are used by NIST radio stations WWV, WWVH and WWVB. For example, one of the most common formats, IRIG B122: IRIG B122 transmits one hundred pulses per second on an amplitude modulated 1 kHz sine wave carrier, encoding information in BCD. This means that 100 bits of information are transmitted every second. The time frame for the IRIG B standard is 1 second, meaning that one data frame of time information is transmitted every second. This data frame contains information about the day of the year (1–366), hours, minutes, and seconds. Year numbers are not included, so the timecode repeats annually. Leap second announcements are not provided. Although information is transmitted only once per second, a device can synchronize its time very accurately with the transmitting device by using a phase-locked loop to synchronize to the carrier. Typical commercial devices will synchronize to within 1 microsecond using IRIG B timecodes. Timecode structure IRIG timecode is made up of repeating frames, each containing 60 or 100 bits. The bits are numbered from 0 through 59 or 99. At the start of each bit time, the IRIG timecode enables a signal (sends a carrier, raises the DC signal level, or transmits Manchester 1 bits). The signal is disabled (carrier attenuated at least 3×, DC signal level lowered, or Manchester 0 bits transmitted), at one of three times during the bit interval: After 0.2 of a bit time, to encode a binary 0 After 0.5 of a bit time, to encode a binary 1 After 0.8 of a bit time, to encode a marker bit Bit 0 is the frame marker bit Pr. Every 10th bit starting with bit 9, 19, 29, ... 99 is also a marker bit, known as position identifiers P1, P2, ..., P9, P0. Thus, two marker bits in a row (P0 followed by Pr) marks the beginning of a frame. The frame encodes the time of the leading edge of the frame marker bit. All other bits are data bits, which are transmitted as binary 0 if they have no other assigned purpose. Generally, groups of 4 bits are used to encode BCD digits. Bits are assigned little-endian within fields. Bits 1–4 encode seconds, and bits 6–8 encode tens of seconds (0–59) Bits 10–13 encode minutes, and bits 15–17 encode tens of minutes (0–59) Bits 20–23 encode hours, and bits 25–26 encode tens of hours (0–23) Bits 30-33