## 2 Characterisation Test Set-Up  
### Energy Calibration Hardware Set-Up  
Initially, the measurements to determine the channel-energy relation of GMOD were performed. The main goal of these measurements was to maximise the number of counts during the integration interval. The spacecraft was secured in a vertical support stand for the energy calibration source measurements. The EIRSAT-1 FM was connected to the power and data acquisition ground-support equipment (GSE) via the Umbilical Interface Unit (UIU), as labelled in Figure 2 (Panel B). The UIU interfaced with a bench power supply unit (PSU) to provide a battery voltage of 8.35 V for the spacecraft. The UIU also provided an interface between the spacecraft and the laptop used to command the satellite via the MCS v21.2 and Python v3.7 scripts and retrieve the experimental data stored on the OBC. On the  
Figure 1: The flight model of EIRSAT-1 including the location of major subsystems and the GMOD detector assembly and motherboard. The coordinate system of the spacecraft is also detailed.  
laptop, a terminal was used to monitor the debug log from the spacecraft throughout the test activities.  
In the first test, different radiative sources (enumerated 1-6, see Table 1) were placed close to GMOD. As in Figure 2, sources 1-5 were positioned on the cross-hairs, which identified the centre of GMOD's CeBr\({}_{3}\) crystal, on a cap on the +Z face of the spacecraft (Panels A and C of Figure 2). The distance from the end cap to the centre of the crystal (CoC) is 49.6 mm. The position of source 6 is centred on the X face of the crystal, 222 mm from the +Y face and 119 mm from the -Z face (see Panel D of Figure 2). It was found that, with some of the sources in this position, the OBC could not manage the rate at which GMOD was generating data for long durations. To determine the GMOD data rate that the OBC could successfully handle, the sources were placed at different distances from the CoC.  
For each source position, the GMOD data production rate, in pages/min, was monitored via the MCS. The address of the GMOD sum page received by the OBC was determined at the same time as the address of the most recent page of generated summed data multiple times over a few minutes. If the difference between these was increasing as time progressed, the rate at which GMOD was producing data was greater than the rate at which the OBC was reading data. The source was moved until the difference between these two parameters remained the same over time, indicating that the OBC could manage the data rate produced by GMOD. This was found to be \(\sim\) 250 pages/min. Each page contains 31 TTEs, so the TTE rate that the OBC can keep up with for long-duration experiments (\(\sim\) hours) is \(\sim\) 130 TTEs/s or 130 counts/s. As GRBs are on much shorter timescales, this issue will not affect GMOD's ability to detect these events. Table 2 contains the exact sources used for the energy calibration measurements, their location with respect to the CoC in GMOD and the integration times.  
Figure 2: **(A)** EIRSAT-1, including co-ordinate system, on vertical integration stand for energy calibration measurements with a source positioned on the +Z endcap. **(B)** GSE for measurements; on the left is the FM, in the centre is the UIU and on the right is the PSU powering the spacecraft. **(C)** Source positioned on the +Z endcap on cross-hairs marking the centre of the CeBr\({}_{3}\) crystal. **(D)** Position of source 6 in Table 2, the source is centred on X of the crystal, 222 mm from the +Y face and 119 mm from the \(-Z\) face.