Summary 

The GNSS-based Upper Atmospheric Realtime Disaster Information and Alert Network (GUARDIAN) is an ionospheric monitoring software system that relies on Global Navigation Satellite System (GNSS) data from NASA's Jet Propulsion Laboratory (JPL) Global Differential GPS (GDGPS) network to detect natural hazards. GUARDIAN provides near real-time (NRT) total electron content (TEC) time series data that allows users to explore ionospheric perturbations resulting from natural and anthropogenic events. GUARDIAN’s aim is to supplement the existing natural-hazard early warning systems and is currently the only software capable of providing multi-Global Navigation Satellite System (GNSS) NRT TEC time series data over the Pacific region to the public and scientific community.

Data Quick Facts

Dataset Name: GUARDIAN Near-Real-Time Ionospheric Total Electron Content data product

doi:10.5067/GNSS/guardian_ionotec_001

Format: Comma-Separated Values (CSV)

Spatial Coverage: 90.0 to -90.0, 180.0 to -180.0 (∼1200 kilometers (km) around monitored stations)

Temporal Coverage: September 15, 2022, to the present

Temporal Resolution: 5 seconds

File Size: ∼ 20 MB/file

Platforms: Multiple GNSS

Introduction

Earth’s ionosphere—a thin, electron-rich ionized plasma in the upper atmosphere—is created by the interaction of solar radiation and (mostly) neutral particles at the top of Earth's atmosphere. It stretches from 50 to 400 miles (80 to 644 kilometers) above Earth’s surface and intermingles with the beginning of space.

When radio waves transmitted by GNSS satellites pass through plasma in the ionosphere on their way to receivers on the ground, they experience a phase delay that is proportional to the electron density of the plasma and dependent on the frequency of the radio wave. This allows scientists to measure the delay of different frequencies and then use these differences to infer changes in ionospheric density.

It’s well known that events in space like solar and geomagnetic storms can impact the electron density of the ionosphere, but so can natural hazards on Earth’s surface. Earthquakes, volcanic eruptions, and tsunamis can cause significant displacements of air that produce vertically propagating atmospheric acoustic and gravity waves. These waves typically reach the ionosphere within 8 to 40 minutes after an event and mechanically couple with it, causing fluctuations in ionospheric electron density.

Because a GNSS satellite signal passing through these fluctuations in the ionosphere will experience a delay proportional to the ionospheric electron density along its line of sight, scientists can use this information to calculate the TEC.

Given that GNSS satellites typically travel in medium Earth orbit, approximately 20,000 km or 12.4 miles above the surface, GNSS systems are well suited for detecting fluctuations in ionospheric density. 

Further, because ground stations can detect GNSS satellites from such a significant distance (up to 1,200 km), it isn’t necessary to have a GNSS station close to the location of a hazard event for its ionospheric perturbations to be detected. This makes GNSS particularly beneficial for in detecting natural hazards in Earth’s more remote locations, including the Pacific’s Ring of Fire. 

Science Objectives  

Tsunamis, large oceanic surface waves triggered by submarine events (i.e., earthquakes, landslides, and volcanic eruptions), can travel thousands of kilometers with relatively little attenuation. Due to conservation of energy in the shallow depths, the height of tsunami waves is amplified, leading them to become catastrophic near coastlines. As shown by many tragic events, tsunamis can impose an immense human and economic cost, which makes the development and enhancement of tsunami early warning systems a worthwhile field of research.

There are also a number of international tsunami warning centers and several types of early warning systems that are instrumental in confirming the presence of tsunamis, all of which rely on ground-based or oceanic in-situ instruments such as buoys. Given that it takes 8 to 40 minutes for the atmospheric fluctuations to reach the ionosphere, ocean buoys are quite beneficial for detecting tsunamis, particularly when an earthquake occurs nearby. Nevertheless, these instruments are subject to technical or practical limitations, such as providing sparse coverage, being difficult and expensive to maintain, or lacking the ability to accurately model the far-field propagation of tsunamis. GNSS-based ionospheric monitoring overcomes most of these limitations. 

First, extremely large data volumes are already available at no additional cost, thanks to the multiple satellite constellations orbiting earth and the numerous ground-based GNSS receivers spread around the globe. Ionospheric measurements can typically be obtained up to 1,200 km from a given ground station, ensuring excellent spatial coverage. Second, waves traveling in the ionosphere are a direct proxy for the characteristics of the event that generated them, and the inversion of wave parameters is particularly straightforward for simple tsunami waves. Finally, NRT TEC analyses can be performed within minutes of the atmospheric wave reaching the ionosphere. Taken together, these attributes make NRT GNSS-based monitoring of the ionosphere an attractive approach to augmenting existing natural hazard early warning systems.

Augmenting these existing early warning hazard systems is GUARDIAN's goal. Its current contributions involve collecting GNSS measurements in NRT, computing TEC time series data, and displaying these data on a public website. At present, the GUARDIAN system uses more than 70 GNSS ground stations distributed around the Pacific Ring of Fire and monitors 4 GNSS constellations (the United States' Global Positioning System (GPS), Europe's Galileo, China's BeiDou Navigation Satellite System, and Russia's GLObal NAvigation Satellite System [GLONASS]). According to JPL, GUARDIAN is currently the only software available and capable of providing multi-GNSS system NRT TEC time series data over the Pacific region and openly sharing it with the public and scientific community. (However, there are similar versions of the software under development in the European Union.)

