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AFLT - Advanced Forward Link Trilateration
By:Bruce Bahlmann
Christine Martz
A type of handset-based position location technology that uses a time difference of arrival technique using the handset's receiver.

Unlike Assisted GPS (A-GPS), AFLT does not use Global Positioning System (GPS) satellites to determine location. To determine location, the phone takes measurements of signals from nearby cellular base stations, or towers, and reports the time/distance readings back to the network, which are then used to triangulate an approximate location of the handset. In general, at least three surrounding base stations are required to get an optimal position fix. AFLT requires new phones or software in phones with precise timing, systemwide base station synchronization, and Time Division Multiple Access (TDMA) traffic channel resources to transmit location data.

AFLT, a Code Division Multiple Access (CDMA) method of triangulation that includes 1/8 chip resolution reporting, uses Transaction Based Roaming Protocol (IS-801) messaging, requires handset software changes, accuracy of 50-200 meters, is commonly uses as a hybrid with A-GPS, and is used by Verizon, Sprint, and Qwest.

Major carriers in the United States have to implement the Enhanced Observed Time Difference (E-OTD) in Global System for Mobile communications (GSM) networks and decided hybrid solution AFLT / Assisted-GPS in CDMA. AFLT is one of the modes used to measures signals from nearby cellular towers and report the time/distance readings back to the locating towers to determine an approximate location of the GPS receiver.


AFLT, which is particularly useful in cases of limited satellite visibility (e.g., indoors), can be enhanced in High Rate Packet Data (HRPD) to help the AT determine its location more accurately and with higher reliability.

AFLT makes use of the arrival times of cellular signals at the phone. AFLT is supported by CDMA networks. Since the cell towers are all synchronized to GPS time, AFLT works in very much the same way as GPS, with cell towers replacing satellites. The Third Generation Partnership Project 2 (3GPP2) standards (Section 9.3) provide details for supporting AFLT.

AFLT is a method for cell phone location developed for phones using CDMA technology. The technique is based on forward link, or the path from the cell tower to the phone and uses measurements at the handset receiver to form pseudoranges between the tower and the phone. This succeeded because CDMA signals are synchronized to a common time base (currently GPS). Measurements are then collected at the handset and set to a location server through the cell phone messaging system where the pseudoranges are combined into a location estimate. A minimum of three signals from cell towers are necessary to estimate the two-dimensional location of the handset along with the clock uncertainty. The advantage of using cellular signals is that compared to GPS, cell signals are received at power levels that are 60-dB higher. This allows substantially increased availability of location solutions in a variety of challenging environments such as indoors where cell signals may be received by Global Navigation Satellite System (GNSS) signals are unavailable.

Other Related Definitions for AFLT
"A-FLT is a handset-based position location technology (primarily used on CDMA networks) that works by using measurements, taken by the handset, of signals from nearby base stations, and reporting the time/distance readings back to the network, which are then used to triangulate an approximate location of the handset. In general, at least three surrounding base stations are required to obtain an optimal position fix. Unlike GPS (or A-GPS), AFLT does not use GPS satellites to determine location but rather a time difference of arrival (TDOA) technique. TDMA (Time Division Multiple Access) is required to transmit location data and is commonly paired with A-GPS to form a hybrid solution; an alternative to purely handset-based or network-based location technologies. (As AFLT is well-suited for urban environments with close proximity of base stations whereas A-GPS is more advantageous in suburban and rural areas having low base-station densities) " [Deloitte Telecom SIG]

"Advanced Forward Link Trilateration. AFLT is a type of device-based position location technology. Trilateration is a method for determining the intersections of three sphere surfaces given the centers and radii of the three spheres. Unlike A-GPS, AFLT does not use GPS satellites to determine location. To determine location, the cell phone takes measurements of signals from nearby cellular base stations (towers) and reports the time/distance readings back to the network, which are then used to triangulate an approximate location of the cell phone. In general, at least three surrounding base stations are required to get an optimal position fix. For successful results, AFLT requires an accurate Base Station Almanac consisting of cell sector identity information, cell sector GPS coordinates, a proper AFLT calibration, and values for uncertainties in measurements." [Johnbminor]

"AFLT (advanced forward link trilateration) is a time based geolocation technique that utilizes the mobile stations measured time of arrival of radio signals from the base stations. To use this technique, the clock between BS and MS has to be synchronized in time. And then MS measures the time of arrival of BSs pilot signal. The pilot phase measurement technique is very important. Because the accuracy of AFLT position location depend on the chip resolution of receiver." [Hyun-Soo Kim, Jung-Hoon Park, Gyu-In Jee]
Links Related to AFLT
Autonomous GPS, Assisted GPS or AFLT - What and Why?
CDMA2000 Location Based Services - Frequently Asked Questions
Location Technologies - An Overview
Assisted GPS - A-GPS
Technical Resources for AFLT
Position Determination Concepts - Technology Guide
Hybrid Positioning - CDMA Networks
Location Based Services - Technologies in Wireless Networks
Products and Solutions for AFLT
Sprint Developer SandboxTools and capabilities
SKYTRX - Real Time GPS Systems
InterTek - GPS Testing

Other AFLT Related Products

Blogs, Videos, and Social Networking about AFLT
Location Based Technology Blog -Localization Technology for Mobile
Technowizz Blog - Wireless Technologies Blog
More Information About AFLT
Books about AFLT
A-GPS: Assisted GPS, GNSS, and SBAS - by Frank van Diggelen
Global Positioning System: Signals, Measurements, and Performance - by Pratap Misra, Per Enge
Understanding GPS: Principles and Applications - by Elliott D. Kaplan, Christopher Hegarty

Other AFLT Related Books

Used and New AFLT for Sale

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