The Helios Faraday Rotation (FR) Experiment, a passive radio science investigation requiring no on-board hardware other than the existing spacecraft radio subsystem, was designed to study the dynamic and quiescent structure of the magnetic fields and electron density in the solar corona. Measurements of coronal Faraday rotation were derived from the linearly polarized S-band downlink carrier signal, which probed otherwise inaccessible regions of the corona in the radial range from 2 to 15 solar radii during the regularly recurring solar conjunctions. More than 1250 hours of Helios FR data were recorded over the duration of the Helios 1 (1974-84) and Helios 2 (1976-80) missions. The time scales of FR variations provide information on various physical phenomena: (a) slowly-varying rise and fall associated with the changing ray path offset, combined with the rotation of the quasi-static corona; (b) ubiquitous random oscillations with higher fluctuation amplitude at smaller solar offset distances, probably caused by coronal Alfven waves; (c) occasional nearly discontinous jumps in the polarization angle, most likely caused by transient events such as coronal mass ejections (CMEs). The Helios FR data, aspects of which have been reported in more than forty publications to date, have now been systematically collected in a data archive for public dissemination. A brief review of the main results of the Helios FR Experiment are presented, together with some suggestions for possible use of the archive for continued solar wind research.

The operating noise temperature (Top )o fradio frequency (RF) receiving systems can be calculated using measured power ratios obtained when switching between calibration loads at different temperatures. One method uses the Rayleigh-Jeans (R-J) approximation to determine the noise temperature of the calibration loads (4-6). An exact calculation uses Planck's radiation law (4-6). We show that small receiver (Te) and antenna (Ti) noise temperature errors resulting from the use of the R-J approximation are self-compensating, and the simpler approximation can be used with an insignificant Top error. The accuracy of Top, consisting of the sum of the calibrated antenna noise temperature and the receiver noise tempera- ture, is adequate using the simplified R-J approximation (physical temperature) at frequencies as high as 100 GHz.

The operating noise temperature (Top) of radio frequency (RF) receiving systems can be calculated using measured power ratios obtained when switching between calibration loads at different temperatures. One method uses the Rayleigh–Jeans (R-J) approximation to determine the noise temperature of the calibration loads [4–6]. An exact calculation uses Planck’s radiation law [4–6]. We show that small receiver (Te) and antenna (Ti) noise temperature errors resulting from the use of the R-J approximation are self-compensating, and the simpler approximation can be used with an insignificant Top error. The accuracy of Top, consisting of the sum of the calibrated antenna noise temperature and the receiver noise temperature, is adequate using the simplified R-J approximation (physical temperature) at frequencies as high as 100 GHz.

One of the defining accomplishments of the 20th century is the beginning of our civilization's exploration of the solar system.

This article presentsthis road map, describes how it supports an increasing mission set while also providingsignificantly increased science data return, summarizes the current state ofkey Ka-band and optical communications technologies, and identifies critical pathitems in terms of technology developments, demonstrations, and mission users

Large axially-symmetric ground-based dual-reflector antennas are used in a variety of applications simultaneously requiring very high gain and very low noise (e.g., satellite communications, radio astronomy, deep-space communications, and radar). In these systems, reducing the noise by 10% is equivalent to increasing the antenna gain by roughly 0.5 dB. Since the early days of radio astronomy this fact has continuously driven efforts to reduce the noise of front-end low-noise amplifiers-a major noise contributor. As the performance of the front-end amplifiers improved, the relative importance of the noise generated by the surrounding warm ground increased, causing the antenna noise to become a major factor in the overall system sensitivity. The results of Moreira, Prata and Thorburn (see IEEE Trans. Antennas Propagat., vol.44, no.4, p.492-9, 1996) are used to implement a low back-scattering cross section on a single strut of the Jet Propulsion Laboratory Deep Space Station 13 (DSS-13) antenna-a research and development 34-meter diameter deep-space communication beam-waveguide shaped Cassegrain antenna, located in Goldstone, California. The antenna and associated noise measurements are discussed, and the improvement on the system noise temperature is presented.

Ground antennas are the major visible components of NASA's Deep Space Network (DSN). The role, key characteristics, and performance of these antennas in deep-space telecommunications are described. The system analyses and tradeoffs to optimize the overall ground-to-spacecraft link and to define future missions are elaborated from an antenna perspective. Overall performance of receiving systems is compared using the widely accepted G/T figure-of-merit, i.e., net antenna gain divided by the operating system noise temperature. Performance of past, present, and future antennas and receiving systems is discussed, including the planned development of a worldwide network of 34-m-diameter beam-waveguide antennas. The need for multifrequency operation, presently at S- and X-bands, and in the future at Ka-band, is discussed. The resulting requirements placed on antenna technology are highlighted. Beam-waveguide antenna performance to further improve performance and operational advantages is discussed.

