Characteristic Of Navigation Signal And System Under Plasma Sheath Of High Speed Aircraft

The high speed aircrafts travel in the atmosphere, tremendously compressing the air around the crafts and surrounding by an envelope of plasma sheath consisting of ionized air which cause the electromagnetic signal transmission with the outside absorbed, reflected, refracted, scattered, seriously interrupting the communication and navigation, namely the blackout. Expecially the satellite navigation system which has low carrier frequency and low transmission power is easily affected in the high speed aircrafts, with the influence of large scape of doppler frequency and its change rate, seriously the receiver doesn’t work. The current mainstream solutions are to improve the communication frequency or to reduce the concentration of the plasma sheath through some sort of intervention methods, such as injection electrophilic materials, application electric field and magnetic field, delayed retransmission, using Ka communication frequency. However the effects of plasma sheath has not been completely alleviated and the real-time communication has not been yet achieved. Therefore, the further research of characteristic of navigation signal and its receiver system has important theoretical significance and practical value.Amplitude attenuation and phase shift are caused by plasma sheath to navigation signals. The influence are mainly ralated with electron density, collision frequency, electromagnetic wave frequency, modulation and other parameters. Most studies indicate that amplitude attenuation of plasma sheath is the major cause for navigation interruption. When attenuation is greater than the power threshold, the receiver fails to work. But the conclusion is not systematicly studied and experimentally verified. Current GPS navigation system is phase modulated. The effect of phase shift on electromagnetic wave and navigation is not validated. Meanwhile the greater dynamic doppler and its change rate also impact the receiver synchronization loop. Thus characteristics of combining amplitude attenuation and phase shift of plasma sheath on navigation signal and system would be studied. Also high dynamic which brings huge doppler and its rate could be considered.To solve these problems, the two plasma factors aiming at the plasma and the ultra high speed are combined to study the effect on the C/A signal of GPS and its receiver. The main innovations and contributions of this paper are outlined as follows:1.The carrier to noise ratio calculation model of GPS signal is proposed in the plasma sheath. The effect of attenuation of GPS signal and its receiver is studied in the plasma. The calculation of attenuation of electromagnetic wave in the plasma is expounded. The extent of plasma attenuation and GPS C/N0 and the relation between plasma attenuation and receiver power threshold are therotically analyzed. Ultilizing the plasma device in the laboratory environment, we design the experiment study on the ground of GPS receiver performance by GPS modulated signal propagating in the plasma. Based on the experiment system, continuously experience process of GPS communication in the plasma and by use of attenuator normally received, disappeared, received again are made. Finally theoretical analysis results of plasma attenuation, C/N0 and receiver power threshold are consistent with the experimental measurement results.2.The analysis model of intermediate frequency GPS signal and navigation system is made. The phase characteristic of GPS signal and its receiver is studied in the plasma sheath. The mechanism of electromagnetic transmission in plasma sheath is expounded that the phase is unchanged in down-converting. In the paper we calculate the plasma phase shift by the method of transfering matrix method, establish the GPS signal model affected by phase shift using Hilbert, and also make the response of plasma phase shift on the GPS navigation by the theory analysis and calculation emulation. Then the phase shift transforming after the loop in the receiver exist in the form of cosine in the signal, they change very little, only cause the outcome change of the navigation and positioning, and will not result in the system blackout. The effect of phase shift in the breakpoints can be ignored.3.The synchronization loop is improved in the high dynamic environment. In the paper we calculate the doppler frequency shift and its change rate caused by the high speed, compare the normal capture method and tracking method, and make the conclusion that main factors are both decreasing signal C/N0 caused by plasma amplitude attenuation and doppler changes resulting from crafts hypersonic velocity. Aiming at two factors, we use inertial aided to provide satellite number, doppler frequency and navigation data, reducing the search scope,enhancing the integration time, improving the cumulative gain. Then lower C/N0 signal cound be captured. Meanwhile tracking loop is improved, third-order filter improve for the second order loop filter with the loss of signal C/N0. Finally 19 d BHz C/N0 signal cound be captured and tracked with the accuracy of the satellite signal decoding and synchronicity. The improvement of the receiver internal structure can combat the plasma, further delay the communication blackout.