Lecture 6 Multispectral Remote Sensing Systems. Overview Overview
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- Slide 1
- Lecture 6 Multispectral Remote Sensing Systems
- Slide 2
- Overview Overview
- Slide 3
- Satellite orbits Satellite orbits are designed according to the
capability and objective of the sensors they carry. The velocity of
satellite can be calculated by the formula: where G is the
universal gravitational constant and M is the mass of the Earth.
The orbit period (p) can be determined by the formula: The height
of a satellite above the earths surface can be expressed as: h = R
o R e = R o 6371 Below 180 km, the Earths atmosphere is too dense
for satellites to orbit without burning as a result of frictional
heating. Above 600-800 km, there is little atmosphere drag that a
satellite will remain in high orbit indefinitely.
- Slide 4
- Satellite orbits: Geostationary The geostationary satellites go
around the Earth at speeds which match the rotation of the Earth so
they seem stationary relative to the Earth's surface. Geostationary
satellites complete a orbit in 24 hours. The orbit is circular. And
its inclination is zero degrees, which means that it is above the
Earth's equator. Weather and communications satellites commonly
have these types of orbits. Due to their high altitude some
geostationary weather satellites can monitor weather and cloud
patterns covering an almost entire hemisphere. It can frequently
and repetitively observe and monitor the same portion of the Earth
for the purpose of detecting, tracking and predicting the weather
or natural hazards. Ideal for making repeated observations of a
fixed geographical area centred on the equator, polar areas are
always covered poorly. Geostationary satellite images of the polar
regions are distorted because of the low angle the satellite sees
the region. Meteosat (ESA, covering Europe and Africa) GOES-EAST
(NOAA, covering North and South America) GOES-WEST (NOAA, covering
Eastern Pacific) GMS (Japan, covering Japan and Australia, Western
Pacific) Fengyun-2 (China, covering China and the Indian Ocean)
GOMS (Elektro) ((Russia, covering Central Asia and the Indian
Ocean) INSAT (India)
- Slide 5
- Satellite orbits: Polar orbits sun-synchronous Most of the
remote sensing satellite platforms are in near-polar orbits. They
pass over or near the north and south poles each revolution. Polar
orbiting satellites can provide global coverage of the atmosphere
and Earth surface. Polar satellites circle at a much lower altitude
(~800km) providing higher quality remote sensing data (more
detailed information) than geostationary satellites. Short orbital
periods - 98 to 102 minutes. As the earth rotates to the east
beneath the satellite, each pass monitors an area to the west of
the previous pass. These 'strips' can be pieced together to produce
a picture of a larger area (mosaic). Typically, near polar orbit
satellites are also designed in sun-synchronous orbits. In a sun
synchronous orbit a satellite passes over each area of the Earths
surface at a constant local time of day called local solar
time.
- Slide 6
- Sun-synchronous polar orbits Ascending and descending passes
Ascending passes of the orbit corresponds to that portion of the
orbit when the satellite is moving from south to north, while
descending passes of the orbit corresponds to north to south
movement. Most sun-synchronous polar orbiters have the ascending
pass is on the shadowed side of the Earth, while the descending
pass is on the sunlit side. Sensors recording reflected solar
energy only image the surface on a descending pass, when solar
illumination is available. Active sensors that provide their own
illumination or passive sensors that record emitted radiation can
also image the surface on ascending passes. Descending pass
Ascending pass
- Slide 7
- Remote Sensing Systems Two major categories of remote sensing
systems - Framing system and Scanning system. Framing systems
instantaneously acquire an image of a large area (or frame) on the
terrain. Cameras are common examples of such systems. A scanning
system employs detectors with a narrow field of view that is swept
across the terrain in a series of parallel scan lines to produce an
image. Generally electro-optic sensors are used in scanning
systems.
- Slide 8
- Electro-optical Sensors In contrast to photographic cameras
that record radiation reflected from a ground scene directly onto
film, electro-optical sensors use non-film detectors.
Electro-optical detectors record the reflected and/or emitted
radiation from a ground scene as electrical signals, which are
converted into the image DN values.
