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Tech Explained: Polar Satellites

Tech Explained

How polar satellites operate and why they are becoming so useful

As the name suggests, polar satellites orbit in a path that closely follows the Earth’s meridian lines, passing within 20 or 30 degrees of the North and South Poles once with each revolution. The Earth rotates to the east beneath the satellite, and the satellite monitors a narrow strip running from north to south.

With each pass, the satellite monitors a section to the west of the previous pass. Each of these ‘strips’ can then be pieced together to produce a picture of a larger area. So, what are these images used for, and why are polar satellites expanding in use?

Uses and advantages

A satellite in polar orbit takes around an hour and a half for a full rotation. As the satellite is in orbit, the Earth is rotating beneath it. As a result, a satellite can observe the entire Earth’s surface over a 24-hour period.

Polar satellites circle at a low altitude, between 200 and 1000 km above the surface, as compared to an altitude of around 35,800 km for geostationary satellites. This means that polar satellites can take much higher resolution images that geostationary satellites.

Their ability to take high-resolution images that cover large areas makes polar satellites especially useful in reconnaissance, but they are also commonly used to provide detailed information about storms and cloud systems, crops and for mapping the Earth. Some communications satellites, such as the Iridium satellite constellation, also use a polar orbit. One reason for this is that their close proximity to the Earth means they can handle high data rates with very low latency.

Drawbacks

The big disadvantage to a satellite in polar orbit is that it cannot continuously communicate with or sense a single spot on the Earth’s surface. Because of this, when they are used for communications, a large number of satellites are needed, and they need to be able to connect with each other so that every satellite has a data path to the target network on the ground. This is why they are often used in a ‘constellation,’ where a large number of small satellites are placed together in orbit.

Sun-synchronous satellites

A related type of satellite is one placed in a Sun-synchronous orbit. This is a nearly-polar orbit in which the satellite passes over any given point of the planet’s surface at the same local mean solar time. For example, it might cross the equator 12 times a day, but at a local mean time of 2:00 pm with each pass.

This type of polar orbit can be used to keep the satellite in constant sunlight, providing power to its solar panels. Because the satellite passes over a given point at the same time each day, the angle of surface illumination is nearly the same each time. This consistent lighting is very useful for innovative remote sensing applications such as mapping the bottom of the sea, determining soil moisture, locating illegal construction, predicting retail earnings by counting cars in mall parking lots, detecting oil spills, and, of course, for spying.

The future of polar satellites

Beginning in 1993, the Indian Space Research Organisation developed the capability to launch polar satellites, including very small satellites, from its Polar Satellite Launch Vehicle (PSLV).

Today, that programme is allowing innovations such as the launch of up to 100 nano satellites at one time, using one launch vehicle, and the launch of small, affordable ‘cube sats’ by universities and schools. Launch vehicles like the PSLV are ushering in a new age of innovation in polar satellites.