SATELLITE SATELLITE ORBITSORBITS

The monitoring capabilities of the sensor are, The monitoring capabilities of the sensor are, to a large extent, governed by the parameters to a large extent, governed by the parameters of the satellite orbit.of the satellite orbit.

Different types of orbits are needed to Different types of orbits are needed to achieve different data:achieve different data:

Continuous monitoring – meteorologicalContinuous monitoring – meteorologicalGlobal mapping- Land cover mappingGlobal mapping- Land cover mappingSelective imaging- Urban areaSelective imaging- Urban area

What is orbit?

The course of the motion of an object around a primary body in space or the path traced by it is called an orbit.

Elliptic orbit is an orbit with the eccentricity greater than 0 and less than 1.

Our Moon and the planets travel in orbits that are very close to being circular. A circle is a special kind of ellipse.

Orbits of artificial satellites can be elliptical or circular.

In general an orbit is a circular path described by the satellite in its revolution about the earth

For Remote Sensing purpose the following For Remote Sensing purpose the following orbit characteristics are relevant:orbit characteristics are relevant:

AltitudeAltitude Inclination angleInclination angle PeriodPeriod Repeat CycleRepeat Cycle

Orbit Altitude- Distance between the satellite and the Orbit Altitude- Distance between the satellite and the earth surface.earth surface.

The altitude influences the area that can be viewed The altitude influences the area that can be viewed (Coverage) and the details that can be observed (Coverage) and the details that can be observed (resolution)(resolution)

LEO the Low earth orbit- 150 to 1000kmLEO the Low earth orbit- 150 to 1000km GEO the Geostationary orbit – 36,000kmGEO the Geostationary orbit – 36,000km

Orbit Inclination AngleOrbit Inclination Angle

Angle in degrees between the orbital plane Angle in degrees between the orbital plane and equatorial plane.and equatorial plane.

It decides together with the field of view the It decides together with the field of view the latitude up to which the earth can be latitude up to which the earth can be observedobserved

If inclination is 60If inclination is 6000, the satellite flies over , the satellite flies over the earth between latitudes 60the earth between latitudes 6000 north and north and 606000 south. south.

If it is LEO, it can not observe parts of earth If it is LEO, it can not observe parts of earth at latitude above 60at latitude above 6000 north and 60 north and 600 0 south. It south. It means it can not observe polar region.means it can not observe polar region.

Orbital periodOrbital period

Time, in minutes, required to complete one Time, in minutes, required to complete one full orbit.full orbit.

Orbit period and the mean distance to the Orbit period and the mean distance to the centre of the earth are interrelated (Kepler’s centre of the earth are interrelated (Kepler’s third law)third law)

Example: If a polar satellite orbits at 860km Example: If a polar satellite orbits at 860km mean altitude, mean altitude,

its orbital period is 101 minutesits orbital period is 101 minutes Its ground speed is 23,700km/h = 6.5km/secIts ground speed is 23,700km/h = 6.5km/sec Speed of an aircraft is 400km/h i.e. 60 times Speed of an aircraft is 400km/h i.e. 60 times

slower than satelliteslower than satellite

Repeat CycleRepeat Cycle

The time, in days, between two successive The time, in days, between two successive identical orbits.identical orbits.

The revisit time, the time between two successive The revisit time, the time between two successive images of the same area, is determined by the images of the same area, is determined by the repeat cycle together with the pointing capability repeat cycle together with the pointing capability of the sensor.of the sensor.

Pointing capability means possibility of the sensor-Pointing capability means possibility of the sensor-platform to look to the side or fore and aft.platform to look to the side or fore and aft.

