Global Navigation Satellite System (GNSS) Radio Occultation (RO) is a novel detection technique that can provide global ionospheric products with high vertical resolution, high precision, and low cost. In recent years, China has launched the FY3 series of meteorological satellites carrying the first RO payload to simultaneously receive GPS (Global Positioning System) and BDS (BeiDou Navigation System) signals. In the accuracy assessment of RO products observed by GNOS (GNSS Occultation Sounder), the maximum F2-layer electron density (NmF2) of GPS occultation and BDS occultation have a standard deviation (std) of less than 20% in comparison with that of ionosondes. The std of F-layer worst-case ionospheric scintillation index ( 4) between COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) and FY3/GNOS is less than 0.1. The above results prove the high precision of FY3 ionospheric RO products. The RO products have been applied to preliminary scientific research and applications, e.g., the process of main phase and recovery phase of magnetic storms revealed by the NmF2 observed by FY3C/GNOS, the premidnight dynamics of F-layer strong scintillation during magnetic storms revealed by GNOS scintillation data, the ionospheric perturbation driven by Tonga volcano eruption revealed by FY3/GNOS, applications of the RO data for the research of sporadic E layers, evaluation of IRI model in statistics and ionospheric climatological characteristics, etc. With the successive network observation and continuous deployment of FY3 meteorological satellites, the continuous improvement of GNOS payload and the BDS system, massive high-precision ionospheric RO products will be developed and show more significant value.

The current work employs the HMC830 phase-locked loop chip to design a frequency synthesizer operating in the L-band. The frequency synthesizer can provide a local oscillation signal for the RF receiver front end. This article employs the phase-locked synthesis technique to describe the design scheme. Due to the advantages of the passive loop filters, such as simplicity, low cost, and low phase noise, a passive fourth-order RLC loop filter is proposed to improve the output signal quality and reduce phase noise. The performance of this loop filter is compared with the passive fourth-order RC loop filter. The effects of these two loop filters on phase noise, loop capture time, and spur suppression are analyzed. Subsequently, the design scheme, simulation analysis, and test results of the frequency synthesizer are presented under these two loop filters. The test results indicate that the passive fourth-order RLC loop filter outperforms the passive fourth-order RC loop filter; its output signal phase noise is higher than −100 dBc/Hz@1 kHz, loop capture time is less than 100 us, and spur suppression is better than 60 dBc. This frequency synthesizer can provide high-performance local oscillation signals for wireless communication equipment such as transmitters and receivers. It meets the application requirements of many radio communication circuit structures and has good application prospects.

The development of global navigation satellite systems (GNSSs) and multi-system compatible radio occultation (RO) techniques provides favorable conditions and opportunities for increasing the number of occultation events and improving their spatiotemporal coverage. The performance of the multiple GNSS RO event number, spatiotemporal coverage, and uniformity need assessments by robust and functional approaches. Firstly, a simulation system of RO events, which took the orbit perturbations into account, was established, and the concepts of global coverage fraction and uniformity of RO events were defined. Secondly, numerical experiments were designed to analyze the GNSS RO performances of a single-receiving satellite and satellite constellations under the condition of using current multiple GNSSs as transmitting satellite systems, in which the Earth was divided into 400 × 400 km2 grids. Finally, the number, timeliness, global coverage fraction, and uniformity of GNSS RO events for a single-receiving satellite and receiving satellite constellations were numerically calculated and analyzed. The results showed that ➀ multiple GNSS integration improved the number of GNSS RO events and their global coverage for a single polar-orbit satellite significantly, e.g., the 24 h multiple GNSS RO event number was about 7.8 times that of the single GNSS system, BeiDou navigation satellite system-3, while the corresponding 24 h global coverage fraction increased nearly 3 times. ➁ In the multiple GNSS integration scenario, the constellation composed of 12 polar-orbit low-Earth-orbit satellites achieved 100% RO event global coverage fraction within 24 h, of which the RO detection capability was comparable to the 100 Spire weather satellites and global positioning system (GPS) RO system. ➂ More GNSS RO events of the polar-orbit constellations were distributed in the middle- and high-latitude zones. Therefore, multiple GNSS integration could increase the RO event number and global coverage significantly to benefit the global climate monitoring and global numerical weather prediction, and the polar-orbit constellations were more favorable to atmospheric detection in middle- and high-latitude regions.