Wolfgang Macher

Cassini model rheometry

Rheometry serves as a method for the determination of effective length vectors of short antennas by means of electrolytic tank measurements. This paper reports on the application of rheometry to the three linear monopoles mounted for the purposes of the Radio and Plasma Wave Science Experiment on the Cassini spacecraft, which will fly to planet Saturn. The voltage signals induced by incoming waves from the Saturnian radio emissions will be recorded for further evaluation. By direction-finding techniques one will trace back from the collected data to the source regions of the received radio waves and determine the wave polarization. An accurate direction finding is only possible if the effective length vectors of the antennas, which are affected by the spacecraft body, are known to a certain degree of accuracy. It is investigated how rheometry enables the determination of the effective length vectors with the help of a scale model. After a detailed discussion of the fundamentals of rheometry, the application of rheometry to the Cassini scale model is described. The results of the measurements are graphically depicted and discussed with the requirements for direction finding taken into consideration. Finally, an overview of the inflight antenna calibration is given, which will be possible by utilizing the strong Jovian radio emissions during Cassinis Jupiter flyby.

Analysis of electromagnetic wave direction finding performed by spaceborne antennas using singular-value decomposition techniques

By using two rotating noncollinear antennas or three spatially fixed noncoplanar antennas on a spacecraft, full information on the polarization and the direction of arrival of an electromagnetic wave can be obtained by measuring the voltages created by the electric field of the incident wave. The physical parameters (polarization and direction of arrival) of the incoming wave are related to the received voltages on the antenna system by the so-called direction-finding equations. Since the used antennas are generally of small directivity (electrically short monopoles or dipoles), the resulting system of equations is numerically close to singular, and generally no unique solution can be obtained for the physical parameters of the wave throughout the inversion process. However, there exists a very powerful tool for dealing with sets of equations that are singular or close to singular, known as singular-value decomposition (SVD), which precisely focuses the problem. For illustration, this paper analyzes the direction-finding equations for the Radio and Plasma Wave Science (RPWS) experiment on the Cassini spacecraft by using SVD techniques. It also compares the expected performances of RPWS with those of the Ulysses Unified Radio and Plasma Wave (URAP) experiment achieved at Jupiter for the kilometer and hectometer emissions. The RPWS experiment on Cassini, which will be launched in 1997, is supposed to observe wave phenomena between a few hundred Hertz and 16 MHz in the Saturnian magnetosphere.

Rheometry of multi-port spaceborne antennas including mutual antenna capacitances and application to STEREO/WAVES

Electrolytic tank measurements were applied often to determine the antenna properties of spaceborne monopole antennas. This technique, called rheometry in the present context, yields the effective length vectors of antennas for the quasi-static frequency range. When receivers or preamplifiers are connected to the antennas, their input impedances and the capacitances of the cables change the effective length vectors. In the evaluation of former rheometry measurements this effect has been taken into account for each antenna individually, tacitly neglecting the mutual antenna capacitances. Our analysis shows that this neglect can cause errors in the measured effective axes (directions of effective length vectors) of up to about 10°. However, in order to apply direction finding techniques with a triaxial antenna system an accuracy of about 2° is required. We therefore extend rheometry to the measurement of the complete antenna capacitance matrix. An application of the presented technique to the WAVES antenna system onboard the STEREO spacecraft verifies that the mutual capacitances are crucial in this context. With the extended rheometry technique the antenna transfer and capacitance matrices can be completely determined, thereby yielding a full representation of the reception properties of the antenna system as part of an electronic circuit.

Accelerometry measurements using the Rosetta Lander's anchoring harpoon: experimental set-up, data reduction and signal analysis

