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An interesting feature of GPS-based positioning solutions

Network -Assisted GPS (A -GPS) uses network -based GPS tracking device for cars to help the terminal measure GPS data. These receivers are placed around the mobile network in 200 to 400 km intervals and collect GPS satellite data on a regular basis. This can then be requested from the GPS-enabled terminal and enable the receiver to make timing measurements without having to decode the actual messages from the satellites. This process reduces the TTFF to one to eight seconds and makes GPS a much more compelling positioning solution. An interesting feature of GPS-based positioning solutions is that they enable user locating in three dimensions. For some specific applications, such as 911 rescue operations in the mountains, this feature might be of value because the rescuers can immediately see at what height the user is located.

A concern with GPS tracker mini is that it requires new hardware in the receiver, which is always something that device manufacturers are reluctant to include. Therefore, some argue that a software-based solution is preferable and is more costeffective for the consumer. Enhanced Observed Time Difference (E-OTD) is a solution that calculates the time difference that it takes to receive data from different base stations and estimates the position based on that information. For the measurements to be valid, however, the signals that are used for the calculation have to be sent at the same time (or the distances measured would have been measured at different times, and the user might have moved). In order for this process to work, an overlay network of Location Measurement Units (LMUs) needs to be deployed in order to provide an accurate timing source for the measurements. The E-OTD-enabled handset notes the time difference between the signals from the measured base stations. This time difference is then a measure of the distance between each of them, and we can use triangulation to calculate the position.

As with GPS tracking solutions, the measurements are made in the terminal, but the calculations can take place in either the terminal or on the network. Again, making the calculations on the network makes the process less power consuming, but E-OTD still adds new requirements to the terminal. In order to use E -OTD algorithms both in idle mode and in dedicated/ready mode, the terminal needs to have additional memory, processing power, and battery power (compared to other handsets). The handset has to work harder during those periods of time when it otherwise would have been resting and saving battery power (idle mode). At this writing, it is unclear how many handsets will support E-OTD. GSM 03.71, Annex C, describes E-OTD.

E-OTD uses triangulation based on downlink measurements in the mobile terminal, and which you can accomplish by using the network as well as the uplink. The Uplink Time of Arrival (UL-TOA) measures the received signal from a mobile station by using three different base stations. In addition to having an LMU that measures the time, UL-TOA relies on synchronized base stations. The synchronization of base stations is crucial and mostly takes place through GPS receivers or atom clocks in the base stations. Because cdmaOne/cdma2000 base stations are already synchronized from the start, UL-TOA is more compelling to use in these systems than the asynchronous GSM/TDMA/EDGE/WCDMA systems.

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