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The global positioning system meteorology (GPS/MET) experiment was the first practical demonstration of global navigation satellite system (GNSS)-based active limb sounding employing the radio occultation technique. This method measures, as principal observable and with millimetric accuracy, the excess phase path (relative to propagation in vacuum) of GNSS-transmitted radio waves caused by refraction during passage through the Earth's neutral atmosphere and ionosphere in limb geometry. It shows great potential utility for weather and climate system studies in providing an unique combination of global coverage, high vertical resolution and accuracy, long-term stability, and all-weather capability. We first describe our GPS/MET data processing scheme from excess phases via bending angles to the neutral atmospheric parameters refractivity, density, pressure and temperature. Special emphasis is given to ionospheric correction methodology and the inversion of bending angles to refractivities, where we introduce a matrix inversion technique (instead of the usual integral inversion). The matrix technique is shown to lead to identical results as integral inversion but is more directly extendable to inversion by optimal estimation. The quality of GPS/MET-derived profiles is analyzed with an error estimation analysis employing a Monte Carlo technique. We consider statistical errors together with systematic errors due to upper-boundary initialization of the retrieval by a priori bending angles. Perfect initialization and properly smoothed statistical errors allow for better than 1 K temperature retrieval accuracy up to the stratopause. No initialization and statistical errors yield better than 1 K accuracy up to 30 km but less than 3 K accuracy above 40 km. Given imperfect initialization, biases >2 K propagate down to below 30 km height in unfavorable realistic cases. Furthermore, results of a statistical validation of GPS/MET profiles through comparison with atmospheric analyses of the European Centre for Medium-range Weather Forecasts (ECMWF) are presented. The comparisons indicate the high utility of the occultation data in that very good agreement of upper troposphere/lower stratosphere temperature (better than 1.5 K rms, Key words. Atmospheric composition and structure (pressure; density and temperature), Meteorology and atmospheric dynamics (instruments and techniques), Radio science (remote sensing)

For the Uranus smooth low frequency emission (SLF) two different (and contradictory) source locations (one polar, one equatorial) have been reported in the literature. Visibility studies show that polar source locations within a limited magnetic longitude range can be singled out, whereas a polar source distributed along the whole range of magnetic longitudes is compatible with observations. In this study we have computed intensity contours for comparison with the observed emission. Profiles obtained by a geometrical beaming model favor the equatorial source location.

By using PRA (Planetary Radio Astronomy), PWS (Plasma Wave System) and MAG (Magnetometer) data from Voyagers 1 and 2, essential features of the nightside Jovian plasma sheet are derived, and the density gradient of the corotating plasma structure in the middle Jovian magnetosphere is estimated. The PRA experiment gives information about the plasma wave polarization. The emission was found to be left hand polarized when the spacecraft was in the northern magnetic tail lobe, and right hand in the southern lobe. The PWS experiment enables the determination of a density profile of the Jovian plasma sheet from the knowledge of the low frequency cutoffs of nonthermal continuum radiation observed at 3 frequencies (f=562 Hz, f=1.0 kHz, f=1.78 kHz). The results are discussed and compared with those obtained from other investigations.

International audience; One major objective of the Cassini mission is the analysis of Saturnian radio emissions of magnetospheric (auroral) as well as atmospheric (lightning) origin. The Radio and Plasma Wave Science (RPWS) experiment is designed to measure the full polarization and the wave vector of the incoming radio waves, allowing us to retrieve information on source locations and emission modes. For that purpose, RPWS uses a two-channel receiver, connected to two electric monopoles (selected among three), which measures the voltages induced by the electric field of the incident waves and their various correlations. The accuracy of retrieved source locations depends directly on the precise knowledge of the orientation of the three effective monopole axes and lengths, which do not coincide with the physical ones owing to interaction with the spacecraft body. Antenna calibration aims at determining the so-called effective length vector of each antenna (combining orientation and length information). For that purpose, roll maneuvers of the Cassini spacecraft were performed before and after the Jupiter flyby, at distances such that Jovian radio sources can be identified with the planet's center but still provide a high signal-to-noise ratio. The resulting modulations of the measured signals allow us to derive the orientation and length of the effective antennas. The analysis is performed in two steps: first, the Stokes parameters (wave polarization) are determined using approximate antenna orientations derived from laboratory measurements on a scale model of the spacecraft. Second, measurements with high signal-to-noise ratio and pure circular polarization are selected and used for the determination of the effective length vectors of the RPWS antennas. Two methods have been developed for inverting the system of equations relating antenna parameters, wave parameters, and measurements (least squares fit and analytical inversion), both of which provide consistent results and present different advantages and limitations which are discussed. A final set of antenna parameters to be used for direction finding studies with the RPWS experiment is obtained.

