# Initial Assessment of the LEO Based Navigation Signal Augmentation System from Luojia-1A Satellite
## 2 The Principle of a LEO Navigation Augmentation System
With the dynamic smoothed algorithm, the satellite is capable of providing sub-meter level accuracy orbit in real-time, which achieves comparable or even better accuracy than the orbit in the GNSS broadcast ephemeris. With properly handled orbital and clock biases, the real-time onboard LEO orbit determination with the broadcast ephemeris reaches 0.3-0.4 m precision [42]. By autonomous orbit determination, Luojia-1A can maintain its coordinates in Beidou or GPS coordinate systems without setting up its own ground stations as if moving a Beidou or GPS satellite down to a lower orbit. The heart of the navigation satellite is the high-performance atomic clock, which is the bottleneck of the navigation satellite performance and lifetime. Currently, most navigation satellites are equipped with 2-4 rubidium or cesium atomic clocks. The BDS3 and Galileo satellites even carry high-performance active hydrogen masers [43]. The one-second frequency stability of current GNSS clocks is around \(10^{-12}\)[44]. The daily stability of the Beidou satellite reaches \(10^{-13}\)-\(10^{-14}\)[45; 46]. These high-performance atomic clocks are often very large, heavy, and expensive, so they are not suitable for most micro/nano-satellite platforms. An alternative solution for the navigation system is equipping atomic clocks on the ground facilities rather than the space segment, such as the Chinese Area Positioning System (CAPS), but this method is only suitable for these GEO and inclined geo-synchronized orbit (IGSO) satellites [47; 48]. Due to cost/size/weight constraints, the Luojia-1A satellite has to employ an oven-controlled oscillator (OCXO) rather than the high-performance atomic clock. The temperature control system of the satellite platform is used to ensure the temperature condition of the onboard OCXO. The short-term stable oscillator generates low phase noise, short-term  
Figure 1: The physical configuration of the Luojia-1A satellite (+z side). stable impulses in order to maintain a sufficiently high-quality carrier phase generation. The oscillator is disciplined by the GPS/Beidou signals in-orbit, which calibrates the long-term frequency drift and maintains the frequency accuracy. With the in-orbit discipline technique, the time system of the Luojia-1A satellite is aligned to the GPS time or Beidou time at the level of tens of nanoseconds, which is satisfactory from the performance and cost perspectives. With the orbit and the clock obtained, Luojia-1A is capable of generating its own ranging signals, which is driven by the onboard OCXO. The orbit and clock information is encoded and modulated on the carrier phase, and then broadcasted via the transmitting antenna. Driven by the same OCXO, the clock information determined by the receiver can be delivered to users via the transmitted signals. The hardware delay caused by the circuit is modeled as a constant bias. The phase noise is controlled within a low level by the low phase noise oscillator, which avoids phase lock loop fluctuation in the ground receiver. As a satellite-based augmentation system, the interoperability and compatibility of current GNSS systems is an important issue. Although the Luojia-1A satellite is a scientific experiment satellite., we still rigorously evaluated its potential impact on other domestic and international satellite operators and we also coordinated with the satellite operators and potential ground users before satellite launch. We strictly controlled its emission power, spurious emission, experimental location, and duration to avoid unnecessary interference to other systems. Particularly, the Luojia-1A ranging signals are designed compatible with current GNSS signals and have no negative interference on current GNSS users. A strict electromagnetic compatibility (EMC) test on the payload was performed in an anechoic chamber before satellite assembly. The Luojia-1A ground receiver is also developed, which is capable of tracking the GPS/Beidou and Luojia-1A augmentation signals simultaneously.