The ionosphere at work: a new source of GPS data and new mathematical models – Inside GNSS

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A new source of data to help scientists better understand the ionosphere and its potential impact on communications and PNT is now publicly available. The The data was collected by sensors on GPS satellites in 2018, and released through a collaborative effort of the Los Alamos National Laboratory (LANL) and the National Oceanic and Atmospheric Administration (NOAA).

[Image above: A layer of charged particles, called the ionosphere, surrounds Earth, extending from about 50 to 360 miles above the surface of the planet – shown in purple and not-to-scale in this image. Because the ionosphere swells in response to incoming radiation from the sun, the exact size and shape of the ionosphere can change throughout the day, and the daytime ionosphere is always larger than the nighttime ionosphere. Image credit: NASA’s Goddard Space Flight Center/Duberstein]

“Radio signals from satellites or transmitters on the ground can pass through or bounce off the ionosphere, so ionospheric conditions have the potential to disrupt communications depending on the density of electrons,” said Erin Lay, scientist in remote sensing in Los Alamos. who was the technical manager of the project. “This new data set will help us better model and predict the behavior of the ionosphere and possibly improve the reliability of our communication and positioning, navigation and synchronization services, which are essential for both daily life. and national security. ”

Markov chain search

In a separate but related development, earlier this year, researchers published an article in the American Geophysical Union’s open access journal Space Weather on “Markov chain based stochastic modeling of deep signal fading: aviation availability assessment based on dual frequency GNSS under ionospheric scintillation,Describing how they developed a new mathematical model to more accurately capture how irregularities in the Earth’s atmosphere disrupt GNSS signals.

The mathematical model accurately emulates interruptions in GNSS signals caused by space weather, especially some irregular, low density areas in the ionosphere.

The plaques, called equatorial plasma bubbles, usually form above the equator around dusk and cause the signal distortion called ionospheric scintillation.

The researchers, Andrew K. Sun, Hyeyeon Chang, Sam Pullen, Hyosub Kil, J iwon Seo, Y. Jade Morton and Jiyun Lee, write that: “GNSS navigation can be lost when several satellites are briefly unusable for receiver calculations. GNSS due to the signal fades due to scintillation. Using dual frequency signals can lessen the impact of scintillation by providing a backup telemetry source on one frequency. However, frequency diversity is only partially useful because the effect of scintillation is correlated between frequencies, which means that a receiver can always lose satellites when their signals on both frequencies are simultaneously plagued by one. deep fainting. Here we propose a new stochastic model to represent a more precise description of the correlated fading processes observed in real scintillation data. We incorporate scintillation effects into a simulation of aviation availability. The simulation results provide estimates of the impact of scintillation on the availability of single frequency L1 and L1 / L5 dual frequency aviation and quantify the benefits of using dual frequency signals during a strong scintillation. Further development of this model will allow assessment of the effects of other scintillation conditions and scenarios on availability for aviation and other GNSS applications.

This approach could improve understanding of other effects of ionospheric scintillation on GNSS signals and ultimately help make GNSS satellites more resistant to scintillation and other forms of space weather.

Los Alamos and NOAA files

The files mentioned at the top of this article, published jointly by LANL and NOAA, contain data products derived from LANL wideband radio frequency (RF) sensors on GPS satellites that measure very high frequency transient events ( VHF, 30-300 MHz). When an RF signal from broadband transient lightning events passes through the ionosphere, it is scattered, so that lower frequencies arrive later than higher frequencies. This scatter can be used to determine the total oblique electron content (STEC, or integrated electron density along the line of sight between the location of the lightning strike and the GPS satellite in medium earth orbit (MEO)).

A mapping factor is used to project the tilted TEC (along the line of sight) to a vertical TEC (VTEC), which is the electron density integrated vertically over a given location. VTEC measurements of lightning events are combined in latitude, longitude, and time intervals, and the median VTEC at a given location is provided in these data files. This grouping creates a sparse time and space averaged global TEC product from lightning events that can be used to compare to other TEC measurement products such as ground-based GPS receivers.

The ionosphere is the boundary between Earth’s atmosphere and space, stretching from about 40 to over 250 miles above the Earth’s surface. It is made up of a thin atmosphere and charged particles (ions and electrons) which interact with the passing radio waves. The behavior of the ionosphere reacts to weather conditions on Earth, such as thunderstorms, wind and hurricanes, as well as space weather created by solar winds impacting the Earth’s magnetic field.

NOAA Space Weather Forecast Center (SWPC) serves a broad customer base interested in the effects of space weather on communications and GPS-based technologies, ”said Bill Murtagh, program coordinator at SWPC. “We expect that access to these data sets from Los Alamos will improve the development, validation and testing of models used at SWPC to characterize and predict ionospheric disturbances. “

The new data comes from unique measurements of lightning events, each of which produces a flash of radio waves that scatter throughout the ionosphere before being detected on satellite receivers. Each measured flash provides a snapshot of ionospheric conditions at that instant, and many lightning measurements accumulated over time provide a unique view of ionospheric time. It is the world’s first ionospheric electron density dataset to use a natural source phenomenon.

Prior to this release, the data available to power the ionosphere models came mainly from arrays of ground-based receivers, which are limited because they only monitor fixed locations. According to Lay, “the new data is being gathered from lightning strikes occurring all over the world and will give scientists the opportunity to study the ionosphere in ways previously impossible.”

Disseminating underutilized datasets was an established priority in 2019 National Space Weather Strategy and Plan of Action. Los Alamos processed data from its radio frequency sensors onboard GPS satellites used for monitoring nuclear treaties, then worked with an interagency group, called Space Weather Operations, Research, and Mitigation (SWORM), to facilitate dissemination. public. . NOAA’s National Environmental Information Centers will house data on existing sites that serve land and space meteorological resources.

Link to the data: https://www.ncei.noaa.gov/archive/accession/0241206


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