What Is a Geosynchronous Orbit?

The geosynchronous orbit has an orbiting man-made satellite orbit equal to the Earth's rotation period (23 hours, 56 minutes, and 4 seconds). When orbital perturbation is not taken into account, satellites operating in geosynchronous orbit pass over the same place on the earth at the same time every day. For the ground viewer, satellites appear in the same direction at the same time every day. A circular geosynchronous orbit with an inclination of 0 is called a geostationary satellite orbit. ^[1]

Chinese name: Geosynchronous orbit
Foreign name: Geosynchronous orbit, Geostationary Earth Orbit or Geostationary Orbit
nickname: 24 hours orbit

Orbital period: 23 hours 56 minutes 4 seconds
Orbit eccentricity: 0
Orbit radius: 42164km
Track height: 35786km

Basic information on geosynchronous orbit

Geostationary Earth Orbit

The orbital period of the satellite is equal to the rotation period of the earth in inertial space (23 hours, 56 minutes, 4 seconds), and the direction is the same. The satellite's sub-satellite point trajectory at the same time every day is the same. The geostationary orbit, that is, the position of the satellite and the ground remains relatively unchanged.

A circular geosynchronous orbit with zero inclination is called a geostationary orbit, because satellites operating in such orbits will always be located over the equator and are stationary relative to the surface of the earth. The orbiting satellite's ground height is about 36,000 kilometers. Its coverage is very wide, using 3 such satellites evenly distributed on the earth's equator, can achieve global communications except a small part of the north and south poles.

To achieve geostationary orbit, the following conditions must be met:

The satellite is moving in the same direction as the Earth's rotation;
Orbit inclination is 0 ° ;
The orbit eccentricity is 0, that is, the orbit is circular;
The orbital period is equal to the Earth's rotation period, and the altitude of the geostationary satellite is 35,786 kilometers.

It is quite difficult and complicated to launch geostationary satellites into geostationary orbit. Due to the limitation of the rocket's carrying capacity and the launch site is generally not at the equator, most of the launch vehicles cannot directly send satellites to the synchronous orbit, and must be divided into three stages to get into orbit.

In the first step, the launch vehicle sends the satellite to a docking orbit 200--300 kilometers above the ground;

The second step is to accelerate the satellite to the tangent of the transfer orbit and the synchronous orbit at the orbiting speed of the parking orbit, that is, the far point of the transfer orbit;

The third step is to ignite the engine at an apogee to bring the satellite into geosynchronous orbit, and use the small engine on the satellite to adjust the attitude of the satellite so that the satellite completely enters into synchronous orbit.

Geosynchronous orbit

A geosynchronous satellite is a geosynchronous orbit satellite, also known as a geostationary satellite. It is an artificial satellite operating in a geosynchronous orbit. The so-called synchronous orbit satellite refers to the satellite's height from the earth to 35786km. The satellite's running direction is the same as the earth's rotation direction. The orbit is a circular orbit on the earth's equatorial plane. The time is equal to 23 hours, 56 minutes, and 4 seconds. The orbiting speed of the satellite in orbit is about 3.07 km / s, which is equal to the angular velocity of the earth's rotation. By laying 3 communication satellites in geosynchronous orbit, global communications can be achieved except for the two poles.

Classification of geostationary satellites

Geosynchronous satellites are divided into geostationary orbit geostationary satellites, inclined orbit synchronous satellites, and polar orbit synchronous satellites. When the inclination of the orbital plane of a synchronous orbiting satellite is zero degrees, that is, when the satellite is operating above the Earth's equator, because the operating direction is the same as the rotation direction of the earth and the operating cycle is synchronized with the earth, people look at the satellite from the earth as if suspended in space Does not move, so the zero-tilt synchronous orbit is called a geostationary orbit, and satellites operating in the geostationary orbit are called geostationary satellites.

Geostationary geostationary satellite

The radio waves radiated by the antenna on the geostationary satellite are basically stable to the coverage area of the earth. In this coverage area, any earth station can realize 24-hour uninterrupted communication. Therefore, geostationary satellites are mainly used for land-based fixed communications, such as telephone communications and television program rebroadcasts, but also for maritime mobile communications. However, they do not have as many base stations as land-based cellular mobile communications, only satellites. It is a large base station. The mobile service switching center is still located on shore (called the shore station). Communication between maritime mobile terminals (that is, ships and ships) can be achieved after two hops of satellites. For example, if A Vessel needs to contact Vessel B. Then, Vessel A sends a signal to the satellite, and the satellite arrives at the mobile service switching center on the shore station. Then, the shore station sends the signal to the satellite, and then arrives at Vessel B through the satellite. . The oblique orbit and polar orbiting satellites are mobile from the earth, but they can pass through specific areas every day. Therefore, they are usually used for scientific research, meteorological or military information collection, and for communication in polar regions and high latitudes.

