What do you mean by Sun Synchronous Orbits (SSO)?

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Sep 9, 2022

Sun Synchronous Orbits are earth-centered orbits which has an altitude of 600-800 km above the earth’s surface and an inclination of about 20-30 degrees from the north-south poles, with the satellites traversing this orbit being synchronous with the sun.

SSO Orbit is synchronized with the sun, which keeps the satellite in the orbit, exactly aligned with the position of the sun. This allows satellites to make observations of a specific region at a fixed time in a day. The below illustration shows 3 consecutive orbits of an SSO Satellite with an equatorial crossing time of 10:30 am for different regions.

A satellite in a sun synchronous orbit (SSO) completes around 15-20 orbits per day from the north to south poles which indicates that they move at an incredibly fast rate around the earth. As the satellite in an SSO passes through the equatorial line always at the same time during the day, reconnaissance, imaging, and weather forecasting can be carried out seamlessly due to surface illumination by the sun. 

For a satellite to remain in this orbit, it needs to travel at a speed of 17,500 miles per hour (7.8 kilometers per second). This speed which is required for a satellite to maintain its orbit around the earth is known as orbital velocity. 

If the satellite’s orbital velocity is higher than the optimal value, it faces the risk of flying out of orbit and into space, rendering the satellite out of bounds. Additionally, If the orbital velocity is kept too low, the satellite will be pulled back to earth due to gravity causing it to crash and burn. Hence, if the correct orbital velocity is maintained the gravity of the earth balances the inertia of the satellite, pulling it down to the earth’s surface just enough to enable the satellite to traverse in its intended orbit. At higher altitudes, the speed required to keep a satellite in orbit changes. The speed of the satellite in an orbit is inversely proportional to the altitude from the earth’s surface.

Launching a Satellite in the SSO orbit requires a larger launch vehicle because the satellite cannot take advantage of the earth’s rotational velocity as it needs to be placed over the equator at an inclination of 20-30 from the north-south poles. The satellites operational in the polar orbit, under GNSS are Aqua, Aura & Cartosat-2A.

Key parameters of a Polar Orbit:

Parameters

SSO Orbit Attributes

The altitude of SSO orbits From Earth

600 – 800 Km

Orbital Velocity for a satellite in a Polar orbit

17,500 mph (7.8 km/s)

Orbits Completed in a Day for a Satellite

15-20 Orbits per day

Satellite Life

3-7 Years

Propagation Loss

Low

Network Complexity

Complex

Broadband Capability

Poor