VeRa: Venus Radio Science
The Venus Radio Science experiment (VeRa) performs the following experiments:
The radio links of the spacecraft communications system are used for these investigations. A simultaneous and coherent dual-frequency downlink at X-band and S-band via the High Gain Antenna (HGA) is required to separate the effects of the classical Doppler shift due to the motion of the spacecraft relative to the Earth and the effects caused by the propagation of the signals through the various dispersive media in the signal path.
The experiment relies on the observation of the phase, amplitude, polarisation and propagation times of radio signals transmitted from the spacecraft and received by ground stations on Earth. The radio signals are affected by the medium through which the signals propagate (atmospheres, ionospheres, interplanetary medium, solar corona), by the gravitational influence of the planet on the spacecraft and finally by the performance of the various systems involved both on the spacecraft and on ground.
Radio sounding of the atmosphere and ionosphere
The sounding of the neutral and ionised atmosphere is performed in the periods just before the spacecraft enters occultation by the planet. The High Gain Antenna is pointed toward the Earth before the approach to occultation. The radio link is a two-way dual-frequency downlink with unmodulated carriers. The radio link passes through a vertical swath of the ionosphere and atmosphere.
Bistatic radar investigation of planetary surface
A bistatic radar configuration is distinguished from a monostatic configuration by the spatial separation of the transmitter (the spacecraft) and the receiver (ground station). The HGA is inertially pointed toward the surface of Venus and an X-band signal without modulation is transmitted. Several passes above specific targets are made. The reflected and/or scattered signal is received by the ground station, for which the preferred choice is the Deep Space Network because of the higher signal to noise ratio that is available.
Solar corona sounding
Solar corona sounding is performed when Venus is within 10° elongation either side of the solar disk. The operational radio link for the sounding of the solar corona is the two-way dual-frequency radio with an S-band uplink. The experiment can be performed whenever the spacecraft is being tracked for data return.
The VeRa experiment will make use of the radio transponder and antenna carried by the spacecraft for communication with Earth. Frequency, amplitude and polarisation information can be extracted from the received radio signal by the ground station.
Uplinks are provided either at X-band, for routine operations, or at S-band for coronal sounding. In the coherent two-way mode the received frequency is used to derive the downlink frequencies by using the constant transponder ratios 880/221 and 240/221 for X-band and S-band downlink, respectively.
The spacecraft transmits a dual-frequency downlink at X-band and S-band simultaneously and phase coherently via the HGA. Typically, the X-band and S-band frequencies are related by a factor of 11/3. If an uplink exists, the downlinks are also coherent with the uplink in their respective transponding ratios.
The dual-frequency downlink is used to separate the classical Doppler shift, due to the relative motion of the spacecraft and the ground station, from the dispersive media effects, due to the propagation of the radio waves through the ionospheres and interplanetary medium. Both frequencies are transmitted via the High Gain Antenna to maximise the signal-to-noise ratio.
The two-way dual-frequency radio link is used for coronal investigations. The radio link benefits from the superior frequency stability of the ground station afforded by the use of hydrogen masers as reference sources.
The New Norcia ground station is used to monitor the spacecraft. A tracking pass consists of typically eight to ten hours of visibility. Measurements of the spacecraft range and carrier Doppler shift can be obtained whenever the spacecraft is visible. In the two-way mode the ground station transmits an uplink radio signal at S-band or X-band and receives the dual-frequency simultaneous downlink at X-band and S-band. The information about signal amplitude, received frequency and polarization is extracted and stored together with the time of receipt.
The ground stations employ a hydrogen maser frequency standard to achieve a frequency stability in the order of 10-15 over long integration time (> 100 seconds). This stability is required for precise two-way tracking in order to achieve velocity accuracy better than 0.1 mms-1 at one second integration time.