Transcript Document

Maa-57.2040
Kaukokartoituksen yleiskurssi
General Remote Sensing
Autumn 2007
Markus Törmä
[email protected]
Lectures: 10 – 12 M2
1.
2.
3.
4.
Tu 18.9. Imaging platforms, satellites, orbits etc.
Tu 25.9. Landsat
Tu 2.10. TERRA and AQUA -satellites
Tu 9.10. Other instruments and satellites: Spot, high
resolution satellites, NOAA, METEOSAT, ADEOS,
SAR, coming missions
5. Tu 16.10. Image restoration: geometry and radiometry
6. Th 18.10. Image enhancement I:
–
filtering, arithmetic operations, color coordinates, texture,
principal component analysis
7. Tu 23.10. Image enhancement II:
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soil-line, vegetation indexes, tasselled cap-transformation,
examples of whole processing chain
Exercise 1
Preprocessing and visual interpretation of satellite image
• Each student chooses own image from 7 Landsat-5 TM or
3 Aster images
• Preprocessing in this course, proper information extraction
and classification in ”Käytännön kaukokartoitus”-course
• Tasks in preprecessing
– read image to PCI Geomatica
– geometric correction: test different methods (polynomial models,
ortho, etc.) and interpolation methods
– cloud interpretation
– atmospheric correction: ATCOR 2/3, test different parameters
Exercise 1
Preprocessing and visual interpretation of satellite image
• Compare spectra measured by spectrometer in previous
course or from spectral library to image in order to locate
areas of different surface material
• Write report
• Person in charge: ?
• Kick-of meeting mid-October
• Deadline: end of December
Exercise 2
Seminar of Remote Sensing Club of Finland
• 8.-9.11. at Finnish Environment Institute,
Mechelininkatu 34a
• Choose topic: snow, water, land, ...
• Write summary based on presentations
– send to Markus
• Deadline end of November
• Alternative:
– Choose topic
– Search 2-3 scientific articles
– Write summary
Remote sensing
Definition:
• Information acquisition from target or
object without touching it (and using
elecromagnetic radiation as information
carrier).
Information acquisition using
remote sensing: parts of system
A. Source of electromagnetic
radiation
B. Atmosphere
C. Radiation interacts with
target / object
D. Instrument detects
radiation
E. Data transmission and
preprocessing
F. Interpretation /
classification
G. Applications
Imaging platforms
• On the ground:
– tripod, roof, handheld...
• The detailed study of target,
e.g. determination of
characteristic reflectance curve
• Measurements for comparison
with satellite data,
determination of atmospheric
influence
• Image: Antenna of microwave
radiometer
Imaging platforms
• Analytical Spectral Devices FieldSpecspectrometer, wavelenght 350 - 2500 nm
Imaging platforms...
Gas balloons:
• Maximum height about 50 km
• Stable
• Not that controllable in xy-plane
• Used mostly in atmospheric sounding
… Imaging platforms
• Airplane / helicopter
– when more accurate information is needed that
is possible using satellite instruments
– covered area is larger that using ground-based
instruments
– also for comparison data for satellite
measurements and substitution for satellite data
Imaging platforms
Helicopter:
• Low altitude and slow speed
• Imaging of small areas, strips and details
• Instrument development
Airplane:
• Maximum flying height about 20 km
• Pressurized cockpit needed if flying height over 3 km
• Pros: it is easy to change flying height and speed, as well
as time of flight
• Cons: movement of airplane due to changing wind
• Recommended 2 motors and minimum speed 200 km/h
for mapping purposes
Airplane
• Research plane Short
Skyvan of HUT / Space
Laboratory
AISA-spectrometer in the front
Airplane
• Different kinds
of radiometer
antennas…
…and antenna
of radar
Airplane
• Rockwell Turbo
Commander 690A of
National Land Survey
• Aerial mapping camera Wild
RC-10 in use
• Nowadays WILD/LEICA
RC 20 + FMC installed
Satellites as instrument platforms
• Satellites circle their
object using path called
orbit
• Orbital parameters like
height describe orbit
Satellite
• Consists of payload and bus or subsystems
• Payload:
– Instruments
• Bus and subsystems:
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Attitude control
Propulsion system
Electrical power unit
Temperature control
Supporting structures
Telemetry, tracking, command and communications system
Ground station
Satellite
Attitude control
• Takes care that satellite follows correct orbit
• Height is measured using GPS-satellites, gravity measurements or
sun radiation pressure
• Attitude is measured using gyroscopes, magnetometers or stellar
