# Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite
## 2 The Principle of a LEO Navigation Augmentation System  
The Luojia-1A satellite is a lightweight scientific LEO designed by Wuhan University, intended for the experiment of signal augmentation based on a LEO platform. It has two major functions: the night light remote sensing and LEO signal navigation augmentation experiment. The satellite was launched on 2 June 2018 from the Jiuquan Satellite Launch Center, China. Luojia-1A employs a sun-synchronized orbit with 645 km orbit height with a local time of the descending node at 10:30 p.m. The external configuration of Luojia-1A is as shown in Figure 1. The satellite is about 19.8 kg and the envelop size is 870 mm \(\times\) 520 mm \(\times\) 390 mm with solar panel unfolded. The physical configuration of the satellite put strong constraints on the weight, size and power consumption of the navigation augmentation payload. The satellite equipped with three L band microstrip antennas on the satellite. Two of them are used to receive the GPS/Beidou signals and one for broadcasting the navigation augmentation signals. Distinguished from the ground-based signal augmentation system, such as the Locata system [35; 36; 37], the Luojia-1A satellite installed two antennas on \(+\)z and \(-\)z sides, respectively, to receive the GPS/Beidou dual-frequency signals. The satellite is tri-axis stabilized and the solar panels (\(-\)z side) face to the sun in standby mode. In this mode, \(-\)z side of the satellite faces to the outer space when the satellite locates between earth and sun. When the satellite moves to the other side of the earth, \(-\)z side faces toward the Earth and the \(+\)z antenna faces to the outer space. The two receiving antennas work simultaneously so that the Luojia-1A satellite is also capable of tracking the GNSS signals from the opposite side of the Earth. A similar GNSS antenna configuration can be found on COSMIC satellites [38; 39]. The GNSS signals from the opposite antenna pair tracked GNSS signals independently and combined processing for orbit determination. The augmentation signal transmitting antenna is installed on the \(+\)z side of the satellite. During the transmission period, the \(+\)z side faces to the Earth. The diameter of the GNSS antennas is less than 10 cm and no choke rings are equipped on the antennas to reduce their weight. Luojia-1A satellite is capable of transmitting dual-frequency band ranging signals, which are denoted as H1 and H2 in this study. There are very few publications documenting the signal augmentation techniques or results so far. As a low-cost experimental satellite, the principle of the navigation augmentation system is different from those navigation satellites, such as the Beidou or GPS. For the navigation satellites, the coordinate system, the time system and the quality of the ranging signals are particularly critical for the positioning/timing function. In the following section, the difference between the Luojia-1A satellite and current navigation satellites are discussed from these three aspects. At first, the orbits of all the navigation satellites are observed by the ground monitoring stations. The broadcast ephemeris in the form of orbital elements are generated using the tracking data from the ground monitoring stations and then uploaded to the satellites [40]. Establishing and maintaining the coordinate framework for the navigation satellite system requires a geographically well-distributed tracking network, which can be costly and difficult to realize on a non-solid surface or inaccessible locations [41]. The onboard computation unit on the Luojia-1A satellite can automatically determine its orbit from received GPS/BDS dual frequency signals as if moving a ground station into space. From this prospect, Luojia-1A is a user of the current GNSS systems. The characteristic of the Luojia-1A satellite can be described by the orbit dynamics, so the precision of the geometrical orbit can be improved by incorporating the satellite dynamic models.