Comparing the coverage of the ionospheric tec from Madrigal database and phone-based mappings for Android users

To compare the coverage of the ionosphere between stations and phones, we calculate the proportion of cell-minutes for Madrigal database and phone-based maps. Over large portions of the globe (particularly Asia, Africa, Eastern Europe and South America), phones provide measurements of the ionospheric TEC at times and places missed by monitoring stations (Extended Data Fig. 2). Over the entire time period, 14% of the ionosphere was measured by stations alone, while 21% was measured by phones alone. If phones are used with monitoring stations you will see a doubling of the coverage of the ionosphere.

Mitigating ionospheric error in satellite-based positioning for Android users is the primary aim of this work. The increased detail and coverage of phone-based TEC maps leads to improved location accuracy, particularly in parts of the world with few monitoring stations (Extended Data Figs. 6 and 7 and Methods). Efforts are underway to deliver the location-accuracy improvements enabled by this research to Android users.

A crest of ionization north and south of the equator can be seen in the TEC maps from phone measurements, as can the day–night cycle.

When the monitoring stations and phones measure the ionosphere in the same way, we can compare the STEC implied by phone to the one measured by the monitoring stations. The average phone-based TEC is a tad higher than the Madrigal station measurements, with the mean offset of 3.4 TECU. Small offsets of this magnitude are typically found between published ionosphere models25,26 (see discussion in Methods).

The VTEC maps on our phones are compared with the Madrigal database. This database combines STEC data from over 9000 stations in the US and Russia at 30-second intervals with carrier phase smoothing to reduce noise. Our VTEC maps are temporally discretized into 1-minute windows. We compare the VTEC estimates in each cell minute with the data from the Madrigal database over a period of three months to see if they are accurate.

Weighted least squares are used to solve the unknown VTECtrue and DCBphone. Roughly 15,000– 30,000 S2 cells with valid measurements are present in a typical time window. To make this tractable, we exploit the sparsity and block structure. Our solutions show that DCBphone clusters are by phone models.

DCB values for satellites are published, but we need to calculate the DCB for each phone. The angle of incidence at the ionosphere piercing point is the only factor we need to determine the linear equation for each measurement.

The differential code bias occurs when hardware in satellites and mobile phones introduces different latencies. The biases shift the measured STEC from its true value.

Although ionospheric error presents a challenge for navigation, this same effect allows us to use a GNSS receiver to measure ionospheric TEC. The delay on signals with higher frequencies can be caused by the ionosphere. By measuring the discrepancy in travel time between signals of different frequencies, we can estimate the TEC along a line of sight between a dual-frequency GNSS receiver and a satellite. This approach has long been used with high-quality ground-based GNSS stations8, but these stations are expensive to install and maintain. Some regions are sparsely covered, leaving gaps in the picture of the ionosphere (Fig. 1, orange dots).

where c is the speed of light and re is the classical electron radius. Some values can go up to 200 TECU. The noise in the raw phone measurements is more than twice as large as the measurement noise for traditional monitoring stations, according to our analysis. Carrier-phase smoothing19 will normally reduce the noise to 0.2 TECU. but carrier-phase measurements are often unreliable on phones. Figure 2 compares the TEC measured by a particular station with the measurements from nearby phones. They agree with the station measurements even though their phone noise is far larger.

Wang says that scientists thought of phones as end users of navigation services. Flipping this on its head to use phone measurements as input data is “new territory”, he says. The paper marks an exciting shift.

Ionosphere Mapping with Google and the Madrigal Database: How many Android smartphones do we need to release to help map Earth’s upper atmosphere?

According to Coster, who works on the Madrigal Database, a community resource for data that includes ionosphere data from thousands of ground stations, there will need to be release of data for science to benefit. Nature’s news team was told that the data behind the study will be released with the paper, but there is no plans to give fresh data in real time.

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. Already, the map has revealed bubbles in ionized gas, known as plasma, over South America that had not previously been observed in detail.

Anyone with an Android phone — and who allows Google to collect sensor data to improve location accuracy — was eligible to contribute to the study. Individual devices are not identifiable because of the aggregated data, according to the firm.

The data obtained by the team made up part of the success. “When combined in large numbers, the noise averages out and you still get a clear signal,” says Williams. It’s like a scientific monitoring station in every city where there are phones.

When the Sun emits charged particles that increase electron density, gps would be out by around 5 metres and by 100,000 metres during solar storms. Many areas of the world don’t have the ground based stations used to make these maps.

When the air is partly ionized, some of the electrons in the air slow down the radio signals that travel to Earth. 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

Anthea Coster: The Massacre Map of Atmospheric Chemistry in the ’60s and 70s, and Where Are We Going?

Anthea Coster is an atmospheric physicist with the Massachusetts Institute of Technology in Cambridge. It fills in the map in areas where we desperately need more information.