Schematic layout of the CIRCE payloads within the lead and trail satellite buses. The UK’s IRIS suite of payloads occupies the front 2U of the leading satellite, and during night-time, the rear of the trailing satellite. The bus avionics are located in the centre 2U of each bus. The US Tri-TIP payloads occupy the rear 2U of the leading satellite, and during night-time, the front of the trailing satellite. This enables the Tri-TIP instruments to image the same volume of the ionosphere to support tomographic reconstruction. (Adapted from Nicholas et al., 2019, their Fig. 1).