GEO (Geostationary Orbit)

Understanding GEO

Why a GEO satellite appears to stand still

The trick is resonance with Earth's spin. At a radius of 42,164 km from the planet's centre — 35,786 km above the surface — an orbit's natural period is 23 hours 56 minutes 4 seconds, one sidereal day, the true time Earth takes to turn once relative to the stars. A satellite there travels at about 3.07 km/s, sweeping around the planet at exactly the rate the ground rotates beneath it, so from the surface it seems pinned to a single point. Placed directly over the equator and moving west to east, it never rises or sets. The engineer and writer Arthur C. Clarke popularised the concept in a 1945 paper, and the ring is still sometimes called the Clarke Belt. Just three evenly spaced GEO satellites can cover almost the whole planet, which is why the orbit has long underpinned global broadcasting.

Geostationary vs geosynchronous — what's the difference?

Every geostationary orbit is geosynchronous, but not every geosynchronous orbit is geostationary. A geosynchronous orbit is any orbit whose period matches Earth's rotation; it may be tilted or elliptical. Geostationary is the special case that is also circular and sits exactly over the equator at 0° inclination, so the satellite holds a genuinely fixed position. Give a geosynchronous orbit some inclination and, viewed from the ground, the satellite traces a slim figure-of-eight — an analemma — over 24 hours, wandering north and south of the equator while returning to the same longitude each day. Such inclined orbits, and highly elliptical Molniya orbits, are used deliberately to serve high latitudes that a true GEO satellite, sitting low on the horizon, cannot reach well.

Holding a slot: station-keeping and the ITU

Left to itself, a GEO satellite will not stay put. The Sun and Moon tug on its orbital plane, tilting the inclination by about 0.85° per year, while the slightly out-of-round equator nudges it east or west toward one of two stable longitudes near 75°E and 105°W. Operators fight back with station-keeping manoeuvres of two kinds: north-south burns to cancel the inclination drift, which consume about 45–50 m/s of delta-v a year and dominate the fuel budget, and much cheaper east-west burns to hold longitude, under 2 m/s a year. Usable longitudes are limited and each is paired with radio frequencies, so the ITU coordinates them as orbital slots — with priority for most bands going to the earliest filing — and records every assignment in its Master International Frequency Register. When the propellant nears empty, the satellite is retired above the belt to free its slot.

GEO in context: how the orbit regimes compare

GEO trades proximity for permanence. Sitting far higher than low or medium orbits, a single satellite sees almost an entire hemisphere at once, at the cost of a longer signal delay and a weaker received signal. Here is how the main Earth-orbit regimes compare: