Multi- and Hyperspectral Remote Sensing

1. Multi- and Hyperspectral Remote Sensing

2. Multi- vs. Hyperspectral RS • Multi- and Hyperspectral RS • Collection of reflected, emitted, or backscattered energy from an object or area in multiple regions of the EM spectrum • Typically collection of energy in digital format • Multispectral: • Multiple (a few; > 2), wide, separated wavelength bands • Hyperspectral: • Multiple (hundreds), narrow, contiguous wavelength bands • Major difference: • Not so much the number of measured wavelengths but the narrowness and contiguous nature of the measurements

3. Multi- vs. Hyperspectral RS • Spectral profiles for tumbleweed (USGS) Green – Hyperspectral Red – Multispectral (Landsat 7 ETM+)

4. Multi- vs. Hyperspectral RS • Spectral Resampling

5. Data Collection • Detection of EM energy from AOI at sensor • Recording of energy as analog electrical signal • Onboard conversion of analog electric signal into digital value through analog-to-digital (A-to-D) conversion • Aircraft platform: digital data “flown” back to Earth • Satellite platform: digital data “telemetered” to Earth receiving stations directly or indirectly via tracking and data relay satellites (TDRS) • Ground: • Data preprocessing • Information extraction • Distribution and use

6. Digital Image Terminology • Pixel • 2D picture element that is the smallest non-divisible element of a digital image • Digital RS data are stored as a matrix (array) of digital numbers, whereby each pixel has a location value (row i and column j) in the matrix and a brightness value (BV) for each of the individual spectral bands (k)  BVi, j, k • n spectral bands are registered to one another • i and j for a pixel are the same in all bands; only a pixel’s BV may/should vary from band to band

7. Digital Image Terminology • BV range (quantization level of sensor system) • 8 bits  BVs range from 0 to 255 • 12 bits  BVs range from 0 to 1023 • The more bits, the more precise the measurement of radiance

8. Many Types of RS Systems

9. Detection & Recording of EM Energy • The Basics • Most RS instruments (sensors) measure photons • Photoelectric effect at the detector • Electrons (negatively charged particles) are emitted when a negatively charged, light-sensitive plate (detector) is subjected to a beam of photons • Emitted electrons (numbers, intensity) can be collected and counted as a signal • Magnitude of electric current (number of photoelectrons per unit time) is proportional to light intensity • Kinetic energy of released photoelectrons varies with wavelength of the impinging radiation • Note: detector material determines the EM wavelengths over which the detector will operate (e.g., silicon for visible light)

10. Spatial information Imagers Altimeters Sounders Imaging Radiometer Imaging Spectrometer Polarimeters Scatterometers Radiometers Spectro-Radiometer Spectrometers Intensity information Spectral information Sensor Classes • Sensor classes according to principle parameter measured • Passive Sensors: • Radiometers • Spectrometers • Spectro-radiometers • Imager • Polarimeter • Sounders • Active Sensors: • Radars • Lidars • Scatterometers • Altimeters • Sounders

11. Sensor Classes – Passive • Radiometer • Instrument that quantitatively measures the intensity of EM radiation in some interval of the EM spectrum • Spectrometer • Device that detects, measures, and analyzes the spectral content of incident EM radiation • Has a dispersing element (e.g., prism) that breaks radiation extending over part of the spectrum into discrete wavelengths and disperses them at different angles to detectors • Spectro-radiometer • Radiometer that measures the intensity of EM radiation in multiple spectral bands rather than at discrete wavelengths

12. Sensor Classes – Passive • Imager • Device that detects EM radiation with spatial resolution (e.g., CCD); scans either mechanically or electronically • Polarimeter • Instrument that measures the state of polarization (related to vibration of EM waves) of EM radiation • Sounder (passive and active) • Instrument that measures vertical distributions of atmospheric parameters (e.g., temperature) from multispectral information

13. Sensor Classes – Active • RADAR • Active Radio Detection and Ranging sensor • Provides its own source of EM energy (i.e., emits microwave radiation) • Detects, measures, and times the backscattered microwave radiation • LIDAR • Light Detection and Ranging sensor • Uses a laser (light amplification by stimulated emission of radiation) to transmit a pulse and a receiver with sensitive detectors to measure the backscattered or reflected light

14. Sensor Classes – Active • Scatterometer • High-frequency microwave radar designed specifically to measure backscattered radiation • Altimeter • Instrument that measures the height of the platform (aircraft, spacecraft) above the surface

15. non-scanning imaging Passive Image plane scanning scanning imaging Object plane scanning non-imaging non-scanning imaging Active Image plane scanning scanning imaging Object plane scanning Sensor Types non-imaging Sensor Type

16. Passive vs. Active Sensors • Passive Sensors • Detect EM radiation that is naturally reflected or emitted by objects (energy source: sun) • Active Sensors • Detect EM radiation that is reflected from objects that are irradiated from artificially generated energy sources (e.g., radar, lidar)

17. Scanning vs. Non-Scanning • Scanning System • System that senses a scene point by point (e.g., small areas within the scene) along successive lines over a finite time • Involves movement of either the entire sensor or of one or more of its components • Non-Scanning System (~ Framing system) • Sensors that either don’t sweep (e.g., laser) or that produce an image instantaneously (e.g., camera, eye, TV)

18. Imaging vs. Non-Imaging • Imaging • System that measures the intensity of radiation as a function of position on the Earth’s surface so that a 2D-image of radiation intensity can be generated (e.g., cameras, scanners) • Non-Imaging • Either does not measure the intensity of radiation OR does not do so as a function of position on the Earth’s surface (average of signal strength, etc.; 1D)

19. Scanning, Imaging Systems • Sensors that sweep (mechanically or electronically) over terrain to produce an image Two broad categories: • Optical-mechanical: • contains essential mechanical component (e.g., moving mirror) that aids in scene scanning • Optical-electronic: • sensed radiation moves directly through the optics onto the linear or array detectors

20. Scanning, Imaging Systems • Object plane scanner (~ Optical-mechanical): • Images one target pixel at-a-time, and all pixels in a sequential fashion, from the object plane to the image plane • Scanning mechanism (e.g., mirror) “points” the scanner to different target pixels in a sequential fashion • Image plane scanner (~ Optical-electronic): • Images an entire scan line or frame at-a-time on the image plane • Scanning takes place on the image plane • Has larger array of detectors in the image plane than an object plane scanner

21. Scanning, Imaging Systems Image plane scanner Object plane scanner

22. Passive Sensor Types • Passive, non-scanning, non-imaging • Microwave radiometer, magnetic sensor, gravimeter, Fourier spectrometer, etc. • Passive, non-scanning, imaging • Camera: monochrome, natural color, infrared, color infrared, etc. • Passive, scanning, imaging, image plane scanning • TV camera, etc. • Passive, scanning, imaging, object plane scanning • Optical-mechanical scanner, microwave radiometer

23. Active Sensor Types • Active, non-scanning, non-imaging • Microwave radiometer, microwave altimeter, laser water depth meter, laser distance meter • Active, scanning, imaging, image plane scanning • Passive phase array radar • Active, scanning, imaging, object plane scanning • Real aperture radar, synthetic aperture radar

24. Motion of Sensors • Two types of forward-moving scanners: • Cross-Track Scanners • Along-Track Scanners • Orbit or Flight path • Forward-moving track • Swath width • Area monitored from side to side of path

25. Sensor Types • Cross-Track Scanners • Use a rotating (spinning) or oscillating mirror to sweep along a line (long and narrow) or a series of adjacent lines traversing the ground • Optical-mechanical • Whiskbroom

26. Sensor Types • Along-Track Scanners • Use a line of detectors (charged-coupled devices, CCDs) – as platform advances along the track, radiation is received simultaneously at all detectors • Optical-electronic • Pushbroom

27. Sensor Types • Discrete detector • Have a single active area • Linear arrays • Have a few to several thousand detectors lined up in a row • Area arrays • Have two-dimensional area arrays

28. Sensor Type I • Analog Frame Camera and Film • Acquires traditional aerial photography • Film with silver halide crystals (emulsion) instead of detectors

29. Sensor Type II • Digital Frame Camera Area Array • Each spectral band has a filter and a separate area array • Number of detectors = # of rows  # of columns  # of bands Light B3 Each one of these is a single detector. B2 B1

30. Sensor Type III • Linear Array (“Pushbroom”) • Similar to area array, but has only 1 row (line) of detectors • Array is moved in a single direction, and a radiance reading is taken at regular intervals • 1 linear array per spectral band; number of pixels contained in one row of an image equals the number of detectors • Filters are used to restrict the wavelengths IFOV (1 detector) Linear array Objective lens Angular field of view Sensor movement direction • Pixel width = easily calculated • Pixel length = function of IFOV, sensor speed and detector sampling.

31. Sensor Type IV • Scanning Mirror and Single Discrete Detectors (“Whiskbroom”) and Filters • 1 detector per spectral band • Rotating mirror changes the angle of the incident light source (hence what portion of the ground is being detected) • Filters restrict the wavelengths for each band Rotating mirror Swath width Detector Angular field of view • Pixel width = function of mirror rotation rate and IFOV • Pixel length = function of IFOV, sensor speed and detector sampling rate Direction of sensor movement

32. Sensor Type V • Scanning Mirror and Multiple Discrete Detectors (“Whiskbroom”) and Filters • Linear array of detectors for each spectral band • Mirror angles the light across multiple detectors instead of one • Filters restrict the wavelengths for each band Pushbroom sensors: may have thousands of detectors per spectral band Scanning mirror sensors: usually only have a few detectors per spectral band (e.g., if there are 6 detectors per array, every 6th pixel in the image is from a given detector) MSS scanning arrangement

