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Lecture 5 Content

Lecture 5 Content. Sensors: Electro-optical imaging sensors Microwave sensors. Sensors In remote sensing a sensor is an instrument which records the energy emitted and reflected from the earth’s surface Two general types: Electro-optical imaging sensors Microwave sensors.

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Lecture 5 Content

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  1. Lecture 5 Content • Sensors: • Electro-optical imaging sensors • Microwave sensors

  2. Sensors • In remote sensing a sensor is an instrument which records the energy emitted and reflected from the earth’s surface • Two general types: • Electro-optical imaging sensors • Microwave sensors

  3. Electro-optical imaging sensors • Also called passive sensors • Operate by focusing energy onto a photo sensitive surface that is sensitive to a designated part of the spectrum • The corresponding electrical signals are either stored in analogue or digital form or transmitted to a processing center

  4. Use an optical system similar to a telescope to view the terrain • Instead of film the image is created by light sensitive detectors that produce electrical signals proportional to the brightness of the light energy • A single detector can be made to view a strip of terrain by using a rotating mirror to direct its field-of-view (FOV) • This is known as scanning

  5. Usual scanning is by swaths perpendicular to the flight path • In practice 20 to 40 detectors or more are used to scan the terrain • Each value recorded is a sample of voltage level at a particular instant in time. The area detected is the instantaneous FOV (IFOV). This is the smallest ground area for which digital measurement is collected • Size of IFOV depends upon the altitude and resolution of the sensor

  6. Swath • Sensors collect 2D images of the surface in a swath below the sensor (similar to that of an IFOV) • Example: Landsat has a 185 km swath

  7. There are various types of electro-optical imaging sensors: • Frame sensor • Pushbroom sensor • Mechanical sensor

  8. Frame sensor • Originally developed out of television equipment • Makes use of Return Beam Vidicon (RBV) cameras • Imagery is obtainable only in the US • in RBV, three television cameras each instantaneously photograph exactly the same part of the earth’s surface • Band 1 – the green band of the spectrum (0.46μm to 0.6 μm) • Band 2 – the lower red band of the spectrum (0.56μm to 0.68 μm) • Band 3 – the red/infrared band of the spectrum (0.66μm to 0.82 μm)

  9. Pushbroom sensor • Consists of a linear array of detectors behind a wide angle optical system • Each detector is typically sensitive to different parts of the spectrum • As the sensor platform moves forward each detector signal is sampled at regular intervals, depending on the speed of forward motion and on the flying or orbiting height • major problems have been to make the detectors small enough and sufficiently uniform in response

  10. Multiple sensitive Detector elements Flight direction Refractive Objective Lens Earth surface IFOV Pushbroom scan principle i.e. electronic scanning across flight path with a large number of discrete photo elements

  11. Mechanical sensor • IFOV is made to scan rapidly in a series of scan lines which are perpendicular to the direction of the forward motion of the scanner platform • Rotating mirrors reflect the radiation received in the IFOV on to detectors • The beginning and end of the scan lines determine the width of the ground swath covered • The electronic signals may be stored for later processing or transmitted directly to a ground receiving station • Product is an analogue image or digital data which require further image processing

  12. Two types of mechanical scanners: • Multispectral scanner • Thematic scanner • Multispectral scanner data have several advantages over aerial photographs: • Very high radiometric (wavelength) resolution in narrow and simultaneously recorded wavebands • These wavebands span a wider portion of the electromagnetic spectrum than photogrammetry (0.3μm to 14m) • Data is displayed in analogue form, but also available for digital image processing

  13. Multispectral scanner: MSS configuration incorporates thermal infrared detectors with digital recording

  14. Thematic scanner: • Thematic scanner is also called Thematic Mapper (TM) • The TM is a multispectral scanning system much like the MSS, except that the TM sensor records reflected/emitted electromagnetic energy from the visible, reflective-infrared, middle-infrared, and thermal-infrared regions of the spectrum. TM has higher spatial, spectral, and radiometric resolution than MSS. • Useful for vegetation type and health determination, soil moisture, snow and cloud differentiation, rock type discrimination, etc.

  15. Microwave sensor • Operate in the wavelength (1mm to 1m) • Detection is by an antenna system • Wavelength sensed can pass through haze, clouds and smoke • Two types of sensor: • Microwave radiometer • Radar

  16. Microwave radiometer • Passive sensor • Operates in the wavelength 20 to 100cm • A narrow beam antenna can be made to scan mechanically or electronically • Resolution is poor because of: • Low energy levels of the radiation in the wavelengths used • High speed of scanning • A large area must be sensed at any instant to gather enough energy for satisfactory measurement • Used to determine soil moisture in geological explorations

  17. Radar (radio detection and ranging) • Operates in the wavelength between 1mm to 1m • An active sensor because it furnishes it own energy • Can operate during darkness as well as daylight • All weather capability has enabled images to be obtained where weather conditions have prevented aerial photogrammetry • Radar transmits a particular band of electromagnetic energy towards an object and detects a portion of the energy reflected from the object • Besides establishing the direction to the target, the system also determines the distance by measuring the elapse time between the transmitted pulse and the return of the reflected energy

  18. Each pulse yields such a small amount of information, thousands of transmissions per second are necessary to produce a sample that can be converted to an image • Radar technology is used in Side-looking radar (SLR) • Applications of SLR are determined by its relatively high cost, its rapid rate of data acquisition and its sensitivity to both surface roughness and moisture content

  19. Limitations with SLR: • Resolution of SLR imagery is best at close range and it can be improved by the use of longer antennae or by operating with shorter wavelengths which is a affected by clouds by obscuring objects • There is a limit to the length of the antennae which can be carried by an aircraft

  20. Improvements in SLR resolution: • Synthetic Aperture Radar (SAR): -- makes use of a short antenna that transmits and receives signals as the aircraft moves along the flight line -- using mathematics the short antenna is transformed into an array of such antennae which is part of the data recording and processing procedures -- The stored signals are later analyzed mathematically so as to give a high resolution image, which corresponds to an effective antenna length of hundreds of meters -- details of the SLR and SAR is advanced and out the scope of this course SAR SLR

  21. … The End …

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