Non-synchronous Orbit
A non-synchronous orbit is any orbit in which a satellite’s orbital period is not synchronized with the rotation period of the Earth. In satellite communications, the term is most often used to distinguish satellites that move relative to the Earth’s surface from geostationary or geosynchronous satellites, whose orbital period is approximately one sidereal day. A satellite in a non-synchronous orbit does not remain fixed over one point on the Earth and is seen by an Earth station as moving across the sky.
Low Earth orbit (LEO), medium Earth orbit (MEO), and most highly elliptical orbits are non-synchronous orbits. A LEO communications satellite may complete one orbit in roughly 90 to 120 minutes. A MEO satellite may take several hours. A highly elliptical satellite may have a period chosen for a particular coverage pattern, such as a 12-hour Molniya orbit, but unless that period matches the Earth’s rotation in the required way, it is still non-synchronous. By contrast, a geostationary satellite has an orbital period equal to the Earth’s sidereal rotation period and appears fixed above the equator.
The most obvious feature of a non-synchronous satellite is apparent motion. From the viewpoint of a user terminal or Earth station, the satellite rises above the horizon, moves across the sky, and eventually sets. The time during which the satellite is visible is called the pass duration or access time. A single non-synchronous satellite can therefore provide service to a particular location only for part of each orbit, unless the satellite is in a special orbit with a long dwell time over the region of interest.
Because a single non-synchronous satellite cannot normally provide continuous service to one location, communications systems using these orbits often require constellations. As one satellite moves out of view, another satellite must come into view and take over the connection. This process is called handover or handoff. The constellation must be designed so that enough satellites are visible at the required elevation, with sufficient capacity and suitable geometry, to maintain service. The number of satellites needed depends on altitude, inclination, beam coverage, minimum elevation, traffic demand, and whether the system provides regional or global coverage.
Non-synchronous orbits offer important advantages. Lower-altitude satellites have much shorter path lengths than geostationary satellites, which reduces free-space path loss and propagation delay. This is one reason LEO and MEO systems are attractive for broadband internet, voice, interactive data, and low-latency services. Non-synchronous orbits can also provide better coverage at high latitudes than geostationary satellites, which appear low on the horizon or are not visible at all near the poles. Inclined LEO and MEO constellations can therefore serve polar, maritime, aeronautical, and remote-area users more effectively.
However, non-synchronous systems are more complex in several respects. User terminals and gateways may need to track moving satellites, compensate for Doppler shift, manage changing path loss, and perform handovers. The network must know satellite positions accurately and schedule beams, frequencies, and capacity as the geometry changes. If the system uses inter-satellite links, routing must also adjust as the constellation topology changes. These requirements make non-synchronous systems more dynamic than traditional geostationary satellite systems.
Doppler shift is a particularly important issue in non-synchronous orbits. Because the satellite moves relative to the Earth station, the received frequency may be shifted upward as the satellite approaches and downward as it recedes. The effect is greater for lower orbits and higher frequencies. Modems and network control systems must account for this frequency variation to maintain synchronization and avoid interference.
Non-synchronous satellites may use fixed beams, steerable beams, or electronically formed beams. In LEO broadband systems, beams may sweep across the Earth, be steered toward users, or be handed from satellite to satellite. Gateways may also need multiple tracking antennas or electronically steered arrays so that they can maintain simultaneous links with several satellites. In some systems, inter-satellite links allow traffic to be routed through space to another satellite with access to a gateway or destination region.
Non-synchronous orbit should not be understood as inferior to synchronous orbit. It is a different architectural choice. Geostationary systems provide continuous coverage from a fixed apparent position, which simplifies Earth station pointing and broadcast delivery. Non-synchronous systems provide lower delay, often lower path loss, improved high-latitude coverage, and flexible constellation design, but require more satellites and more complex network control.
In satellite communications, a non-synchronous orbit is therefore an orbit in which the satellite moves relative to the Earth’s surface rather than remaining fixed in the sky. It is central to modern LEO and MEO communications constellations and is one of the main reasons such systems differ so much from traditional geostationary satellite networks.