Before starting, familiarize yourself with the following terms.
- Ascending node: When a satellite moves from the southern hemisphere towards the north, the point on the orbital plane (the imaginary disc passing through the earth with the satellite’s path as its circumference) is known as the ascending node.
- Descending node: When a satellite moves from the northern hemisphere towards the south, the point on the orbital plane is known as the descending node.
Why are the Sun-Synchronous and Molniya orbits important?
Sun-synchronous and Molniya orbits are extremely important for two reasons. They’re different and have very specific applications. Without the Molniya orbit, people in the polar regions, or to be specific, people in Russia, would have had a lot of issues with connectivity. Whereas without the sun-synchronous orbit, it would be very difficult to identify or track certain parts of the Earth under different lighting conditions.
A sun-synchronous orbit is a special type of a low-earth orbit. As the name suggests, it has something to do with syncing up with the sun. In this orbit, the satellite is placed in a way so that the angle between the orbital plane and the line joining Earth and the sun remains constant. Take a look at the image below to understand this arrangement.
What do you think could be the benefit of this orbit? If you notice closely, whenever a sun-synchronous satellite is above, say, New York, the solar time and illumination in NYC will almost always be constant. This means that a satellite in a sun-synchronous orbit will pass any given location on Earth at the same local solar time.
Note that this does not mean that the satellite will pass through that location every day, but whenever it does, the solar time will be the same. Also, it does not mean that the satellite will always be in the sunlight. It will pass through dark regions too.
The advantage of such an orbit is constant surface illumination angles. The shadows and the way objects/regions are lit will always be the same. Hence its applications include imaging, spying, and weather.
How is this orbit achieved?
Our planet revolves around the sun. Every day it moves along by 1 degree in its path around the sun. To maintain a constant angle with the Earth-Sun direction, the satellite also moves by about a degree per day. This is called precession. Precession is achieved by launching the satellite at a particular height and tilt.
In one orbit across the planet, the satellite covers a thin strip of the planet. Due to precession, it covers another thin strip westwards and this gradual shift continues. The distance between successive strips is the tracking interval. When the satellite reaches the first strip, it counts as one orbital cycle. This is the total number of orbital revolutions that the satellite has to take to reach the same point with the same direction of movement.
The sun-synchronous satellites traverse their descending nodes in the sun-lit areas of their path. And ascending nodes in the dark areas. These orbits are also almost polar and slightly retrograde.
As we have seen in earlier posts, polar regions are a bit tricky to cover. You need highly elliptical orbits that stay focussed on the polar area for most of their orbital period, i.e. highly eccentric (from Kepler’s Second Law of planetary motion). The Molniya orbit is a special kind of orbit that achieves exactly these requirements.
Russia and its neighboring countries up in the north frequently use the Molniya orbit. This orbit acts a geosynchronous orbit for this region. It has an orbital period of 12 hours. And out of these 12 hours, it spends 8 hours beaming signals to the north. Subsequently, it is highly eccentric (thin ellipse) and has a perigee of just 400km and an apogee of 40000km. Its eccentricity is 0.75 and it has an orbit inclination of 65 degrees. Three satellites placed at different phases of the same Molniya orbit are enough to provide uninterrupted service to the regions in the north. For permanent coverage, each satellite of the three satellite constellation is active for about 8 hours centered around the apogee.
The satellites in Molniya orbit are quite easy to launch in comparison to their geostationary counterparts. However, the antennas on land need require steerable antennas to keep sight of the satellites.
Check out this interesting video by Scott Manley explaining different orbits: