Originally posted in the Android Developers Blog, the following is reprinted with permission from authors Frank van Diggelen, principal engineer, and Jennifer Wang, product manager, Google.

At Android, we want to make it as easy as possible for developers to create the most helpful apps for their users. That’s why we aim to provide the best location experience with our APIs like the Fused Location Provider API (FLP). However, we’ve heard from many of you that the biggest location issue is inaccuracy in dense urban areas, such as wrong-side-of-the-street and even wrong-city-block errors.

This is particularly critical for the most-used location apps, such as rideshare and navigation. For instance, when users request a rideshare vehicle in a city, apps cannot easily locate them because of the GPS errors.

The last great unsolved GPS problem

This wrong-side-of-the-street position error is caused by reflected GPS signals in cities, and we embarked on an ambitious project to help solve this great problem in GPS. Our solution uses 3D mapping aided corrections, and is only feasible to be done at scale by Google because it comprises 3D building models, raw GPS measurements, and machine learning.

The December Pixel Feature Drop adds 3D mapping aided GPS corrections to Pixel 5 and Pixel 4a (5G). With a system API that provides feedback to the Qualcomm Snapdragon 5G Mobile Platform that powers Pixel, the accuracy in cities (urban canyons) improves spectacularly.

Pictures above show a pedestrian test, with Pixel 5 phone, walking along one side of the street, then the other. Yellow = Path followed, Red = without 3D mapping aided corrections, Blue = with 3D mapping aided corrections.

Why hasn’t this been solved before?

The problem is that GPS constructively locates you in the wrong place when you are in a city. This is because all GPS systems are based on line-of-sight operation from satellites. But in big cities, most or all signals reach you through non line-of-sight reflections, because the direct signals are blocked by the buildings.

The GPS chip assumes that the signal is line-of-sight and therefore introduces error when it calculates the excess path length that the signals traveled. The most common side effect is that your position appears on the wrong side of the street, although your position can also appear on the wrong city block, especially in very large cities with many skyscrapers.

There have been attempts to address this problem for more than a decade. But no solution existed at scale, until 3D mapping aided corrections were launched on Android.

How 3D mapping aided corrections work

The 3D mapping aided corrections module, in Google Play services, includes tiles of 3D building models that Google has for more than 3850 cities around the world. Google Play services 3D mapping aided corrections currently supports pedestrian use-cases only. When you use your device’s GPS while walking, Android’s Activity Recognition API will recognize that you are a pedestrian, and if you are in one of the 3850+ cities, tiles with 3D models will be downloaded and cached on the phone for that city. Cache size is approximately 20MB, which is about the same size as 6 photographs.

Inside the module, the 3D mapping aided corrections algorithms solve the chicken-and-egg problem, which is: if the GPS position is not in the right place, then how do you know which buildings are blocking or reflecting the signals? Having solved this problem, 3D mapping aided corrections provide a set of corrected positions to the FLP. A system API then provides this information to the GPS chip to help the chip improve the accuracy of the next GPS fix.

With this December Pixel feature drop, we are releasing version 2 of 3D mapping aided corrections on Pixel 5 and Pixel 4a (5G). This reduces wrong-side-of-street occurrences by approximately 75%. Other Android phones, using Android 8 or later, have version 1 implemented in the FLP, which reduces wrong-side-of-street occurrences by approximately 50%. Version 2 will be available to the entire Android ecosystem (Android 8 or later) in early 2021.

Android’s 3D mapping aided corrections work with signals from the USA’s GPS as well as other GNSS: GLONASS, Galileo, BeiDou, and QZSS.

Our GPS chip partners shared the importance of this work for their technologies.

Francesco Grilli, vice president of product management at Qualcomm Technologies Inc.:
“Consumers rely on the accuracy of the positioning and navigation capabilities of their mobile phones. Location technology is at the heart of ensuring you find your favorite restaurant and you get your rideshare service in a timely manner. Qualcomm Technologies is leading the charge to improve consumer experiences with its newest Qualcomm Location Suite technology featuring integration with Google’s 3D mapping aided corrections. This collaboration with Google is an important milestone toward sidewalk-level location accuracy.”

Charles Abraham,…