Basic knowledge of satellite communication

1. Basic concepts and characteristics of satellite communication

Definition: Satellite communication refers to the communication between two or more Earth stations using artificial Earth satellites as relay stations, forwarding or reflecting radio waves. Satellite communication is also one of the forms of space radio communication, and space communication refers to radio communication targeting spacecraft. It has three forms:

(1) Communication between the space station and the earth station; (Direct communication)

(2) Communication between space stations; (Direct communication)

(3) Communication between Earth stations through the forwarding or reflection of space stations. (Indirect Communication)

The third communication method is usually referred to as satellite communication, and when the satellite is a geostationary satellite, it is called geostationary satellite communication. Most communication satellites are geostationary satellites (geostationary satellites: satellites are relatively stationary with the Earth at a certain altitude). A geostationary satellite refers to a satellite whose orbit is within the equatorial plane. The height of the track above the ground is approximately 35800 km (commonly referred to as 36000 km for simplicity).  

Characteristics of geostationary satellite communication

(1) Advantages of geostationary satellite communication

The communication distance is long, and the cost is independent of the communication distance (as long as the transmission between two stations within the satellite beam range is independent of the distance)

B has a large coverage area (three satellites can cover all places) and can perform multiple access communication (one transmitter, multiple receivers)

The communication bandwidth is wide (with a bandwidth of 500M) and the transmission capacity is large

High signal transmission quality and stable and reliable communication lines

Establishing communication circuits with flexibility and good maneuverability (as long as the satellite coverage is available, ground stations can be established for communication)

F can spontaneously collect and monitor

(2) Disadvantages of geostationary satellite communication

The launch and control technology of geostationary satellites is relatively complex (so there are few satellite launches in China).

The polar regions of the Earth are communication blind spots (the orbit is parallel to the equator, and it is impossible to reach the poles in the tangential direction), and the communication effect is poor in the high latitude regions of the Earth. 

There are interruptions in the phenomenon of star eclipses (satellites between the Earth and the Sun) and solar eclipses (Earth between the Sun and satellites). Currently, this phenomenon can be shortened by addressing it

D has significant signal transmission delay (transmission and reception time) and echo interference.  

2. Composition of Satellite Communication System

(1) Composition of Satellite Communication System

Typically, a satellite communication system consists of four main components: an earth station, communication satellites (the first two being the main components responsible for satellite transmission and reception), a tracking, telemetry, and command system, and a monitoring and management system (the latter two providing auxiliary functions such as monitoring satellites and attitude adjustment), as shown in the figure.

(2) Composition of Satellite Communication Lines

The composition of the satellite communication line for two earth stations to communicate through communication satellites is shown in the figure, which is composed of the originating earth station, the uplink and downlink wireless transmission paths, and the receiving earth station.  

3. Satellite communication earth station equipment

Technical requirements for earth stations

Generally speaking, there should be the following requirements for an earth station.  

① The transmitted signal should be wideband, stable, and high-power, capable of receiving weak signals forwarded by satellite transponders (which can be amplified and demodulated).  

② It can transmit signals for multiple services such as telephone, telegraph, fax, as well as high-speed data, television, etc.  

③ Stable and reliable performance, easy maintenance and use.  

④ The construction and maintenance costs should not be too high.  

(1) Performance indicators of earth stations - quality factor (G/T)

G/T is the ratio of the gain G of the earth station receiving antenna to the equivalent noise temperature T of the earth station receiving system. It characterizes the earth station's ability to receive weak signals and is called the quality factor of the earth station.  

(2) Effective radiated power and its stability

In order to ensure the quality of the transmitted signal, it is required that the transmitter of the earth station can transmit a large power, usually several hundred watts to tens of kilowatts, and the transmitted RF signal power is very stable.

(3) Stability of RF frequency

The frequency of the radio frequency signal emitted by the earth station must be very accurate. If there is significant drift, it will not only affect the effective utilization of the satellite repeater frequency band, but also generate intermodulation noise in the satellite repeater.  

(4) Diffusion of Radio Frequency Energy

To reduce intermodulation interference, it is necessary to limit the energy density of the RF spectrum emitted by the earth station when the load is light (i.e. the number of calls is low).

(5) Limitations on interference wave radiation

To prevent interference waves from interfering with satellite transponders and other microwave communication systems, it is stipulated that the intermodulation interference caused by multiple carriers and the total effective omnidirectional radiation power outside the band of the earth station should be less than the limit value.  

Composition of Earth Station

The composition of a standard earth station mainly consists of six subsystems: antenna subsystem, transmitter subsystem, receiver subsystem, communication control subsystem, channel terminal equipment subsystem, and power supply subsystem.

(Fixed stations involve channel terminal equipment subsystems, and in general, products involve transmission systems, reception systems, and antenna systems)

Transmitter subsystem

The transmitter subsystem consists of an up converter (as shown in the above figure, the channel terminal outputs the L-band, while the antenna system's frequency band is Ku/Ka, so frequency conversion processing is required. Therefore, it is called an up converter from the upper part of the above figure), an automatic power control circuit, a transmission wave synthesis device, an exciter, and a high-power amplifier. 

Receiver subsystem

LNA: Only amplifies constant frequency, sometimes a down converter needs to be added

After the satellite repeater passes through the antenna, the signal will be amplified slightly, but if it is not enough, LNB needs to be added for further amplification processing

Due to the low transmission power of satellite transponders, ranging from a few watts to tens of watts, and the low gain of the antenna, the signal arriving at the earth station is extremely weak after a downlink loss of about 200dB.  

