PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM PRODUCER_ID = "ISAS/JAXA" LABEL_REVISION_NOTE = " 2017-03-29, K. McGouldrick, S. Murakami: Initial version; 2017-05-05, S. Murakami: Revised; 2017-10-04, T. Imamura: Revised; 2017-10-06, S. Murakami: Revised; 2017-11-15, S. Murakami: Revised; 2018-11-01, T. Imamura, S. Murakami: Revised; " OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = "VCO" INSTRUMENT_ID = "RS" OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "RADIO SCIENCE" INSTRUMENT_TYPE = "RADIO SCIENCE" INSTRUMENT_DESC = " This summary of the RS Ultrastable Oscillator is compiled primarily from [IMAMURAETAL2011]. Further details may be found in that publication and an initial performance report [IMAMURAETAL2017]. Instrument Overview =================== The Ultra-Stable Oscillator is onboard Venus Climate Orbiter (also known as PLANET-C and AKATSUKI) and is used for Radio Science experiment to investigate Venus atmosphere. Specification of Ultra-Stable Oscillator (USO) ---------------------------------------------- The USO provides a stable reference frequency (38.22567759 MHz) for the transponder. It was manufactured by TimeTech GmbH in Germany and is a heritage from the USOs flown onboard the ESA's Rosetta [PAETZOLDETAL2007] and Venus Express spacecraft [HAEUSLERETAL2006]. We made several modifications such as the redesign of the thermal insulating and mechanical shock protection system to adapt to the higher mechanical loads during the launch, resulting in dimensions (17 cm x 13 cm x 16 cm) and mass (1.9 kg) which are greater than for previous USOs. The USO is characterized by an excellent frequency stability; the Allan deviation is on the order of 10**-13 at the integration time from 1 to 1000 s. Specification of the ground stations ------------------------------------ The primary ground station used in the experiment is the 64-m antenna of Usuda Deep Space Center (UDSC) of JAXA, which is located at 1382154 East longitude, 360744 North latitude. In addition to UDSC, the 32-m antenna of Indian Deep Space Network (IDSN) of Indian Space Research Organization (ISRO), which is located at 772208 East longitude, 125411 North latitude, is used from March 2017, and the 30-m antenna at Weilheim (WHM) of German Aerospace Center (DLR), which is located at 110442 East longitude, 475252 North latitude, is going to be used from 2019. The signals are recorded by open-loop receivers at UDSC and IDSN, and by close-loop at WHM. Scientific Objectives ===================== The primary goal of Akatsuki RS is to characterize the meteorological processes that might drive the planet-wide easterly wind (super-rotation) as well as to understand the cloud dynamics. For this purpose the temporal and spatial variability of the Venus atmosphere will be studied by retrieving the vertical profiles of the temperature, the H2SO4 vapor density, and the intensity of small-scale density fluctuations. The advantage over the previous missions is the near-equatorial orbit of the spacecraft and a suite of multiband cameras dedicated to meteorological study [NAKAMURAETAL2007, NAKAMURAETAL2011B]. The orbit around Venus is a 10.5 days-period elliptical orbit near the ecliptic plane. The direction of orbital motion is westward, which coincides with the direction of the atmospheric super-rotation. The apoapsis altitude is ~360,000 km, or 59 Venus radii, and the periapsis altitude is variable in the range 1000-8000 km. The uniqueness of the Akatsuki RS is that probed points cluster around the equatorial region. Since the onboard cameras are designed to observe wide areas centered at the equatorial region, the locations probed by RS can be observed by the cameras a short time (minutes to hours) before or after the occultations. The vertical profiles taken by RS and the horizontally-resolved information at several height levels by the cameras complement each other in developing three-dimensional models of the atmosphere. The almost complete local-time coverage in the low latitude enables the studies of equatorial cloud dynamics which is poorly understood. For example, cell-like structures have been observed near the subsolar region in ultraviolet [ROSSOWETAL1980, MARKIEWICZETAL2007]. The origin of these structures has been unclear, because their horizontal scales of up to several hundreds kilometers seem to be too large for convective cells and the cloud top region is considered to be basically stably stratified [BAKER&SCHUBERT1992, TOIGOETAL1994]. The diurnal variation of the temperature profile to be revealed by RS, especially the possible variation in the depth of the neutral-stability layer, should become a key to understanding the cloud dynamics in this region. Excursions of the probed points to the high latitude occur several times and enable observations of the latitudinal structure. Since the Venus atmosphere is considered to be in cyclostrophic balance, the meridional distribution of the zonal wind can be derived from the temperature distribution by integrating the thermal wind equation with an appropriate lower boundary condition [NEWMANETAL1984]. This zonal wind distribution will become the basis for understanding the momentum balance of the atmospheric general circulation. The combination of data from Akatsuki RS with those from ESA's Venus Express radio science VeRa [HAEUSLERETAL2006, PAETZOLDETAL2007B, TELLMANNETAL2009], which conducts dense sampling in the high latitude by virtue of the polar orbit, would also enable studies of meteorological processes over broad latitude regions even when the two spacecraft do not orbit at the same time. Another goal of Akatsuki RS is to study the spatial and temporal variabilities of the ionosphere in response to the variations in the solar wind and the neutral atmosphere. Although Akatsuki is not equipped with a magnetometer and particle instruments, Venus Express has those instruments and would provide supplementary information [ZHANGETAL2006,BARABASHETAL2007] if Venus Express mission is extended till Akatsuki's next Venus orbit insertion. Such a configuration is very optimal for studying the response of the ionosphere to the solar wind condition, because the upstream solar wind conditions and their sudden changes such as interplanetary shock passages and coronal mass ejections can be monitored simultaneously. Sudden changes in the solar wind modify the global characteristics of the Venusian ionosphere, which may increase the ion escape flux from the upper atmosphere [FUTAANAETAL2008, EDBERGETAL2010]. Energetic electrons at energies approximately 30 eV (KLIOREETAL1991), considered as one of the generation mechanisms of the nightside ionosphere, may also be observed by an electron spectrometer onboard Venus Express. Extensive measurements of the ionosphere at small SZA allow us to understand how the upper atmosphere interacts with the solar wind in the subsolar region. The investigation of the subsolar region is important for understanding the transfer of energy and momentum from the solar wind to the upper atmosphere. In situ observations of the subsolar region have been difficult, because spacecraft normally pass through very quickly at ionospheric altitudes. Studying the properties of the solar wind plasma is also an objective of Akatsuki RS. When the spacecraft moves into superior conjunction with the sun, radio waves transmitted from the spacecraft toward the tracking station suffer bending, scattering and attenuation in the near-sun plasma. The temporal variations of the phase and the intensity of the received signal provide information on the spatial spectrum (power law) of density inhomogeneities, the anisotropy of inhomogeneities, the inner turbulence scale, and the bulk velocity of plasma [PAETZOLDETAL1996, IMAMURAETAL2005, EFIMOVETAL2010]. Based on the noise level estimation and previous occultation results [WOO&ARMSTRONG1979], we expect the phase power spectrum will be obtained up to ~10 Hz for the solar offset distance of 10 solar radii; such a spectrum would enable us to study, for example, the excess power at small scales (100-600 km) observed by the Nozomi radio occultation (IMAMURAETAL2005). The minimum solar offset distance of ~1.5 solar radii will occur on June 25, 2011 in the renewed cruising plan. Observation System ================== Radio communication system -------------------------- The downlink frequency is 8.410926 GHz (X-band) only. During the experiments, one of the X-band transponders (XTRP) is locked to an Ultra-Stable Oscillator (USO), and the transponder signal is amplified by the 23.4 W traveling wave tube amplifier (TWTA). The 8.7 W solid state power amplifiers (SSPA) will not be used in the experiments unless we have trouble with the TWTA. The gain of the 0.9 m-diameter flat-type slot array antenna (XHGA-T) is 35 dBi, corresponding to a 3 dB full beam width of ~2. Since the ray bending far exceeds the beam width, the spacecraft will perform attitude maneuvers to compensate for the changing direction of the signal path. The equivalent isotropically radiated power (EIRP) is 42.5 dBW when the TWTA is used. Signal processing ----------------- The frequency variation is retrieved from the recorded signal in the following manner (IMAMURAETAL2005). We first subtract the Doppler shift calculated from the orbital information and a model atmosphere from the original signal by heterodyning, thereby suppressing the frequency variation and enabling narrow-band filtering. Then, approximate carrier frequencies are determined for successive time blocks by fitting a theoretical signal spectrum (sinc function) to the discrete Fourier transforms [LIPA&TYLER1979]. The resultant frequency variation is subtracted from the signal by heterodyning in order to apply another narrow-band filtering. The signal frequency and intensity time series are obtained by successively fitting a sinc function to the narrow-band filtered data. Atmospheric profiles are obtained from the frequency variation caused by the Venus atmosphere observed at the tracking station, termed ``atmospheric Doppler shift''. The atmospheric Doppler shift is combined with the trajectory data to calculate the bending angle and the