Lecture 6
Multispectral Remote Sensing
Systems
Overview
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:
v
GM
Ro
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:
2Ro
p
v
• The height of a satellite above the earth’s
surface can be expressed as:
h = Ro − Re = Ro − 6371
• Below 180 km, the Earth’s 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.
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)
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 Earth’s
surface at a constant local
time of day called local
solar time.
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.
Descending
pass
• 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.
Ascending
pass
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.
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.
Flight line/path, scan lines, ground
resolution cell, and pixels
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
IFOV and ground resolution cell
IFOV (Instantaneous field of view)
is the cone angle within which the
incident energy is focused on the
detector
Determined by the instrument’s
optical system and size of the
detectors.
All energy propagating towards the
sensor within the IFOV contributes to
the detector’s 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
D = H′β,
where β 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
Nadir
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)
Digital imaging
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.
DNs
The ground distance
between adjacent
sampling points need
not be exactly equal
to IFOV projected on
to the ground
Remote Sensing Raster (Matrix) Data
Format
Lines or
rows (i )
3
4
18
Co lumns ( j)
2
3
4
5
1
1
2
10
15
17
20
15
16
18
18
20
21
22
20
22
17
24
23
21
1
2
22
25
B ri g htness val ue
rang e
(typica lly 8 bit)
255
white
3
B a nds (k )
127
gray
4
0
X axis
blac k
Pictu re elemen t (pi xel) at lo cat io n
Lin e 4, Col umn 4 , i n Ban d 1 h as a
Bri gh tn ess Val ue o f 2 4 , i.e.,B V4 ,4 ,1 = 2 4 .
Ass o ci ated
g ra y-sca le
Detector Configurations
Used for Panchromatic,
Multispectral and
Hyperspectral Remote
Sensing
Across-the-track and along-the-track scanning
Linear detector array
Optics
Platform
motion
Rotating mirror
Scan line
Along-the-track/push
broom
Across-the-track/whisk
broom
Across-the-track
(whiskbroom)
scanning
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 Earth’s surface.
Rotating mirror
• 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 (1020°) to cover a broad region.
Jensen,
2000
Jensen,
2000
• 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.
Characteristics of across-the-track scanner imagery
Dwell Time
PxlSize PxlSize
 Td 

Vs
SWidth
Compare the dwell times of HRVIR, ETM and ASTER sensors in the VNIR band.
Polar radius of the earth: 6356 km
Satellite
Sensor
SPOT
TERRA
LANDSAT
HRVIR
ASTER
ETM
IFOV
(microradians)
24.3
21.5
42.5
Altitude
(Km)
822
705
705
Orbital period
(minutes)
101.4
98.88
98.8
Total field of
view (degrees)
4.3
4.8
15
Geometric characteristics of across-the-track scanner
imagery
Tangential-scale Distortion
Constant angular
velocity of the
rotating mirror
Resulting variations in linear velocity over a ground resolution cell
Geometric characteristics of across-the-track scanner
imagery
Tangential-scale distortion correction
yp
ymax
p

 max
y p max
p 
ymax
Where,
yp is the distance of the
image point from the nadir
ymax is the distance of the
image edge from the nadir
θmax is the one half of the
total field view of the
scanner
Yp  H  tan p
Geometric characteristics of across-the-track scanner
imagery
Resolution cell size variation
Since
D = H′β,
θ
β
H′
The values of D
would increase
as the distance
from the nadir
increases