Orbital Altitude
Orbital altitude is the height of a satellite above the surface of the body it is orbiting. In satellite communications, the term normally refers to the height of a satellite above the Earth’s surface. It is therefore sometimes called orbital height. Orbital altitude is one of the most important characteristics of a satellite system because it strongly influences coverage, path loss, propagation delay, orbital period, satellite speed, antenna pointing, launch requirements, and the number of satellites needed to provide continuous service.
Strictly, orbital altitude is measured above a reference surface, usually the Earth’s mean sea level or an idealized Earth radius. This is different from orbital radius, which is measured from the center of the Earth to the satellite. For example, a geostationary satellite has an altitude of about 35,786 km above the equator, but its orbital radius is about 42,164 km from the center of the Earth. The distinction is important in orbital mechanics because Kepler's laws use the distance from the center of the attracting body, not just the height above its surface.
Satellite orbits are commonly grouped by altitude. Low Earth orbit (LEO) satellites operate relatively close to the Earth, typically from a few hundred kilometers to about 2,000 km in altitude. Medium Earth orbit (MEO) satellites operate above LEO but below geostationary altitude, often in the range of several thousand to about 20,000 km. Geostationary Earth orbit (GEO) is a special circular equatorial orbit at approximately 35,786 km altitude, where a satellite appears fixed over one longitude on the Earth’s equator. Highly elliptical orbits do not have a single constant altitude; instead, they have a low-altitude perigee and a high-altitude apogee.
Altitude has a direct effect on coverage. A satellite at a higher altitude can see a larger portion of the Earth’s surface because the horizon is farther away. A geostationary satellite can cover roughly one third of the Earth’s surface, although practical coverage is limited near the poles and at very low elevations. By contrast, a LEO satellite sees a much smaller region at any instant. Consequently, LEO communications systems require constellations of many satellites to provide continuous regional or global coverage.
Altitude also affects signal strength. The farther the satellite is from the Earth station, the greater the free-space path loss. GEO links therefore require higher antenna gain, higher satellite power, or more sensitive receivers than comparable lower-altitude links. However, GEO satellites have the advantage of appearing fixed in the sky, allowing Earth stations to use fixed pointing antennas. LEO satellites have lower path loss and lower delay, but they move rapidly across the sky and require tracking antennas, electronically steered antennas, or handover between satellites.
Propagation delay is another important consequence of orbital altitude. A signal traveling to and from a geostationary satellite must cover a very long path, giving a one-way delay of roughly 120 ms and a round-trip delay of about 240 ms before allowing for terrestrial routing and processing. LEO and MEO systems have much shorter delays because the satellites are closer to Earth. This makes lower-altitude systems attractive for interactive broadband, voice, gaming, and other delay-sensitive services.
Altitude determines orbital period through Kepler's third law. A satellite in a higher orbit takes longer to complete one revolution. A typical LEO satellite may orbit the Earth in about 90 to 120 minutes, while a geostationary satellite completes one orbit in one sidereal day. This relationship explains why only a particular altitude, combined with a circular equatorial orbit and motion in the same direction as the Earth’s rotation, produces a geostationary satellite.
Orbital altitude is therefore not merely a descriptive property. It is a central design parameter that shapes the architecture, cost, performance, and applications of a satellite communications system.