Unified Radio & Plasma Wave Investigation
URAP is designed to detect both distant radio emissions, as well as locally generated plasma waves. The sensors consist of a 72.5 m electric field dipole antenna in the spin plane, a 7.5-m electric field monopole along the spin axis and a pair of orthogonal search coil magnetic antennas. The various receivers, designed to encompass specific needs of the investigation, cover the frequency range from DC to 1 MHz and are summarised below together with the relaxation sounder and DC measurements:
Radio Astronomy Receivers (RAR)
The radio receivers consist of four superheterodyne receivers whose frequency stepping is programmed in ROM memory and controlled by telecommand. Two receivers, ZL and ZH, are connected to the spin-axis (Z) preamplifier; the two others, SL and SH, are connected to the combination of the signals from the spin-plane (±X) preamplifiers and the Z preamplifier to form the electronically synthesized tilted dipole. In each case, one receiver is tuned to low frequencies (1.25-48.5 kHz), and the other to high frequencies (52-940 kHz).
Plasma Frequency Receiver (PFR)
The PFR is intended to monitor the wide spectrum of plasma phenomena with constant frequency coverage, large dynamic range, and good frequency resolution. Two such receivers are supplied, one for Ex and one for Ez. The frequency range, 0.57 to 35 kHz, is covered in 32 logarithmic frequency steps, with a corresponding separation of 14%.
Wave Form Analyser (WFA)
The WFA or FFT-DPU (Fast Fourier Transform Data Processing Unit) provides spectral analysis in the frequency range from 0.08 to 448 Hz of signals received from the plasma wave and magnetic preamplifiers. The spectral analysis is performed separately for frequencies below 10 Hz and between 10 Hz and 448 Hz. The > 10 Hz processing is done by three microprocessors, each dedicated to one of the Ex, By, and the selected Bz or Ez sources. The signals are analysed in 12 logarithmically-spaced bands. Signals for the < 10 Hz processing are obtained from the Ex and the selected By or Bz sources which have been low-pass filtered and converted with a 10-bit analog-to-digital converter. The signals are also analysed in 12 logarithmically-spaced bands.
Fast Envelope Sampler (FES)
The purpose of the FES is to capture transient, rapidly varying phenomena, at sample rates up to the order of one sample per millisecond, and store them in a memory for later telemetry. Two filter channels (Hi: 600 Hz – 60 kHz; and Lo: 10 Hz – 20 kHz) are used concurrently, with respectively 3 and 4 commandable filters for the isolation of one phenomenon in the presence of others, but otherwise FES has no frequency resolution, and only samples the envelope of the detected signal.
The major purpose of a relaxation sounder is to provide a reliable measure of the local electron plasma density, through the detection of the resonance excited close to the electron plasma frequency. The sounder was designed around use of the low frequency radio receiver, with the only specific hardware items being two simple transmitters, small additions to the radio receivers, and extra ROM for the microprocessors. The signal is analyzed on board by the URAP Data Processing Unit, using a modified Walsh transform.
DC voltage measurements
DC signals refer to measurements of the instantaneous potential or potential difference of the antennas. Three potentials are measured: the potential on the Z antenna with respect to the spacecraft EZDC, the potential of the +X antenna with respect to the spacecraft EXAN, and the potential difference between the two X antennas EXDC. DC voltage measurements are made using the plasma wave preamplifiers and some signal conditioning within the same box and are digitized by the Spacecraft Data Handling System. There are two sets of measurements made for XDC and ZDC each, fixed and scan. Fixed means that the samples are made at fixed angles with respect to the spacecraft rotation and scan means that the sample angle is changed each time by an amount such that after 512 samples each angle (512th of a spin) has been seen. This permits a reconstitution of the average field around the spacecraft.
Summary of Objectives
The scientific objectives of URAP experiment are twofold:
- the determination of the direction, angular size, and polarization of radio sources for remote sensing of the heliosphere and the Jovian magnetosphere
- the detailed study of local wave phenomena, which determine the transport coefficients of the ambient plasma
The tracking of solar radio bursts, for example, can provide three dimensional snap-shots of the large scale magnetic field configuration along which the solar exciter particles propagate.
URAP observations of Jovian radio emissions greatly improve the determination of source locations and consequently our understanding of the generation mechanism(s) of planetary radio emissions.
The study of observed wave-particle interactions will improve our understanding of the processes that occur in the solar wind and at Jupiter and of radio wave generation.