RBU is a time code radio station located in Moscow (56.7333°N 37.6633°W). It transmits a continuous 10 kW time code on 66⅔ kHz.[1] This is commonly written as 66.66 or 66.666 kHz, but is actually 200/3 kHz. Until 2008, the transmitter site was near Kupavna 55.7344°N 38.15°W and used as antenna three T-antennas spun between three 150 metres tall grounded masts. In 2008, it has been transferred to the Taldom transmitter at 56.7333°N 37.6633°W.[2]
RBU is controlled by All-Russian Scientific Research Institute for Physical-Engineering and Radiotechnical Metrology. It is operated by Russian Television and Radio Broadcasting Network.[3]
Every 100 ms, synchronized to the UTC second, one bit is transmitted:
| Duration | Signal | ||
| +0 ms | 10 ms | Unmodulated carrier | |
| +10 ms | 80 ms | Carrier PM modulated with 100 Hz or 312.5 Hz tone, modulation index 0.698 | |
| +90 ms | 5 ms | Unmodulated carrier | |
| +95 ms | 5 ms | Carrier off |
100 Hz modulation encodes a binary 0, while 312.5 Hz modulation encodes a binary 1.
Each UTC second consists of 10 such bits. 6 of them are fixed, two encode minute boundaries, and two provide time code information:
| Significance | ||
| 0 ms | Time code data bit 1 | |
| 100 ms | Time code data bit 2 | |
| 200 ms | Always 0 (100 Hz tone) | |
| 300 ms | ||
| 400 ms | ||
| 500 ms | ||
| 600 ms | ||
| 700 ms | Always 0, except 1 before start of minute. Minute marker | |
| 800 ms | ||
| 900 ms | Always 1 (312.5 Hz tone). Second marker |
Each minute, the two bits of time code encode the local time of the following minute (like DCF77) and some additional information. Because the time code starts with two 1 bits, the top of the minute is uniquely marked by 5 consecutive 1 bits.[4]
| Data bit 1 | Data bit 2 | Second | Data bit 1 | Data bit 2 | |||||
|---|---|---|---|---|---|---|---|---|---|
| Weight | Meaning | Weight | Meaning | Weight | Meaning | Weight | Meaning | ||
| 00 | 1 | Always 1 | 1 | Always 1 | 30 | 4 | Year (00–99) | 8 | Truncated MJD (0000–9999) |
| 01 | 0 | Unused, zero | +0.1 | DUT1 (+0.1–+0.8 s) Unary encoding, bit set if DUT1 ≥ Weight | 31 | 2 | 4 | ||
| 02 | 0 | +0.2 | 32 | 1 | 2 | ||||
| 03 | 0.02 | dUT1 (±0.02–±0.08 s) Bit set if mod(dUT1) ≥ Weight | +0.3 | 33 | 10 | Month (01–12) | 1 | ||
| 04 | 0.04 | +0.4 | 34 | 8 | 0 | Unused, zero[5] | |||
| 05 | 0.06 | +0.5 | 35 | 4 | 0 | ||||
| 06 | 0.08 | +0.6 | 36 | 2 | 0 | ||||
| 07 | ± | +0.7 | 37 | 1 | 0 | ||||
| 08 | 0 | Unused, zero | +0.8 | 38 | 4 | Day of week 1=Monday 7=Sunday | 0 | ||
| 09 | 0 | −0.1 | DUT1 (−0.1–−0.8 s) Unary encoding, bit set if DUT1 ≤ Weight | 39 | 2 | 0 | |||
| 10 | 0 | −0.2 | 40 | 1 | 0 | ||||
| 11 | 0.02 | dUT1 (±0.02–±0.08 s) Bit set if mod(dUT1) ≥ Weight | −0.3 | 41 | 20 | Day of month (1–31) | 0 | ||
| 12 | 0.04 | −0.4 | 42 | 10 | 0 | ||||
| 13 | 0.06 | −0.5 | 43 | 8 | 0 | ||||
| 14 | 0.08 | −0.6 | 44 | 4 | 0 | ||||
| 15 | ± | −0.7 | 45 | 2 | 0 | ||||
| 16 | 0 | Unused, zero | −0.8 | 46 | 1 | 0 | |||
| 17 | 0 | 0 | Unused, zero | 47 | 20 | Hour (00–23) | 0 | ||
| 18 | ± | ΔUT Moscow time minus UTC Fixed +3 since 26 Oct 2014 | 8000 | Truncated Julian Day (0000–9999) Last 4 digits of Modified Julian day number | 48 | 10 | 0 | ||
| 19 | 10 | 4000 | 49 | 8 | P1 | TJD bits 18–25 | Even parity over | ||
| 20 | 8 | 2000 | 50 | 4 | P2 | TJD bits 26–33 | |||
| 21 | 4 | 1000 | 51 | 2 | 0 | Unused, zero | |||
| 22 | 2 | 800 | 52 | 1 | 0 | ||||
| 23 | 1 | 400 | 53 | 40 | Minute (00–59) | P3 | ΔUT bits 18–24 | ||
| 24 | 0[6] | Unused, zero | 200 | 54 | 20 | P4 | Year bits 25–32 | ||
| 25 | 80 | Year (00–99) | 100 | 55 | 10 | P5 | Month/DoW bits 33–40 | ||
| 26 | 40 | 80 | 56 | 8 | P6 | Day bits 41–46 | |||
| 27 | 20 | 40 | 57 | 4 | P7 | Hour bits 47–52 | |||
| 28 | 10 | 20 | 58 | 2 | P8 | Minute bits 53–59 | |||
| 29 | 8 | 10 | 59 | 1 | 0 | Unused, zero | |||