impact parameter. The relationship between the bending angle and the impact parameter is converted to a refractive index profile through Abel transform. The deviation of the refractive index from unity is the sum of the contributions from the neutral atmosphere and the ionosphere. The neutral and ionospheric contributions are almost separated in altitude with the boundary around 100 km altitude; this enables us to retrieve the vertical profiles of the neutral atmospheric density and the electron density separately. The vertical profile of the neutral atmospheric pressure is derived from the density profile by integrating the equation of hydrostatic equilibrium. The top boundary condition for the temperature, which is needed for integration, is determined empirically. The temperature profile is calculated from the density and the pressure using the ideal gas law. Link budget and error estimation -------------------------------- The distance between the Earth and Venus at occultation opportunities ranges from 0.35 AU (Astronomical unit) to 1.73 AU. Table 1 presents the link budgets including the defocusing and absorption losses, for the minimum and maximum Earth-Venus distances, and for the heights of the ray closest approach of 40, 60 and 80 km. The absorption loss is taken from [HAEUSLERETAL2009]. The carrier-to-noise density ratio (C/N0) of the received signal decreases significantly at low altitudes (40 km). This low C/N0 will be partially compensated by a long integration time, since at low altitudes the vertical movement of the ray path slows down due to the bending of the ray path. Moreover, the greater reflectivity of the lower atmosphere results in a higher sensitivity of the bending angle to the atmospheric temperature. Table 1. Link budget of Akatsuki RS including losses for two Venus-Earth distances and for the minimum probed altitudes of 40, 60 and 80 km. +---------------------------------------+------+------+ |Distance (AU) | 0.35 | 1.73 | +---------------------------------------+------+------+ | EIRP of transmission (dBW) | 42.5 | | Absorption and rain loss (dB) | -0.8 | | Receiver pointing loss (dB) | -0.2 | | G/T of ground station (dB/K) | 52.7 | | C/N0 without Venus atmosphere (dBHz) | 57.5 | 43.6 | +---------------------------------------+------+------+ |Defocusing loss for D = 2 Rv (dB) | | +---------------------------------------+-------------+ | 40 km | -25.9 | | 60 km | -12.6 | | 80 km | -2.1 | +---------------------------------------+-------------+ |Absorption loss (dB) | +---------------------------------------+-------------+ | 40 km | -14.7 | | 60 km | -0.5 | | 80 km | 0.0 | +---------------------------------------+-------------+ |Overall C/N0 (dBHz) | +---------------------------------------+------+------+ | 40 km | 16.9 | 3.0 | | 60 km | 44.4 | 30.5 | | 80 km | 55.4 | 41.5 | +---------------------------------------+------+------+ The statistical (random) uncertainty in the temperature should be on the order of 0.1 K or smaller for a prescribed vertical resolution of 1 km in the altitude region from 40 km to 80 km so that various atmospheric waves can be detected. To estimate the C/N0 required for this purpose, we have computed the response of the downlink frequency to Gaussian-type temperature perturbations with a height of 0.1 K and a full-width of 1 km assuming an occultation geometry in which the ray transverse velocity relative to the Venus limb is its maximum, i.e. the case where the maximum time resolution is needed. The results showed that such temperature perturbations cause frequency perturbations roughly localized around the perturbed region. For altitudes around 80 km, a frequency resolution of ~0.03 Hz and a time resolution of ~0.1 s are needed to discriminate perturbations imposed on adjacent two layers separated by 1 km; this corresponds to a phase resolution of sigma~0.019 with a bandwidth of B ~ 10 Hz, giving a required C/N0 of B/2/sigma**2 ~ 41.5 dBHz. This value will be achieved according to the link budget analysis (Table 1). Similar analyses for 60 km and 40 km altitudes showed that the required C/N0 is lower than the predicted C/N0 for these altitudes. From these results we expect that the statistical error of the order of 0.1 K is possible. The influence of the fluctuation in the USO output frequency is negligible. The accuracy of electron density measurement is limited at least by the fluctuation of the terrestrial ionosphere and is estimated to be on the order of 100 cm**-3 based on the measurement of the ionospheric column density [IMAMURAETAL2010]. The density fluctuation of the interplanetary plasma can also be an error source for small solar offset distances of the ray path. Based on the results of solar occultation measurements [WOO&ARMSTRONG1979, IMAMURAETAL2005], the error is estimated to be ~3000 cm**-3 for the solar offset distance of 0.1 AU, and decreases to ~300 cm**-3 for 0.3 AU, given the typical time scale of traversing the Venus ionosphere of 10 s. It should be noted that these uncertainties vary with time. 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