Remote Sensing Theory & BackgroundGEOG370Instructor: Christine Erlien

Overview

What is remote sensingBrief remote sensing history

Photography enables remote sensingFilm, then digital; balloons satellites

Satellite remote sensingResolutionsScanner typesPlatforms

What is Remote Sensing?

Remote far awaySensing 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 & whyEyes: 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

http://www.remotesensingart.com/

Silk Road, China

Grand Canyon

Waterless plains of southern Algeria

Type Based on source of the energy recorded by the sensor

1. 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

2. Active Remote Sensing: Energy collected by sensors is actively generated by a man-made device. Examples: Radar, LIDAR (Light Detection and Ranging)

Types of Remote Sensing

AVHRR Thermal Imagehttp://www.coml.org/edu/tech/count/srs1.htm

QuikSCAT radar imagehttp://nsidc.org/seaice/study/active_remote_sensing.html

Active and Passive Remote Sensing

Solar Radiation

Electromagnetic radiation energy: Wave-particle duality.

Particle=photon Wavelength

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: = hfh=Planck’s constant (6.62610-34 Js)

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

Solar Electromagnetic Radiation

Atmospheric windows

First Remote Sensing Image

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

Tree

Rooftop

http://www.artlex.com/ArtLex/p/images/photo_niepce.lg.jpg

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

http://rst.gsfc.nasa.gov/Front/overview.html

Remote sensing a critical source of military intelligence for WWI & WWII, Cold WarRemains a critical source of intelligence today

Examples: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

Military Intelligence

Cold War: U-2 AircraftBalloons 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

http://www.fas.org/irp/imint/resolve3.htm

Military Intelligence & Image Resolution

1 meter 5 meter2.5 meter

10 cm

10 meter

50 cm25 cm 100 cmhttp://rst.gsfc.nasa.gov/Intro/Part2_26e.html

Satellite Remote Sensing

ResolutionsSpatial: Area visible to the sensorSpectral: Ability of a sensor to define fine wavelength intervals

Temporal: Amount of time before site revisitedRadiometric: Ability to discriminate very slight differences in energy

Scanner typesAlong-trackAcross-track

Across-track scanningScan 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 dataIFOV 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).

http://ccrs.nrcan.gc.ca/resource/tutor/fundam/chapter2/08_e.php

Along-track scanningUses 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”

http://ccrs.nrcan.gc.ca/resource/tutor/fundam/chapter2/08_e.php

Civil Remote Sensing

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 MapperDecom: decommissioned

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

1st satellite launched for peaceful purposes (1972)

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

Landsat

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

Landsat Orbit

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

Landsat Temporal Resolution

185 km

Field of View

175kmscene

Landsat

Satellite ground tra

ck705km

Spatial Resolution

Pixel size=(30x30m)

Landsat Swath Width & Field of View

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

Landsat 7 ETM+ Spectral Bands

Radiometric Resolution

The number of levels of DN values is determined by the radiometricresolution of the instrument. For example, 8-bit system can differentiate256 (0-255) levels of radiance

MinimumRadiance

MaximumRadiance

Dig

ital n

umbe

rs (

DN

)

Radiance intensity0

255

Alaska’s Aleutian Islands Mississippi River Delta

Landsat Images

http://earthasart.gsfc.nasa.gov

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 daysRadiometric resolution=8-bit

SPOT Imagery

http://www.spotimage.fr/automne_modules_files/gal/edited/r444_santiago3D_800x600.jpg

IkonosOwner: Space Imaging

Temporal resolution: 11 days

Radiometric resolution: 11-bitSpectral bands spatial resolutionBlue (0.45-0.52 4mGreen (0.51-0.60) 4mRed (0.63-0.70) 4mNIR (0.76-0.85) 4mPanchromatic (0.45-0.90) 1m

Swath width: 11kmOrbit: Sun-synchronous; equatorial crossing time of 10:30am

IKONOS