Determination of the Refractive Contribution to GPS Phase “Scintillation”

Abstract

As L‐band radio waves travel through the ionosphere, such as those transmitted by the Global Positioning System (GPS) satellites, changes in the electron density along the ray path may induce refractive and/or diffractive variations in the signal's phase; where refractive variations are deterministic and diffractive variations are stochastic. Typically, the refractive component of these variations is thought to be slow varying, associated with frequencies less than 0.1 Hz. Therefore, if the refractive contribution is assumed to be associated with frequencies less than 0.1 Hz, the frequencies greater than 0.1 Hz are then assumed and treated as diffractive. These variations are usually referred to as scintillation. In scintillation studies the deterministic refractive variations are very often ignored. We propose that rapid changes in the electron density, and therefore changes in the refractive index along the ray path of the GPS signal, can induce dominantly refractive variations at frequencies greater than 0.1 Hz. The increased drift speeds observed in the high‐latitude region create conditions suitable for these high‐frequency refractive variations; the GPS ray path will sweep through large‐scale irregularities at higher speeds, resulting in high‐frequency refractive variations. Using recent advances in GPS, most importantly an improved signal tracking technique, we present examples of rapid refractive variations in the GPS signal's phase. These high‐frequency variations are shown to be refractive using a combination of techniques, one adapted from a previous technique used for the low‐frequency refractive contributions and a new technique only possible with advances in GPS tracking.

Automatic detection of ionospheric scintillation‐like GNSS satellite oscillator anomaly using a machine‐learning algorithm

“…The phase deviation caused by an oscillator anomaly is proportional to the carrier frequency, while ionospheric scintillation is the result of combined refractive and diffractive effects as the signal propagates through ionospheric plasma irregularities (Carrano et al., 2013). The magnitude of the ionospheric‐phase scintillation is approximately inversely proportional to the carrier frequency for weak and moderate scintillation events (diffractive contribution dominates in strong scintillation and thus breaks the inverse proportion relationship) (McCaffrey & Jayachandran, 2019). These different characteristics of phase disturbances at different carrier frequencies can be utilized as features to distinguish the two types of events.…”

“…Ionospheric scintillation is typically due to a combination of refraction and scattering or diffraction of the signal propagating through plasma structures. The diffractive contribution introduces additional disturbance and cannot be removed by the dual‐frequency measurements or ionospheric models (Carrano, Groves, McNeil, & Doherty, 2013; McCaffrey & Jayachandran, 2019; Morton et al., 2020). Therefore, this approach will not be effective.…”

Simultaneous Rocket and Scintillation Observations of Plasma Irregularities Associated With a Reversed Flow Event in the Cusp Ionosphere

“…Note that the amplitude scintillation is caused only by the diffractive effect, while the phase scintillation can be caused by both refractive and diffractive effects (De Franceschi et al, 2019;McCaffrey & Jayachandran, 2019;Wang et al, 2018). These indices were calculated by detrending the raw (50 Hz) data with a cutoff frequency of 0.2 Hz.…”

Plasma density gradients at the edge of polar ionospheric holes: the absence of phase scintillation

“…Wang et al (2018) showed that rapid variations in the phase of a trans-ionospheric signal can arise as a result of plasma structures moving rapidly relative to an observer at ground level and can therefore give the appearance of phase scintillation. Rapid changes in the spatial distribution of electron density can also introduce similar effects, as the GNSS satellite-toreceiver ray path can sweep through these irregularities at high speed, resulting in high-frequency refractive variations (McCaffrey and Jayachandran, 2019).…”