Description
Time of Week (TOW) is a continuous time count used within the framework of Global Navigation Satellite Systems (GNSS), most notably the Global Positioning System (GPS). It is a scalar value that counts the number of seconds that have elapsed since the start of the current GPS week. A GPS week is defined as 604,800 seconds (7 days * 24 hours * 3600 seconds). The TOW count resets to zero at the transition between GPS weeks, which occurs every Sunday at 00:00:00 GPS Time (which is approximately aligned with UTC but does not include leap seconds). The TOW is transmitted within the navigation message from each GPS satellite, specifically in the Handover Word (HOW) of each subframe.
Architecturally, TOW is a core component of the GNSS signal structure. Each satellite broadcasts its navigation data in frames and subframes. The HOW in every subframe contains the TOW count for the start of the next subframe. This allows a GNSS receiver, once it has decoded the HOW from one satellite, to synchronize its internal time base to the precise GPS system time. The receiver uses this TOW information to compute the exact time of transmission for the ranging codes (the pseudo-random noise codes) from each satellite. The accuracy of these timestamps is paramount for calculating the signal travel time, or pseudo-range, from the satellite to the receiver.
In operation, the GNSS receiver performs correlation to acquire the satellite signals. Once a signal is acquired and tracked, the receiver decodes the navigation message to extract the TOW and other ephemeris data. The TOW is used to resolve the integer millisecond ambiguity in the measured code phase. By knowing the precise TOW corresponding to the received signal, the receiver can calculate the full, unambiguous travel time of the signal. This process is essential for computing an accurate position. The Time To First Fix (TTFF) is heavily dependent on how quickly the receiver can obtain a valid TOW. For a cold start, the receiver must decode at least a full subframe (6 seconds) from one satellite to get the TOW, which is a primary factor in the initial TTFF. Assistance data, like predicted ephemeris and time models, can provide an approximate TOW to the receiver, drastically reducing this acquisition time.
Purpose & Motivation
TOW exists as a fundamental timing mechanism within GPS and similar GNSS architectures to provide a common, precise, and continuous time reference for all satellites and users. Before the establishment of a system-wide time like TOW, coordinating signals from multiple, independently moving satellites for precise ranging would be immensely complex. TOW solves the problem of synchronizing the transmissions from the entire satellite constellation and provides users with the critical timestamp needed to measure signal propagation delay.
The historical motivation stems from the core principle of satellite navigation: position is derived from precisely measured distances (ranges) to known satellite locations. To measure distance, one must know exactly when the signal left the satellite and when it arrived at the receiver. TOW provides the "when it left" timestamp in a universal system time. The limitations of not having such a synchronized time would be catastrophic—ranging measurements would be incoherent, and position calculation would be impossible. TOW, along with the GPS week number, creates a continuous, unambiguous timeline for the system.
Its role in TTFF is particularly crucial. The receiver must determine the full signal travel time, which includes an unknown integer number of milliseconds. The TOW value, broadcast every 6 seconds, allows the receiver to resolve this ambiguity. The need to decode this TOW from the relatively slow (50 bps) navigation data link was a primary bottleneck for fast positioning. This limitation directly drove the development of Assisted-GNSS (A-GNSS) technologies, where the network provides the current TOW (or a close estimate) to the UE as assistance data, enabling sub-second TTFF and enabling location services for emergency calls and consumer applications.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (29 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-4, normative work from Rel-15.
In Release 15, the specifications for the Time of Week (TOW) function were refined within the A-GNSS requirements for NR, introducing specific test procedures and parameters. This included defining the use of the 'GPS TOW msec' field in assistance data for Fine Time Assistance tests and establishing conditions for its random offset in Time to First Fix (TTFF) test cases. The corrections ensured proper alignment between the TOW signaling and the new radio access technology.
In Release 16, the new TOW-related capabilities included the introduction of the BDS B1C signal for A-GNSS assistance and support for GNSS Integer Ambiguity Level Indications. The specifications were also updated to reference the newer B1I signal ICD file version 3.0 for the BeiDou system. These enhancements expanded the supported GNSS signals and measurement precision available to the UE for calculating position using reference time assistance.
