## 4 Functionality Demonstration of the Mission Design Configurations in the ISL Network
### Demonstration of Signal Time Delay Detection and Mission Determination  
The 1 PPS GPS reference time stamps from the GPS modules mounted on each satellite were locked to more than 10 satellites and synchronized as shown in Figure 10. On a duty cycle of 1 s, each pulse was detected to be 0.9 s ON and 0.1 s OFF. These 1 PPS reference time stamps were marked on both the incoming and the outgoing signals. The saved wave file data shown in Figure 11 was analyzed to visualize and to detect the time stamp and possible signal delay. The horizontal and vertical axis represent the time and amplitude of the signal, respectively. Three data frames recorded for 3.675 s were observed with a duty cycle of 1 s (0.9 s ON and 0.1 s OFF). Since both satellite payloads were stationary and there was no influence of _TWVC_ and _TEC_, the time stamps of the transmitted and the received signal  
Figure 11: Sample of data at 1 MSPS at 460.00 MHz. Figure 10: Synchronized 1 PPS GPS signal for _Tx_ (yellow) and for _Rx_ (blue). were superimposed at the same positions. This result signifies that the time stamp function was successful and no delay was detected at both 400.15 MHz and 460.00 MHz frequencies since the signals were free from _TWVC_ and _TEC_ delays. During the table satellite test, the SDR hardware suffered from a 35 \(\upmu\)s clock jitter error because its internal clock was not fast enough to support the transfer of data. The clock jittering distorted the signals by flipping the bits, inducing noise data or causing time offset by shifting the positions of the bits. To solve the problem, a 10 MHz GPS clock was integrated to the system to eliminate the effects of clock jittering. After the transmission of SS signal from one satellite (Sat _A_), three data frames received on the second satellite (Sat _B_) were analyzed. Figure 12 shows parts of the received data frames when the RF switch was turned ON and OFF. Observations were made on the received data in an interval of every 1 s. The data frames which contain the same information showed that there was no signal distortion due to clock jittering. The positions, patterns, and time for three data frames were analyzed at 0.77330 s, 1.77330 s, and 2.77330 s, respectively. From the analysis, the bit patterns were correctly received without the flipping or shifting of bits, or unwanted noise. Furthermore, the sections of the time stamps when the RF switch is OFF were completely cut off as anticipated. These results demonstrated the efficiency of the system for calculating the _TWVC_ data without distortion of information or incorrect predictions due to clock jittering. Since the position of satellites changes continuously in orbit, the signal passes through a different part of the atmosphere and ionosphere each second. Therefore, a new true range and new propagation time delay should be calculated onboard the satellite. If the signal time shift caused by _TWVC_ and _TEC_ can be isolated in each second, _TWVC_ and _TEC_ can be measured from the time delay. In that 1 s, frequency manipulation must be achieved to obtain a high resolution of _TWVC_ from _TEC_. To simulate the _TWVC_/_TEC_ time delay (s), a delay pulse generator was connected to the receiving side of each satellite as shown in Figure 6. In this study a time delay of \(\delta T\) = 0.3 s was assumed in order to clearly visualize the delay simulation functionality and positions of 1 PPS time stamps on both the  
Figure 12: Analysis of data frames from the received signal at 450 MHz and 1 MSPS. transmitting and receiving side. This delay was added to a GPS 1 PPS signal (yellow) by a delay pulse generator and a delayed pulse (blue) was formed as indicated in Figure 13.  
As observed on the oscilloscope, the time between the rising or falling edges of the 1 PPS pulse and the corresponding delayed pulse was 0.3 s as set on the delay pulse generator.