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en:hf_tx_subsys

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The radio communications system can be included among key systems of the orbiter. However, as is the case with many engineered systems, there is no ideal solution for it. Various designs demand precise trade-offs between data transmission rate and power draw of the orbiter radio equipment as well as between the directional pattern of the orbiter antenna and its efficiency. Every new orbiter design calls for novel approach to the problem of establishing data communications between the orbiter and Earth.

In a run-of-the-mill solution the orbiter would be fitted with low data rate communications equipment for downstream telemetry transfers over the orbiter-Earth link as well as upstream control command transfers over the Earth-orbiter link. As a rule in this case the orbiter is equipped either with an omnidirectional antenna or a system of narrow-beam onboard antennas. A system of this kind enables communications with spacecraft regardless of its orientation and rotation speed; it draws little power but the drawback is insignificant data transfer rate (about 9600 bps).

Another option is to use the orbiter-Earth link with a high-frequency, high data rate transmitter with a narrow-beam antenna to transfer payload information to the Earth. This data transfer mode enables data transfer rates in the order of tens and hundreds of megabit per second. On the flip side, such an architecture is power-hungry and requires precise orientation of the orbiter antenna (and hence the orbiter as a whole) toward Earth for the narrow beam to hit a receiver station on the ground.

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High-Frequency Transmitter

Virtual power draw: 6100 mA

The high-frequency (HF), high data rate transmitter comes in the construction set as a separate module based on a simple LED with particular blinking frequency and pattern. When light from this “transmitter” is detected by a high-frequency receiver on the ground it is considered that the communications link has been established and imagery can be sent from the satellite to Earth (the actual transfer takes place over Wi-Fi).

High-Frequency Receiver

The high-frequency (HF), high data rate receiver is a part of the construction set that simulates the ground receiving station with a high-speed data link. During the experimentation stage similar receivers will be mounted directly on the Earth surface i.e. on the globe. Nevertheless this module should be used for preliminary tests of the “high-speed radio link”.

This module operates in combination with the Mission Control Center (MCC) software. When a “beam” from the orbiter’s HF transmitter hits the ground HF receiver the latter signals communications link establishment to the MCC. It is important that the link is not only established but sustained by stabilizing the “satellite” precisely – only in this case can data be transferred from orbiter’s payload to “Earth”. An important consideration is the quality of the communications link that depends on the precision of pointing orbiter’s HF transmitter to the MCC: displayed in the range of 0 to 100 in the top right in MCC software GUI, it tells whether the image can be transferred successfully.

Considering that the HF link is designed for “transferring” images from the camera, sample code for operating the link can be found in the Earth Observation Camera section.
en/hf_tx_subsys.1579508388.txt.gz · Last modified: 2020/03/25 16:29 (external edit)