A sensor is a device that collects energy (EMR or otherwise), converts it into a signal, and presents it in a format suitable for gathering information about the target under investigation. Depending on the source of energy, these can be active or passive.

Remote sensing sensors are broadly divided into two types: those that operate in the optical infrared (OIR) region and those that operate in the microwave region. OIR and microwave sensors are further classified as passive and active.

Sensors

  • So far we have discussed the nature and properties of electromagnetic radiation
  • Sensors – gather and process information detect and measure photons.
  • Most air/space sensors are spectroradiometers
  • The term spectroradiometer is reserved for sensors that collect the dispersed radiation in bands rather than discrete wavelengths.
  • Spectroradiometry is the measurement of absolute radiometric quantities in narrow bands of wavelength.  

Sensor Technology

  • EMR is reflected or emitted from the target, through the atmosphere, and monitored by the sensor.
  • Sensors measure photons.
  • Critical component – the detector.

Photoelectric effect (Albert Einstein)

  • The release of electrons that occurs when electromagnetic radiation comes in contact with a metal.

  • Emission of electrons when a negatively charged plate of light-sensitive material is subjected to a beam of photons.
  • Electrons flow from the plate, are collected, and are counted as a signal.
  • The magnitude of electric current is proportional to light intensity.

Types of Remote Sensing Sensors

Optical Sensors used in remote sensing systems

  • MSS
  • TM 
  • HRV
  • LISS I.II
  • LISS III
  • LISS IV
  • PAN
  • WIFS

Active sensors:

Active sensors generate their power. The Earth’s surface is illuminated by energy emitted by its source, a portion of which is reflected by the surface in the direction of the sensor, and this information is gathered.

Passive sensors: 

Solar electromagnetic energy reflected from the surface or energy emitted by the surface itself is received by passive sensors. Except for thermal sensors, these sensors do not have their source of energy and cannot be used at night. Again, sensors (active or passive) can be imaging, such as cameras or sensors that acquire images of the area, or nonimaging, such as non-scanning radiometers or atmospheric sounders.

Imaging (Image forming):

Imaging (Image forming) (Image forming) Image forming systems are divided into two types: framing and scanning. The entire frame of the image is acquired instantly in the basic image unit in the framing type, such as a frame camera used in photography. The information is acquired sequentially from the surface in bits of picture elements or pixels, point by point and line by line, and is then arranged into a frame format.

Nonimaging type of sensors:

Nonimaging sensors are used to measure a spectral quantity or a parameter as it varies with time or distance ( such as Gamma radiation, magnetic field, temperature measurement, etc.) They are mostly used for ground observation as well as atmospheric and meteorological research. These sensors do not produce images and thus are not used in operational remote sensing, but they do provide detailed information on the target’s spectral characteristics. This information is gathered by a sensor system in a satellite and transmitted to Earth, where it is received and recorded at a Ground Station.

Sensors that operate in this region are :

  • Aerial cameras: 0.38 um to 0.9 um
  • Thermal scanners : 3 um to 5 um
                                  : 8 um to 16 um
  • Multi spectral scanner : 0.3 um to 1.1 um
  • Microwave wavelengths : 1mm to 1 meter (Sensors)

which operate in these wavelengths / frequencies are mostly active systems like RADAR)

  • Multispectral Scanner (MSS)

Multispectral Scanner (MSS) used in Landsat series satellites, Multispectral scanner (Optical Mechanical Scanner) onboard Landsat series of satellites (L1, L2, L3, L4, & L5) provides line scan type imagery by continuously scanning the earth surface perpendicular to the spacecraft velocity using an oscillating mirror. For each mirror sweep, six lines are scanned simultaneously in each of the four spectral bands. The scan lines progress along the track due to spacecraft motion. An array of six detectors, each with four spectral bands ranging from 0.5 to 1.1 micrometers, detects radiation at the same time. The outputs of the detectors are sampled, encoded, and formatted into continuous digital data.

  • Thematic Mapper (TM)

Thematic Mapper (TM) used in Landsat series satellites Landsat 4 and 5 have onboard a new payload called “Thematic Mapper” with 7 spectral bands and a ground resolution of 30 meters. This is in addition to the MSS payload, which is identical to that carried by Landsat 1 and 2 and replaces the RBV payload. TM is an Optical Mechanical Scanner, similar to MSS; however, as a 2nd generation line scanning sensor, it ensures better performance characteristics in terms of
i) improved pointing accuracy and stability,
(ii) high resolution,
(iii) new and increased number of spectral bands,
(iv) 16 days of repetitive coverage,
(v) high scanning efficiency using bi-directional scanning, and
(vi) increased quantization levels. A scanline corrector (SLC) is installed between the telescope and the camera to enable bi-directional scanning. and the focal plane. The SLC ensures parallel lines of scanning in the forward and reverse directions.

  • High-Resolution Visible (HRV)

SPOT Satellite employs a High-Resolution Visible (HRV) Imager. The French SPOT-1 spacecraft is equipped with two nominally identical High-Resolution Visible (HRV) imagers that can operate independently or in a variety of coupled modes. In contrast to the Landsat imaging system’s oscillating mirror design, HRV cameras use a Charge Coupled Devices (CCD) array as the sensing element for the first time in space. The two cameras can be used in either multispectral (20 m resolution) or panchromatic (10 m resolution) mode. The cameras can be tilted offset up to 27° on either side of Nadir, covering a 60-kilometer swath. Thus, any point within 950 km of the satellite track can be observed using programmed camera control. SPOT-1 has stereo coverage capability in orbit with tiltable cameras, providing stereo image pairs that are nearly identical to metric camera air photos.

