Amateur radio satellite

Amateur radio satellites OSCAR 69 and OSCAR 70

Establishment of connection between the American amateur radio stations WA9WUA and NZ3M via OSCAR 7 in voice radio

Amateur radio satellites are satellites or space probes that are built or operated by radio amateurs and use the amateur radio bands for communication . Some of these satellites contain transponders that can be used as relay stations for various operating modes. Others contain experiments or cameras whose data is transmitted to Earth. Some amateur radio satellites orbiting the earth are known as OSCAR ( Orbiting Satellite Carrying Amateur Radio ).

purpose

The amateur radio satellites are used for satellite communication between radio amateurs or for experimental purposes. Early satellites also used frequencies in the 15- and 10-meter bands . Today, most radio traffic takes place in the 2-meter and 70-centimeter bands , but higher amateur radio bands are also used.

Satellite operation requires an optical view of the satellite in almost all cases . The currently active amateur radio satellites mostly fly in low orbits of up to about 1200 km ( Low Earth Orbit ). This enables connections over a few 1000 km during a max. 20 minutes long time window possible. The transmission power of the satellites is typically less than 1 W , due to the energy yield of the solar cells and the radiation options for the power loss in a vacuum . In the 2 m range, reception is also possible with omnidirectional antennas . On the higher bands, due to the shorter wavelength, the effective area of the antennas is smaller and therefore the path attenuation is higher. There reception is practically only possible with directional antennas .

history

Radio amateurs have been involved in space travel since the 1960s . They construct small or medium-sized satellites that are taken along as secondary payloads on commercial or scientific flights. Cubesats , which have a standardized, cube-shaped housing with a side length of 10 cm, have been enjoying great popularity for some time .

The first amateur radio satellite was launched on December 12, 1961 under the name OSCAR 1 , just 4 years after the launch of the Soviet first satellite Sputnik 1 . OSCAR 1 was the first satellite that was launched as a second payload together with another satellite and was nevertheless launched into an independent orbit. Although the satellite only stayed in orbit for 22 days , the project was a great success, with more than 570 radio amateurs in 28 countries reporting their observations on the OSCAR project. Many amateur radio satellites have been launched over the years. These satellites have often contributed to significant breakthroughs in satellite research, as you could safely try out new techniques in an amateur radio satellite, if you fail you have significantly fewer obligations to provide a replacement satellite at short notice and with the radio amateurs you have a large number of competent, free observers .

Innovations include the launch of the first satellite with a voice transponder and the development of store-and-forward digital messaging via satellite.

Usual television satellites have a fixed position in the sky due to their geostationary orbit , which means that the terrestrial antennas can be permanently mounted. All previous amateur radio satellites except for the Es'hail-2 , which was launched in 2018, have different orbits, i.e. they move across the sky. Accordingly, complex antenna tracking is necessary.

The largest project carried out by the international association AMSAT was the OSCAR 40 satellite , which only allowed limited operation for a short time due to technical problems. Simpler satellites, such as OSCAR 7 or OSCAR 10 , are still ready for use after decades under favorable conditions. Usually the service life of amateur radio satellites is limited by the accumulators. Cell short-circuits silence a satellite, while interruptions u. May allow limited operation with electricity directly from the solar cells.

To date, well over 100 amateur radio satellites have been launched.

Communication via amateur radio satellites

Amateur radio satellites enable z. B. intercontinental voice and data radio. In addition, most amateur radio satellites transmit measured values of the operating data of the satellite, such as B. the supply voltage , CPU utilization and temperature or from flying experiments or images from an outdoor camera. Current satellites enable radio amateurs to operate in many modes, e.g. B. FM -Sprachübertragung, SSB -Sprachübertragung, CW - telegraphy , SSTV -Standbildübertragung well as digital communications, for example, AX25 - U ( Packet Radio ) or PSK-31 .

