Lec-4 RS Platforms, Obits,Swath

8/19/2019 Lec-4 RS Platforms, Obits,Swath

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ACQUISITION PLATFORMS

• Vehicle or carrier of remote sensing sensors is called

a platform

• Airborne

– Trained pigeons, kites, balloons

– Aircrafts ,( UAV, unmanned)

• Spacecrafts

– Manned

– Un-manned

• Ground –based, ladder trucks

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Remote Sensing Platforms

Ground-based Airplane-based Satellite-based



REMOTE SENSING PLATFORMS

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Current remote sensing systems

• Classified by technology – Active remote sensing

– Passive remote sensing

•

Classified by platforms – Airborne systems

– Spaceborne systems

• Classified by sensors –

Photographic remote sensing – Digital remote sensing



SELECTION OF PLATFORMS

• Geostationary or polar orbiting

– Altitude (determines the ground resolution)

– Inclination

– Attitude

– Coverage- Single or repeat

•

Purpose

• Atmospheric conditions depending on the altitude

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AIRCRAFTS

• First widespread use world war-1 for military intelligence

• A well developed and refined technique

• Currently air crafts flown with several type of sensor

• Flexibility of operation over any desired area at any time

• Most easily available RS data

• Data of higher resolution

• Instrument can be tuned and repaired

• Image can be taken with any solar illumination angle 7



AIRCRAFTS

• Limitations –

– Smaller area coverage

– Higher cost

– Possibility of systematic distortions (roll, pitch, yaw)

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Mid-altitude

U-2 military spy aircraft that flies near 30,000 meters

High-altitude

a remotely piloted aircraft (RPA) flying at a very slow speed

and low altitude of 30 meters

light two-seater aircraft that is capable of flying at a

slow speed and low altitude of 100 meters

Remote Sensing Aircraft



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AIRCRAFTS

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AIRCRAFTS

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Space-borne Remote Sensing

• Space-borne remote sensing systems include:

– Satellites: sun-synchronous or geo-stationary orbiting satellites

or spacecraft

– Space shuttle missions

– Recoverable satellites, manned space stations

– Mostly digital sensors

– Multi-spectral or hyper-spectral scanners, digital cameras

– Synthetic Aperture Radar (SAR)

– Mainly orthographic views

– Mainly regular data acquisition at fixed revisit periods



SPACECRAFTS

• MANNED – Soviet’s Cosmonauts

– Gemini, Apollo

– Space shuttle-the latest

• UNMANNED

– TIROS-1,

– LANDSAT ,

– SPOT, IKONOS

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ADVANTAGES-UNMANNED

• Do not require complex and heavy life support

system

• Possibility of complete global coverage (whole earth)

• Can operate for very long periods subject to

deterioration of equipment

• Repeated coverage

• large area Coverage at lower cost

• Disadvantages

– Atmospheric effects

– Fix repeat cycle16



PLATFORM TYPES

PLATEFORM ALTITUDE OBSERVATION REMARKS

GEOSTATIOANRY

SAT

36000 km FIXED POINT

OBSERVATION

GEOSYNCHRONOUS

METEOROLOGICAL

SAT

CIRCULAR ORBIT

SAT

500 –1000 KM REGULAR OBSERVATION LANDSAT,SOPT

SPACE SHUTTLE 240-350 KM IRREGULAR OBSERVATION

SPACE EXPERIMENT

HIGH ALTITUDE JET

PLANE

100-120KM RECCE- WIDE AREA

INVESTIGATION

LOW ALTITUDE JET

PLANE

500-800M VARIOUS INVESTIGATION-

AERIAL SURVEY

GROUND

MEASUREMENT

CAR

0-30 M GROUND TRUTH CHERRY PICKER

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MISSION CLASSIFICATION

• Earth resource imaging missions

– LANDSAT, SPOT, IKONOS

• Imaging of other celestial objects (planetary

missions)

– HUBBLE SPACE TELESCOPE, APPOLO

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SATELLITE ORBITS

• Path followed by a satellite is referred to as its orbit

• Satellites are placed into orbits tailored to match

– The objectives of each satellite mission

–

The capabilities of the sensors they carry

– A large area is periodically under observation

• The particular type of orbit into which a satellite is deployed is

predetermined by what we require the satellite to observe

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Satellite Orbits

• Normal orbits, based on the assumption that the Earth’s gravitationalfield is spherical

