SonarDistance

CopSonic has created SonarDistance app that allows measuring the distances between multiple devices using BLE and ultrasound (US) signals.
This is a universal solution, devices just use their in-built microphones and speakers.
allows calculating distances between nearby devices with an accuracy of +-10 cm. Once the 2 smartphones discover each other, the calculation time is around 10 seconds. The distance measurement between the devices in close range is performed by ultrasounds, thus confirming the absence of physical barriers such as walls, doors or windows.