## 6 Discussion  
The interference measured over non-populated areas has similar behaviour to the AWGN behaviour both in frequency and time. The in-orbit spectrum measured (Figure 6b) shows the frequency response of the system that was measured in the lab (Figure 3a). The measurements over non-populated areas can be used as an in-orbit relative noise floor and justify the use of heatmaps over these areas since the behaviour is close to Gaussian. In the power spectrum, there is a variation that can be caused by the tumbling of the satellite that introduces a variation in antenna gain and polarisation, as well as a variation in internal temperature. On several of the measurement tracks, the satellite moved from daylight (highest internal temperature) to eclipse (lowest internal temperature). This could also be the explanation why the temporal behaviour is not completely Gaussian; perhaps there is a slow variation of internal temperature causing this difference. For the populated areas, the power levels of the interference are higher than the noise floor. In these areas, there can be a lot of interference sources, such as ground stations for different satellites and amateur radio activity. The results show that the measured interference over different regions has different frequency and time behaviour. There are different types of interference: some have AWGN statistics, others show CW temporal behaviour, and the last type has a pulsed tendency (both band-limited and wide-band). The AWGN is predominant on the edges of the frequency response where no other signals are present. The narrow-band signals (CW) can be be beacons from radio amateurs and would be possible to locate with more measurements. The interference that has roughly 300 kHz bandwidth has higher power than the noise floor and different statistics than CW and AWGN. This signal requires a longer time window to obtain a consistent estimate of the average envelope than the AWGN. It is consistently present over the areas where there are known type A radars. The frequency band measured in this paper is 435 MHz and is exactly in the middle of the band for ground radars (420-450 MHz), so it is expected to see some of that behaviour. The bandwidth also matches the chirp bandwidth for the search mode of type A radars used for space-tracking (100-350 kHz) [21]. Figure 13: Detection of band-limited interference, estimation of bandwidth, and CV. (**a**) Measurement location where band-limited interference (red crosses) and wide-band interference (black circles) with time structure was detected by visual analysis, rest of locations (blue stars), ground radar locations (black triangles with numbers) [21; 31]. 1: Massachusetts (U.S.), 2: Texas (U.S.), 3: California (U.S.), 4: Georgia (U.S.), 5: Florida (U.S.), 6: North Dakota (U.S.), 7: Alaska (U.S.), 8: Thule (Greenland), 9: [PERSON] (United Kingdom), 10: Pirinclik (Turkey). (**b**) Automatic bandwidth estimation using two parameters: the ratio of moments, \((m_{1k})^{2}/m_{2k}\); and the average power spectrum (\(m_{2k}\)). The cases with zero bandwidth corresponds to when the automatic detection failed. (**c**) The median of the CV over the bandwidth of the automatically detected band-limited interference. The cases with zero correspond to when the automatic detection failed. A high coefficient indicates high spread in the measured envelope. There was also another type of interference with time structure detected over central Europe. However, this interference has a bandwidth higher than what can be seen with the configuration of these measurements (higher than 500 kHz). This interference could be caused by the radar chirps from the tracking mode (1 MHz wide), but it can also be a different source. The time behaviour measured shows high variability within five seconds. Average power is a useful measure to design a system that works for the average interference environment, but it does not take the temporal structure and dispersion of the interference into account.