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EETE JULAUG 2014

NAVIGATION & GEOLOCATION Enabling sub 10cm positioning accuracy By Mickael Viot and Matt Gross GPS was the first major revolution to happen in the domain of navigation in centuries, allowing us to find our way around without maps or landmarks. Unfortunately, GPS does not operate indoors, where 85% of our time is spent. This is the start point of navigation and geo-location for indoor positioning. The first examples of indoor positioning are already around us, with many shopping malls and public places offering services and apps to help consumers and visitors navigate their way around. However, people are used to very accurate and reliable outdoor GPS systems. What they require – and expect - is the same level of user experience indoors. While being able to locate a 50m wide shop in a large mall is nice, Fig. 1: Narrowband time based. being able to find a pair of shoes within a store is much better. And this was simply impossible with existing indoor positioning technologies. Many technologies currently being used for indoor positioning are based on measuring RF signal strength. The positioning is based on the assumption that signal strength and distance have a deterministic relationship, according to the Friis equation. Unfortunately, the Friis equation is only applicable in free space. In an indoor environment, multi-path interference and lack of sight channels can cause the range estimate to have an accuracy of tens of meters. With post-filtering and fingerprinting, these systems are able to improve reliability and accuracy to a few meters in a friendly environment, but any change in the floor plan will require a new round of calibration. This approach will never reach the accuracy required by many applications. The idea of building indoor location systems on the time of flight of the RF signal is relatively new. By simply measuring the time of flight, you can accurately estimate the distance between an RF transmitter and RF receiver. There have been attempts to build time of flight systems using standard narrowband RF Wi-Fi or other 2.4 GHz signals. The problem here is that due to the narrow bandwidth, the rising edge of the signal is slow and it is difficult to determine the exact time of arrival in multi-path and low signal to noise ratio environment - see figure 1 - resulting in accuracy of several meters, with reliability still very dependent on the environment. IR-UWB: a game changer Impulse Response Ultra Wideband (IR-UWB) and it implementation as per the IEEE802.14.4-2011 strandard finally offers the performance needed for accurate and reliable indoor positioning. This UWB signal consists of narrow pulses, typically no more than 2ns wide. This makes it highly immune to multi-path and interference - see figure 2. The IEEE802.15.4 UWB technology supports 10cm of geolocation accuracy with a reliability of over 99%, with a range varying from up to 35m in Non Line of Sight, up to 250m in Line Of Sight. The high-ranging makes possible to achieve accurate positioning, precise enough to pinpoint an object in a drawer. The time-based ranging also allows a deployment that is cost effective without time-consuming calibration and environment dependent processes. With IR-UWB, it is possible to implement a real-time location system (RTLS) that can offers two different location services, separately or simultaneously. Tracking of people and assets In tracking, the location of an object or a person is provided to the system via tags that transmit data packets to be received by fixed beacons. As the beacon clocks are synchronized, it is possible to determine the Time Difference of Arrival (TDOA) to the beacons. By using TDOA and multi-lateration algorithms the system virtually draws hyperboles - see figure 3. The intersection of those hyperboles determines the exact position of the object that transmitted the blink. This localization can be achieved in 2D or in 3D with an accuracy of 30cm in 3D and a reliability of 95% or better. It is now possible to locate in which drawer an ECG machine resides or to determine whether staff has stopped to wash their hands before entering the surgery room. Navigation Navigation provides location information to a moving object such as a person, a robot, or a forklift. Unlike tracking, where the information flows from the mobile device to the system, the location information is flowing the opposite way. Mickael Viot is Marketing Manager at Decawave - www.decawave.com Matt Gross is responsible for Technology Partnerships at Red Point Positioning RTLS systems - www.redpointpositioning.com www.electronics-eetimes.com Electronic Engineering Times Europe July/August 2014 25


EETE JULAUG 2014
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