Low Earth Orbit

Satellite Orbits

       There is only one main force acting on a satellite when it is in orbit, and that is the gravitational force exerted on the satellite by the Earth. This force is constantly pulling the satellite towards the centre of the Earth.

       A satellite doesn't fall straight down to the Earth because of its velocity. Throughout a satellites orbit there is a perfect balance between the gravitational force due to the Earth, and the centripetal force necessary to maintain the orbit of the satellite.

The formula for centripetal force is: F = (mv²)/r
The formula for the gravitational force between two bodies of mass M and m is (GMm)/r²

The most common type of satellite orbit is the geostationary orbit. This is described in more detail below, but is a type of orbit where the satellite is over the same point of Earth always. It moves around the Earth at the same angular speed that the Earth rotates on its axis.

We can use our formulae above to work out characteristics of the orbit.

(mv²/r) = (GMm)/r²
=> v²/r = (GM)/r²

Now, v = (2πr)/T.

=> (((2πr)/T)²)/r = (GM)/r²
=> (4π²r)/T² = (GM)/r²
=> r³ = (GMT²)/4π²

We know that T is one day, since this is the period of the Earth. This is 8.64 x 10⁴ seconds.
We also know that M is the mass of the Earth, which is 6 x 10²⁴ kg.
Lastly, we know that G (Newton's Gravitational Constant) is 6.67 x 10^-11 m³/kg.s²

So we can work out r.

r³ = 7.57 x 10²²

Therefore, r = 4.23 x 10⁷ = 42,300 km.

So the orbital radius required for a geostationary, or geosynchronous orbit is 42,300km. Since the radius of the Earth is 6378 km the height of the geostationary orbit above the Earth's surface is ~36000 km.

 A low earth orbit (LEO), or medium earth orbit (MEO) describes a satellite which circles close to the Earth. Generally, LEOs have altitudes of around 300 – 1000 km with low inclination angles, and MEOs have altitudes of around 10,000 km.

A special type of LEO is the Polar Orbit. This is a LEO with a high inclination angle (close to 90degrees). This means the satellite travels over the poles.

      Satellites that observe our planet such as remote sensing and weather satellites often travel in a highly inclined LEO so they can capture detailed images of the Earth’s surface due to their closeness to Earth. A satellite in a Polar orbit will pass over every region of Earth so can provide global coverage. Also a satellite in such an orbit will sometimes appear overhead (unlike a GEO which is only overhead to ground stations on the equator). This can enable communication in urban areas where obstacles such as tall buildings can block the path to a satellite. Lastly, the transmission delay is very small.


       Any LEO or MEO system however, for continuous operation, requires a constellation of satellites. The satellites also move relative to the Earth so widebeam or tracking narrowbeam antennas are needed.


         Low Earth Orbit (LEO) satellites operate in orbits of around 100 km to 1,000 km above the Earth’s surface – much lower than traditional communications satellites – which brings them into frequent radio contact with ground stations. LEOs are used for a variety of civil, scientific and military roles including Earth observation, radar, optical, telecoms and demonstrator.