator Fig. 6

  23. Atmospheric Conditions • In most regions with significant vegetation, there is a diurnal variation in atmospheric moisture and cloud cover that is driven by evapo-transpiration by plants • Cloud cover is lowest in the early morning hours, then increases as the day progresses • In many areas, the resulting cloud formation hinders viewing of the earth’s surfaces over land areas by the early afternoon • Because of this, many sensors schedule fly-over times between 10 am and noon

  24. Earth’s circumference = 39,350 km At 800 km, the satellite has 16 orbits per day at 90 minutes per orbit 39,350 km / 16 = 2,460 km per orbit Question – how big of a swath does a satellite need to image the earth’s surface once per day when the sun is out?

  25. Question – if a satellite has a swath width of 172 km, how long does it take to image the entire earth’s surface? 16 orbits per day Total area covered per day is 16 x 176 = 2,816 km Covering the entire earth: (39,350 km) / (2,816 km/day) = 14 days

  26. CONSIDERATIONS FOR SATELLITE DEPLOYMENT OF A RADIOMETER • What is the size of the patch being detected by the satellite? • How frequently can a satellite view the same piece of ground on the earth’s surface? • How large an area is imaged by the sensor? • 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?

  27. Approaches to Recover Data from Satellite Remote Sensing Systems • Ground receiving station within direct view of the satellite (to acquire global coverage requires a large number of stations) • On-board data recorders (requires reliable, large volume recorders) for delayed transmission to ground receiving station • Using data relay satellites – e.g., the TDRSS - Tracking and Data Relay Satellite System

  28. 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 High resolution, wide swath – pixels per orbit for all channels 155 billion Data per day 2.5 trillion Data per month 743 trillion

  29. Lecture Topics • Measuring radiance • Key questions for designing spaceborne radiometers • Considerations for deploying a spaceborne radiometer • Problems in imaging over wide swaths • Summary of system tradeoffs  categories of satellite radiometers • Types and key features of multi-spectral scanning systems • Key Spaceborne Remote Sensing Systems • Land Surface Remote Sensing • Water Surface Remote Sensing

  30. CONSIDERATIONS FOR SATELLITE DEPLOYMENT OF A RADIOMETER • What is the size of the patch being detected by the satellite? • How frequently can a satellite view the same piece of ground on the earth’s surface? • How large an area is imaged by the sensor? • 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?

  31. Problems with imaging over wide swaths • The size of your ground footprint gets bigger as the angle off nadir increases • Atmospheric effects increase • The bidirectional reflectance at the surface often changes, e.g., the emittance from the surface for the same surface cover type changes

  32. Fig. 9

  33. Fig. 10

  34. Problems with imaging over wide swaths • The size of your ground footprint gets bigger as the angle off nadir increases • Atmospheric effects increase because of the increase in path length through the atmosphere • The bidirectional reflectance at the surface often changes, e.g., the emittance from the surface for the same surface cover type changes

  35. 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 atmospheric correction of the data

  36. Problems with imaging over wide swaths • The size of your ground footprint gets bigger as the angle off nadir increases • Atmospheric effects increase • The bidirectional reflectance at the surface often changes, e.g., the emittance from the surface for the same surface cover type changes

  37. Lecture Topics • Measuring radiance • Key questions for designing spaceborne radiometers • Considerations for deploying a spaceborne radiometer • Problems in imaging over wide swaths • Summary of system tradeoffs  Categories of satellite radiometers • Types and key features of multi-spectral scanning systems • Key Spaceborne Remote Sensing Systems • Land Surface Remote Sensing • Water Surface Remote Sensing

  38. 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 (+) advantage, (-) disadvantage

  39. Categories of satellite radiometers • Wide swath, low resolution • 1000-2600 km swath, 250 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

  40. Lecture Topics • Measuring radiance • Key questions for designing spaceborne radiometers • Considerations for deploying a spaceborne radiometer • Problems in imaging over wide swaths • Summary of system tradeoffs  categories of satellite radiometers • Types and key features of multi-spectral scanning systems • Key Spaceborne Remote Sensing Systems • Land Surface Remote Sensing • Water Surface Remote Sensing

  41. Types of VIS/RIR scanning systems • Multispectral scanners –scanning systems that detect and record reflected, emitted or backscattered energy from an area of interest in a limited number of 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

  42. 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) • Imaged an area of 185 by 185 km

  43. 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)

  44. 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)

  45. Landsat 1 focused on a small part of the EM spectrum In the visible and reflected infrared region

  46. Common 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 • Mechanism to scan the earth’s surface • Data recording system • Data transmission system

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