Statistics – Applications
Scientific paper
Dec 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996esasp.392..189k&link_type=abstract
Environment Modelling for Space-based Applications, Symposium Proceedings (ESA SP-392). ESTEC Noordwijk, 18-20 September 1996. E
Statistics
Applications
Scientific paper
Several common problems occur in measurement techniques and interpretation of plasma natural emissions and antenna impedance data. In both cases the voltage measured at antenna terminals is detected and related to properties of the plasma. In both cases antenna properties in plasma are of prime importance in equivalent circuit analysis. The main difference stems from e.m.f. driving respective circuits. In impedance measurements, the voltage provided by instrument usually dominates e.m.f.so the determination of circuit impedances controlled by plasma parameters is of primary concern. The main goal of plasma natural emission measurements is to determine contribution to e.m.f. due to natural processes and the case when it is the only contribution is the best one. Spacecraft - environment interaction results in variability of equivalent circuit impedances and e.m.f. components and imposes severe constrains on usefulness of simplified interpretation of experimental data. Simulation of impedance and wave data recorded on the ACTIVE APEX and CORONAS satellites addresses these problems. All three satellites have been launched into the orbits with inclination of 82.5deg and provided data from altitudes 440km up to 3100km. The self impedance of a monopole of 7.5m length was measured with the mesh of 200 frequencies evenly distributed in frequency band 0.1-10MHz. The plasma emission spectra were measured on the same mesh on the ACTIVE and APEX satellites and up to 30MHz on CORONAS satellite with the dipole antenna of the 15m length. In stable ionosphere the emission and impedance spectra were satisfactory matched with the equivalent circuits consisting of known preamplifiers' components and monopole impedance calculated according to Balmain cold plasma formula supplemented by vacuum sheath. Peculiar spectral structures were registered in naturally unstable equatorial and polar ionosphere and under the influence of real time telemetry transmitters and ground based high frequency transmitters. Several spectral features were matched after instruments input circuit characteristics were allowed to change due to excessive voltages imposed on protecting diodes. It follows from the differences in implemented input circuits design that spectral content of the disturbing voltages ranged from hundreds of MHz due to real telemetry operation down to a fraction of Hz probably due to differential charging by particle fluxes. It follows from the performed simulations that by proper design and monitoring of input circuits the HF spectra may provide very rich information about internal properties of the environment and its influence on spacecraft operation. Such divers data as reliable values of local density of cold one component as well as multicomponent electron plasma, critical frequency beneath the spacecraft, voltage transients of very different origin in very broad frequency range extending far beyond the nominal instrument band, local and remote generation of plasma emissions can be registered with high temporal resolution. In extreme implementations these objectives can be achieved either by feeding the antennas with HF voltage which exceeds only slightly the cosmic noise level in the case of electromagnetically very clean platforms or by relying totally on the voltage generated by the noisy spacecraft. In both cases the proposed measurement and monitoring technique perturbs environment at impressively low level.
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