Instruments/Techniques Used

The GUARDIAN system relies on the JPL’s Global Differential GPS (GDGPS) network, which collects and processes GNSS data from hundreds of globally distributed stations associated with the GPS, Galileo, GLONASS, and BeiDou constellations. Included in this array are a number of stations located along the Pacific Ring of Fire. Depending on the station, the data are transmitted via the Crustal Dynamics Data Information System (CDDIS) Networked Transport of RTCM via Internet Protocol (NTRIP) with either a BINEX (Binary RINEX) or RTCM-3 format to GDGPS servers.

Next, at a GDGPS server dedicated to GUARDIAN processing, data are eventually selected according to data type and a subset list of stations. A GNSS Data Editor developed at JPL then separates arcs in the data, flags disjointed arcs, and attempts to correct cycle slips (caused by a temporary loss of lock in carrier tracking). Finally, these data samples are pushed to a shared memory slot in the memory of the dedicated GUARDIAN GDGPS’ server, allowing other processes to access it.

For each new satellite-station link, a buffer is initialized to keep track of six fields: time, both carrier phases, and satellite positions in Earth Centered Earth Fixed convention. The buffer is filed by continuously fetching data from the real-time GDGPS memory (carrier phase and orbits). When a link’s buffer has been filled, the GUARDIAN team proceeds with corrections, the computation of TEC and IPPs, and writing to disk.

First, cycle slips and data gaps are corrected. Then TEC is calculated using the carrier phase time series, and the IPPs using the ECEF satellite and ground station coordinates. Finally, the TEC product is output to the relevant file. The file structure consists of one file per station, containing observations for all available satellites.

The GUARDIAN system ingests this NRT data and combines the phase delay measurements at different frequencies to generate assessments of TEC. Then, the TEC data are buffered for a few minutes to allow the system to perform quality checks on the signal. After the data have been analyzed, they are displayed in NRT on the GUARDIAN website. Data generated in CSV format by the backend system are converted to JSON (JavaScript Object Notation) files for web plotting and manipulation by a converter script, which is run automatically every 6 minutes. In addition, the data are also made available through NASA’s CDDIS, where they are updated every five minutes. Making these data available in these ways allows the geodetic community to explore them and conduct their own analyses.

Major Findings 

To ensure its NRT TEC data can effectively augment the existing natural-hazard early warning systems, GUARDIAN scientists have analyzed GNSS system data from stations in the vicinity of several noteworthy natural hazards to ensure they can successfully capture the ionospheric perturbations from natural hazard events.

For example, after the Tohoku earthquake and tsunami in Japan 2011, scientists from JPL analyzed data from GNSS receiving stationed across the country and successfully captured fluctuations in the ionospheric density. In doing so, they provided very clear evidence of the existence of hazard-caused perturbations in the ionosphere and proved that GNSS networks provided a sufficient way to detect them.

At present, GUARDIAN’s data collection system is fully functional and ingesting NRT data, which it combines with the phase delay measurements at different frequencies to generate assessments of TEC. These TEC data are then validated and made publicly accessible online via the GUARDIAN website, so users can explore ionospheric TEC perturbations due to natural (and anthropogenic) events on Earth and characterize potential natural hazards. According to GUARDIAN scientists, it is the first near-real-time ionospheric monitoring network, and as of October 2023, it is monitoring 61% of the total area in the Pacific Ocean. This corresponds to almost two-thirds of the maximum possible coverage (75%) and implementing additional stations in strategic locations would increase this coverage even more.

In the future, GUARDIAN plans to implement artificial intelligence-based automatic detection algorithms to pinpoint TEC perturbations of interest for either tentative automatic inversions or manual investigation by analysts. By optimizing its NRT run-time and leveraging computer parallelization (which would enable the system to process data faster), GUARDIAN could possibly to extend its coverage to the entire set of stations available to GDGPS around the entire globe.

Related Links 

Martire, L., Krishnamoorthy, S., Vergados, P., Romans, L. J., Szilágyi, B., Meng, X., Anderson, J. L., Komjáthy, A., Bar-Sever, Y. E. (2023) The GUARDIAN system - a GNSS upper atmospheric real-time disaster information and alert network, GPS Solutions 27(32). doi:10.1007/s10291-022-01365-6.

Martire, L., Runge, T.F., Meng, X. et al. (2024) The JPL-GIM algorithm and products: multi-GNSS high-rate global mapping of total electron content. Journal of Geodesy 98, 44. doi:10.1007/s00190-024-01860-3

GUARDIAN—Monitoring Earth’s Ionosphere for Natural Hazards in Near Real-Time (NASA Earthdata Webinar)

The International Union of Geodesy and Geophysics (IUGG) established a special commission on Geophysical Risk and Sustainability to foster research on geophysical hazards and their mitigation measures. In particular, the IUGG’s Resolution IV following its 2015 General Assembly urges to use GNSS as enhancing method to tsunami early warning systems.