Maa-57.2040 Kaukokartoituksen yleiskurssi

General Remote Sensing

Autumn 2007

Markus Törmä

Markus.Torma@tkk.fi

Lectures: 10 – 12 M21. Tu 18.9. Imaging platforms, satellites, orbits etc. 2. Tu 25.9. Landsat 3. Tu 2.10. TERRA and AQUA -satellites4. 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 radiometry6. Th 18.10. Image enhancement I:

– filtering, arithmetic operations, color coordinates, texture, principal component analysis

7. Tu 23.10. Image enhancement II: – soil-line, vegetation indexes, tasselled cap-transformation,

examples of whole processing chain

Exercise 1Preprocessing 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 1Preprocessing 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 FieldSpec-

spectrometer, 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 platformsHelicopter: • 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:– Attitude control

– Propulsion system

– Electrical power unit

– Temperature control

– Supporting structures

– Telemetry, tracking, command and communications system

– Ground station

SatelliteAttitude 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

SatelliteElectrical 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

SatelliteSupporting 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• Satellite orbits planet using circular or elliptical orbit• Satellite passes planet using parabolic or hyperbolic orbits• Kepler’s laws:

1. 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.

2. 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.

3. 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– 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:– 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 metreskilometer

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