Orbital Velocity
Orbital velocity is the speed a satellite must have to remain in orbit around a central body such as the Earth. In satellite communications, the term usually refers to the speed of a communications satellite as it travels around the Earth. Orbital velocity is important because it affects orbital period, satellite visibility, Doppler shift, tracking requirements, handover, constellation design, and the distinction between low Earth orbit, medium Earth orbit, and geostationary systems.
A satellite remains in orbit because its forward motion and the Earth’s gravitational attraction are in balance. Gravity continuously pulls the satellite toward the Earth, while the satellite’s sideways velocity causes it to keep falling around the Earth rather than falling into it. This does not mean that the satellite is free of gravity. On the contrary, gravity is the force that keeps the satellite in orbit. The satellite is in continuous free fall, but its forward motion carries it around the curved Earth.
For a circular orbit, orbital velocity depends mainly on the orbital radius, which is the distance from the center of the Earth to the satellite. A satellite in a smaller orbit must travel faster than a satellite in a larger orbit. A typical low Earth orbit satellite may travel at about 7.5 to 7.8 km/s. A medium Earth orbit satellite travels more slowly, depending on its altitude. A geostationary satellite, much farther from the Earth, travels at about 3.1 km/s. Although this is still very fast, it is slower than a LEO satellite because the gravitational acceleration is weaker at geostationary radius and the required orbital speed is lower.
Orbital velocity is closely related to orbital period. Since a higher-altitude satellite has both a larger orbital path and a lower orbital speed, it takes longer to complete one revolution. This is why LEO satellites orbit the Earth in roughly 90 to 120 minutes, MEO satellites take several hours, and a geostationary satellite completes one orbit in one sidereal day. The special case of geostationary orbit occurs when the satellite’s orbital period matches the Earth’s rotation and the orbit is circular, equatorial, and in the same direction as the Earth’s rotation. The satellite then appears fixed over one longitude.
For elliptical orbits, orbital velocity is not constant. The satellite moves fastest at perigee, where it is closest to the Earth, and slowest at apogee, where it is farthest away. This behavior follows from Kepler's second law, which states that a line from the central body to the satellite sweeps out equal areas in equal times. Highly elliptical orbits, such as Molniya orbits, take advantage of this effect. The satellite moves slowly near apogee and can therefore spend a long time over high-latitude regions, even though it moves rapidly through the lower part of its orbit.
Orbital velocity also affects the apparent motion of the satellite as seen from the ground. A LEO satellite moves rapidly across the sky and may be visible to a user terminal for only a few minutes during each pass. The terminal must track the satellite or use an electronically steered antenna, and the network must hand over to another satellite as the first one moves out of view. By contrast, a geostationary satellite has the same angular rate as the Earth’s rotation and appears stationary to an Earth station, allowing the use of fixed pointing antennas.
Doppler shift is another important consequence of orbital velocity. When a satellite moves toward an Earth station, the received frequency is shifted upward. When it moves away, the received frequency is shifted downward. Doppler shift is particularly significant for LEO systems because the relative velocity between the satellite and the terminal can be high. The effect must be allowed for in modem design, synchronization, frequency planning, and handover procedures. It is much less significant for a geostationary satellite as viewed from a fixed Earth station, although small station-keeping motions can still produce minor frequency variations.
Orbital velocity should not be confused with launch velocity or escape velocity. Launch velocity is the speed a launch vehicle provides during ascent and orbit insertion. Escape velocity is the speed required to leave the Earth’s gravitational influence without further propulsion. Orbital velocity is the speed required to remain in a particular orbit. If the satellite’s speed is too low for the orbit, it will descend to a lower orbit or re-enter the atmosphere. If its speed is increased appropriately, it can transfer to a higher orbit or eventually escape Earth orbit.
In satellite communications, orbital velocity is therefore a central orbital parameter. It explains why low-altitude satellites move quickly, why high-altitude satellites move more slowly, why elliptical satellites change speed during each orbit, and why non-geostationary systems require tracking, handover, and Doppler compensation.