Satellite anatomy 101
Since the meeting at Electronica, I have had the same discussion with two other companies, and several readers of Out-of-this-World Design have also emailed asking if I could help them understand the 'anatomy' of a satellite and the basic principles of satellite communications.
Satellites are effectively radio relays that operate in real time over a given footprint above the Earth's surface. This bird's-eye view allows them to see larger areas of the planet at any one time, enabling spacecraft to receive and transmit more information than terrestrial, ground-based communications.
A satellite-communication system comprises space, control, and ground segments, where the former contains orbiting spacecraft organized into a constellation and the latter comprises fixed base stations or hubs, as well as mobile customers such as ships and aircraft. The control segment includes all the ground facilities for monitoring the satellites, as well as managing traffic and on-board resources.
A forward communication link is from a ground hub to a fixed or mobile customer, and comprises both an uplink from the base station to the satellite and a downlink from the spacecraft back down to the user.
A return communication link is from a fixed or mobile customer to a ground hub, and comprises both an uplink from the user to the satellite and a downlink from the spacecraft back down to the base station. The video below shows the concept of a mobile-services, telecommunication satellite capable of transceiving directly from mobile customers and indirectly via fixed, ground stations.
For telecommunication or broadcast applications, RF, uplink carriers from the ground are captured in a set frequency band with a given polarization, amplified by the repeater electronics, and then changed to a downlink frequency before being sent back down to a specific region on the Earth's surface.
The repeater usually includes several channels, also called transponders, which are allocated to specific sub-bands within the overall, frequency spectrum. Traditionally, analog bandpass filters have been used to channelize the input bandwidth, but an increasing number of telecommunication operators use on-board, digital processors to offer more flexible channelization and routing options to dynamically match link capacity to varying traffic demands. The payload still remains a transparent repeater as the uplink carriers are not demodulated.
The performance of the repeater receiver is characterised by the G/T figure-of-merit, where, G, is the antenna receive gain and, T, the system-noise temperature comprising all the noise sources in the uplink. The operation of the transponder transmitter is measured by its Effective Isotropic Radiated Power (EIRP) which is the power fed to the antenna multiplied by its gain.
A satellite consists of a payload and a platform with the former containing a repeater and the receiving and transmitting antennas. The platform consists of all the subsystems necessary to operate and power a spacecraft and the following figure shows a typical bus structure.
The platform or service module contains all the subsystems necessary to support and power the spacecraft and typically includes the following:
- The Electrical Power Subsystem manages the charge-discharge cycle of the batteries to ensure the required amount of power is generated and distributed reliably and efficiently to the various subsystems during all stages of a mission.
Satellites use solar arrays to generate power when exposed to direct radiation from the sun or the albedo reflection from Earth. During each orbit, energy is collected from the solar panels, stored in batteries, and distributed to the spacecraft to power the various subsystems.
When the satellite passes through the shadow of the Earth, the energy harvested from the solar cells is insufficient to supply the spacecraft's systems and power is taken from the on-board batteries during this eclipse. During each orbit, the batteries are replenished when the spacecraft is directly exposed to solar radiation and discharged when the satellite enters the Earth's shadow.
- The Attitude Control Subsystem senses the orientation of the spacecraft sustaining a stable trajectory and ensuring the antennae are accurately pointing towards Earth for communications. The Propulsion Subsystem contains the fuel tanks and the rockets that, when directed by the Attitude Control Subsystem, are fired to maintain the correct orbit.
- The Telemetry, Tracking, and Command (TT&C) Subsystem receives control signals from the ground to initiate spacecraft manoeuvres or to configure or change the state of the payload instruments. The TT&C unit also sends back telemetry information to the ground about the state of the spacecraft including measurement data from equipment and sensors.
- The On-Board Data Handling Subsystem comprises a central computer responsible for all spacecraft housekeeping, data processing and formatting, together with traffic and time management. Figure 2 shows the payload and platform for the Alphasat spacecraft prior to mating.