Lecture 5: Sensors And Scanner

1. Lecture 5: Sensors And Scanner Professor Menglin Jin San Jose State University

2. The Afternoon Constellation“A-Train” • The Afternoon constellation consists of 7 U.S. and international Earth Science satellites that fly within approximately 30 minutes of each other to enable coordinated science • The joint measurements provide an unprecedented sensor system for Earth observations

3. Sensor types (classification) in the following two diagrams

5. Most remote sensing instruments (sensors) are designed to measure photons • we concentrate the discussion on optical-mechanical-electronic radiometers • and scanners, leaving the subjects of camera-film systems and active radar • for consideration elsewhere

6. Non-Photographic Sensor Systems • 1800 Discovery of the IR spectral region by Sir William Herschel. • 1879 Use of the bolometer by Langley to make temperature measurements of electrical objects. • 1889 Hertz demonstrated reflection of radio waves from solid objects. • 1916 Aircraft tracked in flight by Hoffman using thermopiles to detect heat effects. • 1930 Both British and Germans work on systems to locate airplanes from their thermal patterns at night. • 1940 Development of incoherent radar systems by the British and United States to detect and track aircraft and ships during W.W.II. • 1950's Extensive studies of IR systems at University of Michigan and elsewhere. 1951 First concepts of a moving coherent radar system. • 1953 Flight of an X-band coherent radar. • 1954 Formulation of synthetic aperture concept (SAR) in radar. • 1950's Research development of SLAR and SAR systems by Motorola, Philco, Goodyear, Raytheon, and others. • 1956 Kozyrev originated Frauenhofer Line Discrimination concept. • 1960's Development of various detectors which allowed building of imaging and non-imaging radiometers, scanners, spectrometers and polarimeters. • 1968 Description of UV nitrogen gas laser system to simulate luminescence.

7. Passive and Active Sensors • Passive Sensor: energy leading to radiation received comes from an external source, e.g., the Sun • Active Sensor energy generated from within the sensor system is beamed outward, and the fraction returned is measured; radar is an example

8. Imaging and non-imaging sensor • Non-imaging: measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance since the radiation is related to specific points in the target, the end result is an image [picture] or a raster display [for example: the parallel horizontal lines on a TV screen])

9. Imaging and non-imaging sensor • Non-imaging: measures the radiation received from all points in the sensed target, integrates this, and reports the result as an electrical signal strength or some other quantitative attribute, such as radiance • Imaging the electrons released are used to excite or ionize a substance like silver (Ag) in film or to drive an image producing device like a TV or computer monitor or a cathode ray tube or oscilloscope or a battery of electronic detectors

11. Principal: photoelectric effect • There will be an emission of negative particles (electrons) when a negatively charged plate of some appropriate light-sensitive material is subjected to a beam of photons. The electrons can then be made to flow as a current from the plate, are collected, and then counted as a signal

12. Principal: photoelectric effect • There will be an emission of negative particles (electrons) when a negatively charged plate of some appropriate light-sensitive material is subjected to a beam of photons. The electrons can then be made to flow as a current from the plate, are collected, and then counted as a signal • Albert Einstein’s experiment (see lecture 3, or next slide)

13. Principal: photoelectric effect • There will be an emission of negative particles (electrons) when a negatively charged plate of some appropriate light-sensitive material is subjected to a beam of photons. The electrons can then be made to flow as a current from the plate, are collected, and then counted as a signal • Albert Einstein’s experiment (see lecture 3, or next slide) • Thus, changes in the electric current can be used to measure changes in the photons (numbers; intensity) that strike the plate (detector) during a given time interval. • The kinetic energy of the released photoelectrons varies with frequency (or wavelength) of the impinging radiation • different materials undergo photoelectric effect release of electrons over different wavelength intervals; each has a threshold wavelength at which the phenomenon begins and a longer wavelength at which it ceases.

14. photoelectric effect –measure photon energy level • the discovery by Albert Einstein in 1905 • His experiments also revealed that regardless • of the radiation intensity, photoelectrons are • emitted only after a threshold frequency is exceeded • for those higher than the threshold value (exceeding • the work function) the numbers of photoelectrons • released re proportional to the number • of incident photons

15. Handout “Detector types” from John Schott “Remote Sensing –The Image Chain Approach”

16. two broadest classes of sensors • Passive sensor energy leading to radiation received comes from an external source, e.g., the Sun • Active Sensor energy generated from within the sensor system is beamed outward, and the fraction returned is measured Example: radar

17. Radiometer is a general term for any instrument that quantitatively measures the EM radiation in some interval of the EM spectrum • spectrometer When the radiation is light from the narrow spectral band including the visible, the term photometer can be substituted. If the sensor includes a component, such as a prism or diffraction grating, that can break radiation extending over a part of the spectrum into discrete wavelengths and disperse (or separate) them at different angles to an array of detectors

18. spectroradiometer • The term spectroradiometer is reserved for sensors • that collect the dispersed radiation in bands • rather than discrete wavelengths • Most air/space sensors are spectroradiometers.

19. Moving further down the classification tree, the optical setup for imaging sensors will be either an image plane or an object plane set up depending on where lens is before the photon rays are converged (focused), as shown in this illustration.

