# Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite
## 1 Introduction  
The Global Positioning System (GPS) has brought a revolutionary way of providing high-accuracy spatial information. It has been successfully applied in many civil and military applications, which brings inestimable benefit. With recognizing the importance of global navigation satellite systems (GNSSs), more GNSS constellations are designed and deployed. In addition to the U.S. GPS system, the Russian GLONASS, the Chinese Beidou system (BDS), and the European Galileo system have also started providing full or initial positioning services. In addition, there are a few emerging regional navigation satellite systems, such as the Indian NavIC and the Japanese QZSS system. Multi-GNSS constellations bring more visible satellites, lower dilution of precision (DOP), and more reliable positioning solutions [1; 2]. However, current GNSS constellations still face a number of challenges,such as the slow convergence in long baseline real-time kinematic positioning (RTK) and precise point positioning (PPP), signal blocking, and severe multipath effect due to city canyon, jamming, and spoofing, etc. With regard to precise positioning applications, the differential positioning techniques, such as the RTK, the network RTK are limited by their service coverage. The non-differential approach, such as the PPP technique, requires 30-40 min to converge to centimeter-level accuracy, which hinders its applications [3]. In order to reduce the convergence time, the PPP ambiguity resolution technique was developed [4; 5; 6] which can solve the rapid re-convergence problem well, but has limited effect on the first convergence [7; 8; 9]. Another issue for the ambiguity fixed PPP technique is the reliability risk. There is no way to guarantee that the PPP ambiguities can be fixed 100 percent correctly. With regard to the spoofing and jamming issues, there are also a few examples. A Chinese team named Unicorn demonstrated a GPS fraud case with only a simple record and playback device on DefCon 2015 hacker conference [10]. GPS signals are also reported to suffer from inference in Syria earlier in 2018 [11]. These problems are difficult to solve with one particular navigation technique. Therefore, next-generation positioning, navigation, and timing (PNT) systems require diverse techniques, including orbit diversity, non-satellite based navigation techniques, etc. [12; 13]. Current GNSS constellations need to be augmented to extend their performance. Generally, there are two types of satellite-based augmentation systems (SBAS): information augmentation and signal augmentation. For these information augmentation systems, the satellites act as the transponder, which retransmits the correction information uploaded from the ground facilities to users. The information augmentation can be achieved by the standard communication satellites, such as the Inmarsat, which have a wider bandwidth for transmitting more information than GNSS broadcast ephemeris [14; 15]. The correction information may include the orbit correction, clock correction, ionosphere correction, integrity information, etc. [16; 17]. The key to the information augmentation system is more focused on how to generate these corrections with the ground tracking stations. Signal augmentation means transmitting ranging signals by the satellite, so they can work together with current GNSSs, or even work stand alone. In order to generate high-quality ranging signals, high-quality oscillator and onboard ranging signal generation circuits are required for the signal augmentation implementation, so the signal augmentation requires specially designed payloads. The U.S. wide area augmentation system (WAAS) is a successful example of augmentation system based on geo-synchronized Earth orbit (GEO) platform, which combined information augmentation and signal augmentation. The Japanese QZSS also designed the LEX (L band experimental) signal for information augmentation. The Beidou system also broadcasts its own zonal correction via GEO satellite [18].