WWVB - Phase Modulated Time Code

Phase Modulated Time Code

The phase-modulated time code has been completely updated and is not related to the amplitude-modulated time code. The only connection is that the marker bits (where only 20% of the second is transmitted at full strength) are not used for essential time code information.

The time is transmitted as a 26-bit "minute of century" from 0 to 52595999 (or 52594559 in centuries with only 24 leap years). Like the amplitude-modulated code, the time is transmitted in the minute after the instant it identifies; clocks must increment it for display.

An additional 5 error correcting bits produce a 31-bit Hamming code that can correct single-bit errors or detect double-bit errors (but not both).

Additional fields encode DST and leap-second announcement bits similar to standard WWVB, and a new 6-bit field provides greatly advanced warning of scheduled DST changes.

The following information is transmitted:

• 14 fixed sync bits {(0,0,1,1,1,0,1,1,0,1,0,0,0)}
• 26-bit binary minute of century (0–52595999 for 36525 days per century)
  • 1 bit duplicate of the lsbit of the time code
• 5 parity (Hamming code) ECC bits over the above
• 5 bits of DST status and leap pending, comprising:
  • 2 bits of DST status, as in the amplitude modulated code
  • 1 bit of leap second warning, as in the amplitude modulated code
  • 2 parity bits for error detection
• 6-bit DST rules code, comprising:
  • 2 bits indicating time of next change (1/2/3 o'clock, or never)
  • 3 bits indicating date of change (which Sunday)
  • 1 additional bit for ECC, with a peculiar encoding
• 1 bit of "NIST notice"
• 2 reserved bits

A receiver that already knows the time to within a few seconds can synchronize to the fixed synchronization pattern, even when it is unable to distinguish individual time code bits.

The full time code (with the amplitude-modulated code for reference) is transmitted as follows:

Bits within fields are numbered from bit 0 as the least-significant bit; each field is transmitted most significant bit first.

The example shows the time code transmitted on 4 July 2012 between 17:30 and 17:31 UTC. The BCD amplitude code shows a time of 17:30, on day 186 of the year.

The binary time code shows minute 0x064631A = 65789 of the century. Dividing by 1440 minutes per day, this is minute 1050 (= 17×60 + 30) of day 4568 of the century. There are 365×12 + 3 = 4383 days in the 12 years before 2012, so this is day 185 of the year. This day number begins at 0 on January 1, rather than 1 like the BSD time code, so it encodes the same date.