Understanding Orbits

Sun-synchronous low Earth orbits

As with consumer cameras, the closer you are to the object you are trying to capture, the more details you can see. Which is how satellites in Low Earth Orbit (LEO) can capture really high-resolution imagery.

To stay at low altitudes without falling back towards the ground and maximize the amount of time passing over areas covered by daylight, Earth observation satellites follow a sun-synchronous orbit, often referred to as a polar orbit or near-polar orbit, going North to South, keeping pace with the Earth’s movement around the sun. For Low Earth Orbits, altitude ranges from 200 km to 1,000 km (120 - 620 mi) and orbital velocity (speed) from ~7.1 km to ~7.8 km per second (4.4 to 4.8 mi/s).

A sun-synchronous orbit takes approximately an hour and a half to two hours before coming back to the same latitude and starting a new ascent. In those 90-120 minutes, the Earth rotated enough to allow the satellite to now fly over a slightly different area of the globe.

This also means:

  1. The local solar time at a specific location will always be the same whenever the satellite passes over. 
  2. It takes hundreds of rotations, and several days for a satellite to fly over the same spot on the globe.

An entire revolution around the Earth, flying back over the same spot on the planet is called the "Temporal resolution".

VISUAL REPRESENTATION

A Low Earth Orbit satellite, flying over the Earth at noon (from the North pole to South pole) and midnight (from the South pole to the North pole).

Source: Pekka Laurila

The tracks covered by the Tropical Rainfall Measuring Mission (TRMM) satellite, which was launched by NASA to monitor rainfall in the tropics. 

Source: NASA

Some satellites do have propulsion mechanisms but these are used to ensure the satellite stays in orbit and correct for height changes or tilts, not to change the course of the satellite. Since the drag of the atmosphere and the tug of gravity from the Sun and Moon alter a satellite’s orbit, it takes regular adjustments to maintain a satellite in a Sun-synchronous orbit.

While the satellite doesn't deviate from orbit, in some cases, they can orientate their sensors to help capture scenes that are slightly off-path, or in satellite lingo, “off-nadir”, nadir being the point right under the satellite. This helps capture more interesting data, or scenes that aren't covered by clouds, in a single temporal resolution.

 

Geo-synchronous orbits 

These satellites orbit the Earth above the equator at an altitude of ~36,000 km. Their orbital velocity is ~ 3 km per second (1.9 mi/s). Geostationary satellites have an orbital period of 24 hours, which allow them to remain fixed around a specific spot on Earth.

Geo-synchronous satellites have a much lower spatial resolution (what can be detected on the ground) but can cover larger swath of the Earth at once. They are most often used to monitor large scale weather patterns and global-scale events. They are also heavily used for communication (phones, television, radio, GPS).

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