TOPCAT: A space-borne dual-Frequency GPS receiver for topside ionosphere and plasmasphere computer assisted tomography

Free electrons in the ionosphere and plasmasphere are the single dominant cause of signal delay in Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS). Extensive research has already been carried out on the effects of the ionosphere on GNSS, and the dynamics of the electron content of the region is well understood. In contrast, relatively less effort has been extended to study the topside ionosphere and plasmasphere that define the outer limits of the ionized atmosphere of Earth. TOPCAT is a dual frequency GPS Ll(1.575 GHz)/L2(1.227 GHz) receiver that allows remote measurement of electron densities of these regions. It is a payload installed in the UK Space Agency (UKSA) led pilot CubeSat 'UKube-1', launched in July 2014 into Low Earth Orbit (LEO). The payload was developed using simple commercial-off-the-shelf (COTS) components to permit a relatively low cost method to study the upper atmosphere. The technology and design were successfully qualified for operating in a LEO environment through a series of ground testing. Phase data collected by the receiver will be used to image the electron density of the topside ionosphere and plasmasphere using ray tomography. TOPCAT serves as a technology demonstration for a future constellation of CubeSats that will enable continuous global coverage and further research of the regions.