20. Field of View (FOV) • Sensors that instantaneously measure radiation coming from the entire scene at once are called framing systems. The eye, a photo camera, and a TV vidicon belong to this group. The size of the scene that is framed is determined by the apertures and optics in the system that define the field of view, or FOV

21. Scanning System • If the scene is sensed point by point (equivalent to small areas within the scene) along successive lines over a finite time, this mode of measurement makes up a scanning system. Most non-camera sensors operating from moving platforms image the scene by scanning

22. Cross-Track Scanner the Whiskbroom Scanning A general scheme of a typical Cross-Track Scanner

29. Essential Components of Cross-track Sensor • 1) a light gathering telescope that defines the scene dimensions at any moment (not shown) • 2) appropriate optics (e.g., lens) within the light path train • 3) a mirror (on aircraft scanners this may completely rotate; on spacecraft scanners this usually oscillates over small angles) • 4) a device (spectroscope; spectral diffraction grating; band filters) to break the incoming radiation into spectral intervals • 5) a means to direct the light so dispersed onto an array or bank of detectors • 6) an electronic means to sample the photo-electric effect at each detector and to then reset the detector to a base state to receive the next incoming light packet, resulting in a signal stream that relates to changes in light values coming from the ground targets as the sensor passes over the scene • 7) a recording component that either reads the signal as an analog current that changes over time or converts the signal (usually onboard) to a succession of digital numbers, either being sent back to a ground station Note: most are shared with Along Track systems

30. pixel The cells are sensed one after another along the line. In the sensor, each cell is associated with a pixel that is tied to a microelectronic detector Pixel is a short abbreviation for Picture Element a pixel being a single point in a graphic image Each pixel is characterized by some single value of radiation (e.g., reflectance) impinging on a detector that is converted by the photoelectric effect into electrons

31. MODerate-resolution Imaging Spectroradiometer (MODIS) • NASA, Terra & Aqua • launched 1999, 2002 • 705 km polar orbits, descending (10:30 a.m.) & ascending (1:30 p.m.) • Sensor Characteristics • 36 spectral bands (490 detectors) ranging from 0.41 to 14.39 µm • Two-sided paddle wheel scan mirror with 2330 km swath width • Spatial resolutions: • 250 m (bands 1 - 2) • 500 m (bands 3 - 7) • 1000 m (bands 8 - 36) • 2% reflectance calibration accuracy • onboard solar diffuser & solar diffuser stability monitor • 12 bit dynamic range (0-4095)

32. MODIS Onboard Calibrators Spectral Radiometric Calibration Assembly Solar Diffuser Blackbody Scan Mirror Space View Port Fold Mirror Nadir (+z)

33. MODIS Optical System Visible Focal Plane SWIR/MWIR Focal Plane Scan Track NIR Focal Plane LWIR Focal Plane

34. Four MODIS Focal Planes Visible Shortwave IR/Midwave IR Near-infrared Longwave Infrared

35. MODIS Cross-Track Scan on Terra MODIS_Swath MISR_Swath

36. Along-track Scanner pushbroom scanning the scanner does not have a mirror looking off at varying angles. Instead there is a line of small sensitive detectors stacked side by side, each having some tiny dimension on its plate surface; these may number several thousand

37. Along-track, or Pushbroom, Multispectral System Operation

38. NASA, EOS Terra Launched in 1999 polar, descending orbit of 705 km, 10:30 a.m. crossing Sensor Characteristics uses nine CCD-based push-broom cameras viewing nadir and fore & aft to 70.5° four spectral bands for each camera (36 channels), at 446, 558, 672, & 866 nm resolutions of 275 m, 550 m, or 1.1 km Advantages high spectral stability 9 viewing angles helps determine aerosol by µ dependence (fixed t) Multi-angle Imaging SpectroRadiometer (MISR)

39. Orbital characteristics 400 km swath 9 day global coverage 7 min to observe each scene at all 9 look angles MISR Pushbroom Scanner • Family portrait • 9 MISR cameras • 1 AirMISR camera

40. MISR Provides New Angle on Haze • In this MISR view spanning from Lake Ontario to Georgia, the increasingly oblique view angles reveal a pall of haze over the Appalachian Mountains

41. spectral resolution • The radiation - normally visible and/or Near and Short Wave IR, and/or thermal emissive in nature - must then be broken into spectral intervals, i.e., into broad to narrow bands. The width in wavelength units of a band or channel is defined by the instrument's spectral resolution • The spectral resolution achieved by a sensor depends on the number of bands, their bandwidths, and their locations within the EM spectrum

42. Spectral filters Absorption and Interference. Absorption filters pass only a limited range of radiation wavelengths, absorbing radiation outside this range. Interference filters reflect radiation at wavelengths lower and higher than the interval they transmit. Each type may be either a broad or a narrow bandpass filters. This is a graph distinguishing the two types.

43. Enhanced Thematic Mapper Plus (ETM+) • NASA & USGS, Landsat 7 • launched April 15, 1999 • 705 km polar orbit, descending (10:00 a.m.) • Sensor Characteristics • 7 spectral bands ranging from 0.48 to 11.5 µm • 1 panchromatic band (0.5-0.9 µm) • cross-track scan mirror with 185 km swath width • Spatial resolutions: • 15 m (panchromatic) • 30 m (spectral) • Calibration: • 5% reflectance accuracy • 1% thermal IR accuracy • onboard lamps, blackbody, and shutter • solar diffuser

44. Landsat Thematic Mapper Bands • Landsat collects monochrome images in each band by measuring radiance & reflectance in each channel • When viewed individually, these images appear as shades of gray

45. TRMM Satellite

46. Earth Science Mission Profile1997-2003 eospso.gsfc.nasa.gov

47. Earth Science Mission Profile2004-2010 eospso.gsfc.nasa.gov

48. Satellites in Geosynchronous Orbits are used as Relay Satellites for LEO Spacecraft Imaging System (e.g., Landsat) LEO Communication relay system Ground station GEO Communication relay system (e.g., TDRSS)

49. Sample Calibration CurveUsed to Correlate Scanner Output with Radiant Temperature Measured by a Radiometer

50. Color Composites • The human eye is not sensitive to ultraviolet or infrared light • To build a composite image from remote sensing data that makes sense to our eyes, we must use colors from the visible portion of the EM spectrum—red, green, and blue