Assisted GPS

Assisted-GPS reaches 5-50m accuracy outdoors and Enhanced Assisted-GPS allows even higher accuracies (1-3m) over very large service areas, covering most environments including indoors without having to connect the A-GPS server to a Wide Area Reference Network or having to wait several minutes as is the case with traditional GPS.  Higher accuracy can be achieved through a system where location is tracked by multiple base stations simultaneously. This technique is generically referred to as triangulation. 

GPS systems cannot track an individual holding a GPS receiver.  And, the personal navigation system sends no data back to the GPS satellites.  The GPS system does not put an individual -on a map – for that is the job of the positioning software component – of the personal navigation system. 

Positioning is the process of putting a user and the user’s device on the map.  Device positioning is generated by a software process that uses sensor data or location signal and combines with a map database to create a pair of coordinates that coincide with the map geometry. Positioning software updates data every second or less and then transmits the updated position to navigation components and to the application itself.

In the A-GPS model, the data “assist” is executed through an A-GPS server within a telecommunication carrier’s network.  When a caller makes a location request, the wireless network sends the approximate location of the closest cell site to the location server that is receiving the ping from the device/terminal.  The location server then tells the handset which GPS satellites should be relevant for calculating its position.  The device/terminal reads the GPS signals, calculates distance from line-of-sight satellites, then sends an acknowledgement to the A-GPS server which institutes error corrections and calculates the latitude, longitude, and altitude of the device/terminal.  For 911 calls, the server sends the location data to the best PSAP; for other location-based services, the server sends the location data to a wireless carrier or back to the handset itself. 

How do the location servers determine satellites within a sector?  Determination occurs through a regional reference solution, or a set of regional reference stations (RRS).  In an RRS, the cell sites in the regional area – as defined by a carrier – communicate with approximately seven satellites.   The key to quality of service (QoS) is line-of-site availability of all cell sites within the network. 

However, QoS for A-GPS is compromised when a handset/terminal is closer to the horizon than a reference station, and thus will communicate with the rising satellite before the RS does and trigger GPS without assistance. Installing additional reference stations in a regional network improves this situation only slightly, and only if the reference stations are installed towards the edge of a region.

In addition to the RSS solution, is the full sky solution.  It is designed and operated by Global Locate and uses a global network to communicate with satellites in GPS constellations. This capability can be thought of as providing reference stations outside of the regional coverage area, in all directions. By the time each new satellite rises over the horizon of a regional coverage area, reference stations located outside the region have already begun tracking satellite tracking. This ensures that assistance data is available for rising satellites even before a handset acknowledges satellite presence.

Global Locate World-Wide Reference Network (WWRN) takes this concept one step further by placing reference stations in locations that can communicate with each satellite used in a constellation. This ensures that unexpected obstructions or RS failures do not prevent the WWRN from providing continuous assistance data for all GPS satellites.

For more information, see:

http://www.mindcommerce.com/Publications/UltimateGuide_LBS_2011.php

About Mind Commerce

Analysis of telecom and ICT infrastructure, technologies, and applications.
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