Effect of polarization on the link dynamics of a spinning low-earth orbits satellite aligned with geomagnetic field

Abstract

This paper discusses the dynamic communication link aspects of IIT Madras student Satellite (IITMSAT). It is a Low-Earth Orbits satellite which has limited Attitude Determination and Control System (ADCS) ability to align one of its body axes with the geomagnetic field in orbit. The ADCS of IITMSAT has no control over its spin with respect to the axis along the geomagnetic field. This has a destructive influence on the link aspects between the satellite and the ground station. As the satellite spins over its axis, given the two-axis attitude control, the gain of the downlink antennas in the direction of the ground station vary all through the pass of the satellite over the ground station, which will, in turn, affect the signal received at the ground station. Similarly, the uplink is also affected. A rigorous analysis of the variation of the link strengths for uplink and downlink along typical passes of the satellite above a ground station located in Chennai, India, is presented, taking into account the geomagnetic field at the altitude of the orbit. This analysis helps in choosing the polarization (LHCP or RHCP) for uplink and stresses the need for polarization diversity at the ground station for downlink. This will ensure the link with the ground station remains robust through the pass of the satellite and hence achieving higher amounts of data transfer with the satellite. Further, this work helps design the satellite onboard antennas for better link performance, taking into account the ground station location and assisting in taking system-level decisions for small satellite missions.