Tomoaki Toda

OUTFLOW STRUCTURE OF THE QUIET SUN CORONA PROBED BY SPACECRAFT RADIO SCINTILLATIONS IN STRONG SCATTERING

Radio scintillation observations have been unable to probe flow speeds in the low corona where the scattering of radio waves is exceedingly strong. Here we estimate outflow speeds continuously from the vicinity of the Sun to the outer corona (heliocentric distances of 1.5-20.5 solar radii) by applying the strong scattering theory to radio scintillations for the first time, using the Akatsuki spacecraft as the radio source. Small, nonzero outflow speeds were observed over a wide latitudinal range in the quiet-Sun low corona, suggesting that the supply of plasma from closed loops to the solar wind occurs over an extended area. The existence of power-law density fluctuations down to the scale of 100 m was suggested, which is indicative of well-developed turbulence which can play a key role in heating the corona. At higher altitudes, a rapid acceleration typical of radial open fields is observed, and the temperatures derived from the speed profile show a distinct maximum in the outer corona. This study opened up a possibility of observing detailed flow structures near the Sun from a vast amount of existing interplanetary scintillation data.

Measurements and characterization of ultra wideband propagation within spacecrafts

Ultra wideband (UWB) propagation was measured and characterized within spacecrafts, with a view to partly replacing onboard data buses with wireless connections. Spatial distributions of UWB and narrowband propagation in frequency (from 3.1 to 10.6 GHz) and time domains were measured with a microwave vector network analyzer. While narrowband resulted in a number of dead spots (deep fading points) within the conductive enclosures, UWB yielded none. This implies the UWB systems have an advantage over narrowband ones from the viewpoint of reducing fading margins. It was also found that delay spreads can be suppressed by partially panelling a radio absorber on the inner surfaces to facilitate high data rate transmission.

Measurements and Characterization of Ultra Wideband Propagation within Spacecrafts--Proposal of Wireless Transmission for Replacing Wired Interface Buses

Ultra wideband (UWB) propagation was measured and characterized within spacecrafts, with a view to partly replacing onboard data buses with wireless connections. Spatial distributions of UWB and narrowband propagation in frequency (from 3.1 to 10.6 GHz) and time domains were measured with a microwave vector network analyzer. While narrowband resulted in a number of dead spots (deep fading points) within the conductive enclosures, UWB yielded none. This implies the UWB systems have an advantage over narrowband ones from the viewpoint of reducing fading margins. It was also found that delay spreads can be suppressed by partially panelling a radio absorber on the inner surfaces to facilitate high data rate transmission.

Wireless connections within spacecrafts to replace wired interface buses

This paper describes measurement and characterization of radio propagation and transmission - particularly of ultra wideband (UWB) signals - within spacecrafts with a view to partly replacing on-board data buses with wireless connections. Adaption of wireless technologies within spacecraft could contribute to reduction of cable weight and resulting launching costs, and more reliable connections at rotary, moving, and sliding joints. This paper presents measurements and characteristics of radio propagation and transmission and addresses the effects of apertures perforated on the outer surface of satellites on the UWB propagation and transmission for low- and high-band UWB within a shield box. Channel responses, spatial distributions of UWB and narrowband propagation gains, delay spreads, and throughputs were derived from measurements. On the effects of apertures, the larger total area of apertures resulted in lower UWB propagation gains, shorter delay spreads, and (slightly) higher link throughput. The propagation study was followed up with experimental evaluation of UWB link throughput within a simulated spacecraft. Commercially off-the-shelf UWB devices were used in the experiments of ultra wideband technology to facilitate a high data rate (e.g. maximum of 400 Mb/s per node attained with SpaceWire, equaling the standards of a wired onboard data bus) and to reduce the fading margin.

Experimental evaluation of ultra wideband wireless transmission within a simulated spacecraft for replacing wired interface buses

Ultra wideband (UWB) communication was evaluated within a closed space, with a view to partly replacing onboard data buses with wireless connections. Spatial distributions of UWB propagation in frequency (from 3.1 to 10.6 GHz), time domains and throughput with use of a radio absorber were derived from measured. It was also found that delay spreads can be suppressed by partially panelling a radio absorber on the inner surfaces to facilitate high data rate transmission. Even within the empty shield box, the delay spreads were adequately suppressed to accommodate a data rate exceeding 100 Mb/s, when the absorber panel covered typically 8% of the total inner surface area.