Since the Voyager 2 encounter in early 1986, several investigators have attempted to localize the source regions of the smooth high-frequency radio emission which was observed by the planetary radio astronomy experiment at the nightside of Uranus. The various studies (most of them are based on the offset tilted dipole (OTD) model of the Uranian magnetic field) yielded significantly different source locations around the southern magnetic pole of Uranus. This may be a consequence of the individual a priori assumptions of the source model. However, the simplicity of the OTD model (Ness et al., 1986) also cannot adequately represent the complexity of the magnetic field at the radio source locations near the planet. The aim of this study is twofold. (1) We reanalyze the various source locations given in the literature (most of them are based on the OTD model) in the frame of the Q3 magnetic field model (Connerney et al., 1987). Our analysis moves some of the previously determined source locations from open toward closed field lines; however, the uncertainty due to the nonuniqueness of the Q3 model remains too large to exclude the possibility that open field lines are the source of smooth Uranian kilometric radiation. (2) We calculate the uncertainty of the radio source locations imposed by the nonuniqueness of the Q3 and OTD magnetic field models. We construct solutions by using generalized inversion techniques (Connerney, 1981) to obtain estimates of those magnetic field parameters (spherical harmonic coefficients up to degree and order 6) that are constrained by the magnetometer observations. The nonuniqueness of the resulting magnetic field models translates into an uncertainty about the radio source locations of some 20° in Uranocentric coordinates at altitudes of about 1.5 Uranian radii (RU). The present results are important for radio source locations at all the outer planets whose magnetic field geometries are represented by nonunique magnetic field models.

The observations of the planetary radio astronomy (PRA) experiment aboard Voyager 2 reveal the existence of smooth and bursty radio emission. Both recur in a rather regular pattern with a 16.1-hour period (the Neptunian spin period). We describe the phenomenology of the smooth component in terms of frequency, polarization, and occurrence in magnetic longitude and latitude. The existence of both right-handed and left-handed polarized emissions is consistent with two sources (one in each hemisphere) which radiate independently in the TUX mode. Because Voyager passed Neptune at less than 5000 km from the surface at high northern magnetic latitudes, the radio sources were occulted by the planet near the encounter. We have taken advantage of this occultation to locate the northern hemisphere sources by calculating the radio horizon (based on the offset tilted dipole (OTD2) model) for two spacecraft positions close to the encounter. We find that the northern source is located at high magnetic latitudes δm > 40°. By using a geometrical beaming model which assumes emission in a hollow cone pattern we fit the observed PRA intensity profile. The best fit is obtained for a radio source at L=6, thus confirming δm > 40°. The longitudinal extent of the source is at least 180°, from −90° to +90° magnetic longitude. To locate the southern sources we use the large excursions of Voyager 2 to high southern magnetic latitudes. The source is found to be located also at high magnetic latitudes but is possibly more limited in longitude than the northern source. We estimate the uncertainty in source location due to the very limited knowledge of the magnetic field near the Neptunian surface, and we show that the current OTD2 model is satisfactory for the source locations for the lowest observed frequencies; however, an angular uncertainty of about 20° remains for sources in the northern hemisphere. The observed pattern of the smooth emission is strongly frequency dependent, which is in agreement with the azimuthal asymmetries of the magnetic field as predicted by the OTD2 model.

