TOPCAT II: A CubeSat GPS instrument for ionospheric imaging

Posted in: Department of Electronic & Electrical Engineering, Research

Author: Talini Pinto Jayawardena -

TOPCAT II are two GPS-based satellite instruments developed by the University of Bath for the upcoming US-UK joint space weather science mission, CIRCE. The payloads were developed by a team led by Dr Robert Watson and Prof Cathryn Mitchell at the Department of Electronic & Electrical Engineering’s Invert – Centre for Imaging Science. The project was managed by Dr Ali Hadavizadeh.

CIRCE, which stands for the Coordinated Ionospheric Reconstruction Cubesat Experiment, is a US Naval Research Laboratory (NRL) and Defence Science and Technology Laboratory (Dstl) funded mission that consists of two satellites to study dynamics of the ionised upper atmosphere, known as the ionosphere. Each CIRCE satellite will carry University of Bath’s TOPCAT II payload and four additional instruments from NRL (US), UCL (UK) and SSTL (UK) that will measure the electron density, optical UV, ionised and neutral atmosphere and radiation of the region. The satellites are expected to be launched into Low Earth Orbit (LEO) in 2021.


TOPCAT II – TOPside ionosphere and plasmasphere Computer Assisted Tomography II – from the University of Bath consists of a triple frequency GPS receiver that will measure the electron density between the GPS satellites and the CIRCE satellites at LEO. It is controlled by an interface board that ensures the correct operation and communication between the receiver and the main satellite. The full design, development and test cycle of TOPCAT II from prototype to space qualified payload was all carried out within a short development time of 8 months!

How Does It Work?

Radio waves such as GPS signals that travel through the ionosphere interact with free electrons in the region, which can be measured as a delay in the signal that is proportional to the electron density that the signal passes through. The measurements can be used in a process called tomography, similar to CT scans in medical diagnostics, to image the electron density of the ionosphere.

TOpcat II

Why is it important?

One main error source of satellite navigation signals such as GPS is the ionosphere, which is affected by space weather. Space weather can be considered to be analogous to normal atmospheric weather, but the source of this is emissions from the Sun – both radiation and particles. When these emissions interact with the Earth’s magnetic field, it causes variations in the ionosphere. These variations can disrupt the GPS and other similar Global Navigation Satellite Signals (GNSS), which can result in navigation and timing errors for satellite navigation users.

This can be illustrated using an example such as automated landing of aircraft using GNSS. Variations in the ionosphere can result in GNSS signal scintillation, effectively fluctuating the signal rapidly. When this happens, the aircraft receivers can lose the signals from some satellites and therefore will not be able to use GNSS for automated landing. Disturbance in the ionosphere due to space weather can also disrupt other radio signals such HF, VHF and UHF signals that are used for communication.

TOPCAT II and other instruments of CIRCE will allow us to measure and study the ionosphere and improve our understanding of the region and related space weather dynamics, thereby helping us mitigate their effects on systems such as GNSS and HF communications.

Handing over topcat payload.
The handover of TOPCAT II from the University of Bath to the Defence Science and Technology Laboratory.

Posted in: Department of Electronic & Electrical Engineering, Research

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  • Well done Bath .... Keep up the good work !! .... Best Regards.. Mike.E.

    • Good to see you're still keeping an eye on us Mike!