In the years 2011–2013 the ESA mission Rosetta will explore the short period comet 46P/Wirtanen. The aims of the mission include investigation of the physical and chemical properties of the cometary nucleus and also the evolutionary processes of comets. It is planned to land a small probe on the surface of the comet, carrying a multitude of sensors devoted to in situ investigation of the material at the landing site. On touchdown at the nucleus, an anchoring harpoon will be fired into the surface to avoid a rebound of the lander and to supply a reaction force against mechanical operations such as sample drilling or instrument platform motion. The anchor should also prevent an ejection of the lander due to gas drag from sublimating volatiles when the comet becomes more active closer to the Sun. In this paper, we report on the development of one of the sensors of the MUPUS instrument aboard the Rosetta Lander, the MUPUS ANC-M (mechanical properties) sensor. Its purpose is to measure the deceleration of the anchor harpoon during penetration into the cometary soil. First the test facilities at the Max-Planck-Institute for Extraterrestrial Physics in Garching, Germany, are briefly described. Subsequently, we analyse several accelerometer signals obtained from test shots into various target materials. A procedure for signal reduction is described and possible errors that may be superimposed on the true acceleration or deceleration of the anchor are discussed in depth, with emphasis on the occurrence of zero line offsets in the signals. Finally, the influence of high-frequency resonant oscillations of the anchor body on the signals is discussed and difficulties faced when trying to derive grain sizes of granular target materials are considered. It is concluded that with the sampling rates used in this and several other space experiments currently under way or under development a reasonable resolution of strength distribution in soil layers can be achieved, but conclusions concerning grain size distribution would probably demand much higher sampling rates.

Calibration of Electric Field Sensors Onboard the Resonance Satellite

Strategies and results for calibrating electric field sensors (antennas), as used in radio astronomy, onboard the spacecraft “Resonance” are presented. Calibration is performed for four boom antennas and four cylindrical sensors at the boom tips. These antennas are devised for the measurement of electric fields and plasma parameters. It is shown that the electrical representations of the antennas, the effective length vectors, differ from their mechanical originals and are shortened and tilted by several degrees of angle. The knowledge of the acquired parameters is of great benefit to the Resonance mission. In particular, goniopolarimetry techniques like polarization analysis and direction finding depend crucially on the effective axes. For the first time, this kind of analysis is performed for a space-borne antenna system consisting of boom monopoles and cylindrical tip antennas.

In‐flight calibration of STEREO‐B/WAVES antenna system

The STEREO/WAVES (SWAVES) experiment on board the two STEREO spacecraft (Solar Terrestrial Relations Observatory) launched on 25 October 2006 is dedicated to the measurement of the radio spectrum at frequencies between a few kilohertz and 16 MHz. The SWAVES antenna system consists of 6 m long orthogonal monopoles designed to measure the electric component of the radio waves. With this configuration direction finding of radio sources and polarimetry (analysis of the polarization state) of incident radio waves is possible. For the evaluation of the SWAVES data the receiving properties of the antennas, distorted by the radiation coupling with the spacecraft body and other onboard devices, have to be known accurately. In the present context, these properties are described by the antenna effective length vectors. We present the results of an in-flight calibration of the SWAVES antennas using the observations of the nonthermal terrestrial auroral kilometric radiation (AKR) during STEREO roll maneuvers in an early stage of the mission. A least squares method combined with a genetic algorithm was applied to find the effective length vectors of the STEREO Behind (STEREO-B)/WAVES antennas in a quasi-static frequency range (Lantenna≪λwave) which fit best to the model and observed AKR intensity profiles. The obtained results confirm the former SWAVES antenna analysis by rheometry and numerical simulations. A final set of antenna parameters is recommended as a basis for evaluations of the SWAVES data.

Resonance spacecraft antenna calibration: Rheometry and numerical simulations

We report on the calibration effort for the monopole antennas onboard the Resonance spacecraft. The calibration is performed for four boom antennas and four cylindrical sensors at the boom tips. These antennas are devised for the measurement of electrical fields and plasma parameters. We apply two methods for the antenna analysis: first, electrolytic tank measurements (rheometry), which is a method to determine the effective length vectors of electrically short antennas; second, numerical computer simulations which enable us to study also the transition to higher frequencies. The accuracy of the applied methods is about 1 degree for directions of effective axes and some percent for effective lengths and antenna capacitances. It is shown that the electrical representations of the antennas, the effective length vector, differ from their mechanical originals, are shortened and tilted by several degrees of angle. The knowledge of the acquired parameters is of great benefit to the Resonance mission. In particular, goniopolarimetry techniques like polarization analysis and direction finding depend crucially on the effective axes. For the first time this kind of analysis is performed for a spaceborne antenna system consisting of boom monopoles and cylindrical tip antennas.

The uncertainty of the Uranian radio source location due to the nonuniqueness of the planetary magnetic field model

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.