The communications comprises a Dual Band Transponder (DBT), a Radio Frequency Distribution Unit (RFDU), two Travelling Wave Tube Amplifiers (TWTA), a Waveguide Interface Unit (WIU) and four antennas.
The DBT contains two duplicate transmit/receive chains each of which has a transmitter operating in
Venus Express communicates with Earth using the following antennas:
The LGAs are the same as the antennas on Mars Express. HGA1 is similar to the HGA on Mars Express but has a smaller diameter (1.3 metres instead of 1.6 metres) to take advantage of the smaller maximum spacecraft to Earth distance. HGA2 has been added for the Venus Express mission to enable spacecraft communication requirements and thermal constraints to be met while in orbit around an inferior planet.
The LGAs were used during the Launch and Early Operations Phase (LEOP), which covered the first five days of the mission. After LEOP, during the journey to Venus, communication were in
During operational modes when VeRa is not in use, communications are performed in
When VeRa is operational, uplinks may be performed in either
The received RF uplink signal, which is modulated with packetised telecommands as NRZ/PSK/PM data, is routed towards a diplexer, performing frequency discrimination, and then to a Dual Band Transponder input. The transponder performs carrier acquisition and demodulation, and transmits the extracted signal to the Data Handling System for further processing.
The frequencies for the uplinks are approximately 2100 MHz for
Data rates of 7.8125 bps, 15.625 bps, 250 bps, 1000 bps and 2000 bps can be received by the Venus Express spacecraft, defined by the Control and Data Management Unit design. The bit rate is selectable by means of a Memory Load Command (MLC).
As a baseline, the lower bit rates are used via the Low Gain Antennas in
A high rate data downlink capability is required due to the large volume of data generated by the instruments. However, the downlink capacity is limited by the large spacecraft to Earth distance.
The downlink of the telemetry data to the ground stations can be performed in either S- or
Downlink is performed at a commandable, variable bit rate. The CDMU design allows the generation of a telemetry data stream at any bit rate corresponding to
The bit rate to which reference is made is the bit rate following Reed-Solomon encoding, but prior to convolutional encoding, if any. As with the uplink, the baseline is that the lower bit rates are used via the Low Gain Antennas, whilst the highest ones are used operationally through one of the High Gain Antennas in X Band.
The minimum average bit rate that has been assumed during mission design is 17 kbps, which corresponds to 500 Mb for eight hours of data downlinking per day.
The variable bit rate signal is sent to the Dual Band Transponder as:
This signal is phase-modulated in either S Band or X Band by the Dual Band Transponder.