Characteristics of Optical Sensors

1. Characteristics of Optical Sensors Mirza Muhammad Waqar Contact: mirza.waqar@ist.edu.pk +92-21-34650765-79 EXT:2257 RG610 Course: Introduction to RS & DIP

2. Outlines • Earth Observation Remote Sensing • Physical Basis of Remote Sensing • Platform • Sensor • Characteristics of Optical Sensor • Color Composite

3. Earth Observation Remote Sensing Remote Sensing Celestial RS Terrestrial RS Meteorological Telecommunication Earth Observation Optical Thermal Microwave

4. Physical Basis of Remote Sensing Physical Basis of Remote Sensing Observation Interpretation Conclusion Source of illumination EM Radiations Target Traveling Path

5. Platform Platform Cherry Lifter Balloons Air Crafts Space Craft Low Altitude 2 Km Medium Altitude 2-10 Km High Altitude 10-13 Km Low Altitude 200-00 Km Medium Altitude 500-900 Km High Altitude 36k-45k Km

6. Sensor Non-Imaging Active Sensors Imaging Passive Framing Non-Imaging Non-Framing

7. Sensor Type

8. Passive vs Active Remote Sensing Passive Remote Sensing Active Remote Sensing

9. Discrete Detectors and scanning mirrors-MSS, TM, ETM+, GOES, AVHRR, SeaWiFS, AMS, ATLAS Linear Arrays-SPOT, IRS, IKONOS, ORBIMAGE, Quickbird, ASTER, MISR Liner and area arrays-AVIRIS, CASI, MODIS, ALI, Hyperion, LAC

16. Characteristics of Optical Sensor Pixel Spectral Bandwidth Instantaneous Field of View (IFOV) Field of View (FOV) Dwell Time Altitude Resolution Satellite Orbits

17. Pixels Pixel is picture element It contains • Address (latitude & longitude) • Digital Numbers • Size

18. Spectral Bandwidth of the Detector • The signal is stronger for detectors that respond to a broader bandwidth of energy. • For example, a detector that is sensitive to the entire visible range will receive more energy than a detector that is sensitive to a narrow band. • Such as visible red.

19. Instantaneous Field of View (IFOV) • The instantaneous field of view (IFOV) of any detector is the solid angle through which a detector is sensitive to radiation. • Both the physical size of the sensitive element of the detector and the effective focal length of the scanner optics determine the IFOV. • IFOV is defined as the angle which corresponds to the sampling unit. Information within an IFOV is represented by a pixel in the image. • A small IFOV is required for high spatial resolution but also restricts the signal strength.

20. Instantaneous Field of View (IFOV)

21. Field of View (FOV) • The maximum angle of view which a sensor can effectively detect the electromagnetic energy, is called the Field of View (FOV). • The width on the ground corresonding to the FOV is called the Swath Width.

22. Field of View (FOV)

23. Dwell Time • The time required for the detector IFOV to sweep across a ground resolution cell is the dwell time. • A longer dwell time allows more energy to exposure to the detector, which creates a stronger signal.

24. Altitude • For a given ground resolution cell, the amount of energy reaching the detector is inversely proportional to the square of the distance. • A greater altitudes the signal strength is weaker.

25. Resolutions –Key to Check Image Quality Spatial Resolution Spectral Resolution Radiometric Resolution Temporal Resolution

26. Spatial Resolution • The spatial resolution of a satellite image is based on the pixel size or picture element. • Can only identify objects which are larger than the pixel size. • To accurately determine size and shape, object must be a few pixels long and wide.

