Characteristics, classification and application of satellite orbital inclination

A satellite orbit is the path that an artificial Earth satellite follows as it orbits the Earth in space. The orbit lies in a plane and is elliptical in shape, with its extension greatest at the apogee and smallest at the perigee. According to the laws of physics, as the distance from Earth increases, the satellite's speed in its orbit decreases.

The six orbital elements (elements) of a satellite orbit include the inclination (i), right ascension of the ascending node (Ω), and argument of perigee (w), which determine the orbital position; the eccentricity (e), semi-major axis length (a), and time of perigee (τ), which determine the orbital shape. Based on these six elements, the satellite's position in space at any given time can be determined.

Satellite orbital inclination

Among the elements of a satellite orbit, the angle between the satellite's orbital plane and the Earth's equatorial plane is called the orbital inclination. It is specifically calculated by rotating the satellite counterclockwise from the equatorial plane to the orbital plane during its ascent phase. It is a crucial parameter for determining a satellite's spatial position. Based on the magnitude of the satellite's orbital inclination, several common satellite orbits can be distinguished.

Geostationary orbit：Inclination angle 0 degrees

Geostationary orbit is a circular geosynchronous orbit with an inclination of 0 degrees. It belongs to the same category as geosynchronous orbit, a unique characteristic that sets it apart. The plane of geostationary orbit coincides with the Earth's equatorial plane, ensuring the satellite always flies above the equator.

Geostationary orbit has an altitude of 35,786 kilometers above the Earth's surface, with a satellite orbiting at a speed of 3.07 kilometers per second. A single satellite can cover approximately 40% of the Earth's surface. Meteorological satellites, communication satellites, and broadcast satellites commonly use this orbit. Additionally, some satellites in my country's BeiDou Navigation Satellite System also use this orbit.

Geosynchronous orbit：Tilt angle 0-90 degrees

A geosynchronous orbit is a prograde orbit for artificial Earth satellites, with an orbital period equal to the Earth's rotation period. Regardless of its inclination, a geosynchronous orbit remains synchronized with the Earth's rotation. Because its angular velocity around the Earth is the same as the Earth's rotational angular velocity, it appears stationary to ground-based observers.

The fifth satellite of my country's BeiDou Navigation Satellite System, launched at 5:30 a.m. on August 1, 2010, was in geosynchronous orbit.

Polar orbit：90-degree tilt

A polar orbit has an inclination of 90 degrees, with its plane perpendicular to the Earth's equatorial plane, flying over the North and South Poles. Satellites in this orbit can pass over any latitude and the North and South Poles in each revolution, taking approximately one and a half hours to complete a full rotation. As the satellite enters its orbit, the Earth rotates beneath it.

Polar satellites can observe the entire Earth's surface within 24 hours, making it the most suitable orbit for observing the entire planet. Meteorological satellites, resource satellites, and reconnaissance satellites frequently use this orbit. Near-Earth satellite navigation systems (such as the U.S. Navy's navigation satellite system) use polar orbits to provide global navigation services, and some scientific satellites studying polar physics also use polar orbits.

Sun-synchronous orbit：98-degree tilt

A sun-synchronous orbit is a rather special type of orbit, naturally determined by the Earth's non-spherical shape and uneven mass distribution. A sun-synchronous orbit occurs when the angular velocity of the orbital plane matches the average angular velocity of the Earth's revolution around the sun. The satellite's relative position to the sun remains constant, and the angle of sunlight is the same for all regions the satellite passes through. The satellite passes a point on Earth at almost the same time each time. As the Earth revolves around the sun once a year, to maintain a fixed angle in its orbital plane, the satellite must rotate or precess 360 degrees annually, meaning the orbital plane rotates 0.9856 degrees per day.

A typical sun-synchronous orbit has an altitude of approximately 600-800 kilometers, a period of 96-100 minutes, and an inclination of approximately 98° depending on the eccentricity, to meet mission requirements. Choosing a sun-synchronous orbit ensures that the satellite passes through a designated area at a specific time each day, meaning that the local time (local mean solar time) is the same when passing through the same latitude in the same direction. This is why meteorological and resource satellites typically choose sun-synchronous orbits.

Semi-synchronous orbit：Tilt angle 0-180 degrees

The orbital period of this type of orbit is equal to half the rotation period of the central celestial body it orbits, and the direction of motion of the spacecraft in orbit is the same as the rotation direction of the central celestial body. For Earth, a semi-synchronous orbit is a medium-altitude Earth orbit with a period of approximately 12 hours (since Earth's rotation period is 23h56min4s, the actual period of Earth's semi-synchronous orbit is slightly shorter than 12 hours), and an orbital altitude (circular orbit) of approximately 20,200 km. This places them in the range of medium-altitude Earth orbits, outside of the three types of orbits. The eccentricity of these orbits is close to zero, meaning they are close to circular. An eccentric orbit defines an extended orbit. The closer the eccentricity is to zero, the closer the orbit is to a circle.

Semi-synchronous orbit is a typical GPS satellite orbit.

Reverse track：Inclination angle 90-180 degrees

An inclination angle greater than 90 degrees and less than 180 degrees, where the satellite's direction of motion is opposite to the Earth's rotation, is called a retrograde orbit. To send a satellite into this orbit, the launch vehicle needs to be launched southwestward, which not only prevents it from utilizing part of the Earth's rotational velocity but also requires additional energy to overcome the Earth's rotation. Therefore, except for sun-synchronous orbits, this type of orbit is generally not used.

Today, with the development of technology, the number of artificial satellites launched is increasing year by year. Each satellite has its own specific orbit, and the commonly used non-geostationary orbit resources are also limited. In order to prevent satellites from colliding in space, it is necessary to manage and allocate satellite orbits. To this end, international organizations and individual countries have introduced satellite resource management regulations, which serve as the basic basis and guidelines for coordinating, allocating, and using satellite resources. Adhering to these regulations is a necessary prerequisite for the rational and orderly management of satellite resources.