  • Linear Image Self Scanning (LISS) Camera used in IRS- 1A,1B

The Indian Remote Sensing Satellite (IRS-1A), designed and built entirely by the Indian Space Research Organization (ISRO), was launched by a Russian launcher on March 17, 1988. It has four spectral bands in the visible and near-infrared ranges of 0.45 to 0.86 m (0.45 to 0.53 m to 0.59 m, 0.62 to 0.68 m, and 0.77 to 0.86 m) with two different spatial resolutions of 72.5 and 36.25 meters from one open LISS-1 and two open LISS-2 cameras, respectively. It provides repeat coverage every 22 days. IRS, like all other LANDSAT/SPOT missions designed for global coverage, is in sun-synchronous, polar orbit at approximately 900 km altitude and covers a width of 148 km. on the ground. It uses linear array detectors (CCD) like SPOT.

  • Linear Imaging Self-Scanning Camera-3 (LISS-3)

This camera can capture images in three visible bands as well as a short-wave infrared band. For visible bands, the resolution and swath are 23.5 m and 142 km, respectively. The detector is a 6000-element CCD-based linear array with 10m by 7m pixels. The detector is at the focal point of a refractive-type optical system comprised of eight lens elements with a focal length of 360 mm. The analog output video signal is processed similarly to PAN. This camera employs 7-bit digitization, resulting in a 128-level intensity variation.

  • Linear Imaging Self-Scanning Camera-4 (LISS-4)

The LISS-4 camera serves the dual purpose of acquiring 70 km swath mono images and providing continuity to the 1C/ 1D PAN camera. It acquires 23 km swath 3 band multispectral imagery in normal mode, which can be positioned anywhere within the 70 km coverage of Mono mode. The enhanced dynamic range of 10 bits is intended to meet the global demand for radiometric ranges. The stereo capability of 1C/ 1D is retained to meet the needs of users who require across-track stereo.

  • Panchromatic camera (PAN)

The PAN camera is designed to provide visible-spectrum images of the Earth in a panchromatic band (0.5-0.75 m) with a geometric resolution greater than 10 m and a swath of 70 km. For the required focal length, the camera employs an off-axis reflective type optics system comprised of three mirrors. The detector element is a CCD with a 7m pixel size. Three linear array charge-coupled detectors cover a total swath of 70 km, with each detector covering a swath of approximately The central detector is separated from the other two detectors by a focal plane distance equal to 8.6 km on the ground.

The other two detectors, located adjacent to the central CCD, each cover a 24 km swath. These two detectors are perfectly aligned with a resolution of 30 arc sec-1. On the ground, the central swath overlaps the side swaths by 600 m. Each detector has four analog outputs that are processed independently by video chains, converted to digital, and provide a data handling system for formatting. 6-bit digitization with 64 radiometric gray levels is used for PAN data that is compatible with the expected signal-to-noise ratio.

PAN camera characteristics
5.8 m geometric resolution from an altitude of 817 km Optics effective focal length 980 mm 70 km swath Optical field of view 2.5o (across-track) 0.3o (along the track) 0.5-0.75 m spectral band.

  • Wide Field Sensor (WiFS)

This camera has two operating bands: B3: 0.62 m to 0.68 m (Red) and B4: 0.77 m to 0.86 m. (NIR). Each band employs a 2048-element CCD with a 13 m by 13 m element size. A wide-angle refractive optics system with eight lens elements and a focal length of about 56 mm is used. This payload must cover a swath of 770 km with a resolution of 188 m. The selected 817 km orbit and ground swath can provide the required receptivity for the intended application.

For each band, two separate band assemblies are used to cover the 770 km. As a result, two detectors cover the entire swath in each band. Each detector covers one-half of the swath. The signal processing chain is similar to that of the LISS-3, in that the analog video signal is converted to 7 bits before being sent to the data handling system for formatting. The WiFS camera’s specifications are listed in the table below.

 

WiFS characteristics: Band 3 0.62-0.68 m, Band 4 0.77-0.86 m Swath 810 km Resolution 188.3 m 7 bits of radiometric resolution 0.25 pixel band-to-band registration

  • Advanced Wide Field Sensor (AWiFS)

The Advanced Wide Field Sensor (AWiFS) has a spatial resolution of 56 meters and a swath of 740 kilometers. The camera operates in three spectral bands: visible, near-infrared, and short-wave infrared. AWiFS is a one-of-a-kind camera capable of taking images of the world every 5 days in the fields of agriculture, land and water resource management, and disaster management.

Hyperspectral Remote Sensing
Multispectral – Many spectra (bands)
Hyperspectral – Huge numbers of continuous bands
Hyperspectral remote sensing provides a continuous, essentially complete record of spectral responses of materials over the wavelengths considered

Hyperspectral Platforms

First hyperspectral scanners:

  • 1982: AIS (Airborne Imaging Spectrometer)
  • 1987: AVIRIS (Airborne Visible/infrared Imaging Spectrometer)
  • 1995: Hyperspectral Digital Imagery Collection Experiment (HYDICE)
  • 2000: Hyperion (EO-1)

AVIRIS

AVIRIS Specifications

•224 individual CCD (charge-coupled device) detectors •10 nanometer spectral resolution
• 20-meter spatial resolution (at typical flight altitude)
• NASA ER-2 flight platform (modified U-2)
• Flight altitude: 20,000 to 60,000 feet, but usually flown at 60,000 feet.
• The average swath width is 11 km.
• A diffraction grating is used to disperse the spectrum against the detector array.
• The total range is from 380 to 2500 nanometers (roughly the same as the TM band range).
• image, push broom-like, a series of lines, each with 664 pixels

Hyperspectral Cube

  • Demonstrates the amount of data returned by imaging instruments.
  • demonstrates how imaging instrument data is georeferenced.
  •  Data from various wavelengths can be used to generate a “map” (in either true color or false color infrared formats).
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