The global navigation satellite system (GNSS) has been widely used in the modern information society. But it still faces some problems like ground dependence and accuracy improvement. The advantages of optical technology such as optical frequency reference and free space laser link would help to solve the problems. The United States, Germany and China have respectively proposed concepts such as space-time reference based on optical link (O-STR), the third generation global navigation satellite system Kepler, and new generation global navigation and positioning timing system based on optical technology O-GNSS. This paper summarizes the main research progress of their architecture, prototype, and key technologies, providing a reference for promoting the development of space-time reference and navigation positioning timing technology.

The core idea of O-GNSS which was proposed by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (SIOM, CAS) is to make full use of the outstanding advantages of optical frequency reference and optical inter-satellite link to realize space self-maintenance of the space-time reference. It has an important foundation in the overall design, optical frequency reference, optical frequency comb, space laser link, fiber time-frequency synchronization and many other key technologies. The designed prototype system is based on the Beidou constellation, including inclined geosynchronous orbit (IGSO) and geostationary orbit (GEO) satellites, and integrating low earth orbit (LEO) communication constellation. The GEO/IGSO satellite will be equipped with a cavity stabilized frequency reference, iodine/rubidium long-term stabilized light clock, laser communication-range synchronization integrated terminal (LCT), and space reference interferometry terminal. The medium earth orbit (MEO) satellite is equipped with a cavity stabilized laser, optical frequency comb and LCT. Global synchronization with picosecond level accuracy, absolute ranging with sub-millimeter level precision and communication with Gbit/s level speed among satellites will be achieved through laser links. And the optical frequency and time reference will be transferred to the microwave band through optical frequency combs, and broadcast to the Earth. LEO satellite is equipped with signal monitoring terminal to realize continuous monitoring without atmosphere and relay enhancement of MEO navigation signals. The two ground stations act as backup for each other, and they are equipped with high-performance optical clocks, satellite-to-earth LCT, microwave communication measurement and control terminals, to achieve the alignment between self-maintained constellation and earth rotation, as well as system monitoring and ground maintenance. In addition, based on the ground fiber network, the ground fiber time-frequency synchronization network is constructed, which can cooperate with the constellation to provide high-precision continuous time-frequency and phase information to users with high-performance needs.

In terms of key technologies of optical frequency reference, the output frequency noise of Michelson interferometer (MI) cavity stabilized laser based on optical fiber delay line is better than 6 Hz/Hz at 10 mHz, the linewidth is 0.32 Hz, and the frequency stability is 3.2×10^-15 at 1 s and 1.1×10^-14 at 1000 s. The key technology of optical frequency comb is based on a figure-9 cavity to achieve 0.9 GHz high-frequency output, and based on Brillouin amplification scheme to achieve 1 W single comb tooth amplification. In terms of key technologies of space laser link, it simultaneously achieves 1 Gbit/s communication rate and 5 mm ranging accuracy at a working distance of 40000 km. In terms of the key technologies of ground fiber time-frequency synchronization network, the transmission stability of optical frequency and radio frequency can reach 3.5×10^-20 at 1000 s and 5×10^-19 at 10000 s, respectively. The synchronization jitter peak of 1 pulse per second time signal can reach 3.3 ps at 10000 s.

The new generation of GNSS based on optical technology has a good key technical foundation and feasibility, and can give full play to the advantages of optical technology. It is an important option for the further development of GNSS, which would help to overcome the problems of the current system, such as over-reliance on the ground system and others. And it can realize many advanced functions, such as the space-time reference independently constructed and maintained on the constellation, positioning, navigation, and timing (PNT) with higher precision, integrated communication and navigation, and space-earth integration network for backup and performance enhancement.