zenichi yamamoto

GaN HEMT has been expected to satisfy requirements being used in space as amplifying devices of future high-efficiency onboard SSPAs due to its excellent properties such as robustness in a harsh environment, applicability to high frequency and high power operation. However, in X-band, the most important frequency band for space communication at this moment, evaluation of GaN HEMT to prove the space usage has not been conducted yet because ensuring a steady supply of X-band GaN HEMT devices is still difficult due to current collapse phenomena. Therefore, in this research, evaluation of RF characteristics affected by operational conditions and space environmental testing of X-band HPA using COTS GaN HEMT was conducted to secure the space usage.

The electron density profiles above the lunar surface are being observed by the radio occultation technique during the mission using the Vstar and Rstar sub-satellites. In addition to a traditional technique which uses one orbiter, we are conducting another method which uses two orbiters with the second one being used to measure the terrestrial ionosphere contribution. Previous radio occultation measurements have indicated the existence of an ionosphere with densities of up to 1000 cm^-3 above the dayside lunar surface. These densities are difficult to explain theoretically when the removal of plasma by the solar wind is considered, and thus the generation mechanism of the lunar ionosphere is a major issue, with even the validity of previous observations still under debate. The SELENE radio science experiment will establish the morphology of the lunar ionosphere and will reveal its relationship with various conditions to provide possible clues to the mechanism.