Components of Remote Sensing Systems: Cross-Track, Along-Track, Side and Circular Scanning System (Especially for GATE-Geospatial 2022)

Glide to success with Doorsteptutor material for CTET/Paper-1 : get questions, notes, tests, video lectures and more- for all subjects of CTET/Paper-1.

Examrace Books on Mapping, GIS, and Remote Sensing prepares you throughly for a wide range of practical applications.

Two Main Components of Remote Sensing System

The remote sensing system has two main components

  • Framing systems
  • Scanning systems

Framing Systems

  • Framing systems instantaneously acquire an image of an area, or frame, on the terrain. Cameras and Videocon are common examples of such systems. The human eye can also be considered a framing system. A camera employs a lens to form an image of the scene at the focal plane, which is the plane at which the image is sharply defined.
  • A shutter opens at selected intervals to allow light to enter the camera, where the image is recorded on photographic film. A Videocon is a type of television camera that records the image on a photosensitive electronically charged surface. An electron beam then sweeps the surface to detect the pattern of charge differences that constitutes the image. The electron beam produces a signal that may be transmitted and recorded on magnetic tape for eventual display on film.
  • Successive frames of camera and Videocon images may be acquired with forward overlap, as shown in Figure below. The overlapping portion may be viewed with a stereoscope to produce a three-dimensional view Film is sensitive only to portions of the UV, visible, and reflected IR regions (0.3 to 0.9 pm) . The sensitivity range of special vidicons extends into the thermal band of the IR region.
  • A framing system can instantaneously image a large area because the system has a dense array of detectors located at the focal plane. The emulsion of camera film contains tiny grains of silver halide. A Videocon surface is coated with sensitive phosphors.

Scanning Systems

  • A scanning system employs a single detector with a narrow field of view which sweeps across the terrain to produce an image. When photons of electromagnetic energy radiated or reflected from the terrain encounter the detector, an electrical signal is produced that varies in proportion to the number of photons,
  • The electrical signal is amplified, recorded on magnetic tape, and played back later to produce an image. All scanning systems sweep the detector՚s field of view across the terrain in a series of parallel scan lines.

Types of Scanning Modes

There are four common scanning modes:

  • Cross-track scanning
  • Circular scanning
  • Along-track scanning
  • Side scanning

Track Scanning Systems for Acquiring Remote Sensing Images

Track Scanning Systems for Acquiring Remote Sensing

Cross - Track Scanning System

  • The widely used cross-track scanning systems employ a faceted mirror that is rotated by an electric motor, with a horizontal axis of rotation aligned parallel with the flight direction. The mirror sweeps across the terrain in a pattern of parallel scan lines oriented normal (perpendicularly) to the flight direction. Energy radiated or reflected from the ground is focused onto the detector by secondary mirrors.
  • The angular resolving power of a detector, measured in milli-radians determines the IFOV of the detector. As shown in Figure 3.3, the IFOV subtends an area on the terrain called a ground resolution cell are determined by the detector՚s IFOV and the altitude of the scanning system. A detector with an IFOV of I mrad at an altitude of 10 km has a ground resolution cell of 10 by 10 m.
  • The angular field of view is that portion of the mirror sweep, measured in degrees, that is recorded as a scan line. The angular field of view and the altitude of the system determine the ground swath, which is the width of the terrain strip represented by the image.
  • Ground swath is calculated as: Ground swath
  • The distance between the scanner and terrain is greater at the margins of the ground swath than at its centre. As a result, ground resolution cells are larger toward the margins than at the centre, which results in a geometric distortion characteristic of cross track scanner images. At the high altitude of satellites, a narrow angular field of view is enough to cover a broad swath of terrain.
  • For this reason, the rotating mirror is replaced by a fiat mirror that oscillates back and forth through an angle of approximately 15°. The strength of the signal generated by a detector is a function of the factors such as — energy flux, altitude, spectral bandwidth of the detector, instantaneous field of view and dwell time.
  • For a cross - track scanner, the dwell time is determined by the detector IFOV and by the velocity at which the scan mirror sweeps the IFOV across the terrain. The high scanner speed relative to ground speed is required to prevent gaps between adjacent scan lines. The short dwell time of cross-track scanners imposes constraints on the other factors that determine signal strength.
  • For example, the IFOV and spectral bandwidth must be large enough to produce a signal of enough strength to overcome the inherent electronic noise of the system. The signal-to-noise ratio must be sufficiently high for the signal to be recognizable.

Along - Track Scanning System

  • For scanner systems to achieve finer spatial and spectral resolution, the dwell time for each ground resolution cell must be increased, one method is to eliminate the scanning mirror and provide an individual detector for each ground resolution cell across the ground swath. The detectors are placed in a linear array in the focal plane of the image formed by a lens system.
  • The long axis of the linear array is oriented normal to the flight path, and the IFOV of each detector sweeps a ground resolution cell along the terrain parallel with the flight track direction. Along-track scanning refers to this movement of the ground resolution cells. These systems are also called push-broom scanners because the detectors are analogous to the bristles of a broom pushed along the floor.
  • For along-track scanner, the dwell time of a ground resolution cell is determined by the ground velocity. The increased dwell time allows two improvements:
    • Detectors can have smaller IFOVs, which provide finer spatial resolution.
    • Detectors can have a narrower spectral bandwidth, which provides higher spectral resolution.

Circular and Side Scanning System

Factors Affecting Signal Strength of an Image

Circular Scanning System

  • In a circular scanning system, the scan motor and mirror are mounted with a vertical axis of rotation that sweeps a circular path on the terrain. Only the forward portion of the sweep is recorded to produce images.
  • An advantage of this system is processing and display systems are designed for linear scan data, therefore the circular scan data must be extensively reformatted prior to processing. Circular scanners have short dwell times comparable to those of cross-track scanners.
  • Circular scanners are used for reconnaissance purposes in helicopters and low-flying aircraft. The axis of rotation is tilted to point forward and acquires images of the terrain well in advance of the aircraft position. The images are displayed in real time on a screen in the cockpit to guide the pilot.

Side Scanning System

  • The three types of scanners just described are passive systems, since they detect and record energy naturally reflected or radiated from the terrain.
  • Active systems, which provide their own energy sources, operate primarily in the side-scanning mode.
  • The example in below Figure is a radar system that transmits pulses of microwave energy to one side of the flight path (range direction) and records the energy scattered from the terrain back to the antenna. Another system is side-scanning sonar, which transmits pulses of sonic energy in the ocean to map bathymetric features.

Factors Affecting Signal Strength of an Image

  • Energy flux: The amount of energy reflected or radiated from terrain is the energy flux. For visible detectors, this flux is lower on a dark day than on a sunny day.
  • Altitude: For a given ground resolution cell, the amount of energy reaching the detector is inversely proportional to the square of the distance. At greater altitudes the signal strength is weaker.
  • Spectral bandwidth of the detector: The signal is stronger for detectors that respond to a broader wavelength range 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 bald, such as visible red.
  • Instantaneous field of view: Both the physical size of the sensitive element of the detector and the effective focal length of the scanner optics determine the IFOV - A small IFOV is required for high spatial resolution but also restricts the signal strength (amount of energy received by the detector) .
  • 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 impinge on the detector, which creates a stronger signal.

Remote Sensing System Used for Multi-Spectral and Hyper-Spectral Data Collection (Adapted from Jenson 2007)

  • Multi-Spectral and Hyper-Spectral Data Collection

Developed by: