Q 3.1 What types of satellite orbits are used in satellite communication and what are their respective coverages on the Earth?

Orbit selection for satellite communication depends on the desired coverage area and mission objectives (e.g. global coverage or regional, mobile users or fixed). Geostationary orbit is by far the most preferred.

Categorisation
Orbits can be categorised by orbital characteristics – primarily, inclination, eccentricity and altitude.

By eccentricity

When eccentricity is 0 the orbital shape is circular and when eccentricity > 0, the orbit is elliptical. Large eccentricity orbits are sometimes called highly elliptical orbit (HEO).

By altitudeCircular orbits are categorised by altitude as Low Earth Orbit (LEO), Medium Earth Orbit (MEO) [also known as Intermediate Circular Orbit (ICO)] and Geostationary Earth Orbit (GEO).
Taking atmospheric drag as the lower limit and the first Van Allen radiation belt as the upper, a safe altitude range of LEO is about 750-1,500 km. The ideal altitude range of MEO is about 10,000 and 12,000 km, the window between first and second Van Allen belts. GEO is an equatorial orbit, with an altitude of 35786 km.

Numerous variants of LEO, MEO and highly elliptical orbit (HEO) are in use.Coverage
• The maximum geographical area of GEO satellite coverage depends on satellite’s orbital location; the maximum coverage in latitude is limited to about ±81.3o when user is at the same longitude as satellite; the latitude of maximum coverage is lower at other longitudinal offsets;
• Polar orbits can provide full global coverage;
• Coverage from an inclined orbit depends on its inclination favouring regions below the orbit where the coverage belt depends on satellite antenna’s beam-width;
• Eccentric orbit coverage favours areas below their apogee (highest point of orbit);
• A number of satellites are required to provide global coverage for each orbit; GEO requires the least number of satellites for global coverage provided polar region is excluded.

Hybrid constellations utilise a combination of orbit types.Orbit usage (satellite communications)
GEO: a vast majority of operators (e.g. Intelsat – fixed service, Inmarsat – mobile service)
MEO: a few operators (e.g. O3B – fixed service)
LEO: a few operators (e.g. Iridium – mobile service)
HEO: a few operators (XM Radio’s Sirius component – Radio and multimedia broadcast)
Hybrid: Very few operators (XM Radio uses a combination of HEO and GEO)

Note
A majority of satellite navigation systems use MEO (e.g. GPS, GLONASS)
A majority of remote sensing satellite systems use LEO (e.g. Landsat)