Assisted GPS (2 of 7)

MS/UE Assisted.

In principle, the receiver can be simplified if some of the computation, such as the navigation solution, is performed by the network. This form of operation is referred to as mobile station–assisted (MS-Assisted) or user equipment–assisted (UE-Assisted) in contrast to solutions performed in the user's equipment: MS-Based or UE-Based.

The advantage in terms of simplifying the handset is questionable given the computational capacity of modern cell-phone and GPS chipsets. Furthermore, if the location information is required in the handset, this advantage is offset by the need to communicate back and forth. Nevertheless, such architectures are commonplace.

The network has coarse information about the location of a receiver embedded in a cell phone. If it supplies this to the receiver, along with its uncertainty, the receiver then can use this to determine which satellites to search for, restrict its search ranges, and initialize its navigation solution. The uncertainty typically is supplied in the form of either the size and orientation of the semi-major and semi-minor axes of an ellipsoid of uncertainty, or the horizontal radius and vertical height of a cylinder of uncertainty.

Time Assistance.

The network also can supply time and its uncertainty. This may be supplied precisely using hardware in the handset, or much more coarsely over the air from a server. If it is sufficiently precise (for example, to within a few microseconds), it can be used to restrict the search range for the absolute code phases of the satellite signals. If it is less precise (for example, only to within many microseconds to a few seconds), it can be used to restrict the search range for the relative code phases of the satellite signals once an initial signal has been acquired.

If the time assistance is fairly precise (to a few milliseconds or better), the handset position errors resulting from the uncertainty in the satellite positions corresponding to the time uncertainty will be small enough (a few meters) to be tolerated in most cases.

However, if it is coarser than this, the navigation solution also will have to solve for the time error. Note that the receiver may only have code-phase measurements rather than full pseudorange measurements as used in a normal receiver — requiring a procedure to resolve the one-millisecond code-phase ambiguities and a special algorithm to solve for the time error.

Additional Assistance.

Alternatively to supplying position, time, and satellite-derived data, the network can derive and provide other assistance including approximate code phases at a certain instant in time, with the corresponding uncertainties, as well as the Doppler offsets with their uncertainties.

Assistance can be supplied in the "user plane" or in the "control plane."

In the latter case, assistance is supplied via communication over the signaling channels from a server integrated into the network infrastructure. The standards discussed in a later section relate to this form of assistance.

In the former case, the assistance is supplied from a user server (typically Web-based), using standard communications channels such as Short Messaging Service (SMS) and General Packet Radio Service (GPRS) over Global System for Mobile Communications (GSM) networks, or Single Carrier Radio Transmission Technology (1xRTT) over code division multiple access (CDMA) networks.

CDMA vs. GSM

There are technical differences between GSM and CDMA cellular technologies that impact AGPS implementation in these networks. Equally importantly, the two communities have evolved different AGPS standards discussed in the next section.

Precise Timing.

The first technical difference relates to the fact that precise timing is fundamental to CDMA operation. The handset synchronizes to the communications code very precisely (that is, well below the microsecond level). Using hardware (for example, a pulse and message), precise network time can be transferred to the GPS receiver subsystem. Of course, this time will contain an error equal to the communication latency between the network and the handset, but it is more than adequate as precise time assistance for the AGPS purposes described in the previous section.

GSM does not use spread spectrum codes, and hence this form of precise time assistance is not intrinsically available in a GSM network. To deliver precise time assistance (with uncertainties of 5 or 10 microseconds), GSM networks have to be augmented. Not surprisingly, few network operators are keen to roll out additional infrastructure for this purpose, and GSM deployment of AGPS typically is required to operate with coarse time assistance.

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