Since the early days at Rave, we have been actively involved in location based services and are one of the only companies to have direct interfaces to location networks across the leading U.S. wireless carriers (Note: all location queries are only available on a strictly opt-in basis). We are often asked about how the different technologies used to locate wireless subscribers actually work, so I thought we'd share this primer on location technologies.
First, there are a number of factors that effect the accuracy and availability of location services:
- Handset technology
- Network technology
- Local network build out specifics
- Physical obstructions
Each of these factors intersect to determine the accuracy available for a specific subscriber in a specific incident; however, in general there are 4 methods by which a subscribers device may be located and which will determine the accuracy of the result returned to Smart911 by the carrier. These methods are:
GPS (Global Positioning Satellites)
Based on the amount of time it takes signals to travel between the phones GPS chip and satellites, a very precise position can be calculated. While many of today’s phones have GPS, many still do not. Additionally, GPS requires line of sight to the satellites meaning it may not be available indoors and is adversely effected by obstructions. The first location fix using GPS also requires the network to turn on the GPS chip remotely and have it search for satellites it can access. This process can take from 30 seconds to 1 to 2 minutes.
A-GPS (Assisted GPS)
This method is very similar to GPS but utilizes the known location of towers in communication with the handset to “jump start” the process of finding satellites and determining the handsets location. The result is a shorter time to the first location hit and often a very precise hit even when the handset has line of sight to fewer satellites.
AFLT (Advanced Forward Link Trilateration)
This method determines the handset location by measuring signal transmission time between a minimum of three cell towers. This method is less precise than GPS methods but is less susceptible to physical barriers.
Cell tower or Cell Sector
The subscriber handset location is determined based on the data observed by the cell tower serving the phone. Each tower has defined geographic “sectors” of service, and based on the sector the handset is in the tower is able to get a coarse location fix. There are also Enhanced Cell Id methods that improve the accuracy of these fixes, but are still less accurate then other methods. Cell tower location does provide extremely fast, low latency location returns and is available as long as the mobile device has coverage.
The chart below provides a general overview of the accuracy of each of these methods. It is important to note that the latency times are those seen by the network. They do not indicate the amount of time from pressing the “update location” button on Smart911 and when a response is seen. Because Smart911 always requests the most precise location from the carrier, the response time is often a mix of the different methods.
It might make sense to provide 2 comparable objects for each line item. 500-5000 meters are drastically different sizes.