## 4 Functionality Demonstration of the Mission Design Configurations in the ISL Network
### SS-BPSK Transmitter  
The software for SS-BPSK transmission was implemented on each satellite in the ISL network using GNU Radio digital signal processing software and the Python programming language [51]. The SS signal carries the time and location data of one satellite to another which is needed when computing propagation time delay due to the influence of water vapor and total electron content. Information was transmitted from Sat \(A\) (SS-BPSK transmitter) to Sat \(B\) (SS-BPSK receiver) in the ISL network. As illustrated in Figure 7, the SS-BPSK transmitter is where the SS data packets are transmitted from the vector source block over a wider bandwidth at a bit-rate of 250 bits per second, with 1 bit having 250 chips, and 1 chip recurring at every 16 \(\upmu\)s. The SS signal is composed of the header, satellite ID, GPS time, and location information as well as the footer. The header and the footer are identifiers which show the beginning and the end of each packet of data conveyed in the communication network. For simulation purposes, the transmitted information was composed of header information: F; satellite identification (Sat ID): 01; GPS data of time (s): 04; latitude (\({}^{\circ}\)): 33.53513942; longitude (\({}^{\circ}\)): 130.50418427; altitude (m): 46; and footer  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Bands** & **Band 1 (\(f_{1}\) MHz)** & **Band 2 (\(f_{2}\) MHz)** & **Maximum Frequency Gap** \\ \hline
**Available band pairs** & 400.15–401.00 & 460.00–470.00 & 69.85 \\ \hline
**Selected bands** & 400.15 & 460.00 & 59.85 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Frequency selection table.  
information: A. The positive polarity of the latitude and longitude represent north and east positions of the GPS's geodetic coordinates, respectively.  
Likewise, this would indicate the south and west coordinates of the GPS latitude and longitude position data in the case of negative polarity, respectively. The precision the longitude and latitude data were all obtained with \(\pm 1.11\) mm location accuracy. The data in text was converted to 31 bytes of hexadecimal data (0x46, 0x30, 0x31, 0x30, 0x34, 0x33, 0x33, 0x2E, 0x35, 0x33, 0x35, 0x31, 0x33, 0x39, 0x34, 0x32, 0x31, 0x33, 0x30, 0x2E, 0x35, 0x30, 0x34, 0x31, 0x38, 0x34, 0x32, 0x37, 0x34, 0x36, 0x41) that was transmitted in binary form. The SS source software was implemented to add two more bits (00) to the footer in order to have 250 bits of data transmitted in every second. The original SS data was saved using the file sink block at Sat \(A\) for comparison with the received data at Sat \(B\). The transmitted sequence with GPS data was then modulated with BPSK modulation using a constellation modulator. The modulated data was transmitted with SDR through the USRP Hardware Driver (UHD) sink block. For demonstration purposes sample rates of 1 mega sample per second (MSPS) and 500 kilo samples per second (KSPS) and two frequencies (400.15 MHz and 460.00 MHz) were utilized interchangeably through tuning of the digital signal processing parameters.  
Figure 6: ISL network test bed.  
Figure 7: SS-BPSK transmitter at Sat \(A\).