Simulation of GPS radio occultation signals through Sporadic-E using the multiple phase screen method

https://doi.org/10.1016/j.jastp.2021.105538Get rights and content

Highlights

  • S2-fbEs relationship for sporadic-E intensity is linear only over a select range.

  • A spectral analysis of simulated radio occultation teases out additional relations.

  • Fourier transform of RO disruption from sporadic-E yields a low-frequency plateau.

  • Final frequency of low-frequency plateau directly relates to sporadic-E properties.

Abstract

A phase screen simulation experiment is designed to model radio occultation signals through sporadic-E layers for a GPS transmitter operating at the L1 (1575.42 MHz) frequency. A series of simulations is performed for various sporadic-E parameters in an attempt to validate a linear relationship between the blanketing sporadic-E plasma frequency and the S2 scintillation. The S2-fbEs relationship is found to be linear over a select range of sporadic-E intensities, but shows a nonlinear relationship for lower fbEs values and plateaus at elevated intensities. A spectral method for estimating fbEs from the spatial Fourier transform of the signal intensity is introduced and explored using the final peak frequency of the low frequency plateau. This metric is shown to have direct relationships to key sporadic-E parameters and is resistant to noise in the applied signal. However, it requires strong scatter conditions to form which corresponds to fbEs values above ~4 MHz for typical sporadic-E lengths and thicknesses. A combined approach using the S2 index for moderate events and the final peak frequency of the low-frequency plateau for stronger events will provide improved plasma frequency estimates for the range of observed sporadic-E frequencies.

Introduction

The ionosphere is a key source of signal distortion and propagation path alteration for radio frequency (RF) intra- or trans-terrestrial wireless communication and sensing at frequencies in the L-band and below (Basler et al., 1988). Although considerable progress has been made since signals were first reflected from the bottom of the E-layer (Joly, 1902), we still lack the observational capabilities and theoretical understanding to predict ionospheric conditions with enough granularity for applications such as HF geolocation (Mitchell et al., 2017).

Global Navigational Satellite Systems (GNSS) have become the ideal transmitters of opportunity to measured the state of the ionosphere, either by a proliferation of ground-based receivers (Maeda and Heki, 2014, 2015; Maeda et al., 2016) or through use of the specially designed low Earth orbit (LEO) satellites such as the Constellation Observing Satellite for Meteorology, Ionosphere, and Climate (COSMIC) system. This satellite constellation and its recently launched followon of six additional satellites designated COSMIC II, provide a receive system to pair with GNSS transmitters able to characterize vertical electron density gradients in the ionosphere through radio occultation (RO) measurements. RO provides a measure of distortion as a known signal passes through varying indices of refraction in the ionosphere (Hajj and Romans, 1998; Carrano et al., 2011) and neutral atmosphere (Kursinski et al., 1997; Melbourne, 2004; Schreiner et al., 2007).

Previous research has demonstrated the effectiveness of using GPS-RO to identify sporadic-E (Es) layers through particular amplitude and phase signatures in the received signal (Hocke et al., 2001; Wu et al., 2005; Zeng and Sokolovskiy, 2010; Yue et al., 2015; Niu et al., 2015; Arras and Wickert, 2018). Specifically, a linear relationship between the S2 scintillation index and the blanketing frequency of the sporadic-E layer (fbEs) has recently been proposed by Arras and Wickert (2018) and Resende Chagas et al. (2018) (n.b., here we used S2 instead of S4 following the Briggs and Parkin (1963) terminology). Gooch et al. (2020) applied the linear mapping technique to a larger RO data set and compared the results against ionosonde measurements across the globe. Their results indicate that the relationship between S2 and fbEs may depend on additional sporadic-E characteristics and may not necessarily be linear with respect to fbEs. The simulations of Zeng and Sokolovskiy (2010) also support this claim with a strong dependence of the RO signal amplitude profiles in the measurement plane on sporadic-E width, length, and orientation.

