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Why GPS does not work inside buildings?

/images/blog/nogps.jpg November 2020

7 min

iPhone with the Mapwize application running

Nowadays, getting from point A to point B is fairly simple: we open up our favorite mapping app on our phones, enter the address and GPS takes over, telling us how to get there. But what happens when you go into a building? Your directions app suddenly isn’t so useful. Instead of telling you where to turn next, it displays error messages, or worse yet, inaccurate information.

Why does GPS fail to work inside buildings? Let’s look at the reasons and what other options may suit you better.

What is GPS?

We use GPS technology on an almost daily basis. It is easy to access and free to use. Or at least it seems that way. The truth is that a chip only a few millimeters in size and weighing less than five grams lives inside your device acting as a GPS receiver. To provide you with information about where you are located and directions to reach your destination, it connects to a multi-billion dollar network of satellites orbiting the earth.

Developing GPS took decades, starting from the first satellite launch in 1978, to becoming fully operational in 1993 and continuing to launch replacement satellites as maintenance and damage require. When you think about the cost of launching a satellite into orbit, it becomes easier to understand the astronomical cost involved in providing us with this seemingly free service.

Each satellite contains an atomic clock. It constantly broadcasts its time and position over the planet. The receivers in our devices listen to the signals within range, and use Einstein’s infamous speed of light calculation to compute the distance of the surrounding satellites. By doing so, our devices can determine the intersection point (i.e. where we are at that moment) and translate it into a latitude, longitude and altitude.

Where GPS hits a wall?

Given all of the time, money and engineering that has gone into creating GPS, it seems hard to believe that it fails when it hits a wall, but that is exactly what happens. There are two reasons why GPS does not work inside of buildings.

The first reason is low-power signals. Those thirty-one satellites orbiting above us are constantly emitting signals broadcasting their position. Since they don’t have a nuclear power station built inside of them, scientists had to make them as efficient as possible. The solution was to produce a weak signal in the sky, concentrating more power (and energy) on the receiving ends. Our personal devices, with easier access to recharging points, are designed to search out and pick up the satellite signals, within limits.

Walls, ceilings, insulation, furniture and even our own bodies can absorb the signals before they reach our devices, making it harder or even impossible for our devices to determine our location. Think about large buildings such as shopping centers or airports, where any number of things can stand in the way of the GPS signal trying to reach your phone. The same problem happens when you attempt to turn on your vehicle’s navigation system while parked in an enclosed parking garage. Your navigation system will wait until it has a clear signal before it will attempt to give you directions.

The second reason GPS fails inside of buildings is line of sight. The GPS calculations assume that the receiver and the satellite can see one another, without any obstructions between them. When you add a ceiling and a wall, the signal has to go around these barriers before it can make its way to our devices. This can wreak havoc on its calculations and end up with an estimated distance which is incorrect.

Unless you are in a single-floor building with a glass roof, you should not expect your GPS device to give you the right indication of exactly where you are standing. Even if you are standing close to a window, you may end up with the wrong information. If all of the satellite signals enter the building through the same window, your device will mistakenly think you are standing outside instead. Even if the latitude and longitude isn’t far off, GPS cannot guess which floor of a building you are standing on. Directions telling you to Go straight take on a whole new meaning when you are facing the window on the third floor of a building.

Multi-path can equal multiple problems

We spoke above about the problems associated with signals being absorbed by obstructions before they can reach your device. Obstructions with harder surfaces can lead to a different problem: bouncing and rebounding.

In an open, outdoor environment, satellite signals travel straight to your device, being received once. When you add in walls, roofs, doors and windows, that straight line splits into multiple paths as the signal bounces around and rebounds off the obstacles, eventually making its way to our device. Instead of getting a clear message, your device hears the satellite’s information multiple times, making it very hard for it to determine which one was actually sent first. The end result is a signal which looks blurred.

Let’s put the problem of the blurred signal in perspective. The speed of light is so quick that an error of only one microsecond on the arrival time will make your calculated position wrong by three hundred meters. Three hundred meters, indoors, can be the difference between being in a food court or a department store, or being at the ticket counter versus the other side of airport security.

Academics are researching ways to exploit the multi-path signals, but at the moment, it is far from solved.

New GPS technology won’t solve the problem

Fifty years have passed since the US introduced GPS technology. Since then, many regions have invested in developing their own systems and are launching regional alternatives. However, these systems still rely on the same basic principles as GPS and suffer from the same limitations when trying to map locations indoors. Even the EU’s Galileo system, which became fully operational last year, has not overcome this challenge.

Most don’t realize GPS does not work in buildings

GPS is so widely used, it isn’t a surprise that so many people expect it to work as well indoors as it does outside. What we often overlook is how important it is for our GPS devices to have direct line of sight to the satellites. When our mapping apps fail to accurately determine our location, we tend to blame our devices. In truth, the problem is the technology itself.

The GPS technology we rely upon today to help us find our way around was funded by national security budgets. The original use case was tracking enemy movements, not letting us know the fastest route to our favorite restaurant. It certainly wasn’t developed with the idea that it might one day be used to help students find their way around a university campus. This is the reason technology falls short of being a viable solution for indoor and mixed indoor/outdoor locations.

Wayfinding solutions are better for indoor and indoor/outdoor applications

Although GPS has become the standard for outdoor mapping, the same has not happened for indoor solutions. Instead, there are a number of proprietary solutions available. The right one for your building, set of buildings or even campus will depend on the use case.

Positioning solutions for buildings can be complicated and expensive to deploy. Most solutions require installation of a network of devices which must be configured to work together to provide the user with information on their location and their movements. The layout of your facility, building materials used, number of visitors and traffic patterns must all be considered in order to create a customized solution specifically for your site. Unlike GPS, national security budgets will not cover these costs.

In smaller spaces with more obvious visual clues, it is challenging to justify the additional cost required to provide site visitors with a moving blue dot on a map. The question we should be asking ourselves is: Do we really need GPS inside?

To find out more on GPS and Indoor positioning, you may want to read “Why Blue Dot is accessory for Indoor Wayfinding” and “How does Indoor Positioning works”.

Mathieu GERARD's photo


Mathieu GERARD, Co-founder & CTO

Mathieu leads the product development and writes the company roadmap. He’s driven by a real passion for innovation and technology but also likes to solve problems with simple and pragmatic solutions. His vision is that making buildings smarter will significantly improve comfort, safety, productivity and energy consumption.