Acquiring Measurements from the TargetDevice

Acquiring Measurements from the Target/Device

The access network is one source of location measurements. However, the Target device can, in some circumstances, provide information that can assist in the determination of location. This is already seen in some cellular networks, where a cellular telephone provides measurement information to the network-based server.

The FLAP protocol is designed for network-based measurements and as such is not suited to carrying messages from the device. HELD, on the other hand, establishes a session between the Target and a LIS, which makes it a good candidate for carrying measurements from the device. HELD contains extension points that permit the inclusion of location measurements in a request message. The exact contents of any location measurements depend on the type of access network and the capabilities of the device.

At the time of writing, location measurements are not included in HELD messages, except in LIS-to-LIS interactions where one LIS acts on behalf of a device. However, there are a number of cases where a device could provide measurements.

Device-Assisted A-GNSS or A-GPS

Global Navigation Satellite System (GNSS) location determination methods can provide extremely good precision in location estimates, with results that are typically accurate to within meters of the actual location of the device. The United States Global Position System (GPS) is the predominant GNSS, which provides high precision location determination.

Device-assisted Assisted-GNSS (A-GNSS) is a prime example of a device providing location measurements to a server. Unlike autonomous GNSS, where the device calculates its own location, a device using device-assisted A-GNSS provides pseudorange measurements to the server. The server then uses knowledge about the position of GNSS satellites and a set of algorithms to determine the location of the device.

Other Device-Based Methods

In addition to GNSS measurements, a device can also report network-based measurements. For instance, a device can provide GSM radio information, such as the serving cell identifier, and the Timing Advance and Network Measurement Report values. The LIS takes these location measurements and combines them with data from a cell database and specialized algorithms to determine a location estimate.

Other options include the use of Radio Frequency Identifier (RFID) beacons, where the device sends the identity of nearby beacons to the server, which uses a database of known beacon locations to pinpoint the device. Device-based location determination and location measurements are explained in more detail in Chapter 9 of this book.

Hybrid Location Determination

Hybrid location determination methods are those that combine multiple sources of location measurements to estimate location. A hybrid method can address limitations that might exist where only one method is employed, increasing the probability that location information can be used. This is true for both network- and device-based location determination methods.

For instance, the trilateration used for GPS requires four satellite pseudoranges for an accurate estimate; a hybrid method can combine a smaller set of pseudoranges with other information to determine location.