University of Twente Student Theses


The radio frequency interference environment behind the moon between 0.3 - 30 MHz

MEULMAN, G.R. (2015) The radio frequency interference environment behind the moon between 0.3 - 30 MHz.

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Abstract:In the study of several processes in the universe, observations at low frequencies (0.3 MHz – 30 MHz) will give valuable information. These observations cannot be done on the Earth’s surface due to the obstruction and scintillation by the Earth’s atmosphere at low frequencies. Therefore observations have to be done in space. To protect observing satellites in space from radio interference from Earth, the Moon might be used as a shield. It is however not known how well the Moon can function as a shield, and therefore several sources of interference and propagation mechanism are examined in this literature research. The Radio Frequency Interference (RFI) produced by the Earth has several sources. To determine the amount of RFI behind the Moon, these sources are looked into. The first source that is examined is artificial radio communication. These transmissions are used for broadcasting, navigation systems and many more applications. Another source of RFI that is looked into is lightning. During lightning discharges, which globally occur 40-50 times per second, RFI is produced. The third main source that will be examined is Auroral Kilometric Radiation (AKR) which is produced at a high altitude in the auroral zones. An important aspect which has to be accounted for is the location where the RFI is produced. The AKR is produced above the ionosphere, and therefore does not have to propagate through it. Lightning and artificial transmissions however do have to propagate through the ionosphere, which will attenuate signals especially at the lower frequencies. This attenuation also depends on whether it is daytime or night time. To end up at an observer behind the Moon, the RFI also has to propagate behind the Moon. Various propagation mechanisms are looked into to determine the most important propagation mechanism. First, direct line-of-sight (LOS) is discussed. This is especially important for AKR, since AKR is produced at a high altitude above the Earth. When the Moon is between the source and observer, diffraction may occur. Diffraction can be modelled as Fraunhofer diffraction for the far-field, and Fresnel diffraction for the near-field. The final propagation mechanism that will be discussed is surface wave propagation, which will use the surface and the atmosphere as a waveguide to travel past the horizon. Surface wave propagation can be separated in sky wave propagation and ground wave propagation. It appeared that AKR is the most important source, and ground wave propagation is the most important propagation mechanism to get to an observer behind the Moon. Therefore AKR and ground wave propagation are further examined to determine their influence on the amount of RFI behind the Moon. From the information collected about the different sources and propagation mechanisms it can be concluded that the Moon can act as a shield against RFI produced by the Earth. Due to the very low conductivity of the Moon’s surface, interference will be much weaker than the Galactic Background Radio Noise when not in LOS, and therefore proper observations can be conducted behind the Moon.
Item Type:Essay (Bachelor)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:53 electrotechnology
Programme:Electrical Engineering BSc (56953)
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