Remote sensing

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Introduction

Fig. 1. Use of satellites as remote sensing platforms (Photo credit: ESA-AOES Medialab).

There is an increasing demand for accurate, timely information on natural resources and ecosystems, their spatial distribution and temporal changes and trends, from local to global. In the broadest sense, remote sensing is the measurement or acquisition of information about an object or phenomenon by a sensor that is not in direct physical contact with the object. Information is conveyed by waves radiating from the object. The sensor is mounted on a fixed frame (a mast) or on a moving platform, for example an aircraft, drone, satellite or a frame towed behind a ship.

Advantages of this technology are:

  • Observation of a large geographical area (from aircraft or satellite - Fig. 1)
  • Long-term and fast collection of data (from satellite)
  • Low collecting costs
  • "Inaccessible" regions become accessible (e.g. Antarctica)
  • Object is not being perturbed or destroyed

Disadvantages are:

  • Lower spatial resolution than in situ measurement (depending on the type of platform or sensor)
  • Initial investment in development and installation
  • Collected data need to be calibrated via in-situ data
  • Noise caused by another source than the desired one
  • Atmospheric effects degrade the quality of the images and need to be corrected


Brief history

1826 Invention of photography by Joseph Nicéphore Niépce
1850s Photography with cameras from hot-air balloons
1903 Invention of pigeon photography (used during World War I and the Civil War)
1904 Invention 'telemobiloscope', first radar-type device by Christian Hülsmeyer
1909 Photography from air planes
1913 First acoustic underwater transducer patented by Reginald A. Fessenden, precursor of the sonar (sound navigation and ranging).
World War I Military surveillance: airplanes with cameras
1920 First detailed seafloor maps using sonar by the US Coast and Geodetic Survey.
1922 Borehole radar using pulsed modulation patented by Loewy
1930 Invention of Lidar (acronym of "light detection and ranging" or "laser imaging, detection, and ranging") by E. H. Synge
1942 First ship radars for the fleets of the U.S. and British navy
1960s Space programs and race between the United States (US) and the Soviet-Union (USSR)
04 October 1957 Sputnik I (USSR): first artificial satellite to go into orbit
31 January 1958 Explorer I (US) is launched
1960 TIROS-1: first meteorological satellite. First use of term 'Remote sensing'
1961 First Lidar system developed by the Hughes Aircraft Company
1964 First application of marine X-band (or microwave) radar for ocean wave imaging by Ijima et al. (1964[1])
1972 Landsat-1: launch of the first earth resource satellite
1990s Launch of earth resource satellites by national space agencies and commercial companies



General principles of remote sensing

Remote sensors are devices that measure and record specific types of energy. This energy is electromagnetic radiation in remote sensing via the air; underwater remote sensing usually involves acoustic waves. The electromagnetic spectrum is the distribution of radiation energy over wavelengths. The spectrum can be divided into different spectral ranges: gamma rays, X-rays, ultraviolet, visible light, infrared, microwaves and radio waves (Fig. 2). Remote sensing involves measuring energy in spectral bands - discrete intervals of the EM spectrum. Satellite sensors are designed to measure signals emitted by objects on the Earth's surface within certain spectral bands from which the characteristics of these objects can be inferred. Scientists choose a particular spectral band for data collection depending on what they want to investigate. The data recorded by the sensors must be pre-processed (corrected, enhanced), analyzed (classified) and interpreted to provide useful information. Simple traditional methods of visual interpretation have evolved into advanced mathematical and statistical methods using models and analysis techniques as described in Data analysis techniques for the coastal zone and many other articles in the Coastal Wiki.


Fig. 2. The electromagnetic spectrum (Photo credit: Johannes Ahlmann).


Types of sensors

Remote sensing systems can be divided into 2 categories: active and passive sensors. Active sensors (usually detection and ranging systems such as radar, sonar, Lidar) emit radiation which is directed toward the object or surface to be investigated. The radiation emitted by an active sensor provides a specific spectral energy pulse to the study object that activates a response signal which is detected, measured and timed by the sensor. It thus allows to determine distances (ranges). Passive systems (e.g. photographic camera) record the radiation emitted by an object that receives radiation (energy) from an external energy source (for example the sunlight). The radiation emitted by an object is due to reflection, backscatter (diffuse) and re-emitted back radiation. It can provide information about the shape (via reflected radiation), pattern, type of substance and composition of the object.


