The radio occultation (RO) events can obtain the global atmospheric delay information, effectively make up for the deficiency of ground-based observation, and then realize the real-time dynamic positioning based on the precise point positioning (PPP) model. With the development of low-earth orbit (LEO) constellations, a large number of LEO constellation plans with communication or navigation functions have been proposed. If these LEO constellations can be applied to detect occultation events and build space weather models, the full monitoring space-based without large-scale station building on the ground can realize. In this paper, the LRO events between satellites in the LEO constellation are simulated, and these satellites can be used as both transmitting satellites and receiving satellites. The effect of the LEO constellation’s orbital altitude and inclination on LRO events is also analyzed through simulation. The results show that the LEO constellation’s orbital altitude has little influence on the number, and the occultation events spread from the equatorial region to the polar region with the increase of the LEO orbit inclination from 0° to 75°, the number of LRO events increased. The simulation results can provide some references for the occultation exploration plan of the LEO constellation.
In sea clutter background, spaceborne or airborne surveillance radar with a high-speed moving platform is confronted with the problem of clutter spreading. This poses a considerable challenge for maritime radar systems when detecting slowly moving targets of low signal-to-noise ratio. However, the space-time decorrelation characteristics caused by the internal motions of sea clutter under different sea conditions are mostly neglected in the current detecting techniques, which go against the reliable parameter selection in the clutter suppression process. An innovative multi-channel space-time decorrelation analysis method based on sea clutter is proposed. First, the channel decorrelation factors caused by the channel amplitude-phase errors and the noise are calculated by fusing the calibration signal with echo data. Then, the temporal decorrelation curve caused by the internal motions of sea clutter is obtained by fitting the peak points of multi-channel correlation coefficient curve based on the multi-channel phase center. Finally, the independent spatial decorrelation curve is derived through calculating the multi-channel correlation coefficients of different range bins. Due to the utilization of space-time characteristics decomposition method for sea clutter, the space-time coupling problem can be solved effectively. The experimental results based on the measured data prove the effectiveness of the proposed method.
Rubidium electrodeless lamp is one of the core components of rubidium atomic frequency standard. It plays the role of optical pumping and quantum state preparation, and realizes the particle number inversion between two ground states of rubidium atoms. The RF excitation frequency affects the effective light spectral distribution of rubidium electrodeless lamp, and then determines the pumping efficiency and the limiting frequency stability of rubidium atomic frequency standard. In this paper, the influence of RF excitation frequency on the spectral characteristics of rubidium electrodeless lamp can be optimized based on power compensation. It is found that there is a sensitive dependence between the heating rate and the excitation frequency of the rubidium electrodeless lamp, and the excitation frequency range corresponding to the turning point of the maximum heating rate was given experimentally. When the input power and the bubble wall temperature remain unchanged and the rubidium electrodeless lamp is in the ring-mode state, the spectral characteristics have a depression at the optimum resonance frequency. Therefore, the excitation frequency of the rubidium electrodeless lamp should be properly pulled to 5~9 MHz from the optimum resonant point, which can optimize the spectral characteristics of the rubidium electrodeless lamp and can improve the frequency stability of the rubidium atomic clock.
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