SUMMARY: NavIC (IRNSS) users who obtain Coordinated Universal Time (UTC) from NavIC (IRNSS) receivers should be aware of a Week Number (WN) rollover event due soon. Also, the possible effect such an event may have on the estimate of the reported UTC. The navigation message employs a ten (10) bit parameter that represents WN. Thus, the WN parameter “rolls over” to zero every 1024 weeks starting from the inception time i.e. 00:00 UT August 22, 1999. Consequently, the first WN rollover will now occur on April 6, 2019

BACKGROUND: The NavIC (IRNSS) System Time is based on a weighted average of multiple atomic clocks. The System Time used for user navigation solutions is a 27-bit binary number composed of two parameters Week Number (WN) and Time of Week Count (TOWC).

The Week Number is an integer counter that gives the sequential week number from the origin of the IRNSS time. This parameter is coded on 10 bits appearing in the first sub-frame of the navigation message, which covers 1024 weeks (about 19 years). The System Time start epoch is 00:00 UT on Sunday August 22, 1999 (midnight between August 21 and 22). Thus, the first WN rollover as mentioned above is on April 6, 2019. Each WN rollover event defines a new Time Epoch. The System Time is currently in the First Epoch.

SUGGESTIONS:

1. To investigate and understand possible dependencies on NavIC (IRNSS) for obtaining UTC.
2. To contact the receiver manufacturers and understand their preparedness for the April 6, 2019 WN rollover with respect to the available receiver.
3. To ensure that the firmware of NavIC (IRNSS) receivers is up-to-date.