Geosynchronous orbit

Geostationary satellites are often used in communications, meteorology, radio and television, missile early warning, and data relay to achieve continuous work in the same area. In remote sensing applications, in addition to meteorological satellites, a prominent application is the high-speed transmission of earth resources and the environment obtained by earth observation satellites or space shuttles through four tracking and data relay satellite systems in geosynchronous orbits. Remote sensing data. The first geosynchronous satellite in the world was the "syncom" 3 launched by the United States on August 19, 1964. China launched three geostationary satellites for communication broadcasting on April 8, 1984, February 1, 1986, and March 7, 1988, respectively.

Geosynchronous orbit

Polar orbit: An artificial earth satellite orbit with an inclination of 90 °. Also called polar orbit. Satellites operating in polar orbits can pass over any latitude and over the north and south poles within each circle. Because the satellite can cover a certain area at any position, in order to cover the north and south poles, the orbital inclination does not need to be strictly 90 °, but only around 90 °. In engineering, the inclination angle is often around 90 °, but the orbit that can still cover the world is also called polar orbit. Near-Earth Satellite Navigation Systems (such as the US Navy Navigation Satellite System) use polar orbits to provide global navigation services. Many earth resource satellites, meteorological satellites, and some military reconnaissance satellites use sun-synchronous orbits, and their inclination angles differ only a few degrees from 90 °, so they can also be called polar orbits. Other scientific satellites that study polar physics also use polar orbits.

Geosynchronous orbit launches

The launch of a geosynchronous satellite is difficult and the technology is complex. However, if a country s satellite launch site is built on the Earth s equator, the launch of such a satellite is much simpler. From the west to the east on the equator, the required orbital height is reached, and the problem is solved at a suitable location . Unfortunately, many countries that launch satellites are not on the equator, and it is not possible to establish satellite launch sites on the equator. This brings a lot of difficulties to the satellite launch, and it will take several orbital changes to succeed. Now let's take a look at the following figure to see how the geostationary satellite is launched.

For ease of explanation and understanding, first assume that satellites are launched from the equatorial plane, as shown in Figure 1. When the rocket reaches a certain height after take-off, the first stage rocket stops and automatically separates. The rocket carries the satellite to climb for a period of time, that is, the inertial flight in the picture, and then the final stage rocket fires. When it reaches a certain orbit altitude, the rocket stops. Then look at Figure 2. At this time, the satellite enters an orbit very close to the earth, we call it the initial orbit. When the satellite reaches the apogee of the initial orbit, the engine ignites again to accelerate the satellite to a large elliptical orbit (transfer orbit). At this time, the apogee of the large ellipse orbit is the apogee of the original orbit, and the apogee of the large ellipse orbit is exactly At an altitude of 36,000 kilometers, the final stage of the rocket was separated from the satellite. When the satellite turned to the apogee of the great ellipse orbit again, the apogee engine on the satellite ignited and pushed the satellite into a circular orbit of 36,000 kilometers, which entered the geosynchronous orbit.

Let's look at Figure 3 again, because the launching sites of most countries are not on the equator, in fact, the orbital plane when the satellite is launched and the earth's equatorial plane always have an angle, orbital inclination. It can be seen from the figure that the orbital plane of the satellite does not coincide with the equatorial plane of the earth, so the first important task is to bring the satellite into the equatorial plane, that is, to change its direction of movement. Therefore, before the satellite's apogee engine is ignited, the attitude of the satellite must first be adjusted so that the axis of the engine and the orbital plane form a fixed angle. At this time, when the satellite flies over the equator, the apogee engine just ignites, giving the satellite an additional speed along the axis of the engine, as shown in v1 in the figure, and the satellite has an orbit speed when flying in the original orbit, as shown in the figure. v. These two speeds are combined into a new speed in accordance with the principle of speed synthesis in mechanics. The direction of this new speed is the diagonal direction of the parallelogram composed of two speeds, as shown in v2 in the figure. In this v2, the speed is very critical. The direction of its speed must be exactly along the Earth's equator. At this time, the satellite can fly over the equator. Since then, the satellite has a fixed-point mission, and it has to undergo attitude adjustment and accurate attitude correction. This is because when the apogee engine of the satellite is turned off, various errors occur, and the actual position of the satellite is often inconsistent with the required fixed-point position. Satellite pointing is actually a fine-tuning of the satellite's orbit. In addition to the apogee engine, the satellite is also equipped with small pairs of engines in each specific direction. Different small engines are started according to different errors for orbit control, and the orbit is precisely corrected to bring it closer to the stationary orbit. Stop drifting, at which point the satellite is completely fixed at the intended location. But even if the satellite has been accurately positioned, when the working time is long, the position of the satellite changes due to the influence of the shape of the earth (the earth is not perfectly round), the influence of the geomagnetic field, and the gravity of the sun and even the moon (orbit perturbation) Therefore, from time to time, orbital corrections must be performed, and its status and position must be controlled at any time. This correction is called satellite orbit keeping.

From the introduction above, we can see that launching a geosynchronous orbit satellite is much more complicated than launching an orbit satellite.