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 X-Band, a transmitter with a five-Watt final amplifier operating in S-Band, an X-Band receiver, and an S-Band receiver.
The S-Band transmitter outputs and receiver inputs are connected to the RFDU which combines the transmit and receive pathways using diplexers and switches the combined signal pathways to either of the LGAs or to HGA1.
The X-Band transmitter outputs are connected to the 65-Watt TWTAs via an RF crossover switch. The TWTA outputs are fed to the WIU, where they can be directed to one of two diplexers feeding the HGAs. The receive signals from the diplexers are switched to connect either X-Band receiver to each of the HGAs.
RF communications block diagram
Venus Express communicates with Earth using the following antennas:
- Two Low Gain Antennas (LGA), which allow omnidirectional transmission and reception in S-Band
- One dual band High Gain Antenna (HGA1), used for high rate telemetry transmission and telecommand reception in both S- and X-Bands
- One single band offset antenna (HGA2) used for high rate telemetry transmission and telecommand reception in X-Band
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.
S-Band provides omnidirectional coverage for both up and downlink over small distances, while X-Band provides a high rate data downlink and a telecommand uplink while in Venus orbit.
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 X-Band, via HGA2. During Venus Orbit Injection, communications will reverted to S-Band.
During operational modes when VeRa is not in use, communications are performed in X-Band. HGA1 is used when Venus is in the superior conjunction side of its orbit and the distance to Earth is greatest. In order to be able to keep the spacecraft cold face pointed away from the Sun at all times, HGA2 is used for the portion of Venus's orbit near inferior conjunction (Earth to spacecraft distances up to 0.78 AU).
When VeRa is operational, uplinks may be performed in either X-Band or S-Band, via HGA1. VeRa downlinks in both S- and X-Bands, with the signals generated by VeRa's Ultra Stable Oscillator and fed via the spacecraft transponder to HGA1. The downlink is unmodulated during VeRa operations.
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 S-Band and 7166 MHz for X-Band
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 S-Band, while the higher ones are used operationally through one of the High Gain Antennas in X-Band.
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 X-Band. The frequencies for the downlink are approximately 2296 MHz for S-Band and 8419 MHz for X-Band.
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 227/(4*(N+1)(M+1)). The possible bit rates are selected via Memory Load Command (MLC) and vary from 10.6667 bps as a minimum and up to 262 144 kbps.
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:
- PCM/PSK/PM on a 8192 Hz square wave subcarrier for bit rates lower than 1365.3333 bps
- PCM/PSK/PM on a 262 144 Hz square wave subcarrier for bit rates between 1365.3333 bps and 65 536 bps
- SP-L (no subcarrier) for bit rates greater than 65 536 bps
This signal is phase-modulated in either S Band or X Band by the Dual Band Transponder.
||Attitude and Orbit Control
Last Update: 24 May 2007