Communications Subsystem

The communications subsystem is responsible for ensuring telecommunication between the satellite and another system, which may be either another satellite or a ground station. The signals used to interchange data are nothing but electromagnetic pulses molded or manipulated by the transmitter in such a way that contains information that the receiver can understand. It provides the interface between the spacecraft and ground systems.

Payload mission data and spacecraft housekeeping data pass from the spacecraft through this subsystem to operators and users at the operations center. Operator commands also pass to the spacecraft through this subsystem to control the spacecraft and to operate the payload. As for the communication subsystem itself, it is formed by a set of antennae and transceivers to be able to communicate with the monitoring stations, sending collected data and receiving instructions from them. These instructions are processed by the control system, which could be the satellite’s main computer, or certain components from the OBDH (On-board Data Handling) Subsystem. The communications subsystem receives and demodulates uplink signals and modulates and transmits downlink signals.

The subsystem also allows us to track spacecraft by retransmitting received range tones or by providing coherence between received and transmitted signals, so we can measure Doppler shift. Next three bullet points summarizes the main system considerations which drive the design of communications subsystems.

  • Access: Ability to communicate with the spacecraft requires clear field of view to the receiving antenna and appropriate antenna gain.
  • Frequency: Selection based on bands approved for spacecraft use by international agreement. Standard bands are S (2 GHz), X (8 GHz) and Ku (12 GHz). UHF band (0.4 GHz) is also used.
  • Baseband Data Characteristics: Data bandwidth and allowable error rate determine RF power level for communications.

Communications access to a spacecraft requires a clear field of view for the spacecraft antenna. It also requires sufficient received power to detect the signal with acceptable error rate. Table 4 shows how we size the communications subsystem. To do so, we must identify the data bandwidths of the uplink and downlink, select communication frequencies, prepare RF power budgets for both links and select equipment. The basic communications subsystem consisted of a transmitter, a receiver, an antenna and a RF diplexer

Table 4 Steps to prelimnary size the communication subsystem
Step Definition What’s involve
  1. Identify Data Rate
Selec a Data Rate Value Payload commands and data, Spacecraft bus commands and telemetry
  1. Select frequency Bands
Decide which of the allowed bands to use Data Rate and the proper international institution
  1. Prepare RF Power Budget
Analyze characteristics of RF links Some RF properties to define the communications subsystem
  1. Select the equipment
Define equipment properties and needs Configuration, Power subsystem and others

Identify Data Rate

The data rate is defined as the information flow rate, which means the information bits or bytes per unit of time that the satellite and the ground station exchange. This information may come from three different origins, on which its properties depend:

  • Command data: All the directives from the satellite operation. Usually in a range between 2000 and 8000 bps and with a typical value of 4000 bps.
  • Health & Status telemetry: After a concrete command, the satellite send to the ground station the conditions of all his parameters. The typical value is about 8000 bps, with a range from 40 to 10000 bps.
  • Mission/science: All the payload and mission data rate. It depends on the purpose of the mission and on the payload itself. It be divided into Low ( <32 bps), Medium ( 32 bps to 1 Mbps) or High Data Rate ( >1 Mbps).

Select frequency Bands

Regulatory constrains exist on the selection of frequency band, transmission bandwidth and power flux density. International agreements have allocated frequency bands for space communications, as listed in Table 5. These agreements originated with the International Telecommunications Union (ITU) and the World Administrative Radio Conference (WARC). The system designer must apply for and receive permission from the appropriate agency to operate at a specified frequency with the specified orbit and ground locations.

Frequency band Frequency Range (GHz) Service
Uplink Downlink
UHF 0.2 - 0.45 0.2 - 0.45 Military
L 1.635 - 1.66 1.535 - 1.56 Maritime, Telephone
S 2.65 - 2.69 2.5 - 2.54 Broadcast, Telephone
C 5.9 - 6.4 3.7 - 4.2 Domestic, Comsat
X 7.9 - 8.4 7.25 - 7.75 Military, Comsat
Ku 14.0 - 14.5 12.5 - 12.75 Domestic, Comsat
Ka 27.5 - 31.0 17.7 - 19.7 Domestic, Comsat
SHF/EHF 43.5 - 45.5 19.7 - 20.7 Military, Comsat
SHF/EHF 49 38 Internet Data, Telephone, Trunking
V 60 Satellite Crosslinks

A criterion for frequency band allocation is the potential for one link to interfere with another. Two geostationary satellites in approximately the same orbit location servicing the same ground area may share the same frequency band by: (1) separating adjacent satellites by an angle (typically 2 deg), which is larger than the ground station’s beamwidth, and (2) polarizing transmitting and receiving carriers orthogonally, which allows two carriers to be received at the same frequency without significant mutual interference. Right-hand and left-hand circular polarization are orthogonal, as are horizontal and vertical linear polarization. Commercial systems use these frequency-sharing techniques extensively.

The election of the frequency also affect to the design of the communication link itself.

The bandwidth of the frequency selected determine the Data rate capacity by using the Nyquist theorem (5)

(5)\[R \leq 2C\]

Select the equipment

At the end, available equipment must be selected considering the mission requirements and the values obtained from the Link Budget. COTS are highly preferred.