Technique ========= Radio science instrumentation combines equipment on the ground with on-board spacecraft hardware required to create and maintain a highly stable and precise radio link. Most commonly, two-way radio signals have been generated on the ground and transmitted 'uplink' through the large antennas of the NASA Deep Space Network. These transmissions are received by the spacecraft transponder, shifted in frequency, and then re-transmitted 'downlink' to the Earth where they are received either at the original site or at a second site, possibly located on another continent. Transponder design is such that the frequency of the downlink signal is coherently related to the received uplink frequency by a known integer ratio. Because the downlink signal frequency is derived precisely from that of the uplink, it is possible to measure changes in the radio path length by comparison of the received downlink signal with the ground oscillator that generated the uplink signal originally. An increase in the radio path length decreases the phase of the received downlink signal relative to the ground oscillator, while a decrease in path length has the opposite effect. As hydrogen maser clocks are used for the fundamental frequency reference on the ground, measurement of the downlink phase provides an extremely precise method of determining changes in the round trip propagation time to the spacecraft. A one-Hertz difference between the frequencies of the uplink and downlink signals means that the total radio path length is changing at the rate of one wavelength per second; larger or smaller frequency differences correspond to proportionally larger or smaller rates of path length change. Overall, the short term accuracy of the measurement procedure depends on the signal-to-noise ratio achieved and, ultimately, on the stability of the ground station oscillator over the round trip flight time of the radio signals to the spacecraft and back (Eshleman and Tyler 1975; Lipa and Tyler 1979). On the ground, stations for communication over interplanetary distances are built around the large antennas of 20–100 m diameter. These stations are used primarily for uplink transmission of commands and downlink reception of spacecraft data (Yuen 1983). On spacecraft, however, typical antenna sizes are limited to only a few meters at most, and the transmitted downlink signal power ranges from about 1 to less than 100 W. As mentioned, hydrogen maser atomic clocks are used for the ground station frequency reference, while microwave frequencies in the 2 (S-band) and 8 (X-band) Gigahertz range, corresponding to 12–13 cm and 3–4 cm wavelengths, respectively, are used for the radio signals. Either band can be used separately or both used simultaneously. Use of dual frequencies is advantageous since this permits direct separation of the effects of neutral and ionized gases on the basis of differences in the dispersive characteristics of the two media. Frequency changes as small as about 0.001 Hertz can be measured, corresponding to a fractional accuracy in the range of a few parts in 10^(14) (Tyler et al. 1992). In the absence of other effects, this leads to an accuracy in the measurement of spacecraft velocity, for example, in the range of 30 micrometers/second when the 8 gigahertz band is used. Under the best conditions accuracies better than 10 micrometers/second have been achieved. Similar or slightly lower accuracies are anticipated for Venus Express. The use of a two-way, uplink/downlink radio path is suitable for study of gravity and for spacecraft navigation purposes. The strong atmosphere of Venus has also the effect of a bending of the microwave ray. In consequence the spacecraft HGA has to vary its pointing attitude in order to compensate for the ray bending effect. Occultation observations exhibit considerable signal dynamics, with simultaneous variations in signal frequency and amplitude, as well as the presence of near-forward scattering and diffraction when the radio path passes near a planet's surface. In the case of Mars diffraction from the planetary limb is observed on all occasions while near-forward scattering occurs in roughly 80 percent of occultation events. Observed signals obtained from bistatic scattering experiments are characteristically broadened relative to the illuminating signal as a result of the combination of angular spreading of the waves by the scattering process and the relative motion of the spacecraft and ground station with respect to the planet's surface. Unlike occultation observations, much of the information regarding the surface properties is in the polarization and amplitude of the scattered signal; typically there is no coherent component in the scattered fields. Thus, both occultation and bistatic scattering observations produce dynamic signals occupying a considerably greater bandwidth than the transmitted illuminating waveform. For this reason, the measurement of these signal characteristics requires capture of the time-sampled waveform at a sufficient sampling rate to avoid frequency aliasing effects. This is accomplished with open-loop receivers pre-programmed to track the expected spectral window. The dynamical characteristics of the occultation and scattered signals preclude reliable use of phase-locked loop techniques for reliable radio occultation measurements. For more information see MEX-MRS-RIU-MA-3050.PDF in the DOCUMENT folder.