# A Novel Signal Design and Performance Analysis in NavCom Based on LEO Constellation
## 2 Related Surveys
### Satellite-Based Systems  
Satellite-based augmentation systems broadcast various error terms and other calibration information to end-users via GEO/MEO/LEO constellations to improve users' experience in positioning and timing services. It may be a redundant satellite system independent of GNSS but can also be the communication payload carried on the GNSS satellite. However, the signal interference encountered on the space link between the satellite and user will be the main challenge facing SBAS services, especially LEO SBAS. Figure 2 shows the normalized power spectral density function of the main electronic systems used for navigation, communication, and other services in the target band. However, it seems that the challenges from actual interferences and threats are not limited to RDSS, IRNSS, and Globalstar, etc., but also affect many undisclosed LEO micro-satellites and high-altitude aircrafts. In Figure 2, one can see that the signal normalized PSD of BPSK (4) marked by the dark red solid line has a wide main lobe and side lobes and a large value for the peak sidelobe ratio; all these are beneficial for the receiver's power spectrum detection, tracking, and positioning accuracy performance. The green line marks the BPSK (1) signal with a simple modulation and the main lobe; thus, it is easy for low-cost receivers to detect and track it. The black solid line marks the signal of BOCs (5, 2) and provides  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Types** & **System** & **Freq-Range (MHz)** & **Bandwidth (MHz)** & **Modulation** & **EIRP** & **PWR** \\ \hline \multirow{4}{*}{
\begin{tabular}{c} **  
a restricted service belonging to the IRNSS. It has the characteristics of the main lobe being split and the width being large, but it is not in the middle area of the target band. For the receiver, the bandwidth determines that its accuracy must be higher than that of the standard positioning service, and the offset of the main lobe determines that more advanced receivers must be used to achieve spectrum detection and tracking. The pink line marks SRC (0.2, 1) and has the characteristics of a certain bandwidth and the same main sidelobe power spectrum. The spectral characteristics can be locked according to the position of the center frequency--that is, the spectral tracking of the receiver can be realized. At the same time, it is different from several other signals in that it has a flat peak duration in its NPSD and is very suitable for data transfer; thus, it can be used as a signal model for NavCom. From the perspective of the system, RDSS with a frequency range of 2483.5 to 2500 MHz cannot be independently performed by the user for satellite to receiver ranging and position calculation but rather must be coordinated by the external system through the user's response. According to the regional and frequency allocation regulations in the ITU, RDSS is the main service used in the United States and the secondary service used in Europe, Africa, the Middle East, North Asia, and the Asia-Pacific region. At the same time, ITU-R research supports the use of RDSS/MSS global integration solutions for protecting Globalstar, VICs, and other services [36; 37; 38]. IRNSS is an area navigation system promoted by ISRO and the ICD. It has been shown that the carrier center frequency of IRNSS is 2492.028 MHz; its signal modulation schemes are BPSK (1) and BOC (5, 2); and it provides SPS and RS services, respectively. The minimum received power of the SPS service is \(-162.3\) dBW [39; 40; 41]. Globalstar is different from the previous two in that it is a global system that continuously provides mobile voice and data communication services. It consists of 48 LEO satellites. To achieve the frequency reuse of a 16.5 MHz beam, the beam is divided into multiple sub-beams, where the bandwidth of each beam can reach  
Figure 2: Comparison of normalized power spectral density function of the main satellite–based electronic signals in the S–band.