Satellite attitude effects on the reception of transionospheric HF signals: Examples from the Radio Receiver Instrument onboard e-POP/Swarm-E

The characteristics observed by an antenna onboard a spacecraft depend on the geometry between the receiving antenna and the electric fields of the radio wave. The antenna boresight direction should be along the transmitting source’s line-of-sight direction to be able to completely resolve the wave’s polarization characteristics. However, this is not always the case, especially for the antenna onboard a spacecraft due to the spacecraft orientation restrictions. Hence, in this study, the focus is on the antenna orientation effects on the observed polarization characteristics of the radio wave. Precise quaternion and sp3 orbit data of Swarm-E (formerly known as the CAScade, Smallsat, and Ionospheric Polar Explorer – CASSIOPE) spacecraft are used to determine the boresight of the Radio Receiver Instrument (RRI) onboard, for the coordinated experiments with the NRCAN-Ottawa transmitter (45.4oN, 75.6oW). The observed characteristics of the HF radio waves are determined for two selected cases with different attitude behavior: slew-to-target on 14 May 2017, and nadir mode on 11 August 2017. The effect of the angle between dipoles and radio wave propagation direction should be removed to resolve the physical characteristics of the wave.