- Slide 9
- Flight line the path of the sensor platform (satellite/air
craft) Scan line The line along which the sensor scans the ground
Ground resolution cell The ground segment sensed at any instant
Pixel (picture element ) the radiometric response of the ground
resolution cell on the image Flight line/path, scan lines, ground
resolution cell, and pixels
- Slide 10
- IFOV (Instantaneous field of view) is the cone angle within
which the incident energy is focused on the detector Determined by
the instruments optical system and size of the detectors. All
energy propagating towards the sensor within the IFOV contributes
to the detectors response at any instant The ground resolution cell
within a IFOV can have homogenous (pure pixels) or heterogeneous
composition (mixed pixels). IFOV and ground resolution cell
- Slide 11
- D = H, where is in radians D is loosely referred to as the
spatial resolution of the system. Since H within a IFOV increases
away from the nadir, the ground resolution cell increases away from
nadir. Smaller IFOV >> better spatial resolution, poorer
signal-to-noise ratios Higher IFOV >> better radiometric
resolution, better signal-to-noise ratios, longer dwell time Higher
signal-to-noise ratios for small IFOV can also be achieved by
taking data over larger wave bands thus lowering spectral
resolution (ability to discriminate fine spectral difference)
Nadir
- Slide 12
- Digital images are created by quantizing an analog electrical
signal - (A-to-D conversion. The response of the detector to the
incoming radiance from the IFOV is in the form of a continuous
analog signal. The continuous signal is sampled at and specified
time interval and recorded numerically at each sample point. The
sampling rate is determined by the highest frequency of change in
the signal it should be twice as high as the highest frequency
present in the signal. Digital imaging The ground distance between
adjacent sampling points need not be exactly equal to IFOV
projected on to the ground DNs
- Slide 13
- Remote Sensing Raster (Matrix) Data Format Remote Sensing
Raster (Matrix) Data Format
- Slide 14
- Detector Configurations Used for Panchromatic, Multispectral
and Hyperspectral Remote Sensing
- Slide 15
- Across-the-track and along-the-track scanning Optics Platform
motion Scan line Rotating mirror Along-the-track/push broom
Across-the-track/whisk broom Linear detector array
- Slide 16
- Across-the-track (whiskbroom) scanning Rotating mirror Using a
rotating mirror, this system scan the terrain along lines that are
perpendicular to the direction of motion of the sensor platform.
Scanner repeatedly measure the energy from one side of the
satellite platform to the other. As the platform moves forward over
the Earth, successive scans build up a 2-D image of the Earths
surface. An array of electro-optic detectors are located on focal
plane. The angular field of view (Total Field of View) is the sweep
of the mirror used to record a scan line, and determines the width
of the imaged swath. Across-track scanner can be mounted on both
aircrafts and satellites. Airborne scanners typically sweep large
angles (between 90 and 120), while satellites, because of their
higher altitude need only to sweep small angles (10- 20) to cover a
broad region. Because the distance from the sensor to the target
increases towards the edges of the swath, the ground resolution
cells also become systematically larger and introduce geometric
distortions to the images.
- Slide 17
- Characteristics of across-the-track scanner imagery Dwell Time
SatelliteSensor IFOV (microradians) Altitude (Km) Orbital period
(minutes) Total field of view (degrees) SPOTHRVIR24.3822101.44.3
TERRAASTER21.570598.884.8 LANDSATETM42.570598.815 Compare the dwell
times of HRVIR, ETM and ASTER sensors in the VNIR band. Polar
radius of the earth: 6356 km
- Slide 18
- Geometric characteristics of across-the-track scanner imagery
Tangential-scale Distortion Resulting variations in linear velocity
over a ground resolution cell Constant angular velocity of the
rotating mirror
- Slide 19
- Geometric characteristics of across-the-track scanner imagery
Tangential-scale distortion correction Where, y p is the distance
of the image point from the nadir y max is the distance of the
image edge from the nadir max is the one half of the total field
view of the scanner
- Slide 20
- Geometric characteristics of across-the-track scanner imagery
Resolution cell size variation H = Hsec H Since D = H, The values
of D would increase as the distance from the nadir increases (Hsec
) (Hsec 2 ) H