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Remote Sensing Theory & Background . GEOG370 Instructor: Christine Erlien. Overview. What is remote sensing Brief remote sensing history Photography enables remote sensing Film, then digital; balloons  satellites Satellite remote sensing Resolutions Scanner types Platforms.

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Remote sensing theory background

Remote Sensing Theory & Background


Instructor: Christine Erlien


  • What is remote sensing

  • Brief remote sensing history

    • Photography enables remote sensing

    • Film, then digital; balloons  satellites

  • Satellite remote sensing

    • Resolutions

    • Scanner types

    • Platforms

What is Remote Sensing?

Remote  far away

Sensing things from a distance

Remote sensing is the science and art of obtaining information about a target through the analysis of data acquired by a device that is not in contact with the target under investigation.

What we see & why

Eyes: Sunlight is reflected onto our nerve cells in the retina.

What we see: Visible spectrum (blue, green, red wavelengths)

Remote sensing equipment allows us to sense electromagnetic radiation beyond the visible spectrum

Silk Road, China

Waterless plains of southern Algeria

Grand Canyon

Types of Remote Sensing

  • Type  Based on source of the energy recorded by the sensor

  • Passive Remote Sensing: Energy collected by sensors is either reflected or emitted solar radiation.

    • Reflected – must be collected during daylight hours

    • Emitted – day or night as long as emissions large enough to record

  • Active Remote Sensing: Energy collected by sensors is actively generated by a man-made device.

  • Examples: Radar, LIDAR (Light Detection and Ranging)

Active and passive remote sensing
Active and Passive Remote Sensing

AVHRR Thermal Image

QuikSCAT radar image



Solar Radiation

Electromagnetic radiation energy: Wave-particle duality.

Light speed: c=f 

c = speed of light (186,000 miles/second)

f = light frequency: number of waves passing a reference per unit time (e.g., second).

The amount of energy carried by a photon:  = hf

h=Planck’s constant (6.62610-34 Js)

Note: The shorter the radiations’ wavelength, the higher its frequency  the more energy a photon carries

Atmospheric windows

Solar Electromagnetic Radiation

First Remote Sensing Image



1st permanent photograph (remotely sensed image), by Niepce in 1826.

Remote Sensing of Large Areas

Early remote sensing  limited by means available to put the sensor (i.e., camera) high above the target

The means:

1. Balloons

2. Pigeons

3. Gliders

4. Aircraft

5. Satellite

Military Intelligence

Remote sensing  a critical source of military intelligence for WWI & WWII, Cold War

Remains a critical source of intelligence today


WWI: British reconnaissance aerial photography revealed a major change in direction of the German forces advancing on Paris  allowed the Allied army to fortify its position and hold off the German advance to Paris

WWII: German barges identified in canals near the coast of France in summer of 1940. British launched an air attack on the invasion forces  Germany forced to postpone & eventually abandon invasion

Cold War: U-2 Aircraft

  • Balloons can be easily shot down  high altitude aircraft called the U-2 built to collect remotely sensed data

  • U-2 flies at 70,000 ft, putting it beyond the range of surface-to-air missiles & other aircraft (at that time)

  • Remains a valuable means of collecting remote sensing data today

    • President Bush used it during Gulf War in 1991

    • President Clinton used it in the war in Bosnia in 1998-99

Cuba, 1962

Military Intelligence & Image Resolution

1 meter

2.5 meter

5 meter

10 meter

10 cm

25 cm

50 cm

100 cm

Satellite remote sensing
Satellite Remote Sensing

  • Resolutions

    • Spatial: Area visible to the sensor

    • Spectral: Ability of a sensor to define fine wavelength intervals

    • Temporal: Amount of time before site revisited

    • Radiometric: Ability to discriminate very slight differences in energy

  • Scanner types

    • Along-track

    • Across-track

Across track scanning
Across-track scanning

  • Scan the Earth in a series of lines

    • Lines perpendicular to sensor motion

    • Each line is scanned from one side of the sensor to the other, using a rotating mirror (A).

  • Internal detectors (B) detect & measure energy for each spectral band, convert to digital data

  • IFOV or Instantaneous Field of View (C) of the sensor and the altitude of the platform determine the ground resolution cell viewed (D), and thus the spatial resolution.

  • The angular field of view (E) is the sweep of the mirror, measured in degrees, used to record a scan line, and determines the width of the imaged swath (F).

Along track scanning
Along-track scanning

  • Uses forward motion to record successive scan lines perpendicular to the flight direction

    • Linear array of detectors (A) used; located at the focal plane of the image (B) formed by lens systems (C)

      • Separate array for each spectral band

    • Each individual detector measures the energy for a single ground resolution cell (D)

      • May be several thousand detectors

      • Each is a CCD

      • Energy detected and converted to digital data

    • “Pushed" along in the flight track direction (i.e. along track).

    • “Pushbroom scanners”

Civil Remote Sensing

Earth Resources Technology Satellite (ERTS-1; renamed Landsat 1)

1st satellite launched for peaceful purposes (1972)

Satellite Launched Decom RBV MSS TM Orbit

Landsat-1 23 Jul 1972 6 Jan 1978 1-3 4-7 none 18d/900km

Landsat-2 22 Jan 1975 25 Feb 1982 1-3 4-7 none 18d/900km

Landsat-3 5 Mar 1978 31 Mar 1983 A-D 4-8 none 18d/900km

Landsat-4 16 Jul 1982 -- none 1-4 1-7 16d/705km

Landsat-5 2 Mar 1984 -- none 1-4 1-7 16d/705km

Landsat-6 5 Oct 1993 Launch Failure none none ETM 16d/705km

Landsat-7 15 Apr 1999 -- none none ETM+ 16d/705km

RBV: Return Beam Vidicon MSS: Multispectral Scanner TM: Thematic Mapper

Decom: decommissioned


Data transmission to the ground, allows fast & efficient data delivery

Landsat Orbit

Sun-synchronous orbit: Satellite always crosses the equator at precisely the same local time

Landsat Temporal Resolution

Temporal Resolution: The shortest time needed to repeat the ground track

Landsat Swath Width & Field of View


Field of View


Satellite ground track


Spatial Resolution


185 km

Pixel size=


Landsat 7 ETM+ Spectral Bands

Spectral resolution: The number of bands and the width of spectrum that each sensor covers


Digital numbers (DN)


Radiance intensity






The number of levels of DN values is determined by the radiometric

resolution of the instrument. For example, 8-bit system can differentiate

256 (0-255) levels of radiance

Landsat images
Landsat Images

Alaska’s Aleutian Islands

Mississippi River Delta

SPOT (Systeme Pour l’Observation de la Terre)

  • Along track scanning system (Pushbroom System)

  • Sensors are pointable

    • Allows repeat coverage from different angles

    • Increases potential frequency of coverage of areas where cloud cover is a problem

    • Ability to collect stereoscopic imagery

Temporal resolution=26 days

Radiometric resolution=8-bit

Spot imagery
SPOT Imagery


Owner: Space Imaging

Temporal resolution: 11 days

Radiometric resolution: 11-bit

Spectral bands spatial resolution

Blue (0.45-0.52 4m

Green (0.51-0.60) 4m

Red (0.63-0.70) 4m

NIR (0.76-0.85) 4m

Panchromatic (0.45-0.90) 1m

Swath width: 11km

Orbit: Sun-synchronous; equatorial crossing time of 10:30am