Effects of Inner Volume on UWB Propagation Channels within Closed Spaces

Experimental evaluation of ultra wideband (UWB) wireless transmission was carried out with a view to replacing wired interface buses in spacecrafts. Application of wireless technologies within the spacecrafts could contribute to reduction in cable weight; reduction in the cost of design, manufacture, and test; more flexibility in layout of spacecraft subsystems; and reliable connections at rotary, moving, and sliding joints. However, multipath propagation in semi-closed conductive enclosures, such as spacecrafts, restricts the link performance. Spatial distributions of UWB and narrowband propagation gains, delay spreads, and throughputs were measured with use of four different-sized shield boxes (simulated miniature satellites). Then UWB link throughput was experimentally evaluated in the boxes with use of connecting off-the-shelf MB-OFDM UWB devices. Increase in the inner volume of boxes resulted in higher UWB propagation gains, but wider delay spreads and hence lower link throughput.

Experimental evaluation of ultra wideband propagation and transmission within a spacecraft for replacing wired interface buses

Ultra wideband (UWB) technology has been expected to communicate in very high data rates by wireless within closed environments, such as spacecrafts. With a view to replacing onboard wired interface buses by wireless connection, UWB and narrowband propagation were measured in a UWB frequency band (from 3.1 to 10.6 GHz) within a small spacecraft. While narrowband propagation resulted in considerable spatial variations in path gain due to interferences caused by multipaths, UWB yielded nearly no fading. This suggests that the UWB systems have an advantage over narrowband from a viewpoint of reducing fading margins. Propagation gains were measured with full-, low-, and high-band UWBs, and narrowband (20 MHz bandwidth) for various antenna settings and polarization configurations for transmission and reception. Polarization configurations were found to produce almost no effect on average power delay profiles. Throughputs were also measured with a WiMedia UWB device. More than 85-Mb/s throughputs were observed for all antenna settings and propagation configurations.

Experimental evaluation of ultra wideband wireless transmission within a spacecraft for replacing wired interface buses

Ultra wideband (UWB) transmission was evaluated within a small scientific spacecraft, with a view to partly replacing onboard data buses with wireless connections. Spatial distributions of UWB link throughput were derived from measurements. The results revealed that commercially-available UWB devices were capable of accommodating up to around 90-Mb/s data buses within spacecrafts.

Local Time Dependence of the Thermal Structure in the Venusian Equatorial Upper Atmosphere: Comparison of Akatsuki Radio Occultation Measurements and GCM Results

Plain Language Summary Temperature profiles of the Venus atmosphere obtained by the Akatsuki radio occultation measurements showed a prominent local time dependence above 65-km altitude at low latitudes equatorward of 35 degrees. A zonal wavenumber 2 component is predominant in the temperature field, and its phase (i.e., isothermal) surfaces descend with local time, suggesting its downward phase propagation. A general circulation model (GCM) for the Venus atmosphere, AFES-Venus, reproduced the local time-dependent thermal structure qualitatively consistent with the radio occultation measurements. Based on a comparison between the radio occultation measurements and the GCM results, the observed zonal wavenumber 2 structure is attributed to the semidiurnal tide. Applying the dispersion relationship for internal gravity waves to the observed wave structure, the zonally averaged zonal wind speed at 75- to 85-km altitudes was found to be significantly smaller than that at the cloud top. The decrease of the zonal wind speed with altitude is attributed to the momentum deposition by the upwardly propagating semidiurnal tide excited in the cloud layer. Akatsuki radio occultation measurements showed the local time dependence of the Venus atmosphere in the equatorial region. By comparing the measurements with a general circulation model, it is attributed to the upward propagating semidiurnal tide generated in the cloud layer. And then, we proposed a new method to estimate the zonal wind speed above the cloud layer, where any optical instruments cannot be measured, for the first time.

A X/Ka bands feeder antenna for a planetary exploration high gain reflector antenna

A compact X/Ka bands feeder antenna has been proposed for a Cassegrain type reflector antenna board on a planetary exploration satellite. A sequentially rotated array in X band and a helical array in Ka band are employed for realizing the circular polarization. In the X band array, the unique feeding method has been developed for reducing cross polarization level of the sequential array and in the Ka band array, the compact configuration has been realized by positioning it within the space of the X band elements.