Air pressure sensors in smartphones: Transforming navigation and fitness tracking
Today’s smartphones utilize a wide array of accelerometers, gyroscopes and various other sensors in their designs. Relatively unknown is the barometric pressure sensor, which measures air pressure. These sensors are currently being integrated into premium-grade smartphones, but are also becoming more common in mainstream phones or wearables such as fitness trackers and Internet of Things (IoT) applications.
Why measure air pressure?
A barometric pressure sensor measures air pressure and can, therefore, play a key role in weather forecasting – lower pressure indicates that poor, rainy weather is more likely. Also, air pressure is affected by altitude changes, which means that such a sensor can provide highly accurate information about vertical elevation.
Let’s take a look at some of the most significant applications and the role that barometric pressure sensors perform in consumer equipment.
Indoor navigation is one of the more interesting applications for this technology, where air pressure data is interpreted to determine the user's current floor level in a building – this is useful for finding one’s way through shopping malls or underground parking lots.
GPS-based systems do not generally function inside large modern buildings, which tend to block out satellite signals. To enable navigation in such shielded environments, altitude detection with the help of the pressure sensor can be combined with step counting data from an accelerometer to establish a three-dimensional matrix, otherwise commonly known as 3D navigation.
Indoor localization is essential for accurately identifying a caller’s location when they call emergency services. In the USA, the FCC (Federal Communication Commission) has recently updated their E911 rules, mandating that the location of a 911 caller indoors be automatically transmitted to emergency responders. It is expected that all smartphones will require a pressure sensor for the purpose of accurately pinpointing a caller’s location inside a large building.
An example of this trend is Bosch Sensortec's cooperation with NextNav in the development of accurate floor level detection solutions. NextNav has demonstrated its ability to provide precise altitude information by implementing Bosch Sensortec’s BMP280 pressure sensor within the scope of its wide-area Metropolitan Beacon System (MBS) network. To improve data accuracy for a given metropolitan area, sensor readings are used to compensate for changing weather conditions and the effects of local microclimates. Figure 1 shows a measurement with a smartphone, available on the market, in which Bosch pressure sensor technology is integrated. The graph demonstrates how the Bosch sensor provides data which matches that of high accuracy reference devices.
Figure 1 Bosch sensors are as accurate as reference devices for air pressure tracking
Even in an outdoor environment, GPS technology has its limitations. Its level of accuracy can be considerably compromised by obstacles such as trees or nearby skyscrapers, by poor weather conditions or by an insufficient number of available satellites. Adding measurements from an air pressure sensor enables smartphones to compare these altitude points with known terrain elevations sourced from a database, and then apply this data to compensate for GPS positioning errors.
While GPS can perform certain altitude measurements itself, it is only complete and reliable under optimal signal transmission conditions, which means outdoors under a clear sky. In other scenarios, the coordinates may be off by ten meters or even more. Figure 2 shows an altitude measurement for a person walking down a hill and out the rooftop of a four-story parking lot – the inaccuracy of the GPS reading compared to the barometric pressure sensor is evident.
Figure 2 GPS versus barometric pressure sensor altitude raw signal measurements
(source: Bosch Sensortec)