|The handset and the base-stations interact to implement|
observed TDOA positioning. The RTT measurement
is used for eCID mobile location.
Location measurements involving distances is referred to as 'trilateration' and measurements involving angles is called 'triangulation'. A third method is called multilateration which uses difference in distances or absolute measurements of time-of-flight from three or more sites at known locations. GPS position location uses a type of trilateration. eCID can use one or more of these techniques as a hybrid mobile tracking solution.
For some eCID location techniques, fingerprinting methods are similar to the approach used for Wi-Fi positioning. A central database is created with geotagged cell information, such as received signal strength indication (RSSI), cell ID, real-time difference (RTD), and network measurement record (NMR) in GSM. The mobile tracking device reports currently observed network information from the serving cell and all other nearby cells. This information is used as an input to a fingerprinting algorithm that estimates the mobile device position. Accuracy is 100 to 300 meters.
In time difference of arrival (TDOA) methods (downlink only), the mobile device detects the TDOA of signals between the serving base-station and one or more neighbors. The time differences can be used to estimate the mobile tracking device position. Typically, TDOA methods are more effective in synchronized networks. Accuracy ranges from 25 to 300 m. Common applications include:
- CDMA/advanced forward link trilateration (AFLT): Time differences in pilot signals from the serving cell and the reference cell are detected and used for positioning.
- LTE/observed TDOA (OTDOA): Special cell-specific reference signals, called positioning reference signals (PRS), are used for the time difference measurements.
Positioning must be conducted in connected mode at the same time as various network procedures such as handovers, circuit-switched fall back (CSFB), and cell reselections. Cellular positioning typically does not provide as high-grade accuracy as A-GNSS, but it can assist in deriving a quick, coarse location, as well as provide an alternative for GPS or GNSS when indoors.