14 Data Collection and Scanning systems

Dr. Puneeta Pandey

 

1. Learning Objective

 

The objective of this module is to understand the basic concept of data acquisition by multispectral scanners. The chapter provides an overview on the two broad types of multiple scanners i.e., across and along track scanners based on the method of scanning employed to acquire multispectral image data. The basic principle and processes of these scanners are also presented in this module.

 

2. Introduction

 

The electromagnetic energy used for deriving information about earth surface features can be detected either photographically or electronically. The photography technique makes use of chemical reactions on surface of a light-sensitive film to detect energy variations. Electronic sensors produce an electric signal that corresponds to the energy discrepancies in the original scene. Electronic sensors have enhanced calibration potential, broader spectral range of sensitivity and capacity to electronically store and transmit data.

 

Aircraft and satellite platforms are used as scanning systems in remote sensing. A commonly used scanning system is multispectral scanner (MSS) that is used to collect data over wide wavelength ranges . Generally, the satellite scanners are usually electromechanical scanners, linear array devices or imaging spectrometers that operate in either a ‘pushbroom’ (for example, SPOT) or ‘sweep’ (such as LANDSAT) mode.

 

The satellite scanners are passive systems which detect solar radiation reflected or emitted from the Earth’s surface. The data obtained from multispectral scanners provides information about types and distribution of vegetation, geomorphology, soil types, roads, surface waters, and river networks etc. Reflectance or short wavelength infrared sensors detect reflected energy from surfaces and have been specifically useful for monitoring fires and for studying areas of volcanic and geothermal activities, while thermal or long wavelength infrared sensors have been commonly used for mapping ocean temperatures and study of the dynamics of coastal waters. Examples of passive sensors are Landsat, IRS, SPOT and IKONOS etc.

 

Active systems that operate in the visible spectrum utilize laser technologies such as light detection and ranging systems for oceanographic and forestry applications. Radar systems use active microwave energy for oceanographic, navigation, forestry etc. Active systems are independent of reflected energy from the Sun for image formation. Therefore, they have ability to acquire data during the day or at nighttime. Examples of active sensors are synthetic aperture radar or LiDAR.

 

3. Multispectral scanners

 

Multispectral scanners are designed to collect remote sensing data in numerous spectral bands and over wider range of electromagnetic spectrum (EMR). Multispectral scanners use different types of electronic detectors. Therefore, these scanners can sense the signal in UV, visible, near infrared (NIR), mid infrared (MIR) and thermal infrared (TIR) spectral regions i.e. in the wavelength range of 0.3 to 14 µm of the electromagnetic spectrum. Multispectral scanners also have advantage of sensing in very narrow bands. High spatial and low spectral resolution mapping is done by multispectral scanners.

 

Airborne/space-borne multispectral scanner systems generate two-dimensional images of the terrain for the area beneath the aircraft. Scanning system mainly comprises of two types of scanners:

 

a)      Whisk Broom Scanner

b)      Push Broom Scanner

 

3.1 Whisk Broom Scanner: It is also known as ‘across track’ scanner. Landsat satellites mostly use this type of scanners. Single detector is available for each band of multispectral signal. Using an oscillating or rotating scan mirror in front of a telescope, these systems scan the terrain along scan lines that are perpendicular to the direction of flight line (Figure 1). This permits the scanner to repetitively compute the energy from one side of the aircraft to the other and hence and thus a two-dimensional image is built-up.

 

Data is collected within an arc below the aircraft by airborne scanners at large angles usually between 90° and 120°whereas satellite due to their positioning at higher altitude and broader area coverage, sweep small angles i.e., 10–20°. With the forward motion of aircraft, successive scan lines are covered, giving a series of contiguous narrow strips of observation comprising a two-dimensional image of scan lines. The incoming reflected energy from an oscillating or rotating scan mirror is sensed independently and separated into several thermal and non-thermal spectral components (ultraviolet, visible, near-infrared, and thermal) on the basis of their constituent wavelengths; using a dichroic grating and a prism.

Figure 1: A demonstration of across track scanner.

 

The scanner visualizes the energy within the Instantaneous field of view (IFOV) of system (Figure 2). This IFOV is the cone angle (b) within which incident energy is focused on the detector. The cone angle is calculated by the optical system and size of detectors. The total energy transmitting towards the instrument within the IFOV adds to the detector response at any instant. Therefore, more than one land cover type at any given instant of time can be involved in the IFOV and the composite signal response will be recorded. So, a grouping of pure and mixed pixels is involved in an image that depends on the IFOV and the spatial complexity of the ground features.

Figure 2: Instantaneous field of view and resulting ground area sensed directly beneath an aircraft by a multispectral scanner.

 

The figure 2 describes the area of ground surface covered when the IFOV of a scanner is oriented directly beneath the aircraft. This area can be represented as:

 

D = H’b

 

where,

 

D is the diameter of circular ground area viewed (spatial resolution)

 

b is the instantaneous field of view of a system

 

H’ is the flying height above terrain

 

The ground segment sensed at any instant is known as ground resolution element or cell. The ground resolution of single pixel is given by the IFOV depending on sensor characteristics and flight altitude. The diameter of the ground area detected at any time instant is called spatial resolution of system.

