An error analysis on nature and radar system noises in deriving the phase and group velocities of vertical propagation waves
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 6 October 2007
Accepted: 17 January 2013
Published: 17 September 2013
A Fourier analysis method has been employed deriving both the phase and group velocities of the atmospheric and ionospheric waves observed by various radar instruments. In this paper, a simulation is conducted to estimate errors of the two velocities resulted from the noises of natural background and radar system. The two noises with various amplification factors are placed into both synthetic data and observations of the MU (Middle- and Upper-atmosphere) radar for further investigations and validations. It is found that the errors of the phase and group velocities caused by the system noise are approximately constants, while those by the natural background noise are generally proportional to the amplification factors. The results confirm that the method can sustain the influences from natural background and system noises.
Various sounding techniques ranging from VLF to UHF bands are employed to observe the ionospheric waves (Davies, 1990; Hunsucker, 1991). A Fourier analysis method has been adapted deriving the phase and group velocities of atmospheric and/or ionospheric waves in data of the Doppler velocity by the Chung-Li very high frequency (VHF) radar (Kuo et al., 1993), virtual height (Liu et al., 1998; Lee et al., 2002), true height by ionosondes (Altadill et al., 2001), and echo-power by the Middle- and Upper-atmosphere (MU) radar (Liu et al., 2007). Most of the studies simply adapted this method focus on calculating the two velocities and finding the associated mechanisms of the waves. However, noises of the nature background (the atmosphere and/or ionosphere) and of the sounding systems contributing to the data could easily affect the accuracy in deriving the phase and group velocities.
On the other hand, scientists examined the electron density perturbations in incoherent scatter measurements with the MU radar (Fukao et al., 1993; Oliver et al., 1994, 1995, 1997). These papers report the electron density perturbations showing downward phase propagation and parameters of the observed perturbations obey the dispersion relation of atmospheric gravity waves.
In this paper, based on the characteristic of incoherent scattering observations of the MU radar, a synthetic data is generated to investigate errors in the wave propagation by using the Fourier analysis. Simulations with various amplification factors of the natural background and/or sounding system noises smearing both the synthetic wave packet and the observation are used to estimate the associated errors in the phase and group velocities.
3. Discussion and Conclusion
It is found that the phase velocities above and below 437 km altitude are in the downward and upward directions, respectively, while the group velocities above and below 434–493 km altitude are in the upward and downward directions, respectively (Fig. 1(b)). The group (phase) velocity in the away and toward direction indicates that the wave origin (or energy source) is at about 434–493 km altitude where it is near the F 2 peak, upper F 2 region. Kelley (1989), (2009) observed some common features in several nights of the mid-latitude ionosphere at the Arecibo radar observatory. He reported that the F peak itself displays undulations with a period of 2 hours and the F layer rose and fell many tens of kilometers during these long-period oscillations. Kelley (2009) further cross-examines characteristics of the above features and those of atmospheric tides, and suggests gravity waves are probable the causal. Similarly, the MU radar observation shown in this paper reveals the F-layer electron density undulations with a period of 3–5 hours (about 224.5 minutes). Liu et al. (2007) cross-compare characteristics observed in this study with those of external factors, such as traveling ionospheric disturbances, atmospheric gravity waves, neutral winds, plasma flows, etc. Based on the observed phase velocity and group velocity having opposite directions Hines (1960), they also propose that the pronounced signature is likely to be caused by atmospheric gravity waves. Although, possible causal mechanism might not be yet fully indentified, we focus on error analyses on nature and radar system noises in deriving the phase and group velocities of vertical propagation waves by using the Fourier analysis.
Results show that the error percents of the group velocities are greater than those of the phase velocities. Note that to derive the phase velocity we need to calculate the vertical wave number by differentiating the phase with respect to the altitude and dividing the center harmonic angular frequency by the vertical wave number. The error of the vertical wave number could result in that of phase velocity. In calculation of the group velocity, it needs further differentiating the angular frequency with respect to the vertical wave number. It might be due to two derivative processes instead of one, the errors of the group velocities are larger than those of the phase velocities.
Figures 4 and 6 reveal that for both the synthetic data and observation, the errors of the two velocities caused by the system noise are about constants (i.e. the amplification factor independent), while those by the natural background noise are generally proportional to the amplification factors. Note that the received signal is the product of transmitted power (modulated by the system) and the atmospheric medium (wave packet with natural background noise). It is clear that when the natural background is getting noisy, the associated random noise fluctuation starts growing, even competing with the wave packet amplitude. Because the natural background noise is a random function of space and time, the phase relationships between the adjacent altitudes are disturbed and become less correlated, which results in the errors of the two velocities increasing with the amplification factor of natural noise accordingly. On the other hand, the system noise is simply a random function of time, and therefore the transmitted power together with its noise gives an equal weighting to each altitude, which preserves the phase relationships along the altitude. Therefore, the errors of the two velocities remain about constants and not functions of the amplification factor of the system noise.
Both the white noise and noise with Gaussian distribution are examined. It is found that either in the nature background or system noise simulation, the Gaussian distribution constantly yields the smaller errors of the two velocities than the white noise does. Note that a small value of the half width in Gaussian distribution, the random function issues numbers about a constant, while the white noise function randomly gives the numbers. Therefore, the Gaussian distribution contributes similar amplitude of the noise fluctuations to the wave packet, which results in the smaller errors of the two velocities.
Since the system noise is not a function of altitude, the phase of the observed waves at each altitude can be preserved, and therefore the accuracy in the phase and group velocities won’t be seriously impacted. By contrast, the nature background noise at each altitude could be rather random, which can significantly disturb the phase at each altitude. Consequently, the associated random errors contributed by the nature background noise would severely affect the accuracy of phase and group velocities. In conclusion, when the system noises is under a Gaussian distribution, and the nature background noise level is relatively low, the Fourier analysis is capable to correctly derive the phase and group velocities of pronounced atmospheric and/or ionospheric waves.
The authors thank Professor M. Yamamoto at Research Institute for Sustainable Humanosphere, Kyoto University, for providing the MU radar data.
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