## 4 Experimental Validation
### Experimental Results  
With the major components of the hardware logic are verified for its functionality, the actual operation of the TLE observation system was verified on the ground simulation facility. Figure 13 illustrates a scenario for this experiment. As the targeted TLE is assumed to be a blue jet, a 405 nm UV LED on the bottom left corner blinks every 1 s and is turned on for 100 ms. As soon as the detector IP senses the flash light on the 54 th cell of the MaPMT, it triggers the camera IP with the corner coordinates for the ROI of which the size is predetermined by 640 by 480 px. Then the camera IP scans the internal buffer of the CMOS image sensor and only extracts the ROI pixels out of entire active image pixels.  
The MaPMT's 64 cell data before and after the trigger signal are plotted in Figure 14. The graph displays the time history of the MaPMT data of the 640 successive MaPMT frames at the MaPMT frame rate of 3.012 kHz. The abrupt change of the digitized value of the 54 th cell data clearly shows a pulse and its location on the MaPMT sensing plane. The \(y\)-axis value of each graph represents the electric charge output of the MaPMT during the period of 332 \(\upmu\)s. When the UV LED is off, the output value represents the amount of charge due to the dark current of each cell, which is approximately 0.0143 pC. On the other hand, during the UV LED on period, the electric charge of the 54 th cell increased by about 0.057 pC, which appears to be a drop in the graph. The increased electric charge is due to the radiant power of the UV LED for TLE simulation is arbitrarily chosen between the case of the limb direction and the nadir direction, corresponding to the electric charge of 0.407 pC and 0.0157 pC, respectively. The experimental result shows that the TLE observation system can sufficiently detect TLEs occurring at very long distances. In addition, the latency of the TLE detection algorithm is experimentally confirmed by measuring the time delay between the external command signal to the UV LED and the trigger signal from the detector IP. The experimental result, as shown in Figure 15,  
Figure 13: Scenario of the experiment on the ground simulation facility. First, a 405 nm UV LED blinks every 1 s and is turned on for 100 ms duration to emulate a blue jet. Next, as soon as the detector IP senses the flash light, it triggers the camera IP with the estimated location of the light. Finally, the camera IP captures successive images around the ROI.  
shows that the TLE detection flag occurs 664 \(\upmu\)s after the external UV LED is commanded on. Since the TLE discriminator needs to be run on two consecutive MaPMT frames in order to determine a TLE occurrence, it can be seen that the latency of the system is twice the MaPMT frame period, which is 332 \(\upmu\)s, as described in Section 3.1.  
As soon as the image sensor reconfigured, the camera started to capture images over the LED light and stored it until the user requested the data. As shown on Figure 16, LED light was captured on the four consecutive images since the camera had the frame rate of 40 frame per second.  
Figure 14: Graph of MaPMT measurements for 128 frames from 1 st cell (left bottom corner) to 64 th cell (right top corner). \(x\)-axis is the number of frame and \(y\)-axis is ADC value. On the 54 th cell (red box), the ADC value abruptly dropped as soon as receiving the flash light.  
Figure 15: Latency between the UV LED pulse and the TLE detection flag. The latency was 664 \(\upmu\)s which is caused by the TLE detection algorithm.