Fashion labels

Historically, the uplink (transmission direction to the satellite) and downlink (reception of the satellite) bands were coded with simple letters:

Mode A: 2 m / 10 m
Mode B: 70 cm / 2 m
Mode J: 2 m / 70 cm

New uplink and downlink designations use the combination of two letters with the structure X / Y , where X is the uplink band and Y is the downlink band.

Designation:	H	T	V	U	L.	S.	C.	X	K	Q
Amateur radio band:	15 m	10 m	2 m	70 cm	23 cm	13 cm	6 cm	3 cm	1.2 cm	6 mm

The exact times at which a mode is in operation and whether a voice or a data transponder is switched and which area of the amateur radio band assigned to the amateur radio service via satellites is used is announced for each satellite individually.

Doppler shift

The Doppler effect resulting from the large orbital velocity of the satellite, the uplink - and downlink - frequencies change as the ground station during a flight. As the satellite moves towards the ground station, the downlink frequency appears higher and therefore the receiver must receive above the actual frequency. On the other hand, the satellite receives the uplink signal at a higher frequency than the ground station sent, so the ground station must transmit at a lower frequency in order to be received by the satellite. After the satellite has passed the location of the ground station, i.e. it has moved away from the viewer, the transmission frequency must then be higher; the reception frequency can be set lower.

The result is that the ground station is received by the satellite on the same frequency at all times. This is the only way to communicate via orbiting satellites, as the frequency offset is different for each ground station.

The following mathematical formulas relate the speed of the satellite to the working frequency of the satellite:

Corresponds to:
${\ displaystyle f_ {d}}$	=	Doppler corrected downlink frequency
${\ displaystyle f_ {u}}$	=	Doppler corrected uplink frequency
${\ displaystyle f}$	=	Original frequency
${\ displaystyle v}$	=	Speed of the satellite relative to the ground station in meters / second Positive when approaching, negative when moving away
${\ displaystyle c}$	=	Speed of light in a vacuum ( meter / second) ${\ displaystyle \ approx 3 \ times 10 ^ {8}}$

Frequency change	Downlink correction	Uplink correction
${\ displaystyle \ Delta f = {\ frac {f \ cdot v} {c}}}$	${\ displaystyle f_ {d} = f + {\ frac {f \ cdot v} {c}}}$	${\ displaystyle f_ {u} = f - {\ frac {f \ cdot v} {c}}}$

Since it is very difficult to determine the relative speed of the satellite and the speed with which this correction has to be carried out is very high, these calculations are usually made by special software for tracking the satellites. Usually the antenna system is also carried along with the satellite and the frequency in the radio is adjusted via a CAT interface . Manual correction of the frequency offset is possible, but it is difficult to stay on the same effective frequency. FM is more tolerant of frequency offset than SSB , so operation via FM satellites is much easier.

Numbering and outlook

Many orbiting satellites are assigned an OSCAR number after they have been commissioned ( see List of OSCAR satellites ). Russian and Soviet satellites traditionally have an RS number.

With the P5A project , AMSAT wants to send an amateur radio probe to Mars for the first time under German leadership. The transmission of scientific data and research into radio-technical networks should be the focus here. Other amateur radio satellites that have left earth orbit were, for example, the German Manfred Memorial Moon Mission from 2014 (flyby of the moon) or the Japanese probes UNITEC-1 , Shin'en-2 and ARTSAT2-DESPATCH (solar orbit).

Web links

Commons : amateur radio satellites - collection of images, videos and audio files

Amateur radio satellite in the amateur radio wiki of the German Amateur Radio Club and the Adacom Association for Amateur Data Radio e. V.
Project Mars (AMSAT)
Overview of amateur radio satellites (AMSAT)
RS0RS SuitSat ( Memento from May 31, 2016 in the Internet Archive )
Amateur radio on board the International Space Station ( NASA )
NASA J-Track Real Time Amateur Radio Satellite Tracking (NASA)
Practical introduction to operation via amateur radio satellites
Sounds from Space: sound recordings from amateur radio satellites