Orbits Necessary for Remote Sensing

An important consideration in remote sensing is designing a sensor to provide the necessary data as well as placing the satellite in the proper orbit. There are different types of orbits which can provide various results including continuous monitoring, global mapping, select coverage, or repeat coverage.

Orbits can range in shape and velocity and can be derived by orbital mechanics. The simplest orbit is a circular orbit, which is most common for Earth observing satellites. For circular orbits, the altitude of the satellite is uniform over the body it is orbiting. Elliptical orbits are commonly used for interplanetary missions since they are less demanding on the spacecraft's propulsion system.

Remote Sensing factors which can determine a particular orbit.
Global coverage for observation
polar orbit
Constant illumination
Thermal Observations
12 hour repeat cycle
Continuous monitoring

For example, the Weather satellite images that you see every night on the 10 o'clock news comes from a geosynchronous satellite (GOES: Geostationary Operational Environmental Satellite). There are two GOES satellites that are continuously monitoring weather patterns affecting the United States. One is positioned over the Pacific Ocean at a longitude of 138 degrees West, while the other is positioned over the Atlantic Ocean at 78 degrees West.

Special Cases of Orbits

Polar orbits are commonly used since guarantee that every spot on the Earth will be covered. Satellites placed in a polar orbit can vary in altitude, depending upon the desired satellite period. The NOAA Polar Orbiters or POES (Polar Operational Environmental Satellite) are in a very low altitude and have a period of approximately 90 minutes. This low altitude allows them to have many repeat cycles per day over a given area.

Sun Synchronous
The sun synchronous orbit is a very important orbit for satellites that use optical sensors. It is desired that these sensors continuously be observing the Earth in daylight. The sun sunchronous orbit allows this to happen since the orbital precession exactly matches the rotation of the Earth. All of the Landsat satellites are in a sun synchronous near polar orbit with an altitude of 800 km and an inclination of 98 degrees.

The geosynchronous orbits is a special case of orbital mechnics. These orbits are special because the period (time it takes the satellite to make one revolution) of the orbit is equal to one day. The altitude of a geosynchronous orbit is a whopping 35,800 km above the Earth. These types of orbits are used when it is necessary for the satellite to maintain continuous coverage over a certain area. Satellites which broadcast TV and telephone signals are all in a geosynchronous orbit.