SATELLITE ORBITS The monitoring capabilities of the sensor are, to a large extent, governed by the parameters of the satellite orbit. Different types of orbits are needed to achieve different data: Continuous monitoring – meteorological Global mapping- Land cover mapping Selective 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 orbit characteristics are relevant: Altitude Altitude Inclination angle Inclination angle Period Period Repeat Cycle Repeat Cycle Orbit Altitude- Distance between the satellite and the earth surface. Orbit Altitude- Distance between the satellite and the earth surface. The altitude influences the area that can be viewed (Coverage) and the details that can be observed (resolution) LEO the Low earth orbit- 150 to 1000km LEO the Low earth orbit- 150 to 1000km GEO the Geostationary orbit – 36,000km GEO the Geostationary orbit – 36,000km

Orbit Inclination Angle Angle in degrees between the orbital plane and equatorial plane. Angle in degrees between the orbital plane and equatorial plane. It decides together with the field of view the latitude up to which the earth can be observed It decides together with the field of view the latitude up to which the earth can be observed If inclination is 60 0, the satellite flies over the earth between latitudes 60 0 north and 60 0 south. If inclination is 60 0, the satellite flies over the earth between latitudes 60 0 north and 60 0 south. If it is LEO, it can not observe parts of earth at latitude above 60 0 north and 60 0 south. It means it can not observe polar region. If it is LEO, it can not observe parts of earth at latitude above 60 0 north and 60 0 south. It means it can not observe polar region.

Orbital period Time, in minutes, required to complete one full orbit. Time, in minutes, required to complete one full orbit. Orbit period and the mean distance to the centre of the earth are interrelated (Kepler’s third law) Orbit period and the mean distance to the centre of the earth are interrelated (Kepler’s third law) Example: If a polar satellite orbits at 860km mean altitude, Example: If a polar satellite orbits at 860km mean altitude, its orbital period is 101 minutes its orbital period is 101 minutes Its ground speed is 23,700km/h = 6.5km/sec Its ground speed is 23,700km/h = 6.5km/sec Speed of an aircraft is 400km/h i.e. 60 times slower than satellite Speed of an aircraft is 400km/h i.e. 60 times slower than satellite

Repeat Cycle The time, in days, between two successive identical orbits. The time, in days, between two successive identical orbits. The revisit time, the time between two successive images of the same area, is determined by the repeat cycle together with the pointing capability of the sensor. The revisit time, the time between two successive images of the same area, is determined by the repeat cycle together with the pointing capability of the sensor. Pointing capability means possibility of the sensor-platform to look to the side or fore and aft. Pointing capability means possibility of the sensor-platform to look to the side or fore and aft. The sensors mounted on SPOT, IRS and Ikonos have this capability The sensors mounted on SPOT, IRS and Ikonos have this capability

Orbit Types Common for RS missions Polar orbit Polar orbit Sun- synchronous orbit Sun- synchronous orbit Geostationary orbit Geostationary orbit Other orbits Other orbits

POLAR ORBIT Inclination angle between 80 0 and Inclination angle between 80 0 and An orbit having more than 90 degree angle means that the satellite motion is in westward direction An orbit having more than 90 degree angle means that the satellite motion is in westward direction Launching a satellite in eastward direction requires less energy due to eastward rotation of earth Launching a satellite in eastward direction requires less energy due to eastward rotation of earth Such orbit enables observation of the whole globe, also near the poles Such orbit enables observation of the whole globe, also near the poles Typically placed in orbit at 600km to 1000km Typically placed in orbit at 600km to 1000km

Sun-Synchronous Orbit Near polar orbit chosen in such a way that the satellite always passes overhead at the same time Near polar orbit chosen in such a way that the satellite always passes overhead at the same time The inclination angle is between 98 0 and 99 0 to achieve this. The inclination angle is between 98 0 and 99 0 to achieve this. Most sun synchronous orbits cross the equator at mid-morning at around 10:30 hours local solar time Most sun synchronous orbits cross the equator at mid-morning at around 10:30 hours local solar time At this time the sun angle is low and the resultant shadows reveal terrain relief At this time the sun angle is low and the resultant shadows reveal terrain relief In addition to day-time images, this orbit also allows the satellite to record night-time images (thermal or radar) during the ascending phase of the orbit at the dark side of the earth. In addition to day-time images, this orbit also allows the satellite to record night-time images (thermal or radar) during the ascending phase of the orbit at the dark side of the earth. Examples of Polar and SS orbits: Landsat, SPOT and IRS Examples of Polar and SS orbits: Landsat, SPOT and IRS

Polar/Sun Synchronous Orbits Pass roughly over the north and south poles Pass roughly over the north and south poles Fly over the same place on earth at the same time of day (sun always in same position) Fly over the same place on earth at the same time of day (sun always in same position) Examples: Landsat, AVHRR Examples: Landsat, AVHRR Good for land remote sensing Good for land remote sensing Return time depends on Swath Width Return time depends on Swath Width

Swath Width Swath Width

Geostationary orbit ( GEOS, weather, Geosynchronous orbiting earth satellite) The orbit in which the satellite is placed above the equator i.e. inclination angle is 0 0 The orbit in which the satellite is placed above the equator i.e. inclination angle is 0 0 The altitude of placement is 36,000km The altitude of placement is 36,000km At this distance the orbit period of the satellite is equal to the rotational period of the earth, exactly one sidereal day At this distance the orbit period of the satellite is equal to the rotational period of the earth, exactly one sidereal day Used for meteorological and telecommunication satellites Used for meteorological and telecommunication satellites Combination of geostationary orbit and polar orbit are used in meteorology Combination of geostationary orbit and polar orbit are used in meteorology

Other Orbits Lagrangian or Liberation Points: For a satellite circling two massive bodies it appears that there are five points where pull of gravity of two bodies is at equilibrium (L1 to L5). Lagrangian or Liberation Points: For a satellite circling two massive bodies it appears that there are five points where pull of gravity of two bodies is at equilibrium (L1 to L5). In these points the satellite can be positioned at zero velocity with respect to both bodies. In these points the satellite can be positioned at zero velocity with respect to both bodies. L1 point of sun-earth system (1,5 mkm from the earth) is in use by number of solar observation satellites. L1 point of sun-earth system (1,5 mkm from the earth) is in use by number of solar observation satellites.

DATA TRANSFER The data of space borne sensors need to be sent to ground for further analysis. The data of space borne sensors need to be sent to ground for further analysis. Some old systems utilized film cartridge that fell back to a designated area on earth. Some old systems utilized film cartridge that fell back to a designated area on earth. Now practically all earth observation satellites apply satellite communication technology as a downlink of data. Now practically all earth observation satellites apply satellite communication technology as a downlink of data. The data are directly sent down to a receiving station, or to a geostationary communication satellite that transmits the data to receiving stations on the ground. The data are directly sent down to a receiving station, or to a geostationary communication satellite that transmits the data to receiving stations on the ground.

Orbital Characteristics Geosynchronous Polar Sun synchronous