Call for communications from spacecraft for sophisticated data transmission techniques

The telemetry, control, management and data processing functions ensure the flow of information between a satellite and the Earth. These involve various transmit and receive functions, the tasks of collecting and processing data to prepare it for transmission, and the routing of control data from the ground station.

A telecommunications control system in space manages all data sent and received, including scientific data and payload operations. The system is connected to radiofrequency (RF) transmitter and receiver units, which are the primary circuitry for data entering or leaving the aerospace system.

An essential function of the telecommunications system is contact with ground stations for control, command, communication and sufficient computer processing power to run all spacecraft subsystems with, in many cases, a high degree of autonomy.

introduction

In the world of satellite communications, several technological trends are emerging, including higher bandwidth, spectrum reuse and the application of complex modulation formats. Higher bandwidths allow higher maximum single-channel data transmission speeds and, at the same time, increase the overall capacity of the system, thus increasing the number of available channels. Several channels are multiplexed together, creating a signal with a higher bandwidth which is sent in a single channel via a single transponder.

The so-called spectrum reuse is a practical technique thanks to the use of transmission systems which emit particularly concentrated beams intended for restricted geographical areas. The possibility of generating this type of signal beam makes it possible to focus more power instantly in the most stressed areas.

All satellite radio communication systems are equipped with antennas for transducing / irradiating electrical pulses in the form of electromagnetic signals to be transmitted over the radio channel. The antennas are located both on orbiting satellites to serve the downlink and on earth stations to serve the respective downlink and uplink sections (to and from the satellite).

Typically, these antennas are directional antennas because they must point towards the terrestrial ground of the satellite towards regions of interest. The intersection of the beam from the irradiating antenna and the surface of the Earth determines an area commonly referred to as the “footprint”. Ground stations typically use parabolic antennas aimed at the satellite of interest, while satellites use horn antennas, phased arrays, or antenna patches.

On the uplink (connection from Earth to satellite), the control system receives and decodes all commands and data for communications system platform and payload operations. On a downlink (from satellite to Earth), the system collects different types of data acquired from subsystems of the platform or generated by scientific payloads and multiplexes them into transfer frames for transmission to the ground.

Satellite communications

Operators use various types of satellite systems for different applications, ranging from one-way broadcasting to two-way data transmission, from navigation to remote sensing or cellular communications. Each system consists of two main parts: the space segment and the earth segment.

The ground section of a typical system consists of at least three sub-parts: the satellite control system, gateways and network concentrators (hubs) and, finally, user terminals. The control system performs Tracking, Telemetry, and Command (TT&C) functions, through which the satellite’s orbital position is determined, and commands are sent to adjust altitude, orientation, and track.

The TT&C functions, if they are bidirectional, also transmit information relating to the state and the operating state of the payload of the satellite. Communications, including voice, video and data, are carried between gateways, hubs and user terminals.

Gateways connect satellite communications to landline telephone systems, cellular networks, Internet service providers and other operators. Hubs, in turn, connect multiple elements such as user terminals or other hubs. In all cases, the ground station transmits uplink signals to the satellite and receives downlink signals from the satellite.

Satellites allow telecommunications between very distant places. The electromagnetic waves used for this type of communications are microwaves and radio waves in the range 1 GHz to 300 GHz, or in the UHF, SHF and EHF bands. The equipment on board the satellite receives the signals, amplifies them, and transmits them to a region of the earth where any receiving station can pick them up.

A typical telecommunications satellite has several repeaters. Each transponder consists of a receiving antenna and a power amplifier. The receiving antenna is tuned to a channel, or range of frequencies, in the uplink in a device that matches those frequencies to the frequency range of the output channel (downlink), while the power amplifier provides microwave output and power (Fig. 1 ). The number of transponders, or channels, indicates the capacity of the satellite. Signals traveling from Earth to satellites must pass through the atmosphere.

