OPERA is an electric field experiment designed to measure Ex, Ey, Ez from DC to 200 kHz. OPERA (Onde de Plasma E Radiazioni Aurorali) is part of the ASPI complex devoted to the study of plasma waves. The ASPI package is made up of OPERA itself, the MIF-M experiment (95-039A-13) for magnetic field fluctuations (a fluxgate and two search coils), and the IFPE (95-039A-14) which studies the particle density fluctuations by means of three Faraday cups. Other parts of the ASPI complex are PRAM (95-039A-16), which is a data processor, and ADS (95-039A-15), which is a sort of spectrum analyzer.
The electric field along 3 axes is measured by detecting the potential differences among probe pairs located at the end of long booms that constitute 3 dipoles of 22, 4.4, and 4.4 m, respectively. All the probes are far away from the spacecraft, which is very important in order to minimize spacecraft electromagnetic contamination. Each probe is made of a sphere of aluminum painted with a special conductive carbon colloidal (graphite). The 6 signals are sent to 3 differential amplifiers from which 3 outputs are obtained: the quasi-static electric field DC to 0.2 Hz in 3 components; the potential of each probe with respect to the spacecraft ground; and the 3 AC signals 0.2 Hz to 200 kHz. These last signals are handled in different ways: three bandpass filters (0.2 - 32 Hz) to measure the waveform; a wideband filter 10 - 1000 Hz; and a bank of six filters, each capable of covering 4 adjacent frequency bands. Aside from the data sent directly to the digital telemetry, six channels are provided to the analog telemetry: the DC electric field components and the waveform components.
The DC electric field sensitivity is 0.25 to 5 mV/m, depending on gain and dipole length. The waveform measurement sensitivity ranges from 1 to 0.1 mV/m Hz^0.5 for 0.1 - 32 Hz signals. For the 10-1000 Hz channel, at 10 Hz with a gain of 1 it is 1 to 0.03 mV/m Hz^0.5. At 1000 Hz with gain 10, it is .1 to 3E-3 mV/m Hz^0.5. For the AC signals (10 Hz - 200 kHz) the sensitivity is 1E-8 V/m Hz^0.5 with high gain, and 20 dB less with low gain.
The overall dynamical range for both DC electric field and waveform measurements is +635 to -640 mV/m. The AC dynamical range is 80 dB. Nominal accuracy for all measurements is 2 %.