By Brock Butler
In the next two to four years, mobile device location platforms will be able to provide positioning performance that enables emergency call (E911) and location-based services (LBS) with excellent accuracy (5–10 meters) in all locations. We call this accurate everywhere location, and it will be a significant enabler of indoor navigation applications and for even wider adoption of consumer LBS.
In fact, we may eventually forget how we ever lived without it. This technology can enhance our lives by enabling our mobile devices to know precisely where we are at all times. Armed with this information, our devices can behave in a way that suits our specific situation, and they can do this without us having to do anything other than keep the phone with us.
Text and images will get significantly bigger while driving or walking. Facebook notifications can be automatically disabled while at work. Shopping lists can be automatically displayed when approaching a store that has an item on the list. The potential benefits are endless — provided that the privacy issues associated with location are handled appropriately.
GNSS is the superior technology when a mostly unobstructed sky is available, but it can’t deliver accurate position fixes in all environments — at least not at a cost and in a form factor that works for consumer mobile devices. Accurate everywhere location requires some form of advanced hybrid location technology. Because its definition is constantly evolving, the term hybrid can mean different things to different people. This article aims to clear that up.
Here is an overview of the hybrid positioning technology currently used in mobile devices, as well as what is coming in the next two to four years that will enable accurate everywhere location:
GPS + GLONASS. Multiple GNSS technologies are starting to be more common in new chipsets aimed at mobile devices, and assisted-GPS (A-GPS) + A-GLONASS is right around the corner. The benefit from this hybrid GNSS approach is that with more satellites in the sky, devices are likely to receive more line-of-sight signals in challenging environments where a significant portion of the sky is obstructed (like urban canyons). While this might improve performance on a street in downtown Manhattan, it does not help when you are in the middle of a building or in the subway.
Cellular Multilateration + A-GNSS. Mobile devices with CDMA cellular radios have supported hybrid A-GPS + advanced forward-link trilateration (AFLT) for more than a decade. This concept is now being applied to long-term evolution (LTE) devices, with support for A-GNSS + observed time difference of arrival (OTDOA) being written into the 3GPP standards. Both AFLT and OTDOA are forms of cellular multilateration, which means that devices can make measurements of relative timing offsets between multiple downlink cellular signals, and those measurements can be used in a hyperbolic multilateration formula to compute a position (one signal acts as reference and hyperbolic intersection of 2+ signals are used for position).
Does this sound familiar? It happens to be very similar to GNSS location computation, so it is possible to combine measurements from cellular signals and measurements from GNSS satellites to compute a hybrid position. For example, 2 satellites + 2 cellular measurements can be combined to compute a position, which makes this technique very attractive. Although it is used for both E911 positioning in North America and LBS worldwide, this technology will become even more widespread as LTE adoption increases.
A-GNSS + Wi-Fi Positioning + eCID. Many popular smartphones today support Wi-Fi positioning and enhanced cell ID (eCID) in addition to A-GNSS. This hybrid solution allows coarse positioning in indoor environments where A-GNSS does not work. Solutions for Wi-Fi and eCID positioning are currently very fragmented and proprietary. However, this is the reason you are able to get a semi-accurate position fix on your Android or iOS mobile device when GNSS satellites are impossible to measure (many other devices support this as well). These technologies are going to provide more accurate information as time goes on, but we don’t believe they will achieve accurate everywhere location on their own.
A-GNSS + Wi-Fi Positioning + Cellular Positioning + Sensors. You might have guessed it, but we think accurate everywhere location will be enabled by a combination of all the above hybrid techniques plus one more important technology: sensors. Integrated sensors like accelerometers, magnetometers, and barometers enable devices to sense changes in direction, orientation, and elevation. Given an accurate starting location (for example, GNSS position fix), sensors can track location accurately for several minutes (and this will continue to get better). Location error will accumulate over time, but this can be minimized when Wi-Fi, cellular, and GNSS positioning are used in conjunction to constrain the error. Furthermore, barometers can be used to track elevation changes, thereby allowing devices to know exactly what floor of a building a user is on. Other technologies, or signals of opportunity, may be used in the future to further improve performance, but we think this mix of A-GNSS, Wi-Fi, cellular, and sensor positioning is the key to accurate everywhere location in mobile devices.
With substantial R&D dollars being spent now, and standardized testing for hybrid positioning emerging this year, our best estimate is that the accurate everywhere technology will become commercially widespread by 2015.
Brock Butler is director of Spirent’s Wireless Location Technologies, part of a team that has made major contributions to development of the LBS standards in the 3GPP: Spirent filled the editor and rapporteur roles for the TS 51.010 and TS 34.171 A-GPS Terminal Conformance Specifications, as well as the editor role for the Enabler Test Specification for SUPL in the OMA. Butler holds a BSc in electrical engineering from Villanova University.