Remote Sensing Introduction

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LECTURE 3LECTURE 3
What is Remote Sensing?What is Remote Sensing?
INTRODUCTION TOINTRODUCTION TO
REMOTE SENSINGREMOTE SENSING

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1. What is Remote Sensing?1. What is Remote Sensing?
Lecture Outline (Satellites and Sensors) :Lecture Outline (Satellites and Sensors) :
•• PlatformsPlatforms
•• Satellite Characteristics: Orbits and SwathsSatellite Characteristics: Orbits and Swaths
•• ResolutionsResolutions
-- Spatial, Spectral, Radiometric, TemporalSpatial, Spectral, Radiometric, Temporal
•• MultispectralMultispectral ScanningScanning

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On the Ground, In the Air, In SpaceOn the Ground, In the Air, In Space
PlatformsPlatforms
•• On the groundOn the ground
•• On an Aircraft or Balloon (some other platform within the EarthOn an Aircraft or Balloon (some other platform within the Earth’’ss
atmosphere)atmosphere)
•• On a Spacecraft or Satellite Outside of the EarthOn a Spacecraft or Satellite Outside of the Earth’’s Atmospheres Atmosphere
QUESTION:QUESTION: What advantages do sensors carried on board satellitesWhat advantages do sensors carried on board satellites
have over those carried on aircraft? Are there any disadvantageshave over those carried on aircraft? Are there any disadvantages thatthat
you can think of?you can think of?

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Satellite Characteristics: Orbits and SwathsSatellite Characteristics: Orbits and Swaths
Geostationary OrbitsGeostationary Orbits::
TheseThese satellites are at very high altitudes,satellites are at very high altitudes,
which view the same portion of the Earthwhich view the same portion of the Earth’’ss
surface at all times.surface at all times.
These satellites, at altitudes of approximatelyThese satellites, at altitudes of approximately
36,000 kilometers, revolve at speeds which36,000 kilometers, revolve at speeds which
match the rotation of the Earth so they seemmatch the rotation of the Earth so they seem
stationary, relative to the Earthstationary, relative to the Earth’’s atmosphere.s atmosphere.
QUESTION:QUESTION: Can one satellite cover the entire EarthCan one satellite cover the entire Earth’’s surface?s surface?

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NearNear--Polar OrbitsPolar Orbits::
TheseThese satellites are designed to follow an orbit (basicallysatellites are designed to follow an orbit (basically
northnorth--south) which, in conjunction with the Earthsouth) which, in conjunction with the Earth’’s rotations rotation
(west(west--east), allows them to cover most of the Eartheast), allows them to cover most of the Earth’’s surfaces surface
over a certain period of time.over a certain period of time.
Two types of nearTwo types of near--polar orbits:polar orbits:
•• SunSun--SynchronousSynchronous::
-- Cover each area of the world at a constant local time of dayCover each area of the world at a constant local time of day
-- At any given latitude, the position of the sun in the sky as thAt any given latitude, the position of the sun in the sky as the satellitee satellite
passes overhead will be the same within the same seasonpasses overhead will be the same within the same season
•• NonNon--SunSun--Synchronous:Synchronous: Opposite of SunOpposite of Sun--SynchronousSynchronous

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QUESTION:QUESTION: Discuss advantages and disadvantages of SunDiscuss advantages and disadvantages of Sun--
synchronous satellites.synchronous satellites.

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The satellite travels northward on one side of the Earth andThe satellite travels northward on one side of the Earth and
then toward the southern pole on the second half of its orbit.then toward the southern pole on the second half of its orbit.
These are calledThese are called ascending and descending passesascending and descending passes,,
respectively.respectively.
If the orbit is also sunIf the orbit is also sun--synchronous, the ascending pass is most likely onsynchronous, the ascending pass is most likely on
the shadowed side of the Earth while the descending pass is mostthe shadowed side of the Earth while the descending pass is most likely onlikely on
the sunlit side.the sunlit side.
Sensors recording reflected solar energy only image the surfaceSensors recording reflected solar energy only image the surface on aon a
descending pass, when solar illumination is available.descending pass, when solar illumination is available.
Active sensors which provide their own illumination or passive sActive sensors which provide their own illumination or passive sensorsensors
that record emitted (e.g. thermal) radiation can also image thethat record emitted (e.g. thermal) radiation can also image the surface onsurface on
ascending passes.ascending passes.

