## 1 Introduction
GMOD is based on in-house instrument heritage and will be used to scale up to a 6U mission with detection and localisation capabilities, called Gamma-ray Investigation of the Full Transient Sky (GIFTS) [29; 30; 35; 36; 37]. The characterisation of an instrument in the spacecraft is necessary to investigate its performance and ensure that it is capable of achieving the mission's science goals [38]. Instrument characterisation as presented in this paper provides an in-depth understanding of the behaviour of the instrument that could not be achieved via functional testing. Performing the characterisation of the flight model of GMOD in EIRSAT-1 is essential for interpreting the downlinked science data throughout the mission and identifying intrinsic features in the data products. This characterisation campaign also provides a benchmark of the experiment's performance pre-launch. The results from this campaign can be used to compare with the detector's performance in-orbit, during commissioning and nominal operations. Initially, the ground-based energy calibration of GMOD was performed to map the analog-to-digital (ADC) values to energy. Once the data were in energy space, the properties of the detector could be investigated, including the instrument's resolution. A calibration of the instrument will be performed post-launch using the signature of galactic positron annihilation at 511 keV to determine the shift in the energy spectrum [39]. Additionally, lines from solar activity, e.g., the 2.2 MeV line from neutron capture on hydrogen [40], can be used for in-orbit calibration. In addition to the ground characterisation, in-orbit cross-calibration will be performed when GRBs are detected in coincidence with other missions (e.g., Ref. [41]). The aim of these tests was, first, to evaluate the effective area of the GMOD detector as a function of energy and source incident angle to correctly generate the detector response files [5]. This gave an understanding of the response of the detector and allowed an interpretation of the mission's science data when analysing potential GRB triggers from different sky positions (on- and off-axis). Some triggers were also simulated by bringing a source close to the spacecraft for a short period of time. The aim of these tests was to further validate the correct operation of the trigger algorithm and data processing and protection in the onboard software (OBSW) v18.3; see Refs. [30; 42] for more details. Simulations were performed in MEGAlib, a collection of programs to simulate gamma-ray instrumentation [43], to correlate with measurements in the lab and validate a Geomega EIRSAT-1 model, with the original model detailed in Ref. [29]. Section 2 presents the experimental set-up in the cleanroom, including the angular measurements on a motorised turntable, where all the characterisation tests were performed with the EIRSAT-1 flight model (FM). The methods for producing the calibration and energy resolution are outlined in Sections 3 and 4. The effective area of GMOD as a function of energy and the angular response of GMOD are presented in Section 5. The trigger data analysis is presented in Section 6. The simulations performed in MEGALib are discussed in Section 7 and compared to the experimental data to assess the accuracy of the model.