Indoor GPS - Does it work? Everything you need to know

GPS is one of the most widely used pieces of technology in the world today. While we may not always be conscious of its presence, GPS has revolutionized the way we interact with the world around us. Almost all modern smartphones make use of GPS constantly to provide tracking information. Modern mapping and navigation systems such as Google Maps leverage GPS to provide us with positioning data. Numerous applications use GPS to obtain super accurate timing measurements that, without GPS, would be impossible.

Given GPS’s ubiquity in modern life and its incredible, almost unconscious level of ease of access, it’s no surprise that one of the most common questions we get asked when discussing indoor positioning and mapping applications with clients is ‘Why can’t we just use GPS?’ In this post, we’ll discuss everything you need to know about GPS in terms of how it relates to use for indoor systems and applications.

First things first, a few basics:

What does GPS stand for?

GPS stands for Global Positioning System. The project was begun by the United States government in 1978 and the system became fully operational upon the launch of the system’s 24th satellite in 1993. 

While other nations and organizations, including Russia, China and the EU, have launched similar systems, GPS remains the predominant positioning system, to the extent that the term GPS is often now used as a genericization for any satellite-based navigation or positioning technology.

How does GPS work?

GPS works by sending a number of signals from different satellites in orbit around the Earth to a ground device, such as a smartphone. Currently, there are 32 satellites in operation to make up GPS, which tends to mean that no matter where you are situated on the planet at any one time, you’re likely within ‘sight’ - or range - of at least six satellites, as this graphic demonstrates:

Source

Every GPS satellite is fitted with an atomic clock and a super accurate reading of its own position. When a ground device - such as a user’s smartphone - receives signals from multiple satellites at the same time, it can use the relative times of arrival (TOA) of the different signals to triangulate the ground device’s position. 

When a strong enough signal is available from multiple satellites, GPS is capable of achieving a positioning accuracy of between 5 and 10 meters for the average consumer smartphone.

Does GPS work indoors?

Put simply, GPS can work indoors, but the critical determiner for its effectiveness indoors is the ability of the satellite signals to pass through ceilings and other obstructions in order to reach the ground device. Anything in an indoor environment that impedes or completely blocks GPS signal can have a major impact upon the ability of GPS to ascertain a ground device’s position successfully.

Furthermore, this issue is not going away anytime soon. Modern building materials - such as concrete, steel, and other metals - are all significant impediments to GPS signals. Older materials, such as wood, are more porous when it comes to GPS, but these are gradually forming less and less of modern constructions.

GPS signals can also reflect off objects such as walls, further complicating matters indoors. This means that sometimes, signals can reach a user indoors but, due to the nature of how the ground device interprets the signal, can be read incorrectly and actually result in less accurate results than if the signal had been blocked entirely.

Even in situations where GPS signals are able to penetrate a building and provide a strong connection with a ground device, there is another significant factor to consider when using GPS indoors, which is the overall accuracy that GPS is able to achieve.

As mentioned above, GPS in ideal situations is routinely able to pinpoint a ground device’s position to an accuracy of between 5 and 10 meters. In outdoor environments, this level of accuracy is perfectly sufficient, particularly when combined with other data points, such as the device’s internal compass, which can help the app predict the user’s orientation and direction of travel. In most cases when driving, for example, roads are large enough and turnings spaced wide enough apart that even if the positioning accuracy is off by 5 or 10 meters, drivers will have enough information to be able make the correct decision about where their navigation wants them to go. 

In indoor environments, this level of potential discrepancy between a user’s reported position and their actual position presents enormous issues. This is due to the much smaller dimensions of the average indoor space versus the average outdoor environment. A user being told to turn left immediately when they’re actually 10 meters further down a corridor than their device thinks they are could be led into a room that is multiple rooms away from the one they intended to enter.

The combination of the lack of precision accuracy and the difficulties GPS faces to even provide signal to indoor locations presents a significant impediment to its ability to work indoors.

What determines GPS signal indoors?

GPS’ indoor signal is predominantly determined by the number and density of obstacles and obstructions the signal has to pass through between the orbiting satellites and the ground device, such as a smartphone. The greater the density of material that the signal must travel through in order to reach the ground device, the weaker the signal will be (if it can even reach at all), meaning the weaker the performance of the GPS and the worse the accuracy will be.

Can GPS signal be improved indoors?

Given the various issues with obtaining an accurate GPS signal outlined above, it’s natural to wonder if there’s a way to improve GPS’s indoor signal. 

The good news is that there is. GPS repeater devices are becoming more common and affordable. These devices typically work by affixing an antenna to the roof or outside of a building where GPS signal can reach it, then feeding that signal into the building via cables and transmitting it into indoor environments via one or multiple repeaters, normally fixed to a room’s ceiling.

However, while these repeater devices can help improve GPS’s signal indoors to the point where its performance comes close to mirroring the performance you’d expect outdoors, the limitations of GPS’s general accuracy for use in an indoor environment - namely that even at 5m accuracy, the margin for error in many indoor locations such as retail stores, workplaces or hospitals is too great - remain.

Can GPS be used for indoor positioning or navigation systems?

GPS can technically be used as the technological foundation for indoor positioning and indoor navigation systems. However, for a number of reasons, there are better options now available, such as Bluetooth Low-Energy (BLE).

• GPS signal struggles to reach indoor locations effectively, with signals either reflecting off internal structures such as walls and losing accuracy or being blocked entirely by ceilings and modern building materials such as concrete
• The GPS signal issue can be alleviated somewhat by the installation of devices such as GPS repeaters that can boost the signal. However, doing so is often more time consuming and expensive than competing indoor positioning technologies, such as BLE
• Even with a perfect GPS signal, there are still significant issues to its use for indoor positioning and navigation. Chief among these is the fact that even under ideal conditions, GPS can generally only offer consistent accuracy of between 5 and 10 meters
• This level of accuracy is acceptable for outdoor navigation tools due to the amount of space and landmarks. However, in indoor environments, a 5m discrepancy can mean directions and blue dot location are wildly inaccurate, to the point where the GPS system is unusable