The GPS satellites transmit a ranging code that ultimately allows the handset to determine the distance between the satellite and the handset. When combined with measurements from other satellites, knowledge of the location of each satellite, and application of error correction, the location of the handset can be calculated. When the handset is A-GPS enabled and connected to a network, the position calculation may be performed on the handset or the server. When running in handset-assisted mode, the Rearview Mirror server provides the handset with a compact set of assistance data (normally reference time and acquisition assistance) and the handset locks onto the satellites and returns the measurements to the server. This allows the position calculation to be performed on the server that has more processing power and can optionally include other measurements that have been made by the network to calculate a hybrid location.
With your new understanding of coordinate systems and datums there are some basic GPS concepts to apply. As we all know, Personal GPS calculate our position, and when supplied with a position or coordinate of another location they can direct us on how to get there, as in the case of locating a geocache. Each GPS manufacturer puts a spin on how this action occurs and the set of features and functions that their models include, but a couple basic traits are common to most receivers: waypoints, routes, and tracks. here am I with respect to the current route? What maneuver is next? Today vehicle positioning has become much less complex, since the GPS signals are very precise and digital maps supply a high density and coverage. Once the vehicle has determined its coordinates, the position can be snapped to the road network. The closest map node relative to the current position represents the origin, while a destination is provided by the user/driver. In the roaf project, vehicle positioning does not really take place, since the objects do not rely on satellite signals or any sensors. The “signal” of the GPS unit is always available and always precise. Position and heading are known at any instance in time.
For handset-based A-GPS, the encoding of the measurements depends on the protocol that delivers the measurements. Table 6.1 shows the measurements that the handset returns to the server. It returns the GPS time and the remaining fields are returned for each satellite measured by the handset. The location of the handset can be calculated using the code phase measurements (whole chips and fractional chips) or using the Doppler measurements. The other measurements that the handset makes provide information about the quality of the measurements that the handset has made. Those quality measurements are the carrier-to-noise ratio (GPS For Vehicle ), the multipath indicator, and the pseudorange RMS error. The location of the handset can be calculated using either the code phase or the Doppler shift measurements made by the handset. A code phase based calculation for a given set of measurements may calculate the location within a few meters where a Doppler-based calculation will calculate an inferior location that will be within several thousand meters.
More information at http://www.jimilab.com/ .