If you’ve ever followed a navigation app that only veered off course, the culprit responsible for the confusion may be hovering 50 to 200 miles above your head. That region of Earth’s atmosphere, called the ionosphere, can contain varying levels of free electrons that, when highly concentrated, can slow down GPS signals traveling between satellites and device receivers. That delay, similar to someone arriving late for work after frantically bobbing and weaving through a busy city street, is one of the leading causes of errors in navigation systems.
In an article published this week in NatureGoogle researchers have shown that they could use GPS signal measurements from millions of anonymous Android mobile devices to map the ionosphere. While a signal from a single mobile device is too “noisy” to tell researchers much about the ionosphere, that noise is corrected when there are many other devices to compare it to. Ultimately, the researchers were able to use the vast web of Android phones to map the ionosphere with a level of accuracy equivalent to measuring stations. In areas such as India and Central Africa, Android technology far exceeded the accuracy of measuring stations alone.
Scientists around the world can measure this ionospheric traffic (officially called total electron content (TEC) via a network of high-quality measuring stations on the ground. These detection tools are effective, but also relatively expensive to build and maintain, making them less common in developing regions of the world. This unequal access to measuring stations leads to differences in the accuracy of global ionospheric maps. Google researchers tried to address this mismatch by leaning on something owned by a majority of the world’s population: mobile phones.
Google researcher and co-author of an article Brian Williams told Popular science he had previously seen firsthand how changes in the ionosphere could hinder GPS capabilities while working on Android products. He says he saw this project as an opportunity to simultaneously contribute scientific advances and improve accuracy for everyday mobile device users.
“Rather than thinking of the ionosphere interfering with GPS positioning, we can turn this on its head and think of the GPS receiver as an instrument to measure the ionosphere,” Williams said. “By combining sensor measurements from millions of phones, we create a detailed view of the ionosphere that would otherwise not be possible.”
How millions of Android phones became a ‘distributed sensor network’
Modern smartphones are equipped with GPS receivers that measure radio signals emitted by satellites orbiting the Earth in medium Earth orbit (MEO). The receivers calculate the distance between themselves and the satellites and use that to determine the location, accurate to about 15 feet. These signals travel quickly and unhindered through space until they reach the ionosphere. That part of the upper atmosphere can be more or less crowded depending on variables such as the season, time of day, or distance from the equator, all of which can ultimately contribute to GPS accuracy errors. Most telephone receivers are equipped with a correctional model capable of cleaning up about half of that estimated error.
The Google researchers wanted to see if measurements from receivers in Android phones could essentially replicate the ionosphere mapping that takes place in more advanced monitoring stations. Pound for pound, monitoring stations have a clear advantage over cell phones. For starters, they have much larger antennas than cell phones. They also typically sit under clear open skies, unlike cell phones which are often obstructed by urban buildings or a user’s pocket. Each individual phone also has its own unique measurement deviation that can be off by a few microseconds. But what the phones lack in individual complexity, they make up for in sheer numbers.
For their experiment, the researchers collected satellite navigation signals from millions of Android phones around the world equipped with a dual-frequency Global Navigation Satellite System. Google says the measurements are “aggregated” and “anonymized” and come from a sample of Android devices that already had location and other relevant settings enabled. To further protect privacy, the researcher only used the “rough location” (approximately 10 kilometers) associated with the phones. The sheer size of the phones used in the study allowed the researchers to correct for individual biases by comparing the devices. After combining all these measurements, the researchers were able to create a detailed, global map of free electrons in the ionosphere.
“A massive crowdsourced network of aggregated signals can act as a highly sensitive scientific instrument,” Google researchers write this in a blog post.
Researchers then compared the Android phone card with measurements from a database including measurements from measuring stations all over the world. The telephone method significantly expanded coverage, especially in areas of India and Eastern Europe where there is a lack of reporting stations. In the image above, the blue dots show approximately 100,000 locations around the world where enough phone measurements were available to map the ionosphere. That is compared to only 9,000 measuring stations.
“In many parts of the world, our model’s performance is equivalent to using the state-of-the-art global ionosphere map matched to station measurements,” Google’s blog post said.
Ionosphere maps can make GPS more accurate
All of this could soon translate into more accurate navigation tools for consumers. In the study, the researchers claim that their phone-based ionosphere maps outperformed the standard ionosphere error correction already used in most phones. We are moving forward, Google said Popular science it has plans to bring some of the benefits of location accuracy to future Android devices. That could potentially mean fewer annoying navigation errors when plotting a route. Seemingly small or trivial accuracy improvements could actually play a crucial role when applied in extreme emergency situations. This new level of accuracy, Williams explained, could help better distinguish between a highway and a parallel, rough frontage road, a crucial detail for first responders trying to get to the scene as quickly as possible.
The wide coverage area of the calling card data also gives researchers the opportunity to see and document interesting ionospheric phenomena plasma bubbleswith a limited amount of specialized equipment. In one surprising case, Google researchers were able to use the calling map to identify an equatorial anomaly over South Asia that went undetected by the few monitoring stations in the area.
“When we first achieved the resolution needed to show the plasma bubbles moving over India at sunset, I was blown away,” Williams said. “Scientists have seen equatorial plasma bubbles before, but not at this level of detail in this part of the world. We were thrilled to confirm our observations when one of NASA’s COSMIC-2 satellites passed through one of these plasma bubbles and measured a drop in ion density, just as the phones did.”
There are also other, less immediate benefits of the Android ionosphere maps. When they analyzed the Android reception measurements, the researchers said they were able to detect signals of electromagnetic activity that corresponded to a pair of powerful solar storms that occurred earlier this year, including one that occurred between May 10 and 11, 2024 in North America. The ionosphere, measured by phones in that area, showed a clear spike in activity, followed by a rapid depletion afterwards. Although that storm was picked up by monitoring stations, the study notes that telephone ionosphere measurements in areas without monitoring stations could provide more insight into solar storms or geomagnetic activity that might otherwise be missed. Analyzing this data could help scientists better understand how to best prepare for and respond to potentially dangerous events in the future.
“The ionosphere maps produced using telephone measurements show the dynamics of the ionosphere at some locations in more detail than previously available,” Williams said. “We hope that the new picture of the ionosphere revealed by phone measurements will help scientists better understand the effect of geomagnetic storms on the ionosphere.”