Fig. 3 The basic principle of ocean colour remote sensing: The radiance, which arrives at the satellite consists of 4 components: (1) radiance which is scattered in the atmosphere, (2) radiance which is specularly reflected at the sea surface, (3) radiance which is backscattered by water and its constituents and (4) in shallow water, radiance, which is reflected from the sea bottom. All 3, or in case of shallow water 4, components have to be determined to retrieve the radiance leaving the water, i.e. component 2.

Atmospheric effects

Remote sensors that observe the Earth are looking through the atmosphere. The atmospheric constituents (particles and gases) can affect the incoming light and radiation by causing wavelength-dependent absorption and scattering (Fig. 3). Scattering occurs when particles or large gas molecules present in the atmosphere interact with the electromagnetic radiation and cause it to be diverted from its original path. Absorption causes molecules in the atmosphere to absorb and re-emit energy at various wavelengths. Ozone, carbon dioxide and water vapor are the most important atmospheric constituents that absorb radiation. These atmospheric effects degrade the quality of the images. They need to be corrected for before the images are subjected to further analysis and interpretation. Sunglint, the direct reflection of the sun from the water surface to the sensor, is another effect that must be corrected for.

Remote sensing applications in the Coastal Wiki


Articles related to remote sensing

Satellite remote sensing

Light fields and optics in coastal waters
Optical remote sensing
Plankton remote sensing
Plankton remote sensing North Sea
Satellite-derived nearshore bathymetry
Oil spill monitoring
Use of Lidar for coastal habitat mapping
HyMap: Hyperspectral seafloor mapping and direct bathymetry calculation in littoral zones
Plankton remote sensing
Remote sensing of zooplankton
The Baltic Algae Watch System - a remote sensing application for monitoring cyanobacterial blooms in the Baltic Sea
Bathymetry from remote sensing wave propagation
Use of aerial photographs for shoreline position and mapping applications
SeaWiFS
MODIS
MERIS
Tidal flats from space
Detecting the unknown - novelty detection of exceptional water reflectance spectra
The HIMOM and OFEW approaches to monitoring intertidal flats

Radar remote sensing

Coastal ocean satellite altimetry
Waves and currents by X-band radar
Bathymetry German Bight from X-band radar
Measuring current fields in the German Bight by radar techniques
Application of radar hydrography in the German Wadden Sea
WiRAR - A marine radar wind sensor
WiSAR - Wind retrieval from synthetic aperture radar

Under water remote sensing

General principles of optical and acoustical instruments
Monitoring coastal morphodynamics using high-precision multibeam technology
Optical Laser diffraction instruments (LISST)
Remote sensing of zooplankton
Optical backscatter point sensor (OBS)
Instruments and sensors to measure environmental parameters
Acoustic monitoring of marine mammals
Acoustic point sensors (ASTM, UHCM, ADV)
Acoustic backscatter profiling sensors (ABS)
Currents and turbulence by acoustic methods
Acoustic kelp bed mapping in shallow rocky coasts - case study Helgoland (North Sea)
Instruments for bed level detection
Measuring instruments for fluid velocity, pressure and wave height

Video camera

Argus video monitoring system
Argus video
Underwater video systems
Video technology
Application and use of underwater video
Argus applications


The main authors of this article are Knockaert, Carolien and Job Dronkers
Please note that others may also have edited the contents of this article.

Citation: Knockaert, Carolien; Job Dronkers; (2024): Remote sensing. Available from http://www.coastalwiki.org/wiki/Remote_sensing [accessed on 20-07-2024]

  1. Ijima, T., Takahashi, T. and Sasaki, H. 1964. Application of radars to wave observations. Proc. 11th Int. Conf. Coastal Engineering 30: 10-22