- CR for TS36.171, Introduction of BDS B1C in A-GNSS TS 36.171CR0020
- Introduction of B1C signal in BDS system in A-GNSS TS 37.355CR0248
- Introducing support for GNSS Integer Ambiguity Level Indications TS 37.355CR0252
- CR for TS38.171, Introduction of BDS B1C in A-GNSS TS 38.171CR0011
- Frequency bands for testing of A-GNSS sensitivity requirements TS 36.171CR0025
- Update B1I signal ICD file to v3.0 in BDS system in A-GNSS TS 37.355CR0259
+ 1 more changes
In Release 17, the TOW (Time of Week) function was enhanced to support high-accuracy GNSS location estimates, including the provision of NMEA GGA sentence information. The release also introduced new requirements for NavIC L5 A-GNSS support and clarified various GNSS assistance data fields, such as SSR orbit and clock integrity bounds and tropospheric delay corrections, to improve alignment with external standards like RTCM.
- NMEA GGA sentence info in high accuracy GNSS location estimates [HA-GNSS-NMEA] TS 37.355CR0349
- Correction on the GNSS Orbit and Clock Integrity Bounds in TS 37.355 TS 37.355CR0377
- GNSS SSR BDS orbit emphemeris reference clarification to align with RTCM TS 37.355CR0461
- Field description correction for HA-GNSS metrics TS 37.355CR0474
- Requirements for NavIC L5 A-GNSS support TS 38.171CR0025
- CR on TS 36.171 requirements for support of A-GNSS TS 36.171CR0027
+ 4 more changes
In Release 18, the TOW (Time of Week) function saw enhancements focused on improving assistance data accuracy and correcting system support parameters. The updates included corrections for A-GNSS positioning, specifically regarding the support indications for GNSS-Almanac and GNSS-UTC-Model, and refinements to the signaling of GNSS system time fields. These changes aimed to increase the reliability of the GPS TOW msec field used in reference time assistance for faster and more accurate UE positioning.
- GNSS LOS/NLOS assistance information [GNSS LOS/NLOS] TS 37.355CR0446
- Miscellaneous RIL corrections for GNSS LOS/NLOS [GNSS LOS/NLOS] TS 37.355CR0495
- Correction on GNSS-AlmanacSupport and GNSS-UTC-ModelSupport in A-GNSS positioning TS 37.355CR0518
- Correction on NavIC almanac set IE, and field descriptions under KlobucharModelParamater and GNSS-SystemTime. TS 37.355CR0534
In Release 19, the TOW (Time of Week) function was expanded to support new global navigation satellite systems, specifically introducing A-GNSS assistance for the BDS B2b signal and the NavIC L1 SPS signal within the LPP protocol. This enhancement allows the UE to utilize the GPS TOW msec field and related reference time assistance for these additional constellations to meet TTFF test conditions. The release also included corrections and a new UE capability to request equalIntegerAmbiguityLevel assistance data for improved positioning.
- CR for TS 36.171 to introduce BDS B2b signal in A-GNSS TS 36.171CR0032
- Introduction of NavIC L1 SPS A-GNSS in LPP TS 37.355CR0532
- Introduction of B2b signal in BDS system in A-GNSS TS 37.355CR0545
- UE request for equalIntegerAmbiguityLevel assistance data [GNSS-EqualIntegerAmbiguity] TS 37.355CR0557
- CR for TS 38.171 to introduce BDS B2b signal in A-GNSS TS 38.171CR0031
- Miscellaneous LPP Corrections [GNSS LOS/NLOS] TS 37.355CR0567
Explore further
Broader topics and technologies where TOW plays a role.
Defining Specifications
3GPP specifications that define or reference TOW, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 25.171 vj00 | A-GPS Minimum Performance Requirements for UTRA FDD UE | Rel-19 |
| TS 25.172 vj00 | A-GANSS UE Minimum Performance Requirements (FDD) | Rel-19 |
| TS 25.173 vj00 | A-GANSS Performance Requirements (TDD) | Rel-19 |
| TS 25.305 vj00 | UTRAN UE Positioning Stage 2 | Rel-19 |
| TS 25.453 vj00 | PCAP Protocol Specification | Rel-19 |
| TS 36.171 vj10 | A-GNSS Minimum Performance Requirements for UE | Rel-19 |
| TS 36.355 vj00 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 37.571 vj00 | UE Conformance for Positioning | Rel-19 |
| TS 38.171 vj10 | 5G A-GNSS UE Positioning Requirements | Rel-19 |
| TS 45.005 vj00 | GSM RF Requirements for MS and BSS | Rel-19 |