• In fact, satellites actually follow perturbed orbits, due in part to

– Earth’s gravitational field by the Earth’s oblate shape (flattened at

the poles, and bulging at the equator)

– Due to lunar and solar gravity, tides, solar wind, and other

influences

•

Types – Geostationary orbits

– Polar/ near polar Orbits

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SATELLITE ORBITS

• GEOSTATIONARY

– Stationary with reference to Earth at about 36000 km altitude

•

POLAR ORBITING – Orbiting the Earth at about 1000 km

– Generally in the North –South direction

–

Typical for Earth observation• High Inclination orbit

• Low inclination orbit

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GEOSTATIONARY ORBIT

- Circle Earth at same rate as Earth

spins

- Complete one revolution of the

Earth in the same amount of time as

of one rotation of the Earth about

its axis, moving West to East

- Remains stationary with respect to

the Earth surface

- Orbital altitude ~ 36000 km,22



GEOSTATIONARY ORBIT

- View the same portion of the

Earth's surface at all times

– Almost full hemisphere

observation

– Often used in meteorological and

communication satellites

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SUN-SYNCHRONOUS ORBIT(near Polar)

– Satellite observe every point on the Earth at the same local

time of the day (local sun time)

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SUN-SYNCHRONOUS ORBIT(near Polar)

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SUN-SYNCHRONOUS ORBITS

– Designed to ensure that the angle between the orbital

plane and sun remains constant to ensure consistent

illumination conditions

• Altitude <1000 km

• Θ = 37.5◦

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Perigee

Apogee

Orbit



SATELLITE ORBITS

• Descending path on the sunlit side of the earth and ascending on the

shadowed side

• Sensor that depend on reflected solar radiations are programmed to

acquire data only during the descending path

• Time of day and seasons are responsible for the variations in the

characteristics and intensity of light

• Earth observational sat are usually placed in orbits designed to

provide the best illumination and to minimize cloud cover

• One desirable effect of sun-synchronous orbit is that illumination is

similar in images taken on different orbits

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SATELLITE ORBITS

• An early morning crossing time the sun at a very low angle in the sky and would be good for

emphasizing topographic effects but would result in a lot of shadow in areas of high relief.

• A crossing time around noon would have the sun at its highest point in the sky and would

provide the maximum and most uniform illumination conditions. This would be useful for

surfaces of low reflectance but might cause saturation of the sensor over high reflectance

surfaces, such as ice. Also, under such illumination, 'specular reflection' from smooth surfaces

may be a problem for interpreters.

• In the mid afternoon, a phenomenon called solar heating causes difficulties for recording

reflected energy, will be near maximum at this time of day.• In order to minimize between these effects, most satellites which image in the visible,

reflected, and emitted infrared regions use crossing times around mid-morning as a

compromise.

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Satellite Orbits

• Earth observation satellites are placed in orbits designed to

acquire imagery between 9:30 and 10:30 a.m. local sun time—a

time that provides an optimum trade-off between ideal

illumination for some applications and time of minimum cloud

cover in tropical regions

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SATELLITE ORBITS

• Equatorial crossing

– The points where the path crosses the equator

• Orbital inclination

– Is the angle between the sat track and the equator, (Descending path)

– controls the north-to-south latitude extent of the satellite orbit

• Period

– The time a sat takes to complete one orbit, the period increases with

the altitude

– At an altitude of 36000 km, sat has same period as the earth

• Altitude

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SATELLITE ORBITS

• Orbital inclination

– The inclination angle of the orbit determines, together with

field of view of the sensor, which latitudes can be observed.

– If the inclination is 60˚ then the satellite flies over the Earth

between the latitudes 60˚ South and 60˚ North; it cannot

observe parts of the Earth at latitudes above 60˚.