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Orbit LengthOrbit Length::
Orbit LengthOrbit Length: If we start with any randomly selected pass in: If we start with any randomly selected pass in
a satellitea satellite’’s orbit, an orbit cycle will be completed when thes orbit, an orbit cycle will be completed when the
satellite traces its path, passing over the same point on thesatellite traces its path, passing over the same point on the
EarthEarth’’s surface directly below the satellite (called the nadirs surface directly below the satellite (called the nadir
point) for a second time.point) for a second time.
QUESTION:QUESTION: Is the orbit length the same as the revisit period?Is the orbit length the same as the revisit period?
The revisit period is an important consideration for aThe revisit period is an important consideration for a number ofnumber of
monitoring applications, especially when frequent imaging ismonitoring applications, especially when frequent imaging is
required (for example, to monitor the extent of flooding).required (for example, to monitor the extent of flooding).

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SwathSwath::
As a satellite revolves around the Earth, the sensorAs a satellite revolves around the Earth, the sensor
““seessees”” a certain portion of the Eartha certain portion of the Earth’’s surface. Thes surface. The
area imaged on the surface, is referred to as thearea imaged on the surface, is referred to as the
swathswath..
Imaging swaths forImaging swaths for spacebornespaceborne sensors generallysensors generally
vary between tens and hundreds of kilometers wide.vary between tens and hundreds of kilometers wide.

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QUESTION:QUESTION: Does the revisit period change with latitude?Does the revisit period change with latitude?
In nearIn near--polar orbits, areas at high latitude will be imaged more frequenpolar orbits, areas at high latitude will be imaged more frequentlytly
than the equatorial zone due to the increasingthan the equatorial zone due to the increasing overlap in adjacent swathsoverlap in adjacent swaths
as the orbit paths come closer together near the poles.as the orbit paths come closer together near the poles.

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Spatial Resolution and Pixel SizeSpatial Resolution and Pixel Size
For remote sensors, the distance between the target being imagedFor remote sensors, the distance between the target being imaged andand
the platform plays a large role in determining the detail of infthe platform plays a large role in determining the detail of informationormation
and the total area imaged by the sensor.and the total area imaged by the sensor.
QUESTION:QUESTION: Look at the two images below. One of them is taken from aLook at the two images below. One of them is taken from a
satellite, and the other one from an aircraft. Can you tell whicsatellite, and the other one from an aircraft. Can you tell which ish is
which?which?

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Spatial ResolutionSpatial Resolution of the Sensors: refers to the size ofof the Sensors: refers to the size of
the smallest possible feature that can be detected.the smallest possible feature that can be detected.
Spatial resolution of passive sensors depends primarilySpatial resolution of passive sensors depends primarily
on theiron their Instantaneous Field of View (IFOV).Instantaneous Field of View (IFOV).
IFOV:IFOV: is the angular cone of visibility of the sensor (A) and determinis the angular cone of visibility of the sensor (A) and determines thees the
area on the Eartharea on the Earth’’s surface which iss surface which is ““seenseen”” from a given altitude at onefrom a given altitude at one
particular moment in time (B).particular moment in time (B).
The size of the area viewed is determined by multiplying the IFOThe size of the area viewed is determined by multiplying the IFOV by theV by the
distance from the ground to the sensor (C). This area on the grodistance from the ground to the sensor (C). This area on the ground isund is
called thecalled the resolution cellresolution cell..

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QUESTION:QUESTION: Are the pixel size and resolution the same?Are the pixel size and resolution the same?
Images where only large features are visible are said to haveImages where only large features are visible are said to have coarse or lowcoarse or low
resolutionresolution. In. In fine or high resolutionfine or high resolution images, small objects can beimages, small objects can be
detected.detected.

