Amplitude Spectrum Density Research Articles

Equatorial Ionospheric Scintillation Measurement in Advanced Land Observing Satellite (ALOS) Phased Array-Type L-Band Synthetic Aperture Radar (PALSAR) Observations

Amplitude stripes imposed by ionospheric scintillation have been frequently observed in many of the equatorial nighttime acquisitions of the Advanced Land Observing Satellite (ALOS) Phased Array-type L-band Synthetic Aperture Radar (PALSAR). This type of ionospheric artifact impedes PALSAR interferometric and polarimetric applications, and its formation cause, morphology, and negative influence have been deeply investigated. However, this artifact can provide an alternative opportunity in a positive way for probing and measuring ionosphere scintillation. In this paper, a methodology for measuring ionospheric scintillation parameters from PALSAR images with amplitude stripes is proposed. Firstly, sublook processing is beneficial for recovering the scattered stripes from a single-look complex image; the amplitude stripe pattern is extracted via band-rejection filtering in the frequency domain of the sublook image. Secondly, the amplitude spectrum density function (SDF) is estimated from the amplitude stripe pattern. Thirdly, a fitting scheme for measuring the scintillation strength and spectrum index is conducted between the estimated and theoretical long-wavelength SDFs. In addition, another key parameter, the scintillation index, can be directly measured from the amplitude stripe pattern or indirectly derived from the scintillation strength and spectrum index. The proposed methodology is fully demonstrated on two groups of PALSAR acquisitions in the presence of amplitude stripes. Self-validation is conducted by comparing the measured and derived scintillation index and by comparing the measurements of range lines and azimuth lines. Cross-validation is performed by comparing the PALSAR measurements with in situ Global Position System (GPS) measurements. The processing results demonstrate a powerful capability to robustly measure ionospheric scintillation parameters from space with high spatial resolution.

Effects of the Space Plasma Density Oscillation on the Interspacecraft Laser Ranging for TianQin Gravitational Wave Observatory

AbstractThe TianQin space gravitational waves (GW) observatory will contain three geocentric and circularly orbiting spacecraft with an orbital radius of 105 km, to detect the GW in the milli‐hertz frequency band. Each spacecraft pair will establish a 1.7 × 105 km‐long laser interferometer immersed in the solar wind and the magnetospheric plasmas to measure the phase deviations induced by the GW. GW detection requires a high‐precision measurement of the laser phase. The cumulative effects of the long distance and the periodic oscillations of the plasma density may induce an additional phase noise. This study aims to model the plasma induced phase deviation of the interspacecraft laser signals, using a realistic orbit simulator and the Space Weather Modeling Framework model. Preliminary results show that the plasma density oscillation can induce the phase deviations close to 2 × 10−6 rad/Hz1/2 or 0.3 pm/Hz1/2 in the milli‐hertz frequency band and it is within the error budget assigned to the displacement noise of the interferometry. The amplitude spectrum density of phases along three arms become more separated when the orbital plane is parallel to the Sun‐Earth line or during a magnetic storm. Finally, the dependence of the phase deviations on the orbital radius is examined.

Validation of an ankle-hip model of balance on a balance board via kinematic frequency-content

BackgroundThe ankle-strategy model, where the human body is modeled as a single inverted pendulum hinged at the ankle, has been used for decades to study the dynamics and the stability of the human upright posture (UP). However, the contribution of the hip joints is critical whenever postural disturbances are considered. To account for hip contribution, a double inverted pendulum (DIP) model rotating about the ankle and hip joints has been recently proposed in our previous work but experimental validation efforts are scarce. MethodIn the present study, it is investigated whether the DIP model is able to reproduce the experimentally observed frequency spectrum of the ankle and hip joint kinematic for young and elderly subjects balancing on a compliant surface. The DIP model based and experimental kinematics are compared via Fourier analysis to obtain their corresponding amplitude spectrum density (ASD) functions. Quantitative comparisons of the ASD functions are accomplished through Bland-Altman (B&A) analysis, and Pearson correlation coefficient (PCC). ResultsThe DIP model can reproduce part of the experimental frequency spectrum of the ankle and hip joint angle position and velocity, especially for frequencies larger than 0.35 Hz. Moreover, the model captures the decaying behavior of the experimental ASD functions as frequency increases. With respect to joint angle velocities, the highest PCC between model-based and experimental ASD functions is found for the hip joint of elderly subjects. The B&A analysis shows that the zero-difference between model-based and experimental ASD functions lies between the 95 % confidence interval, especially for the joint angle position results. These suggest that the DIP model reproduces part of the experimentally observed frequency spectrum, which validates the model to study the dynamics and stability of the human upright posture.

On the Relationship Between Surface Backscattering Strength at Low Frequencies and the Ocean Gravity Wave Spectrum

A theory employing the Kirchhoff approximation and other physical assumptions has been used to obtain a theoretical expression for bistatic surface scattering strength 10 log s(φi,φs) appropriate for the case of a vertically suspended separated source and receiver. In backscattering only one grazing angle φ is involved, and s(φi,φs) reduces to s(φ,φ′) = g2k42π2l2⋅2P(F̃)Ω̃3, where 2P(F̃) is the ocean wave amplitude spectrum density at frequency F̃ = Ω̃/2π, Ω̃2 = 2g k cosφ, and l is unit length. At steeper grazing angles where surface shadowing effects are relatively unimportant, the formula provides a good description of the relation between experimental values of 10 log s(φ,φ) and 2P(F) recently reported by Brown and Saenger [J. Acoust. Soc. Amer. 52, 944 (1972)]. Low-frequency s(φ,φ) data of other investigators will be discussed in light of this theoretical relation. [This work was supported by the Office of Naval Research.]