The main requirements for the receiver subsystem are:

① The noise temperature is low, and the noise temperature of the receiver subsystem is very low,

Usually only a few tens of Kelvin (K).  

② The working frequency band is wide, generally requiring a bandwidth of 500MHz. 

③ Gain stability.

Low noise amplifier

The low-noise amplifiers used in the microwave frequency band mainly include low-noise transistor amplifiers, field-effect transistor amplifiers, and parametric amplifiers.

Down converter

The microwave signal amplified by a low-noise amplifier needs to be sent to a down converter for conversion to an intermediate frequency, and then amplified by the intermediate frequency before being sent to a demodulator.  

4. Antenna system of the earth station (based on the earth station and combined with the company's products)

The antenna of the earth station is the most distinctive device in satellite communication and is a massive system. Most earth stations use reflector antennas. The characteristics of a reflector antenna are good directionality, high gain, and ease of long-distance transmission of radio waves. There are many classification methods for reflective surfaces, which can be divided into dual reflector antennas (such as dynamic communication and fixed stations, where the beam is reflected to the main surface, reflected to the secondary surface, and then enters the horn) and single reflector antennas (such as static communication, where the horn is at the front end and the satellite beam directly enters the horn through the antenna surface) based on the number of reflective surfaces; According to the feeding method, it can be divided into forward feeding antenna and offset feeding antenna; According to frequency bands, it can be divided into single band antennas and multi band antennas; According to the shape of the reflective surface, it can be divided into flat panel antennas and parabolic antennas.         

Goal: The axis of the Earth station antenna should always be aligned with the direction of the satellite

The fixed satellite earth station system is mainly composed of the antenna feeder subsystem (speakers, main and auxiliary surfaces, duplexers, etc. mainly complete signal transmission and reception), the servo and receiver subsystem (which can be divided into servo subsystem and receiver subsystem when complex, mainly responsible for motion control), and the transmission subsystem (referring to the turntable part, mainly providing reliable support and functional platform). The antenna feeder subsystem completes the signal transmission and reception functions, while the servo and receiver subsystems control the rotation and tracking of the antenna, completing the function of aligning the antenna with the satellite. The transmission subsystem mainly provides reliable support and working platform for the above systems.

Antenna feeder subsystem

A device that radiates or receives radio wave energy

Antenna classification:

Antennas can be divided into long medium wave antennas, short wave antennas (line antennas: mobile phones, routers), and microwave antennas (surface antennas: company products) depending on the frequency band. Different frequency bands have different types, with the former mostly being linear antennas and the latter mostly being planar antennas. Surface antennas include horn antennas, slot antennas, lens antennas, and parabolic antennas. Parabolic antennas are the most commonly used and can be divided into feedforward parabolic antennas and feedback parabolic antennas (double reflector antennas). Backfeed parabolic antennas are all double reflector antennas, which include Cassegrain antennas, Grigory antennas, and ring focal antennas. In order to improve antenna gain and reduce sidelobes, dual reflector antennas can use reflector correction technology.

composition

Composed of main reflector, secondary reflector, corrugated horn, feed system, duplexer, power divider, multi-channel rotary joint, multiple filters and other components

Single pulse tracking: Multi mode generator, combined network (divided into combining branch, differential branch, and then servo system, with high accuracy; however, the feed source system is complex and costly, requiring the use of a single pulse receiver to demodulate both branches)

Directional diagram (signal strength and energy distribution diagram, vertical axis unit dB, horizontal axis unit angle)

Center frequency (as shown in Figure 14.25GHz)

The beamwidth (X2-X1=1- (-1) corresponding to the parabola when y=-6)=2 is 2dB beamwidth

Side lobe (at the point where X=0, the antenna is aligned with the satellite, and the first highest point deviating from this point is the first side lobe, and so on. The further it deviates from X=0, the weaker its ability to receive signals; The larger the main lobe, the more severe the impact on other satellites

Polarization (when the direction of electromagnetic wave transmission is horizontal to the ground, it is horizontal polarization, and when it is vertical, it is vertical polarization. The utilization rate is higher when using two methods. Another concept is cross polarization. The frequency points of horizontal polarization and vertical polarization - the level value of the spectrometer - are different. The point found by using anti polarization (the level difference between the two) is the polarization direction.

structural composition

There are mainly two forms of mature turntables, A-E antenna turntables and X-Y antenna turntables, both domestically and internationally.

A-E device

X-Y device

(1) The structure is small and the weight is light.

The structure is relatively large and the weight is heavy.

(2) High precision that can be achieved

The difficulty of achieving high precision is significant

(3) The antenna cover (if needed) is smaller

Use a larger antenna cover

(4) The polarization form of the antenna remains unchanged during the tracking process

The polarization form of the antenna is variable during the tracking process, and circular polarization or polarization diversity reception must be used to compensate for this disadvantage

(5) Light and small structure, poor wind resistance

More sturdy structure, capable of withstanding larger wind loads

(6) There is a dead zone of about 5-10 degrees above the head

(such as in motion communication)

When the depression angle is low, there are two dead zones of about 10 °, but the axis direction can be appropriately arranged to allow tracking to avoid the dead zones

(7) Requires a large servo drive power

Low driving power

(8) Antenna feeding requires high-frequency slip rings or rotating joints.