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Spectral ResolutionSpectral Resolution
Broad classes, such as water and vegetation, can usually be sepaBroad classes, such as water and vegetation, can usually be separatedrated
using very broad wavelength rangesusing very broad wavelength ranges –– the visible and near Infraredthe visible and near Infrared
Other more specific classes, such as differentOther more specific classes, such as different
rock types, may not be easily distinguishablerock types, may not be easily distinguishable
using either of these broad wavelength rangesusing either of these broad wavelength ranges
and would require comparison at much finerand would require comparison at much finer
wavelength ranges to separate them. Thus, wewavelength ranges to separate them. Thus, we
would require a sensor with higher spectralwould require a sensor with higher spectral
resolution.resolution.
Spectral resolutionSpectral resolution describes the ability of a sensor to define finedescribes the ability of a sensor to define fine
wavelength intervals. The finer the spectral resolution, the narwavelength intervals. The finer the spectral resolution, the narrowerrower
the wavelength range for a particular channel or band.the wavelength range for a particular channel or band.

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Spectral ResolutionSpectral Resolution
Many remote sensing systems record energy over separate wavelengMany remote sensing systems record energy over separate wavelengthth
ranges at various spectral resolutions. These are referred to asranges at various spectral resolutions. These are referred to as multimulti--
spectral sensorsspectral sensors..
Advanced multiAdvanced multi--spectral sensors calledspectral sensors called hyperspectralhyperspectral sensorssensors detectdetect
hundreds of very narrow spectral bands.hundreds of very narrow spectral bands.
QUESTION:QUESTION: What would be some of the advantages and disadvantages ofWhat would be some of the advantages and disadvantages of
hyperspectralhyperspectral scanners?scanners?

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Radiometric ResolutionRadiometric Resolution
TheThe radiometric resolutionradiometric resolution of an imaging system describes its ability toof an imaging system describes its ability to
discriminate very slight differences in energy.discriminate very slight differences in energy.
The finer the radiometric resolution of a sensor, the more sensiThe finer the radiometric resolution of a sensor, the more sensitive it is totive it is to
detecting small differences in reflected or emitted energy.detecting small differences in reflected or emitted energy.

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You canYou can’’t have it all!t have it all!
There are tradeThere are trade--offs betweenoffs between spatial, spectral, andspatial, spectral, and
radiometric resolutionradiometric resolution which must be taken intowhich must be taken into
consideration when engineers design a sensor.consideration when engineers design a sensor.
QUESTION: For high spatial resolution, what should
be the characteristics of the sensor?
For high spatial resolution, the sensor has to have a
small IFOV (Instantaneous Field of View).
However, this reduces the amount of energy that
can be detected as the area of the ground resolution
cell within the IFOV becomes smaller. This leads
to reduced radiometric resolution

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You canYou can’’t have it all!t have it all!
QUESTION: To increase the amount of energyTo increase the amount of energy
detected (and thus, the radiometric resolution)detected (and thus, the radiometric resolution)
without reducing spatial resolution,without reducing spatial resolution, what should be
the characteristics of the sensor?
To increase the amount of energy detected (andTo increase the amount of energy detected (and
thus, the radiometric resolution) without reducingthus, the radiometric resolution) without reducing
spatial resolution, we would have to broaden thespatial resolution, we would have to broaden the
wavelength range detected for a particular channelwavelength range detected for a particular channel
or band. Unfortunately, this would reduce theor band. Unfortunately, this would reduce the
spectral resolution of the sensor.spectral resolution of the sensor.

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Temporal ResolutionTemporal Resolution
The actual temporal resolution of a sensor depends on a varietyThe actual temporal resolution of a sensor depends on a variety of factors,of factors,
including the satellite/sensor capabilities, the swath overlap,including the satellite/sensor capabilities, the swath overlap, and latitude.and latitude.

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MultispectralMultispectral ScanningScanning
Many electronic (as opposed to photographic) remote sensors acquMany electronic (as opposed to photographic) remote sensors acquire dataire data
usingusing scanning systemsscanning systems, which employ a sensor with a narrow field of view, which employ a sensor with a narrow field of view
(i.e. IFOV) that sweeps over the terrain to build up and produce(i.e. IFOV) that sweeps over the terrain to build up and produce a twoa two--
dimensional image of the surface.dimensional image of the surface.
A scanning system used to collect data over a variety of differeA scanning system used to collect data over a variety of differentnt
wavelength ranges is called awavelength ranges is called a multispectralmultispectral scanner (MSS),scanner (MSS), and is theand is the
most commonly used scanning system.most commonly used scanning system.
Two main modes of scanning:Two main modes of scanning: acrossacross--track scanningtrack scanning, and, and alongalong--tracktrack
scanningscanning..

