Polar Orbiting Environmental Satellites (POES)

Due to the rotation of the Earth, it is possible to combine the advantages of low-altitude orbits with global coverage, using near-polar orbiting satellites, which have an orbital plane crossing the poles. These satellites are launched into orbits at high inclinations to the Earth's rotation (at low angles with longitude lines), such that they pass across high latitudes near the poles. Most POES orbits are circular to slightly elliptical at distances ranging from 700 to 1700 km (435 - 1056 mi) from the geoid. At different altitudes they travel at different speeds.

Fig. 1. Example of a Near-Polar Orbit.

The ground track of a polar orbiting satellite is displaced to the west after each orbital period, due to the rotation of the Earth. This displacement of longitude is a function of the orbital period (often less than 2 hours for low altitude orbits).

Fig. 2. Map of the ground path of one revolution of a typical near-polar orbiting satellite.

 

Fig. 3. The orbit of a near polar satellite as viewed from a point rotating with the Earth.
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Depending on the ground swath of the satellite, it is possible to adjust the period (by varying the altitude), and thus the longitudinal displacement, in such a way as to ensure the observation of any point on the Earth within a certain time period. Most of the near polar meteorological satellites ensure complete global coverage of the Earth, during one day, thanks to a ground swath of about 3300 km.

Fig. 4. The ground paths of the multiple orbital revolutions during one day for a near-polar orbiting satellite.

Sun-synchronous polar orbiting satellites

Depending on orbital altitudes, angular velocities, and inclinations, polar orbiting satellites can be sun-synchronous, that is, they cross the equator southbound about 11 deg. westward (as Earth rotates underneath) with each trip around the world (about 105 minutes long), so that they cross some reference position (e.g., the equator) at the same local time. This time is usually between mid-morning and mid-afternoon on the sunlight side of the orbit. Sunsynchronous satellites pass over any given latitude at almost the same local time during each orbital pass. Thus they image their swaths at about the same sun time during each pass, so that lighting remains roughly uniform. Of course the clouds change with each orbit, but their broad patterns and positions remain mostly unchanged in the short orbital periods involved. From this method, we can make a daily mosaic from the swaths, which is a good general summary of global weather patterns for that period. This same orbital configuration applies to Landsat, SPOT, and some of the other land observers. In addition, for a given latitude and season, sunsynchronous satellites observe the Earth surface with a nearly constant sunlight ratio. This characteristic is useful for measurements in the visible wavelengths, and also for thermal considerations in keeping with the diurnal cycle.

Fig. 5. Example of the positions of a sun-synchronous satellite in 12 hour intervals.
Figure courtesy of the National Space Agency of Japan (NASDA)

Sunsynchronous orbits are made possible by the fact that the Earth is not a perfect sphere. A strictly spherical Earth would have an orbit as described by Kepler's Law, so the orbital plane would have a fixed orientation in a fixed stars orientated frame of reference.

Fig. 6. A non sunsynchronous Keplerian orbit as the Earth revolves around the sun.

This is not the case for sun-synchronous satellite orbits. The orbital plane of a sun-synchronous satellite makes a constant angle with the Earth-Sun direction.

Fig. 7. Sunsynchronous satellite orbit as Earth revolves around the sun.

Advantages of sun-synchronous orbits

The low altitude of a sun-synchronous orbit permits good ground resolution. It also enables easier active measurements with radar or lidar. The circular orbit implies a constant satellite velocity, which is important for having a regular scanning resolution along the satellite ground track. The near polar orbit allows a global coverage for the observation of the whole Earth. Orbit altitudes of between 700 and 900 km permits both a large ground swath, offering a daily global coverage, and a good ground resolution. Most of the Earth observing missions use sun-synchronous satellites in low near polar orbits (NOAA polar orbiting meteorological satellites, Landsat, SPOT, ERS, etc...).

Sun-synchronism produces time-constant illumination conditions of the observed surfaces, (except for seasonal variations). This property is useful for many remote-sensing applications in Earth observation. Another property of interest is the nearly constant sunlight ratio of the satellite on each orbit, which implies a near constant solar energy supply for the satellite platform.

Limitations of sun-synchronous orbits

A continuous temporal observation is not possible with only one sun-synchronous satellite. It passes over polar regions on every orbital period, but much more rarely over equatorial regions (2 times a day for most current meteorological satellites; more generally it depends on the drift and the ground swath). A possibility to ease this difficulty could be to use a constellation of satellites. At present this is only envisaged for telecommunication applications and not for Earth observation.