sensors
• The attitude of satellite is changed using moment wheel or
propulsion system
Propulsion system
• Is used to keep satellite on the correct orbit or change orbit
Satellite
Electrical power unit
• Produces electrical power for other systems
• Solar panels transfer sunlight to electrical power
• They should be pointed toward sun all the time
• Batteries are used for storing
• Some Russian satellites even use small nuclear reactors
Temperature control
• Controls thermal balance and operation of different parts
• Part of satellite is towards sun (hot), other parts away (cold)
• Thermal difference between different parts can be even 200K
• Coating materials, insulators and active heat transfers
Satellite
Supporting structure
• ”Bones”, keeps different parts attached to each other
Telemetry, tracking, command and communications system
• Connection between satellite and ground station
• Transmits measured data to ground station
Ground station
• Receives and stores data send by satellite
• Ground station controls and programs instruments and other
systems
• Antenna system follows the orbit of satellite
• Noise of communication is removed
• Possibly geometric and radiometric correction of data
Orbit
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•
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Satellite orbits planet using circular or elliptical orbit
Satellite passes planet using parabolic or hyperbolic orbits
Kepler’s laws:
1.
2.
3.
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Law of ellipses: The path of the satellite about the planet is elliptical in
shape, with the center of the planet being located at one focus.
Law of equal areas: An imaginary line drawn from the center of the
planet to the center of the satellite will sweep out equal areas in equal
intervals of time.
Law of harmonies: The ratio of the squares of the periods of any two
satellites is equal to the ratio of the cubes of their average distances
from the planet.
The most closest point between orbit and planet is called
perigeum and point farthest away apogeum
Orbit
Orbital parameters:
• a: half lenght of
principal axis of ellipse
• : eccentricity of orbit
• i: inclination angle: angle between equatorial plane and orbit
• : orbit longitude of ascending node (ascending node: intersection
of orbit and equator)
• : angle between ascending node and perigeum
• tpe: time, when satellite is at perigeum
• Circular orbits are used in remote sensing:
principal axis a = minor axis b
Geostationary satellite
• Orbits earth with same
speed as earth revolves on
its axis
• As result, keeps over same
place all the time
• i=0
• Orbital height about 36000
km, so image wide area of
earth surface
• Many weather satellites like
Meteosat
Geosynchronous orbit
• Satellite orbits earth with same speed as
earth revolves its axis
• i0
• As result, place of satellite varies within
time
• Variation in longitudes small, larger in
latitudes
Sunsynchronous orbit
• Satellite on sunsynchronous
orbit pass over same place at
same time of day
• The illumination conditions of
target are same, as
measurements are made
– different years but same dates, or
– succesive days
• Position of sun (azimuth and
zenith angles) varies between
seasons, therefore also
illumination of target varies
Remote sensing satellites
• Worldwide coverage
• Distance to target constant
• circular orbit
• height usually 500 – 1000 km
• Pass over same place at same time of
day
• sunsynchronous orbit
• Inclination angle about 100 degrees
• period 95-100 min
”Polar orbit”
• Sunsynchronous orbit
is also called polar
orbit because satellite
passes polar regions
• Ascending pass:
satellite is flying
from south to north
• Descending pass:
satellite is flying
from north to south
”Polar orbit”
• When target is sunlit, it can be
imaged using passive
instruments which measure
radiation originating from sun
• I.e. satellite is on the same side
as sun
• Usually this happens on
descending pass
”Polar orbit”
• Ascending pass goes on the
other, dark (no sunlight),
side of earth
• Passive instruments cannot
measure reflected sunlight
• Measurements can be done
using thermal infrared or
microwave regions, where
radiation emitted by earth is
measured
• Also, active instruments do
not need sunlight
Swath width
• Swath width is the witdh
of imaged area orthogonal
to flight direction
• Varies from ten kilometers
to thousands of
kilometers, depending on
orbit and instrument
characteristics
Successive orbits
• Earth revolves toward
east as polar orbit
satellite flies over
• The track of orbit on
the ground seems
