Satellites orbit Earth at varying distances, with their altitude chosen based on specific functions and orbital properties. This placement optimizes performance for diverse applications, as a satellite’s altitude directly influences its speed, coverage area, and operational lifespan.
Satellites in Low Earth Orbit
Low Earth Orbit (LEO) satellites are positioned relatively close to Earth, typically from 160 kilometers (100 miles) up to 2,000 kilometers (1,200 miles) above the surface. This proximity requires them to travel at high speeds, averaging around 7.8 kilometers per second (17,500 mph), completing an orbit in approximately 90 minutes.
LEO’s characteristics benefit applications like detailed Earth observation, where closer proximity allows for higher-resolution imagery. The International Space Station (ISS) operates within LEO at about 400 kilometers (250 miles). Global internet constellations, such as Starlink, also use LEO, with satellites typically orbiting around 550 kilometers (342 miles). The rapid movement of individual LEO satellites necessitates large constellations for continuous telecommunications coverage.
Satellites in Medium Earth Orbit
Medium Earth Orbit (MEO) occupies the space between LEO and Geostationary Orbit, with altitudes ranging from 2,000 kilometers (1,243 miles) to 35,786 kilometers (22,236 miles) above Earth’s surface. This region primarily houses navigation satellite systems. Global Positioning System (GPS) satellites are a prime example, orbiting at about 20,200 kilometers (12,550 miles).
GPS satellites complete two orbits around Earth daily, resulting in a 12-hour orbital period. This configuration allows for global coverage, providing accurate positioning and timing services. While navigation is the main function, MEO can also accommodate some communication satellites.
Satellites in Geostationary Orbit
Geostationary Orbit (GEO) is a specific type of geosynchronous orbit located at 35,786 kilometers (22,236 miles) above Earth’s equator. Satellites in GEO appear stationary from the ground because their orbital period perfectly matches Earth’s rotational period, which is one sidereal day (approximately 23 hours, 56 minutes, and 4 seconds).
Satellites in GEO travel at about 3.07 kilometers per second (1.91 miles per second). This speed keeps them above a fixed point on the equator. GEO satellites are widely used for television broadcasting, weather monitoring, and long-distance telecommunications, as their fixed position allows ground antennas to remain pointed in a constant direction. A single geostationary satellite can provide continuous coverage to about one-third of Earth’s surface, with three satellites capable of covering nearly the entire planet, excluding the extreme polar regions.
The Trade-Offs of Satellite Altitude
The choice of a satellite’s altitude involves several engineering and operational considerations. Signal latency, the time it takes for a signal to travel, is directly affected by altitude. Higher orbits, such as GEO, result in greater latency, with round-trip delays reaching around 600 milliseconds. LEO satellites offer lower latency, with delays as low as 5.2 milliseconds.
Coverage area also varies with altitude; higher orbits provide a wider view of Earth. A single GEO satellite can monitor a vast region, reducing the number of satellites required for broad coverage compared to a LEO constellation. However, communicating with satellites at greater distances necessitates more powerful transmitters and receivers, increasing energy demands for higher-orbiting spacecraft.
Launch costs are another factor, as reaching higher orbits requires more energy and expense. GEO satellites are typically more costly to launch than those destined for LEO.
Orbital lifetime and space debris management also differ. LEO satellites experience atmospheric drag, leading to natural deorbiting within a few years or decades. The ISS, for example, requires periodic reboosts to maintain its altitude. In contrast, higher orbits like MEO and GEO experience minimal atmospheric drag, allowing defunct satellites to remain in orbit for centuries, contributing to long-term space debris concerns.