27. 0.6 m Quick Bird (Pan) 5,8 m IRS-1C (Pan) SPOT – 5 (Pan) 2.5 m SPOT – 5 (XS) 10 m LANDSAT TM 30 m LANDSAT MSS 80 m NOAA 1 km Spatial Resolution Resolution Satellite

28. 80 m x 80 m Approximately the size of a hockey field Landsat MSS General Detail

29. 30 m x 30 m approximately 1/3rd of a hockey field Landsat ETM+ Local Detail

30. ASTER 15 m x 15m Point Detail

31. Quickbird (0.6 x 0.6 m)

32. Spectral Resolution • The finer the spectral resolution • the narrower the wavelength range for a particular channel or band.

33. 0.7 mm 0.4 mm Black & White Images Blue + Green + Red Spectral Resolution • Example: Black and white image - Single sensing device - Intensity is sum of intensity of all visible wavelengths Can you tell the color of the platform top? How about her sash?

34. 0.7 mm 0.4 mm Color Images BlueGreenRed Spectral Resolution • Example: Color image • - Color images need • least three sensing • devices, e.g., red, green, • and blue; RGB • Using increased spectral • resolution (three sensing • wavelengths) adds • information • In this case by “sensing” • RGB can combine to • get full color rendition

35. Spectral Response Differences TM Band 3 (Red) TM Band 4 (NIR)

36. Spectral Data • On the basis of spectral resolution we can divide data as follows: • Panchromatic Data • Multispectral Data • Hyperspectral Data

37. Hyperspectral Data Multispectral Data Panchromatic Data

38. Radiometric Resolution • The radiometric characteristics describe the actual information content in an image. • The radiometric resolution of an imaging system describes its ability to discriminate • very slight differences in energy

39. Radiometric Resolution • Number of Shades or brightness levels at a given wavelength • Smallest change in intensity level that can be detected by the sensing system

40. Radiometric Resolution

41. Each satellite revisits the same area after certain time period, which is called Temporal Resolution. Note that the Earth is also rotating to the East, means satellite does not pass over the same path next time. Temporal Resolution

42. Temporal Resolution • It is the revisit frequency of the satellite • More frequency => More Temporal resolution • Different for every satellite • Varies with the altitude of satellite • Temporal resolution is high for upper latitude but lower for equator • The temporal resolution of stereo satellite vary. • Animation in comment box

43. Temporal Resolution • High altitude satellite => High temporal resolution (3-4 days) • For frequent coverage => Satellite to satellite transfer coverage • Data is not on same scale

44. Satellite Orbits Geostationary Orbit Polar Orbit Sun Synchronous Orbit

45. Satellite Orbit Determines... • What part of the globe can be viewed. • The size of the field of view. • How often the satellite can revisit the same place. • The length of time the satellite is on the sunny side of the planet.

46. Satellite Orbits • A satellite follows a generally elliptical orbit around the earth. • The time taken to complete one revolution of the orbit is called the orbital period. • The satellite traces out a path on the earth surface, called its ground track, as it moves across the sky. • As the earth below is rotating, the satellite traces out a different path on the ground in each subsequent cycle.

47. Repeat Cycle • Remote sensing satellites are often launched into special orbits such that the satellite repeats its path after a fixed time interval. • This time interval is called the repeat cycle of the satellite.

48. Geostationary Orbit • If a satellite follows an orbit parallel to the equator in the same direction as the earth's rotation and with the same period of 24 hours. • The satellite will appear stationary with respect to the earth surface. • This orbit is a geostationary orbit. • Satellites in the geostationary orbits are located at a high altitude of 36,000 km.

49. Near Polar Orbit • A near polar orbit is one with the orbital plane inclined at a small angle with respect to the earth's rotation axis. • A satellite following a properly designed near polar orbit passes close to the poles and is able to cover nearly the whole earth surface in a repeat cycle.

50. Sun Synchronous Orbit • Earth observation satellites usually follow the sun synchronousorbits. • A sun synchronous orbit is a nearpolar orbit whose altitude is such that the satellite will alwayspass over a location at a given latitude at the same local solartime. • In this way, the same solarillumination condition (except for seasonal variation) can be achieved for the images of a given location taken by the satellite.