Sun-synchronous orbit

Sun-synchronous orbit or a heliosynchronous orbit very important because of its particular importance to satellites intended for remote sensing and military applications. A sun-synchronous orbit is one that lies in a plane that maintains a fixed angle with respect to the Earth-sun direction. In other words, it combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given point of the Earth’s surface at the same local mean solar time.

We can say that the orbital plane in such a case has a fixed orientation with respect to the Earth-sun direction and the angle between the orbital plane and the Earth-sun line remains constant throughout the year. It is shown by the below diagram:

Features of Sun Synchronous Orbits

• The satellite passes over a given location on Earth every time at the same local solar time.
• Thus, it guarantees the same illumination condition, which varies only with seasons.
• The orbit is Quasi-polar in nature and so ensures coverage of the whole surface of the Earth

Every time a sun-synchronous satellite completes one revolution around earth, it traverses a thin strip on the surface of the Earth. During the next revolution it traverses another strip as shown in the diagram.

Frozen Orbits

We all know that Earth is not perfectly round. This means the gravitation is not exactly same at all the places. Apart from that there is gravitational pull from Sun and Moon too, followed by the solar radiation pressure, air drag and so many other forces. In other words, most satellites experience noticeable variations in orbital eccentricity.

But, fortunately, the distorting impacts of these issues can be induced to cancel each other by expert satellite planners. They choose optimum Orbital altitude, inclination, eccentricity and argument of perigee. The satellites whose orbital parameters are controlled by such techniques is said to be in Frozen Orbits.

Thus we can say that:

• Frozen orbit is a Sun-synchronous orbit in which the precession of the orbital plane around the polar axis of the Earth caused by the oblateness of the Earth is utilized to the benefit of the mission by choosing correct orbital parameters.
• The Earth observation satellites ERS-1, ERS-2 and Envisat are all operated in Sun-synchronous “frozen” orbits

Other Orbits

• Super synchronous orbit is a disposal / storage orbit above GSO. From earth, they would seem drifting in westerly direction.
• Sub synchronous orbit is a orbit close to but below GSO and is used for satellites undergoing station, changes in an eastern direction.
• Graveyard orbit is a Supersynchronous orbit where spacecraft are intentionally placed at the end of their operational life.