The sensors mounted on SPOT, IRS and Ikonos The sensors mounted on SPOT, IRS and Ikonos have this capabilityhave this capability

Orbit TypesOrbit Types

Common for RS missionsCommon for RS missions

Polar orbitPolar orbit Sun- synchronous orbitSun- synchronous orbit Geostationary orbitGeostationary orbit Other orbitsOther orbits

POLAR ORBITPOLAR ORBIT

Inclination angle between 80Inclination angle between 8000 and 100 and 10000

An orbit having more than 90 degree angle means that the satellite An orbit having more than 90 degree angle means that the satellite

motion is in westward directionmotion is in westward direction

Launching a satellite in eastward direction requires less energy due to Launching a satellite in eastward direction requires less energy due to

eastward rotation of eartheastward rotation of earth

Such orbit enables observation of the whole globe, also near the polesSuch orbit enables observation of the whole globe, also near the poles

Typically placed in orbit at 600km to 1000kmTypically placed in orbit at 600km to 1000km

Sun-Synchronous OrbitSun-Synchronous Orbit

Near polar orbit chosen in such a way that the satellite Near polar orbit chosen in such a way that the satellite always passes overhead at the same timealways passes overhead at the same time

The inclination angle is between 98The inclination angle is between 9800 and 99 and 9900 to achieve to achieve this.this.

Most sun synchronous orbits cross the equator at mid-Most sun synchronous orbits cross the equator at mid-morning at around 10:30 hours local solar timemorning at around 10:30 hours local solar time

At this time the sun angle is low and the resultant At this time the sun angle is low and the resultant shadows reveal terrain reliefshadows reveal terrain relief

In addition to day-time images, this orbit also allows In addition to day-time images, this orbit also allows the satellite to record night-time images (thermal or the satellite to record night-time images (thermal or radar) during the ascending phase of the orbit at the radar) during the ascending phase of the orbit at the dark side of the earth.dark side of the earth.

Examples of Polar and SS orbits: Landsat, SPOT and Examples of Polar and SS orbits: Landsat, SPOT and IRSIRS

Polar/Sun Synchronous Polar/Sun Synchronous OrbitsOrbits

Pass roughly over the north and south Pass roughly over the north and south polespoles

Fly over the same place on earth at Fly over the same place on earth at the same time of day (sun always in the same time of day (sun always in same position)same position)

Examples: Landsat, AVHRRExamples: Landsat, AVHRR Good for land remote sensingGood for land remote sensing Return time depends on Swath WidthReturn time depends on Swath Width

Swath WidthSwath Width

Swath

Width

Geostationary orbitGeostationary orbit( GEOS, weather,( GEOS, weather,

Geosynchronous orbiting earth satellite)Geosynchronous orbiting earth satellite)

The orbit in which the satellite is placed above the The orbit in which the satellite is placed above the equator i.e. inclination angle is 0equator i.e. inclination angle is 000

The altitude of placement is 36,000kmThe altitude of placement is 36,000km

At this distance the orbit period of the satellite is equal to At this distance the orbit period of the satellite is equal to the rotational period of the earth, exactly one sidereal the rotational period of the earth, exactly one sidereal dayday

Used for meteorological and telecommunication satellitesUsed for meteorological and telecommunication satellites

Combination of geostationary orbit and polar orbit are Combination of geostationary orbit and polar orbit are used in meteorologyused in meteorology

Other OrbitsOther Orbits

Lagrangian or Liberation Points: For a Lagrangian or Liberation Points: For a satellite circling two massive bodies it satellite circling two massive bodies it appears that there are five points where appears that there are five points where pull of gravity of two bodies is at pull of gravity of two bodies is at equilibrium (L1 to L5).equilibrium (L1 to L5).

In these points the satellite can be In these points the satellite can be positioned at zero velocity with respect to positioned at zero velocity with respect to both bodies.both bodies.

L1 point of sun-earth system (1,5 mkm L1 point of sun-earth system (1,5 mkm from the earth) is in use by number of from the earth) is in use by number of solar observation satellites.solar observation satellites.

DATA TRANSFERDATA TRANSFER The data of space borne sensors need to be The data of space borne sensors need to be

sent to ground for further analysis.sent to ground for further analysis.

Some old systems utilized film cartridge Some old systems utilized film cartridge that fell back to a designated area on earth.that fell back to a designated area on earth.

Now practically all earth observation Now practically all earth observation satellites apply satellite communication satellites apply satellite communication technology as a downlink of data.technology as a downlink of data.

The data are directly sent down to a The data are directly sent down to a receiving station, or to a geostationary receiving station, or to a geostationary communication satellite that transmits the communication satellite that transmits the data to receiving stations on the ground.data to receiving stations on the ground.

Orbital Orbital CharacteristicsCharacteristics

• Geosynchronous

• Polar

• Sun synchronous