Satellite orbit of amateur satellite communication

Man-made objects flying around the earth can maintain the balance between centrifugal force and gravity without falling if their height reaches more than 250 kilometers in theory. The orbital height of artificial satellite in practical application can be roughly divided into the following sections: satellite communication has its own characteristics, such as short communication time, narrow available frequency band, Doppler frequency shift and so on. Therefore, in addition to the general communication methods, there are some unique ways of satellite communication, which are introduced as follows:

◆ Analog satellite (voice, code)

FM mode: It is mostly used in repeaters, such as UO- 14, AO-27, etc. The International Space Station (ISS) also uses this mode for voice QSO and packet communication.

SSB mode: Most transponders of amateur satellites have this mode, which uses cross-band (uplink and downlink use different frequencies) for duplex communication. In order to compensate Doppler frequency shift more easily, LSB uplink and USB downlink modulation methods are usually adopted.

CW mode: This is the oldest communication mode, and it also saves bandwidth, but it requires special skills.

Most of the beacons and telemetry of analog satellites also adopt CW mode.

FM Digital Mode: On Satellite? Speech synthesizer? After the computer data on the plane is synthesized into speech, it is transmitted to the ground radio station by frequency modulation, which is similar to broadcasting functions, such as FO-29 and SO-35.

Tone TLM: Small experimental satellites (such as RS- 17) often convert data (such as temperature) into tones to modulate FM carriers for transmission to ground stations.

SSTV mode: In addition to general SSTV communication, if there is a CCD camera on the satellite, the analog satellite can immediately transmit the captured image in SSTV mode. The SSTV experiment has been completed in the previous Mir and space shuttle missions, and the SSTV communication experiment will be conducted regularly on the International Space Station in the future.

◆ Digital satellite (encapsulated communication)

Packet 1200 baud FSK beacon (such as ISS, NO-44, etc.). )

Packet 1200 Potter PSK beacon & broadcast BBS (such as AO- 16, LO- 19, IO-26, etc. )

Reverse 1200 baud FSK beacon (e.g. UO- 1 1)

Packet 400 baud PSK beacon (such as AO-40)

◆ Meteorological satellite (137 MHz, non-amateur frequency band)

American NOAA series (sample of cloud image is as follows)

Before using satellite communication, Russian Meteor Series (pictured below) needs to predict when the satellite will appear in your sky, which needs the help of satellite tracking program. On the Internet, some websites provide the function of tracking satellites online. By entering the latitude, longitude, time zone and the name of the satellite to be tracked, you can know when and where the satellite will appear in your sky.

Without internet connection or further tracking function (for example, the satellite tracking program can simultaneously control the directional antenna to aim at the satellite you want to communicate with), which requires an independent computer and appropriate satellite tracking program, such as:

1. NOVA for Windows

One of the most commonly used programs for amateur satellite enthusiasts all over the world, which can control antenna steering and transceiver frequency compensation, and has very complete functions. This program can track dozens or even hundreds of satellites at the same time, and there is no limit to the number, but it is usually more convenient for users to display satellites in the form of pages. There are two kinds of tracking display methods: ground projection trajectory and space trajectory, and the relative relationship between satellite and ground radio station is clear at a glance. The software (trial version) can be downloaded from the following website:

2.STS Plus

This is a DOS version of the program, but it can be executed in DOS mode of any version of Windows. This software is used by NASA and NSPO ground tracking station in China.

3. Real-time tracking

This is also a DOS version of the program, because it is very powerful, it can control the antenna steering and transceiver frequency at the same time, and it still has many users. The most commendable feature of this program is that it can be executed quickly, even on an old computer. It is called instant tracking, but it is not free and there is no trial version. For those who are interested in amateur satellite communication, the first question when getting started is always: Isn't the satellite communication equipment very complicated? The fact is not necessarily complicated. It depends on which satellite you choose. The most important key is the antenna. The following are some examples of satellite-specific antennas:

1. Hand-held UV dual-frequency Yagi antenna

This antenna can be assembled in two minutes, with light weight and high portability, and is suitable for satellite communication experiments in the field.

2. Ultraviolet cross-shaped antenna

This antenna has a large number of elements and a large volume, and is suitable for being erected in a base station. Through the direction angle and elevation angle controller, the computer automatically tracks the movement of the satellite. Because of the high gain, almost all amateur satellites can be connected.

3. dish antenna (BAR-B-Q rack, aluminum plate)

When the operating frequency of the satellite is above L-band (1.290 GHz-2.401GHz), it is best to use a dish antenna. Because the higher the frequency, the greater the loss of radio wave propagation in the air, while the gain of dish antenna is higher, and the gain is proportional to the diameter of dish antenna, but it should not be too heavy in practical application. It is usually made of aluminum mesh or barbed wire with a diameter of 65433.

4.Moxon antenna

Moxon antenna is a kind of square antenna, which is similar to Yagi, with high gain and strong directivity. After being made according to the size, almost no debugging is needed, and the impedance is 50 ohms. U-shaped cross-section, small antenna size, easy to carry. It is a kind of high gain antenna which is very suitable for self-made and is superior to binary yagi antenna in practical use. It is said that it is very popular abroad. It has an outstanding front-to-back ratio as high as 20 decibels, which means that the signal from the front is very good, and the antenna attenuates the signal or interference from the rear greatly, so the noise is small, and it has application value in radio direction finding. IARU (International Amateur Radio Union) has certain specifications for the frequencies used in amateur satellite communication, and the frequency ranges applicable in different parts of the world are also different. China is located in the third area, and its specifications are: 10m band is 29.30-29.5 1 MHz, and 2m band is145.80-146.00mhz.

As for the details such as frequency and modulation mode of amateur satellites still in operation, they are divided into analog satellites and digital satellites. Although amateur satellite communication is only an extension of amateur radio communication, it can be regarded as an attempt to replace communication mode and the improvement of personal amateur communication ability because it involves satellite tracking and Doppler frequency shift and other related knowledge and equipment. In addition, the reception of real-time satellite images is like extending people's eyes to hundreds or even tens of thousands of kilometers to see the earth in our lives. Perhaps we can have more experience and understanding of the weather closely related to us.