How do smartphones know where North is?…..

Conversation on Twitter, where someone pulls a statement on you and you KNOW it’s true but at the same time your brain is wondering why, you hadn’t thought about it before….yesterday was one of those days. I made a statement about whether the compass was dead and whether this is the rise of the GPS (mobile) compass, taken from an article written by the Ordnance Survey- Here

Obviously someone pointed out that GPS needs at least 3 points to calculate direction (an m, movement) value….couldn’t argue, completely right but something was telling me that my phone isn’t that stupid to tell me where North is with only 2 points.

I’m not stupid (well……)

A typical smart phone has three magnetic field sensors, fixed perpendicular to each other, which are used to work out the local direction of Magnetic North.

In addition, they have three accelerometers which sense gravity to give tilt information and to help work out which way is down.

Image courtesy of British Geological Survey

Further to this, my phone is even more clever than I realised, when it can’t get a GPS fix it will calculate where I am using its angle of approach to the cell tower, how long it takes the signal to travel to multiple tower & the strength of your signal when it reaches the towers. Since obstacles like trees and buildings can affect how long it takes your signal to travel to a tower, this method is often less accurate than a GPS measurement but it is still pretty clever!

Downside?

Unfortunately mobile GPS is not that accurate compared to a dedicated GPS unit, this is usually due to;

Chipsets. Dedicated GPS, and Smart Phones use a chipset to actually perform the GPS calculation. The accuracy and capabilities vary. Some chipset’s are optimized for urban canyons and are particularly suitable for smart phones (SIRFstar III). Others can track a greater number of satellites concurrently and feature SBAS augmentation. Some cell phone chipsets may outperform those used in dedicated devices. Even some professional grade GPS (Nomad 900G) units ship with less accurate consumer grade chipsets. Antenna. The placement and quality of the antenna effects GPS accuracy. Some dedicated GPS units have external antenna’s instead of the internal antenna typical in smartphones. They may also feature an antenna socket to allow an aftermarket antenna to be added.

How bad?

The difference between smartphone GPS and dedicated GPS is shortening, in 2009 the difference was around 20m, today a mobile GPS will get a fix within 5m of a point, or if ESRI are to be believed, 3m. In this blog ESRI discusses mobile GPS in phones & tablets v dedicated units.

Nick D

 

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