The two Helios spacecraft underwent regular solar occultations during their extended missions from Dec 1974-Feb 1986 (Helios 1) and Jan 1976-Mar 1980 (Helios 2) thereby providing many opportunities for radio propagation experiments in the solar corona. On certain rare occasions over the course of these investigations, Faraday rotation measurements of the linearly polarized Helios signals could be recorded simultaneously at two widely-spaced ground stations. Many of these two-station measurement intervals display clear evidence of wave-like structures with quasi-periods of the order of a few minutes to a few hours. These structures are attributed to coronal Alfven waves. The radial propagation direction and velocity of these waves are estimated from a cross-correlation analysis of the data between the two stations. The majority of the waves appear to propagate away from the Sun, but about 30 percent of the cases indicate a propagation direction toward the Sun.

Provided here is an intersystem comparison of present and evolving Deep Space Network (DSN) microwave receiving systems. Comparisons of the receiving systems are based on the widely used G/T sub op figure of merit, which is defined as antenna gain divided by operating system noise temperature. In 10 years, it is expected that the DSN 32 GHz microwave receiving system will improve the G/T sub op performance over the current 8.4 GHz system by 8.3 dB. To compare future telecommunications system end-to-end performance, both the receiving systems' G/T sub op and spacecraft transmit parameters are used. Improving the 32 GHz spacecraft transmitter system is shown to increase the end-to-end telecommunications system performance an additional 3.2 dB, for a net improvement of 11.5 dB. These values are without a planet in the field of view (FOV). A Saturn mission is used for an example calculation to indicate the degradation in performance with a planet in the field of view.

The discovery of a remarkably strong gravitational lens/Einstein ring in the flat-spectrum radio source PKS1830-211 was reported recently by Jauncey et al. (1991). For the past three years this source has been the subject of an intense radio and optical observing campaign, which we review here.

An analysis of noise-temperature measurement errors of low-noise amplifiers was performed. Results of this analysis can be used to optimize measurement schemes for minimum errors. For the cases evaluated, the effective noise temperature (Te) of a Ka-band maser can be measured most accurately by switching between an ambient and a 2-K cooled load without an isolation attenuator. A measurement accuracy of 0.3 K was obtained for this example.

A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere by the VEGA spacecraft.

Measurements of the Giotto Radio-Science Experiment, consisting of the Doppler frequency shift and the intensity level of the X-band downlink signal of the Giotto spacecraft during Halley encounter, are analysed and interpreted. Continuous radio-science data were recorded throughout the encounter. The Doppler shift observed over a time interval of about 100 s is attributed to a drag effect in the cometary atmosphere causing a deceleration of the spacecraft. The total change of velocity of Giotto was 23.05 ± 0.05 cm s−1, yielding a best estimate for the total cometary mass impacting the spacecraft of 0.32 g. The cometary mass fluence and mean mass density along the Giotto trajectory was found to be 1.2 10−4 kg m−2 and 1.8 10−11 kg m−3, respectively. Within a time interval ± 45 s about closest approach, at least four well-established, sharply confined dust jets are distinguished, for which the characteristic geometries and production rates can be inferred. The total dust production rate is estimated to be 1.5 103 kg s−1. The comet’s jet structure derived from these Doppler data is shown to correlate with measurements on board Giotto.

Coronal Faraday rotation of the linearly polarized carrier signals of the HELIOS spacecraft was recorded during the regularly occurring solar occultations over almost a complete solar cycle from 1975 to 1984. These measurements are used to determine the average strength and radial variation of the coronal magnetic field at solar minimum at solar distances from 3–10 solar radii, i.e., the range over which the complex fields at the coronal base are transformed into the interplanetary spiral. The mean coronal magnetic field in 1975–1976 was found to decrease with radial distance according to r −α, where α = 2.7 ± 0.2. The mean field magnitude was 1.0 ± 0.5 × 10 −5 tesla at a nominal solar distance of 5 solar radii. Possibly higher magnetic field strengths were indicated at solar maximum, but a lack of data prevented a statistical determination of the mean coronal field during this epoch.

On June 11 and 15, 1985, two instrumental balloons were released from the Soviet VEGA 1 and VEGA 2 spacecraft and deployed in the atmosphere of Venus. The VEGA probes flew by the planet on their way to a rendezvous with comet Halley in March 1986. Drifting with the wind at altitudes of 54 km, the balloons traveled one-third of the way around the planet during their 46-hour lifetimes. Sensors on-board the gondolas made periodic measurements of pressure, temperature, vertical wind velocity, cloud particle density, ambient light level, and frequency of lightning. The data were transmitted to Earth and received at the Deep Space Network (DSN) 64-m stations and at several large antennas in the USSR. Approximately 95 percent of the telemetry data were successfully decoded at the DSN complexes and in the Soviet Union, and were provided to the international science team for analysis. Very Long Baseline Interferometry (VLBI) data were acquired by 20 radio observatories around the world for the purpose of monitoring the Venus winds. The DSN 64-m subnet was part of a 15-station VLBI network organized by the Centre National d'Etudes Spatiales (CNES) of France. In addition, five antennas of the Soviet network participated. VLBI data from the CNES network are currently being processed at the Jet Propulsion Laboratory.