This work expands on the Gooch et al. (2020) study by simulating signal propagation through idealized layers using the multiple phase screen (MPS) method (Knepp, 1982; Wu et al., 2005; Zeng and Sokolovskiy, 2010) in order to determine the relationship between S2 and sporadic-E parameters. Additionally, a new metric to estimate fbEs from a spatial Fourier transform of the signal intensity is presented as an alternative approach for extracting sporadic-E characteristics from GPS-RO measurements.

Throughout this document, we use fbEs to characterize the Es intensity following the comparisons between GPS-RO and ionosonde measurements presented in Arras and Wickert (2018); Resende Chagas et al. (2018); Gooch et al. (2020). The fbEs parameter is the maximum blanketing frequency of the Es layer as measured by ionosondes and is related to the electron density through fbEs[MHz]=8.98×106nm,Es[e/m3] where nm,Es is the maximum electron density of the blanketing layer. This conversion does not account for the background E-layer density as calculated by the metallic ion fbμEs parameter (Haldoupis, 2019), but is used as a stand-in approximation for the sporadic-E intensity to analyze trends.

Section snippets

Parabolic wave equation

The MPS method is an iterative numerical approach for solving the parabolic wave equation. Originally described in Leontovich and Fock (1946), the simplifications applied to wave propagation allow for a substantial reduction in computational time and resources compared to full-wave processes (e.g. Finite Difference Time Domain, FDTD, or Method of Moments, MoM). The phase screen parabolic approach can thus be applied to problem sets that would be prohibitively large for FDTD or MoM methods,

S2 simulations

Amplitude scintillation caused by lensing from sporadic-E layers is commonly used to locate Es from GPS-RO measurements (Wu et al., 2005; Arras et al., 2008). One metric commonly used is the S2 scintillation index, which is the standard deviation of the signal amplitude divided by the mean (Briggs and Parkin, 1963):S2=E2E2E2,where the electric field amplitude, E, is used as the signal amplitude in these simulations through the reduced wave construct u. Here we adopt the Briggs and Parkin (1963)

Spectral method

In an attempt to find an additional metric capable of extracting fbEs values from intense sporadic-E layers, a spatial Fourier transform is taken of the final output screen of the phase screen simulation with a sampling frequency of 1/Δz (1/38 cm−1). Fig. 6 reveals the spatial spectrum of the diffracted plane wave through a sporadic-E layer with an effective length of 65 km, a vertical thickness of 1.75 km, and a fbEs of 10 MHz. There are several structural components that can be observed in

Conclusions

The multiple phase screen method was used for simulations of GPS L1 radio occultation signals through various idealized sporadic-E layers to analyze changes in signal amplitude profiles at the measurement plane. An investigation of the S2 scintillation index found a strong dependence on fbEs and Es size (length, width, and vertical thickness). A linear regime between S2 and fbEs was found for lower fbEs values before the S2 plateaued. The fbEs value of the plateau point depends on the effective

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We would like to thank L. J. Nickisch from NorthWest Research Associates for several helpful discussion on the multiple phase screen method and sporadic-E profiles.

References (32)

  • G. Hajj et al.

    Ionospheric electron density profiles obtained with Global Positioning System: results from the GPS/MET experiment

    Radio Sci.

    (1998)
  • C. Haldoupis

    An improved ionosonde-based parameter to assess sporadic E layer intensities: a simple idea and an algorithm

    J. Geophys. Res.: Space Physics

    (2019)
  • K. Hocke et al.

    Global sounding of sporadic E layers by the GPS/MET radio occultation experiment

    J. Atmos. Sol. Terr. Phys.

    (2001)
  • D. Hysell et al.

    Sporadic E layer observations over Arecibo using coherent and incoherent scatter radar: assessing dynamic stability in the lower thermosphere

    J. Geophys. Res.: Space Physics

    (2009)
  • J. Joly

    Mr. Marconi's results in day and night wireless telegraphy

    Nature

    (1902)
  • D. Knepp

    Propagation of wide bandwidth signals through strongly turblulent ionized media

  • Cited by (0)

    View full text