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LANDSAT

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ORBITS VS EARTH ROTATION

• FROM ONE ORBIT TO THE NEXT, THE POSITION DIRECTLY BELOW

THE SAT MOVES A CERTAIN DISTANCE AT THE EQUATOR AS THE

EARTH ROTATE BENEATH IT

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COVERAGE

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ORBITS OVERLAP

To observe the entire globe

2875/159= 18 days

( 20 times a year)

Westward longitudinal shift

159 Km 37

= 2

Tearth

xTorbit

D= distance Earth rotated

between the two orbits

R= Radius of Earth

Torbit = Time to complete one orbit

Tearth =Time earth takes to complete

its one orbit



ORBIT CYCLE

• Orbit cycle is not the same as the revisit period

• Revisit period is important for frequent imaging

requirements

• In near polar orbits, areas at high latitudes will be

imaged more frequently than the equatorial zone due

to the increasing overlap in adjacent swaths as orbit

paths come closer near the poles

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ORBIT CYCLE/PERIOD

• T- Orbit period

• R- radius of the Earth ≈( 6380 km)

• H-Orbital altitude (variable)

• g- Acceleration due to gravity= 0.00918 km/sec

2

)()(

gR

H R H R

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ORBIT CYCLE/PERIOD

• Torbit = Time to complete one orbit

• Tearth =Time earth takes to complete its one orbit

• N= Number of orbits per day

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Torbit

Tearth N



LANDSAT COVERAGE

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SATELLITE SWATH

• Scanning width the

satellite sensors covers is

called the swath

• During the senor’smovement, it sees

certain area on the

surface of earth ,is

referred as swath• it helps determine the

scene size

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SWATH

• As the satellite orbits the Earth from pole to pole, its east west

position wouldn’t change if the earth didn’t rotate

• From the orbit’s pattern it seems that satellite is shifting

westward because earth is rotating west to east beneath it

• This apparent movement allows the satellite swath to cover a

new area with each consecutive pass

• Satellite’s orbit and rotation of the earth work together to allow

complete coverage of earth’s surface after it has completed

one complete cycle of orbit

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Support Subsystems

• All Earth observation satellites require support systems.

• The attitude control subsystem maintains orientation of the

satellite with respect to the Earth’s surface and with respect to

the orbital path

• The orbit adjust subsystem (OAS) maintains the orbital path within

specified parameters after the initial orbit was attained.

•

The OAS also made adjustments throughout the life of the satelliteto maintain the planned repeatable coverage of imagery

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Support Subsystems

• The power subsystem supplies electrical power required to operate all

satellite systems by means of solar array panels

• The thermal control subsystem controls the temperatures of satellite

components

• The communications and data-handling subsystem provides microwave

communications with ground stations for transmitting data to the ground

stations

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Data Transmission

For Aircraft, data is retrieved once it lands, processed and delivery

follows

For Satellite, data need to be electronically transmitted to ground

station, and is done as

Direct Transmission if Satellite and Ground Stations are direct line-

of-sight view of the ground antenna

Data can be recorded on board and transmitted at later time

D T i i



Data Transmission

Data can also be relayed to the Ground Receiving Station through

the Tracking and Data Relay Satellite System (TDRSS)

a series of communications satellites in geosynchronous orbit.

The data are transmitted from one satellite to another until they

reach the appropriate Ground Receiving Station

A network of ground stations has been established all over

the world



Reference/ Reading Assignment

• Chapter 6, Section 6.3, J B Campbell.

• Chapter 6, Remote sensing of the environment-J R Jensen, 2nd

• Handout, Satellite Characteristics-Orbits and Swath

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• Noem ch-3

• Ch=4 Geoinformation

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• Non-Sun-synchronous orbits

• Tropics, mid-latitudes, or high latitude

coverage, varying sampling

• typical altitude 200-2,000 km

• example: TRMM, ICESat

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Satellite Orbits

•

Ideally, all remotely sensed images acquired by satellite would be acquiredunder conditions of uniform illumination, so that brightnesses of features

within each scene would reliably indicate conditions on the ground rather than

changes in the conditions of observation

• In reality, brightnesses recorded by satellite images are not directly indicative

of ground conditions because

– differences in latitude,

– time of day,

– and season

• All these factors may lead to variations in the nature and intensity of light

that illuminates each scene.

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Lec-4 RS Platforms, Obits,Swath