moves westwards on
successive orbits
• Orbits of one day
cover quite large area
Repeat coverage / cycle
• Time interval between successive
observations of the same area of
Earth
• Geostationary satellites:
– imaging frequency, e.g. 30 min
• Sunsynchronous satellites:
– typically varies from 16 – 35 days
– pointing capability of instrument
can decrease repeat cycle
Successive orbits
• Farther away from Equator,
the coverage of neighboring
orbits overlaps more and
more
• We get more observations
from a place from
neighboring orbits
– repeat cycle decreases
• In this sense Finland is
situated in rather good place
Instruments
• Can be divided according to their way of
operation
1. imaging vs. non-imaging instruments, or
2. active vs. passive instruments
• Or according to used wavelenght:
– optical (visible and infrared) vs. microwave
Imaging vs. non-imaging instruments
• Imaging instruments measure data over wide area
• Usually, instruments onboard satellites or
airplanes are imaging instruments
• Non-imaging instruments are used for research
porposes
– development of new instruments
– accurate (radiometrically or spectrally) measurements
Passive instruments
• Measure radiation emitted by
target or sunlight reflected from
target
• E.g. cameras, scanners,
radiometers, spectrameters
• Usually utilize visible, infrafed or
thermal infrared wavelenghts
– sometimes microwaves
(kuva: Canada Centre for Remote Sensing)
Active instruments
• Send pulse of electromagnetic
radiation to the target
• Measure backscattered or
reflected radiation, its amount
and possibly how pulse has been
changed
• Microwave radar, laserscanning
(kuva: Canada Centre for Remote Sensing)
Instantaneous Field Of View (IFOV)
• Angular aperture which sensor is
sensitive to electromagnetic radiation
• As incidence angle of instrument
changes
 distance to target changes
 IFOV different in different parts
of image
• Small IFOV  small objects are
visible (good spatial resolution)
• Large IFOV  sensor collects more
radiation (good radiometric
resolution)
Resolution
• Term defining the smallest discernable
physical unit of an observed signal by a
sensor
• Varieties
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spatial
radiometric
spectral
temporal
Spatial resolution
• Called also geometric resolution
• Separation between two measurements in order for
a sensor to be able to discriminate between them
• Size of pixel / IFOV
• Also target and its background influence
– e.g. road in coniferous forest
• Levels:
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Very high resolution: less than 10 m
High resolution: 20 – 50 m
Medium resolution: 200 – 500 m
Coarse resolution: 1 – 50 km
Spatial resolution
• Examples about different instruments vs. American
football field
Spatial resolution, examples
less than meter
tens of metres
kilometer
Radiometric resolution
• The ability of sensor to detect the
incoming radiation and its minor
variations
• Better radiometric resolution  easier
to distinguish between different targets
Radiometric resolution
• 8 bit data  28
= 256 different
values
• 16 bit data 
216 = 65536
diferent values
2 bit
6 bit
Spectral resolution
• How wide area of
spectrum is covered by
instrument
– wavelenghts of channels
– different areas of
spectrum
• How accurately each
channel is measured
– width of channels
Spectral resolution
• Although spectral resolution is poor, it is easy to
distinguish e.g. water and vegetation
– targets are so different
– red and nir channels needed but they can be wide
• If it is needed to distinguish targets which are
more look-a-like, it is needed better spectral
resolution meaning more and narrower channels
– deciduous vs. coniferous forest
– vegetation species
– properties of water, e.g. clean water vs. polluted water
Temporal resolution
• How often new data is available
• Repeat cycle tells when satellite passes over same
place again
• Can be faster, due to overlapping neighboring
orbits same target can be seen from different orbits
• Typical values:
– Geostationary weather satellites: 15 min – few hours
– Sunsynchronous orbit: 16 – 35 days
• In Finland cloudiness, seasons decrease temporal
resolution
Strenght of measured radiation
• How much radiation arrives to sensor
• Flying height: amount of radiation decreases as
distance increases  weaker signal
• Spectral resolution: worse resolution (wider area
of spectrum is measured)  stronger signal
• IFOV: small  good spatial resolution  less
radiation to sensor  weaker signal
• Integration time (time used to average
measurements): large  stronger signal