Low-orbit satellites can enhance satellite navigation signals as an enhancement and supplement of GNSS (Global Satellite Navigation System); they can also integrate independent communication signals and navigation systems to broadcast independent ranging signals to form backup positioning and navigation capabilities. With its unique constellation and signal advantages, LEO satellites have gradually attracted the attention and favor of the world’s satellite navigation field, and are expected to become a new addition to the development of new generation satellite navigation systems.
1. USA implements backup of GPS positioning time service capability through Iridium system
In order to enhance the capabilities of GPS systems, the United States has realized the integration and development of GPS systems on the new generation of Iridium systems, providing users with Satellite Time and Location (STL) services, provided by Satellites, which can back up and enhance GPS capabilities. The Iridium system includes six orbital planes, each of which is evenly distributed with 11 satellites, forming a complete constellation, which can cover global regions including the polar regions.
In January 2019, the United States completed the deployment of a new generation of Iridium system. In addition to communication services, Iridium provides STL services, which can realize positioning, navigation, and timing in indoor and canyon areas. STL service performance, positioning accuracy of 30-50 meters, timing accuracy of about 200ns, the original signal landing power is 300 to 2400 times stronger than the GPS L1 C/A code signal (24.8 to 33.8dB), indoor availability has been greatly improved, and complex terrain Navigation availability and safety in environmental and complex electromagnetic environments. Through Iridium and GNSS receivers, the STL service can both enhance multi-GNSS navigation services, including GPS, and also serve as a backup when signals are unavailable or difficult to use.
2. European experts propose to greatly enhance the Galileo system capability with Kepler low-orbit system
Kepler system (Kepler) is a new idea put forward by the European Galileo system technology team, which can reduce the dependence on the ground system while achieving the integrity and accuracy enhancement of the Galileo system.
The Kepler system’s technology core is to use 4-6 low-orbit satellites to form small-scale constellations, and the laser intersatellite link (ISL) to improve the existing constellation system. MEO satellites do not need to be equipped with atomic clocks. All satellites are connected by laser intersatellite links, enabling navigation satellites to achieve direct synchronization at a very high level of accuracy, and then further provide high-precision distance measurements instead of pseudoranges for orbit determination in order to obtain The orbital accuracy of the mm level and the phase measurement accuracy of the nm level have greatly improved the capability. At the same time, the LEO satellite constellation observes navigation signals without ionospheric and tropospheric disturbances, which can improve the integrity and accuracy of the MEO system. Under the Kepler system architecture, most of the measurement and communication facilities of the ground operation control system are no longer necessary. Only a small number of ground stations are required to maintain consistency with the Earth coordinate frame and the ability to control the system in special circumstances.
3. China launches low-orbit enhancement research and technology trials
China has carried out related theoretical research, simulation calculations and on-orbit satellite verification of low-orbit satellite enhancement, and has proposed corresponding constellation plans. Communication constellations such as “Hongyan”, “Hongyun” and “Integrated Space and Earth Information Network” all take into account the need for low-orbit satellite enhancements. Micro-space and Arrow Brigade mirroring mainly focus on low-orbit high-precision enhancements; On-orbit technical tests of low-orbit test satellites such as Jia-1, Microspace, and Netcom-1 have accumulated test data for low-orbit satellite navigation signal enhancement technology and accuracy enhancement technology.