There are millions of phones, so you can map the ionosphere

Interference from the Ionospheric Environment Improves Geolocation Errors in the Early Years of GPS and Other Satellite Monitoring Systems

Phone data could reduce GPS errors by 10–20% in some areas, and more in underserved regions, estimates Ningbo Wang, an atmospheric physicist at the Aerospace Information Research Institute of the Chinese Academy of Sciences in Beijing. Even with adjustments, interference from the ionosphere continues to be a problem, especially during solar storms. “The results presented are truly impressive.”

Global Positioning System (GPS) and other GNSS receivers estimate distances to satellites by measuring the time for a radio signal to travel from a satellite to a receiver. Ionospheric TEC affects the propagation speed of radio waves, introducing significant errors in receivers’ calculations of the distance to satellites. This is one of the largest sources of geolocation error, and many GNSS receivers use a coarse spatiotemporal model of the ionospheric TEC to compensate for it. Roughly 50% of the ionospheric error7 is mitigated by most phones using an 8-parameter model. This simple model was developed in the early days of GPS and was optimized for the limited computation and bandwidth available at the time.

The longitudinal structures in the equator were observed by us. These phenomena have been recently observed in nighttime extreme-ultraviolet images from the geostationary GOLD (Global-scale Observations of the Limb and Disk) satellite31. The ionospheric VTEC measured by phones (Fig. 4b) agrees with the observations made by GOLD (Fig. 4c). The plasma bubble structures are telltale signs of plasma instability, which can result in scintillation of GNSS signals4,32 and disrupted radio navigation and communication systems2. Extended Data Fig. 5 shows that a GNSS monitoring station near São Paulo, Brazil, experienced strong scintillation during this time.

The conditions in the ionosphere are changeable, depending on the time and location. Steep spatial gradients in ionospheric TEC often cause plasma density structures that disturb trans-ionospheric radio signals4. The ionospheric TEC refers to the number of free electrons contained in a column of unit cross-sectional area measured in TEC units (1 TECU = 1016 electrons per square metre). TEC is an important indicator of the state of the ionosphere and space weather. The use of global real-time ionospheric TEC maps to create space-weather operational products for a wide range of civilian and security activities is needed. Current space-weather and ionospheric products lack coverage, resolution, and refresh rates.

We evaluate our phone-based VTEC maps by comparing with the Madrigal database23,24. This database combines line-of-sight STEC measurements from over 9,000 monitoring stations for the US GPS and Russian GLONASS constellations at 30-second intervals, with carrier-phase smoothing to reduce noise. Our maps are temporally discretized into 1-minute windows. We compare our VTEC estimates in each cell-minute with measurements from the Madrigal database over the period of 10 September to 6 November 2023 to assess the accuracy and coverage of our measurements.

Over 10-minute time windows, we solve for the unknown VTECtrue and DCBphone using weighted least squares. In a typical time window, there are approximately 100,000– 500,000 phones and 15,000– 30,000 S2 cells. To make this tractable we use sparsity and block structure of the system. Our solutions show the clusters of DCBphone.

Although DCB values for satellites are published22, we must calculate the DCB for each individual phone. Each line-of- sight measurement can have a linear equation for the angle of incidence at the ionosphere piercing point.

The differential code bias is caused by different latencies in the hardware when processing signals of different frequencies. The biases shift the measured STEC from its true value as follows:

A GNSS receiver measures the ionosphere TEC along a line of sight from a satellite using the difference in measured travel times18, t1 and t2, for the satellite signal on two different frequencies, f1 and f2, respectively. The TEC along the path is approximated to first order in the frequencies by:

where c is the speed of light and re is the classical electron radius. The TEC value can be up to 200 TECU. Our analysis shows that the noise in the raw phone measurements has a standard deviation of around 70 TECU, more than 30 times larger than the measurement noise for traditional monitoring stations. Carrier-phase smoothing19 will usually reduce the noise further to 0.2 TECU. Measurement of carrier phase is unreliable on phones. Figure 2 compares the TEC from a particular monitoring station to the TEC from nearby phones. They agree with the station’s measurement, despite the larger noise in individual phone calls.

Until now, scientists have thought of phones as end users of navigation services, says Wang. He says using phone measurements as input data is new territory. The paper marks an exciting shift.

Google uses millions of phones to map Earth’s ionosphere and improve GPS, says a physicist at the University of Arizona

For science to really benefit, Google will need to release the data, says Coster, who works on the Madrigal Database, a community resource for geospacer data that brings together ionosphere data from thousands of ground stations. There are no plans for fresh data to be released in real time, at least according to the source for the data behind the study.

Williams says that efforts are already under way to use this technique to improve location accuracy for Android users. But the data should also be useful for scientific studies of Earth’s upper atmosphere, he says. The map has shown bubbles in ionized gas over South America that had not been seen before.

anyone with an open source device such as anANDROID phone is able to contribute to the study The firm says the data was aggregated and that the individual devices are not identifiable.

The quantity of data obtained was a factor in the success of the team. When you combine the noise with large numbers, you get a clear signal. “It’s like there is a scientific monitoring station in every city where there are phones.”

Without these corrections, GPS would be out by around 5 metres, and by tens of metres during solar storms, when charged particles from the Sun can boost electron density. The ground-based receiver stations used to make these maps do not exist in many parts of the world.

When the air is partly ionized, freely moving electrons slightly slow down the radio signals travelling to Earth from GPS and other navigation satellites. This can affect the nanosecond-precision timing that satellite navigation devices use to pinpoint their locations, with potentially serious impacts on aeroplane landings and autonomous vehicles.

Source: Google uses millions of phones to map Earth’s ionosphere and improve GPS

The Dataset of an Atmospheric Physicist’s Degree of Freedom: How Wonderful is It? Anathematician Ana Coster

Anthea Coster, an atmospheric physicist at the Massachusetts Institute of Technology in Cambridge, describes the data set as amazing. “It fills in the map a lot, in areas where we desperately need more information.”