33. Sensor Type VI • Scanning Mirror and Multiple Discrete Detectors (“Whiskbroom”) and Dispersing Element • Instead of wide band filters, this type has a dispersing element (prism) that breaks the incoming radiation into discrete wavelengths and disperses it across a linear array of detectors • Rotating mirror and forward sensor movement create the spatial arrangement of pixels Advantage of dispersing element (vs. a set of filters): much smaller bands can be detected without a massive amount of additional hardware (there is not 1 filter per band as in the previous sensors)

34. Sensor Type VI • Hyperspectral Area Array • Combines pushbroom linear array with a dispersing element

35. Comparison of Sensor Types

36. Examples for Sensor Types

37. Data Transmission • All sensors record the image data digitally • Data must routinely be transmitted to Earth • Memory buffer (Think about your “hard drive” …) may fill up • We want to use it (Data doesn’t do us any good in orbit.)

38. Sensor Movement • All detector configurations require forward movement of the detectors to create an image • Two exceptions: • Digital frame camera area arrays • Stationary platforms • Satellites and airplanes are ideal platforms on which to mount sensors

39. Sensor Movement • Polar-orbiting satellites • Orbit in north-south direction while Earth spins beneath it in east-west direction  Satellites can scan the entire Earth’s surface (like pealing an orange around and around, one strip at a time) • Relatively low altitude: 300–800 km above the surface • Must move quickly to avoid being pulled in by Earth’s gravitational field • Circle approximately every 90 minutes

40. Sensor Movement • Geosynchronous-orbiting satellites • Orbit roughly at the speed of Earth’s rotation  Satellites are always above the same spot and scan the same region • Relatively high altitude: 36,000 km above the surface • Monitor almost one-third of the Earth’s surface and have full-disk view of the Earth from pole to pole

41. Sensor Movement • Geostationary-orbiting satellites • Appear to remain in the same position above the Earth • Circular orbit with an inclination of zero degrees (i.e., above the equator) • Altitude: ca. 36,000 km • Speed: 3 km/sec • Orbital period: ca. 24 hours • Called geostationary because they appears to be stationary when viewed from Earth • Typical roles: weather observation and broadcasting

42. Sensor Movement • Geostationary-orbiting satellites

43. Sensor Movement • Synchronous-orbiting satellites • Hoe around the Earth once per day and return to their original positions • Circular or elliptical orbit with varying inclinations • Orbital period: 24 hours • Typical role: monitoring of, and providing communications for, areas in higher latitudes

44. Sensor Movement • Recurrent-orbiting satellites • Return to their starting points above Earth’s surface within one day, regardless of how many orbits they have made in that time overall • Circular or elliptical orbits • Orbital period: integral fraction of Earth’s rotation period • Typical roles: communications and observation functions over higher altitudes

45. Sensor Movement • Sub-recurrent-orbiting satellites • Orbit the Earth several times per day but return to their starting points above Earth’s surface a number of days later, at the same time of day • Orbital period: satellite that returns to starting point above Earth’s surface after 16 days would have a 1 16-day sub-recurrent orbit • Typical roles: long-term, regular monitoring of the Earth’s surface

46. Sensor Movement • Sun-synchronous-orbiting satellites • Orbital plane and Sun’s direction are always the same • Direction of rotation of the orbital plane and the period (rotation angle per day) are the same as the Earth’s orbital period (rotation angle per day) • Looking at the Earth from an orbiting satellite, the Sun’s radiation would always be coming from the same angle • Orbital inclinations and altitude must be in tune to generate a sun-synchronous orbit • Orbital period: one year • Typical roles: monitoring of sites that must always be observed under the “same” conditions

47. Sensor Movement • Sun-synchronous-orbiting satellites

48. Sensor Movement • Sun-synchronous-sub-recurrent-orbiting satellites • Combines sun-synchronous orbit with a sub-recurrent orbit • Orbital period: once every few days • Typical roles: Earth observation --- global coverage yet relatively frequent coverage of same broad regions of the Earth’s surface

49. Sensor Movement • Example: Landsat 1-3 • Sun-synchronous, circular orbit • Nominal altitude: 919 km • Orbital inclination: 99º (nearly polar; crosses equator at 9º) • 1 orbit/103 min.  14 orbits/day • Position below spacecraft moves: 2,875 km/orbit  40, 250 km/day • Orbit 15 is displaced from orbit 1 at equator by 159 km 18 days later, orbit 252 falls directly over orbit 1 ~ 26 km of overlap between successive orbits • Path & Row World Reference System (WRS) 57,784 scenes each 185 km wide and 170km long

50. Sensor Movement