Fig. 1: A functional diagram of a transponder. (Source: Maurizio Di Paolo Emilio)

Frequency bands for satellite communications for civilian purposes have been allocated through international agreements. Each band consists of a frequency range for the uplink and a lower frequency range for the downlink due to higher attenuation and atmospheric disturbances. At lower frequencies, rain and atmospheric attenuation have less of an impact on the downlink.

The transmission frequency depends on the system application. The frequency range extends from P-band to Ka-band and beyond:

P-band (0.2–1 GHz)

L band (2–4 GHz)

C band (4-8 GHz)

Ku band (12-18 GHz)

Ka band (26.5-40 GHz)

The power required for satisfactory satellite communications depends on the power required by the receiver on the ground and the fraction of the power emitted by the satellite that is received on Earth. This fraction is equal to the ratio between the area of ​​the terrestrial receiving antenna and the area covered by the microwave beam.

To obtain well collimated beams covering limited areas, the antenna must be large enough on the satellite. Indeed, the width of the microwave beam is proportional to the ratio between the wavelength and the diameter of the satellite antenna.

The strength of the received signal will decrease with increasing transmission distance, so a larger spacecraft in deep space missions almost always uses satellite dishes to be able to focus radio transmissions within a precise directional radius. . Therefore, the spacecraft must be able to point accurately.

The large physical dimensions and the high pointing requirements of a parabolic antenna make it difficult to integrate this antenna with a cubic satellite (CubeSat), for example. The developers looked for alternative solutions, especially for attitude determination and control. An inflatable satellite dish is a proposed solution (2 ).

Fig. 2: Deployable UHF monopole antennas (four) for use on CubeSats. (Source: GomSpace)

One trend that is helping to improve RF communication systems is the development of software-defined radios (SDR). By using Field Programmable Gate Arrays (FPGAs), SDRs have greater flexibility that allows them to be used with multiple bands, filtering and modulation schemes, without many (if any) modifications to the system. equipment. In addition, these features can be changed during flight by loading new parameters from the ground. SDRs are particularly attractive for use on CubeSats because they can be made smaller and more efficient with shrunken electronics that require less power.

For the space market, Microsemi’s RTG4 integrated FPGAs are one example. FPGAs are the company’s fourth generation flash FPGA fabric with high performance interfaces such as serialization / deserialization (SerDes) on a single chip. They find applications in the most difficult radiation environments, such as spaceflight (LEO, MEO, GEO, HEO, deep space), high altitude aviation, medical electronics and nuclear power plant control (3 ).

Fig. 3: A functional diagram of the RTG4 FPGA. (Source: Microsemi)

Optical communication in free space using lasers

The laser is rapidly advancing in satellite communications as a technology of tremendous importance. It offers higher throughput than RF connections and does not require spectrum coordination with other parties. For distances between Low Earth Orbit (LEO) and Geostationary Earth Orbit (GEO), lasers can easily provide gigabit and faster speeds between satellites, serving as high-speed relay connections.

Several broadband companies are planning to use laser crosslinks in their LEO satellite constellations for network routing in space. Until now, laser connections have been attempted by large satellites, but the future shows a clear trend towards miniaturization of satellites to weights less than 10 kg or even less than 1 kg, usually in the form of CubeSats (4 ).

Fig. 4: Laser communication in free space. (Source: NASA)

Conclusion

Space missions and associated communications with Earth require sophisticated data transmission techniques, from antenna configuration to select data processing systems such as FPGAs. New optical technologies with high speed transmissions have been developed for satellites using the power of the laser. There are also new studies which show that the direction of propagation of a laser pulse can be controlled, giving life to the design of new photonic chips consisting of a quantum dot, capable of emitting one photon at a time as a result of the de-excitement. of electrons by a laser.

Related Articles:

Part 1: Power supply systems for space missions require careful consideration

Part 2: Thermal control systems for spacecraft require passive and active design techniques