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AcrossAcross--track Scanners:track Scanners:
•• Scan the Earth in a series of lines. The lines areScan the Earth in a series of lines. The lines are
perpendicular to the direction of motion of theperpendicular to the direction of motion of the
sensor platform (i.e. across the swath).sensor platform (i.e. across the swath).
•• Each line is scanned from one side of the sensor to the other,Each line is scanned from one side of the sensor to the other, using ausing a rotatingrotating
mirror (A).mirror (A). As the platform moves forward over the Earth, successive scansAs the platform moves forward over the Earth, successive scans
build up a twobuild up a two--dimensional image of the Earthdimensional image of the Earth’’s surface.s surface.
•• A bank of internalA bank of internal detectors (B)detectors (B), each sensitive to a specific range of, each sensitive to a specific range of
wavelengths, detects and measures the energy for each spectral bwavelengths, detects and measures the energy for each spectral band.and.
•• TheThe angular field of viewangular field of view (E) is the sweep of the mirror, measured in degrees,(E) is the sweep of the mirror, measured in degrees,
used to record a scan line, and determines the width of the imagused to record a scan line, and determines the width of the imageded swathswath (F).(F).

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AcrossAcross--track Scanners:track Scanners:
•• Because the distance from the sensor to the target increases toBecause the distance from the sensor to the target increases towardswards
the edges of the swath, the ground resolution cells also becomethe edges of the swath, the ground resolution cells also become largerlarger
and introduce geometric distortions to the images.and introduce geometric distortions to the images.
•• The length of time the IFOVThe length of time the IFOV ““seessees”” a ground resolution cell as thea ground resolution cell as the
rotating mirror scans (called therotating mirror scans (called the dwell timedwell time), is generally quite short), is generally quite short
andand influences the design of the spatial, spectral, and radiometricinfluences the design of the spatial, spectral, and radiometric
resolution of the sensor.resolution of the sensor.

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AlongAlong--track Scanners:track Scanners:
•• Instead of a scanning mirror, they use aInstead of a scanning mirror, they use a
linear array of detectors (A) located at thelinear array of detectors (A) located at the
focal plane of the image (B) formed by lensfocal plane of the image (B) formed by lens
systems (C), which aresystems (C), which are ““pushedpushed”” along thealong the
flight track direction (i.e. along track). Theseflight track direction (i.e. along track). These
systems are referred to assystems are referred to as pushbroompushbroom
scannersscanners..
•• Each individual detector measures the energy for a single grounEach individual detector measures the energy for a single groundd
resolution cell (D) and thus the size and IFOV of the detectorsresolution cell (D) and thus the size and IFOV of the detectors
determines the spatial resolution of the system.determines the spatial resolution of the system.
•• A separate linear array is required to measure each spectral baA separate linear array is required to measure each spectral band ornd or
channel.channel.

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Advantages of AlongAdvantages of Along--track Scanners over Acrosstrack Scanners over Across--track Scanners:track Scanners:
•• The array of detectors combined with theThe array of detectors combined with the pushbroompushbroom motion allowsmotion allows
each detector toeach detector to ““seesee”” and measure the energy from each groundand measure the energy from each ground
resolution cell for a longer period of time (dwell time).resolution cell for a longer period of time (dwell time).
•• This allows more energy to be detected and improves the radiomeThis allows more energy to be detected and improves the radiometrictric
resolution.resolution. The increased dwell time also facilitates smallerThe increased dwell time also facilitates smaller IFOVsIFOVs andand
narrower bandwidths for each detector.narrower bandwidths for each detector.
•• Detectors last longer because they have no moving parts.Detectors last longer because they have no moving parts.
Disadvantages of AlongDisadvantages of Along--track Scannerstrack Scanners
CrossCross--calibrating thousand of detectors to achieve uniform intensitycalibrating thousand of detectors to achieve uniform intensity
across the array is necessary and complicated.across the array is necessary and complicated.
QUESTION: Discuss the advantages and disadvantages of along-track
scanners w.r.t. across-track scanners.

Remote Sensing Introduction

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Remote Sensing Introduction