 

The instantaneous field of view for airborne multiple scanner systems lies between 0.5 to 5 milliradian (mrad). Smaller is the IFOV, finer will be the spatial resolution. Besides, large IFOV stands for large quantity of total energy focused on a detector. This allows higher sensitive scene radiance measurements because of higher signal levels. Consequently, there is improvement in radiometric resolution i.e., the capacity to differentiate minor energy variations.

 

Numerous aircraft instruments consist of whiskbroom scanners. For example, the Calibrated Airborne Multispectral Scanner (CAMS), the Airborne Ocean Color Imager (AOCI) and the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS).

 

3.2 Push Broom Scanner: This imaging system is commonly termed as along track scanner due to its direction along the track i.e. similar to the direction of flight.

Figure 3: Demonstration of along-track scanner

 

The multispectral data is collected using linear arrangement of detectors. These detectors are generally charged coupled devices (CDD) or semi-conductive elements of very small size. Several individual detectors (up to 1000) form a single array that records the energy for only one pixel. Each spectral band is measured by a separate linear array of its own. These arrays are positioned at the focal plane of the image formed by the lens systems such that each scan line is viewed by all arrays concurrently. The energy sensed by each detector of single linear array, received from multiple ground resolution cells along the scan line, is sampled electronically and recorded digitally. A two-dimensional image is formed by recording successive scan lines with the forward motion of aircraft. These scan lines are slanting perpendicular to the flight direction.

 

In this along track scanner type, the strength of signal is enhanced by linear arrangement of detectors that facilitates longer dwell time over each pixel. Moreover, improved radiometric resolution is also obtained. The size of the ground resolution cell is estimated by the IFOV of individual detector. Hence, finer spectral and spatial resolution can be attained with no effect on radiometric resolution.

 

The pushbroom pattern is widely used scanning approach among conventional hyperspectral sensors for remote sensing. Aircraft applications such as Hyperspectral Digital Imagery Collection Experiment (HYDICE), Compact Airborne Spectrographic Imager (CASI), Hyperspectral Imager for Low Light Spectroscopy (PHILLS) etc use pushbroom scanners.

Airborne digital sensor ADS40 invented by Leica Geosystems is another example of along-track multiple scanner. Multiple linear arrays of CCDs for multispectral image acquisition are incorporated in these sensors.

 

4. Thermal scanners

 

Multispectral scanners that operate only in the thermal portion of the EMR spectrum are referred to as Thermal scanners. Thermal infrared radiation refers to electromagnetic waves with wavelength 3-14 µm. But due to certain atmospheric effects, thermal scanners are usually limited to 3-5 µm and 8-14µm wavelength ranges.

 

5. Hyperspectral scanners

 

Hyperspectral sensing is another recent development in the multispectral scanning, in which acquisition of images is carried out in hundreds of very narrow, contiguous spectral bands of the visible, NIR, MIR portions of the EMR spectrum. Hyperspectral scanners can also be along or across track scanners.

 

6. Advantages of scanning systems

 

The scanning systems (along track or across track) have several advantages over the photographic systems.

 

The first advantage is that MSS have wider spectral range that extends from visible to thermal infrared wavelengths; while the photographic systems are limited to the visible and near-infrared regions of electromagnetic spectrum.

 

MSS have higher spectral resolution than photographic systems.

 

Photographic systems record the energy detected by photochemical process while MSS detect the energy responses electronically.

 

Photographic systems require a continuous supply of film and processing on the ground after the photographs have been taken; while digital data in MSS systems facilitates transmission of data to receiving stations on the ground and its immediate processing.

 

7.Summary

 

Remote sensing science and technologies using space-based and airborne sensor systems have profoundly changed the practice of environmental monitoring and understanding of the dynamics of estuarine and coastal environments. In remote sensing, the reflected energy is sensed remotely and this sensed energy is transformed into a usable digital form such as pixels which is then interpreted and analyzed for different applications. The remote sensor uses either reflected solar energy (passive sensing) as its source of energy to store the information about an object or it uses its own outgoing source of energy (active sensing) to record the incoming energy after hitting back to the target. The multispectral scanning is widely used scanning system in remote sensing. A multispectral scanner concurrently acquires images in multiple bands of the EMR spectrum.

 

Suggested Readings:

  • Avery, T.E., and G.L. Berlin, Fundamentals of Remote Sensing and Airphoto Interpretation, Macmillan, New York, 1992
  • Campbell, J.B., Introduction to Remote Sensing, 3rd ed., Guilford Press, New York, 2002. Curran, P.J. 1985. Principles of Remote Sensing. Longman Group Limited, London.
  • Estes, J. E., and Loveland, T. R. (1999). Characteristics, sources, and management of remotely-sensed data. Geographical Information Systems: Principles, Techniques, Applications, and Management,, 667-675.
  • Fowler, J. E. (2014, October). Compressive pushbroom and whiskbroom sensing for hyperspectral remote-sensing imaging. In Image Processing (ICIP), 2014 IEEE International Conference on (pp. 684-688). IEEE.
  • Gibson  P.J  (2000)  “Introductory  Remote  Sensing-  Principles  and  Concepts”  Routledge,London.
  • http://www.nrcan.gc.ca/node/9337
  • Keith, D. J. (2014). Coastal and Estuarine Waters: Optical Sensors and Remote Sensing for Management.
  • Lillesand, T. M., Kiefer, R. W. and Chipman, J. W. (2008). Remote sensing and image interpretation, 6th ed., John Wiley & Sons, USA.