## 4 Discussion
### Mission Operations Issues and Status
#### 4.2.5 Spacecraft Status  
The Ten-Koh spacecraft was continuously operated daily since its launch (29 October 2018) until 19 March 2019, when a loss of communication between the spacecraft and the Kyutech ground station occurred. The ground-track of the last orbits of the satellite where the failure occurred are shown in Figure 16. The last data from its beacon were received over Greece on 18 March 2019 between 23:12:33 and 23:25:43 UTC. The decoded data indicated that all the main housekeeping parameters were inside their operating ranges, as shown in Figure 17. According to the same beacon data, the satellite was in the expected mission mode, i.e., nominal CPD mission scheduled between 22:00 on 18 March and 00:20 on 19 March UTC. The mission was supposed to finish when the satellite was coming from above the South Pole and heading northwest towards the southeast of South Africa. After finishing the CPD mission, Ten-Koh traveled for three more orbits, including two of them over the SA anomaly, before the pass over Kyutech ground station on 19 March 2019 at 13:31 JST (19 March 2019 at 04:31 UTC) when no signal was received. Irrespective of the anomaly that made the satellite fail, it occurred during the two orbits following the CPD mission and before the pass over the Kyutech ground station. A review of the geomagnetic activity parameters from the NOAA Space Weather Prediction Center between 15 March 2019 and 18 March 2019 revealed that the Kp-index, an average of K-indices from a network of geomagnetic observatories, showed above the threshold value of 4 on 17 March 2019. As shown in Figure 18, Kp = 4 was observed between 21:00 on 16 March 2019 and 00:00 on 17 March 2019 UTC, and increased to Kp = 5 between 00:00 and 03:00 on 17 March 2019 UTC. Kp then decreased to Kp = 4 between 03:00 and 09:00 on 17 March 2019 UTC. The Kp-index with values bigger than 4 indicates a geomagnetic storm activity. The Ten-Koh anomaly occurred approximately 50 h after the geomagnetic activity (between 18:00 on 18 March 2019 UTC and 03:00 on 19 March 2019 UTC). This shows a direct correlation between the anomaly and the presence of a geomagnetic storm. Space weather can produce temporal variations in the trapped radiation [34], which in turn can cause SEE and charging (surface and internal) that result in anomalies experienced by satellites. The main hypothesis for the failure of the satellite was an error triggered by a single event effect (SEE), which was previously observed as errors in the in-built EEPROM of several microcontrollers. Note that such events were expected to take place over South Atlantic Anomaly due to presence of high-energy protons (in excess of 30 MeV) that could penetrate the spacecraft structure. As in the case of the readings from the magnetometer that were stopped and then recovered after some time, the satellite began to transmit its beacon again after a solar storm that occurred on 10 May 2019 and that arrived at Earth between 14 and 16 May 2019. This correlation between and increased number of particles in LEO caused by the solar storm, and recovery of the satellite functionality corroborates the hypothesis that it was originally disabled by radiation damage. It is thought that the bit changes in the in-built EEPROM or microcontroller were undone by subsequent single event upsets so that the satellite could resume operations. It is also possible that the satellite was originally disabled by a single event latch-up, which did not increase the OBC power consumption enough to trigger the over-current protection. Subsequent latch-ups could have eventually caused the OCP trip, and the OBC to reboot and resume operations. This, however, is not the most likely scenario because multiple reboots were attempted from the ground but to no avail. Figure 16: Ten-Koh orbit ground-tracks when the satellite is suspected to have failed.