The Ulysses unified radio and plasma wave (URAP) experiment is well suited to observe the Jovian hectometric radio emission (HOM) and so complement the observations made by the low-frequency band receiver on Voyager. When the spacecraft encounters Jupiter, in February 1992, observations will be possible from considerably higher magnetic latitudes, in both hemispheres, than those accessible to Voyager. It should also be possible to determine the source location, as URAP can measure the direction of the incoming waves and all four Stokes parameters. Based upon the HOM beaming and source model of Ladreiter and Leblanc (1990a,b), the following predictions can be made: (1) During the inbound trajectory, Ulysses will, at first, be close to the Jovian equatorial plane when equal amounts of right-handed (RH) and left-handed (LH) circularly polarized HOM should be observed from the northern hemisphere and the southern hemisphere, respectively. During the three days immediately before closest approach, the spacecraft will move to a higher northerly magnetic latitude where it will be outside the main HOM emission beam so that a relatively weak RH polarized signal should be observed. (2) At closest approach, Ulysses will be very close to the low-frequency HOM sources and will cross the HOM shadow zone, which extends out to about 20 Jovian radii (RJ) close to the magnetic equatorial plane. (3) During the outbound trajectory, the spacecraft will be at a fairly high southerly magnetic latitude and outside the main HOM emission beam, so that a relatively weak LH polarized signal should be observed.

Ray tracing using realistic magnetic field and plasma models has been performed to locate the Jovian hectometric radiation (HOM) sources. To consider only a few possible source locations, we have assumed that the radiation mechanism is the cyclotron maser instability (CMI), and taken into account the characteristics of the HOM including the recent results by Ladreiter and Leblanc (1989) (hereafter paper 1). The emission is assumed to escape from the planet in the extraordinary wave mode within a thin beam of hollow cones whose apexes are distributed uniformly in longitude at the northern and southern hemispheres at altitudes where the gyrofrequency fc is less but nearly equal to the observed wave frequency. The rays are traced from possible sources located at f/fc = 1.05 and labeled by the dipole shell parameter L. For each source, we have determined the emergence wave normal angle θcrit above which the rays no longer travel within the HOM emission beam nearby the magnetic equator (paper 1). We show that the HOM sources cannot be located on field lines within the Io flux tube. However, wave refraction due to the Io torus plays a crucial role for the propagation and beaming of the waves toward the magnetic equator. It is found that HOM sources located at field lines 15 ≤ L ≤ 30 agree best with the observations when assuming that the cone half angle of emission is about 70°–90° with respect to the local magnetic field. Thus, the source is associated with the tail field aurora where the magnetic field lines connect the polar region to the Jovian magnetic tail and is extended in altitude from 2 to 7 Jovian radii (RJ). Our results explain in detail the radio phenomenology of the hectometric radiation, in particular the strong latitudinal beaming, the lowest observed frequencies (≈ 40 kHz), the solar wind control, the absence of HOM emission when Voyager 1 was within the Io torus, and the absence of Io control on the HOM. The results are discussed in the context of the other Jovian radio sources, namely the sources of the non-Io decametric and kilometric emissions.

By using planetary radio astronomy (PRA), plasma wave system (PWS), and Magnetometer (MAG) data from Voyager 1 and 2 (V1 and V2), essential features of the nightside Jovian plasma sheet are derived, and the density gradient of the corotating plasma structure in the middle Jovian magnetosphere is calculated. The PRA experiment gives information about the plasma wave polarization. To determine the density profile of the plasma sheet, the authors have derived the hinge point position of the plasma disc from MAG data and used the low-frequency cutoffs observed at three frequencies (562 Hz, 1 kHz, and 1.78 kHz) from the PWS experiment. They show that the hinge point position varies with the solar wind ram pressure, and the plasma disc thickness decreases with distance up to about 60 R{sub J}. The average thickness for an isodensity contour corresponding to 1 kHz is 3.29 R{sub J} for V1 and 3.16 R{sub J} for V2.