Initial Location

Description

Initial Location is a concept defined in 3GPP specifications 23.171 and 23.271 for emergency call handling. It refers to the location estimate and the associated time stamp at the commencement of the call set-up for an originating emergency call. This information is critical for emergency services (e.g., PSAPs) to determine where the emergency is occurring and to dispatch appropriate assistance. The initial location is typically obtained through the control plane or user plane location services (LCS) procedures that are triggered when the emergency call is initiated.

The architecture for providing initial location involves several network elements. In the control plane, the Gateway Mobile Location Centre (GMLC) requests the location from the serving network (e.g., MME, SGSN, or AMF) which then interacts with the radio access network to obtain the location estimate. In the user plane, the SUPL Location Platform (SLP) may be used. The location estimate can be based on various positioning methods such as Cell ID, OTDOA, E-CID, GNSS, or hybrid methods. The timestamp ensures that the location is associated with the correct point in time, which is essential for accuracy and for correlating with the call.

The initial location is distinct from updated or periodic location reports that may be provided during the emergency call. It represents the first location estimate available at call setup. The provision of initial location is mandated by regulatory requirements in many regions (e.g., FCC in the US, E112 in Europe) to improve emergency response. 3GPP specifications define the procedures for obtaining and delivering the initial location to the emergency services.

In terms of implementation, the initial location may be provided to the PSAP either embedded in the emergency call signaling (e.g., via IMS) or through a separate location retrieval function. The accuracy and reliability of the initial location depend on the positioning capabilities of the UE and the network, as well as the availability of location services. Over releases, enhancements have been made to support new positioning methods and to improve the speed and accuracy of initial location determination, especially for VoLTE and 5G emergency calls.

The term has remained stable in its definition since R99, reflecting its fundamental role in emergency services. However, the underlying technologies and procedures to obtain the initial location have evolved significantly with each generation of mobile networks.

Purpose & Motivation

Initial Location exists to ensure that emergency services receive the caller's location as early as possible during an emergency call. Without it, emergency responders would have to rely on verbal descriptions or time-consuming location queries, delaying assistance. Regulatory bodies worldwide require that emergency calls provide location information to Public Safety Answering Points (PSAPs). The initial location addresses this by capturing the location at call setup, which is often the most critical moment for dispatching help.

Historically, before the widespread adoption of location services in mobile networks, emergency calls provided only the cell ID, which could be very imprecise. The introduction of more advanced positioning methods and the definition of initial location in 3GPP allowed for more accurate and timely location information. This concept solves the problem of providing location information without requiring the user to speak or take additional actions, which is vital for callers who are unable to communicate.

The initial location also serves as a baseline for subsequent location updates during the call. It enables PSAPs to initiate response even if the call is dropped or if further location updates are delayed. The timestamp associated with the initial location helps in reconstructing the events and ensuring that the location corresponds to the time of the emergency. Overall, the initial location is a cornerstone of emergency call handling in 3GPP networks.