Time Offset Table

Description

The Time Offset Table (TOT) is a standardized data structure specified in 3GPP TS 26.917 for location services. It is a key component in supporting UE-based positioning, where the user equipment itself calculates its position, often using Global Navigation Satellite System (GNSS) signals like GPS or Galileo. The core function of the TOT is to provide precise timing offset information. This information typically describes the relationship between the time scale of a GNSS constellation (e.g., GPS System Time) and a universal reference time like Coordinated Universal Time (UTC), or potentially a cellular network's system time. The table contains entries with parameters such as a reference time (e.g., a specific GNSS week and time of week), the corresponding offset value, and often the rate of change of that offset (drift).

Architecturally, the TOT is delivered to the UE as auxiliary data, typically from a location server like the Secure User Plane Location (SUPL) Location Platform (SLP) or the LTE Positioning Protocol (LPP) server. This delivery can happen via control plane protocols (e.g., LPP in LTE/5G) or user plane protocols (e.g., via SUPL). The UE receives and stores this table. When the UE performs a GNSS-based position fix, it uses the satellite signals which are timestamped in the GNSS system's own time. To compute an accurate position in a universal coordinate system, the UE must convert these timestamps to a common time reference. The TOT provides the necessary conversion parameters.

In operation, the UE's GNSS receiver measures pseudo-ranges to multiple satellites. These measurements are inherently biased by the offset between the UE's local clock and GNSS system time. By applying the corrections from the TOT, the UE can more accurately align its measurements to a known time reference, reducing the time-related unknowns in the positioning equations. This directly improves Time To First Fix (TTFF) and the accuracy of the calculated position, especially for cold or warm starts where the UE has limited prior knowledge. The TOT is part of a suite of assistance data defined by 3GPP, working alongside other data like ephemeris and almanac models, to enable efficient and high-performance positioning for a wide range of services from emergency calls to location-based applications.

Purpose & Motivation

The TOT was introduced to address the need for faster and more accurate UE-based positioning in cellular networks. Prior to standardized assistance data like the TOT, UE-based GNSS positioning could be slow, especially from a cold start, as the UE needed to download full satellite ephemeris data from the satellites themselves, a process taking tens of seconds. It could also be less accurate if the UE's internal clock had a significant drift relative to GNSS time, introducing errors into the position calculation.

The creation of the TOT, particularly within the context of 3GPP's Location Services (LCS) enhancements, was motivated by service requirements for Emergency (E911/E112) services, commercial location-based services (LBS), and regulatory mandates. By providing pre-calculated, network-derived timing corrections, the TOT allows the UE to resolve time ambiguities more quickly. This reduces the number of unknown variables the UE must solve for, leading to a faster convergence on a position fix (improved TTFF) and enhanced accuracy by mitigating errors from clock bias. It solves the problem of the UE needing to derive this timing relationship independently, which is computationally intensive and time-consuming.

Historically, as 3GPP evolved its positioning architecture from network-based methods (like Cell-ID and OTDOA) to more sophisticated UE-based and UE-assisted methods, the provision of standardized assistance data became critical. The TOT, introduced in Release 14, was part of this broader push to optimize positioning performance, reduce UE power consumption (by shortening active GNSS reception time), and meet increasingly stringent accuracy and latency requirements for new use cases, including those emerging in the 5G era for IoT and vertical applications.

Key Features

• Provides conversion parameters between GNSS system time and UTC or network time
• Delivered as part of standardized positioning assistance data (e.g., via LPP or SUPL)
• Contains reference time points and corresponding offset values, often with drift rates
• Enables faster Time To First Fix (TTFF) for UE-based GNSS positioning
• Improves positioning accuracy by correcting for known time reference biases
• Supports multiple GNSS constellations (GPS, Galileo, etc.) through standardized formats

Evolution Across Releases

Defining Specifications