Standard Frequency and Time Signal Services

Item (c) of the Section B syllabus is one line long — “Standard Frequency and Time Signal Services in the World” — and it reliably produces a question or two about which station belongs to which country and on what frequency. It is pure recall, so it is cheap. But the underlying idea matters to you as an operator as well: these stations are the only free, absolute reference against which you can check the dial of a receiver, and hearing one is the quickest honest test of whether a band is open.

What a standard frequency and time signal service is

A standard frequency and time signal station does one job: it radiates a carrier whose frequency is exactly what it claims to be, and marks on that carrier the passage of seconds and minutes, both traceable to a national atomic clock and hence to UTC. It carries no traffic and takes no calls. It is a metre rule broadcast over the horizon.

Two consequences follow for an amateur, and both are examinable.

1. Calibrating your frequency-measuring device

Your licence does not merely permit this, it makes it necessary. The station conditions require you to keep a reliable frequency-measuring device at the station and to check your frequency every time you change it. A device that has never been checked against anything is not reliable — an analogue dial drifts with temperature and age, and even a crystal-controlled digital display is only as good as the crystal underneath it, which ages by a few parts per million a year.

A standard frequency transmission gives you a known point to check against. The frequency measurement lesson covers the instruments themselves; this lesson is about the yardstick they are measured against.

2. Judging whether a band is open

These stations run continuously, at known power, from known places, on frequencies spread across the whole of HF. If at 8 pm IST you hear one clearly on 5 MHz but nothing at all on 15 MHz, the ionosphere is telling you that the maximum usable frequency has fallen below 15 MHz for that path — so 20 m is probably shut and 40 m is probably alive. That is a propagation measurement made with a receiver you already own, and it is more direct than any forecast. The propagation lesson explains why the MUF moves the way it does.

Why 2.5, 5, 10, 15, 20 and 25 MHz

The ITU allocates the standard frequency and time signal service its own narrow slices of spectrum, so that nothing else is supposed to be transmitting there. The allocations are at 20 kHz (19.95–20.05 kHz) and then at 2 500, 5 000, 10 000, 15 000, 20 000 and 25 000 kHz, each with a guard band of about 5 kHz either side. Transmissions in the service must carry identification signals, so you can always tell what you are hearing.

The choice is not arbitrary. Round decade and half-decade numbers are easy to find on any dial and easy to do arithmetic against — an error of 400 Hz on 10 000 kHz is 40 parts per million, which you can work out in your head. They are also spread roughly one per octave across HF, so between them they sample the whole range of ionospheric behaviour.

The principal stations

Learn the first column against the second and third. That is what the paper asks.

A few of the low-frequency entries are worth separating in your head, because they are asked against one another. 60 kHz is used by two different stations — WWVB in Colorado and MSF in Cumbria — on opposite sides of the Atlantic, where they do not interfere. 77.5 kHz is DCF77 and nothing else, and the number is written into the call sign, which makes it the easiest fact in the lesson to keep. None of the LF stations carries voice; they send a slow digital time code that a clock decodes and a human cannot.

India's own position

ATA was the HF standard frequency and time signal transmission of the National Physical Laboratory in New Delhi. It operated on 10 MHz, with earlier transmissions on 5 MHz and 15 MHz, radiating about 8 kW, and allowed a clock anywhere in India to be set to within roughly a millisecond of Indian Standard Time.

It has been discontinued, and India no longer operates an HF time signal station. That is worth saying plainly, because ATA is still widely described as though you could tune it tonight. You cannot. One honest caveat — the exact date of closure could not be established from a primary source for this course, so nothing here claims a year for it. If the paper asks who operated ATA, the answer is the National Physical Laboratory, New Delhi.

NPL has not stopped disseminating Indian Standard Time; it has stopped doing so on shortwave. The current services from CSIR-NPL are:

• a satellite-based Standard Time and Frequency Signal service via INSAT, giving IST to within about ±10 microseconds and frequency calibration to about ±100 picoseconds;
• TELECLOCK, a dial-up telephone time service;
• NTP servers and a web clock, for computers and phones.

IST itself is generated and held by the Time and Frequency Standards Laboratory at CSIR-NPL, New Delhi, from an ensemble of caesium clocks and hydrogen masers that contributes to international UTC. IST = UTC + 5 h 30 min, referred to the meridian 82° 30′ E, which passes near Mirzapur in Uttar Pradesh.

So what does an Indian amateur listen to for an off-air check? In practice WWV and WWVH on 5, 10 and 15 MHz, BPM from China on the same frequencies, and RWM on 4 996, 9 996 and 14 996 kHz — all three are receivable here, and BPM in particular is close and strong. A GPS receiver or an NTP-synchronised phone now gives a better time reference than any of them; but neither gives you a frequency reference you can hear on your own receiver, which is the point of the exercise.

How the signals are put together

The HF stations follow a broadly common pattern, and knowing it lets you use one without a manual.

• Seconds pulses — a short tick every second, a few milliseconds of tone. The tick marks the beginning of the second.
• An omitted or lengthened tick at a particular second, usually the 29th and 59th, so you can find your place without counting from the top of the minute.
• A minute marker — a longer or distinctly different tone at the top of each minute. That is the instant you set a clock to.
• Time announcements: a spoken voice in UTC (WWV, WWVH), Morse identification and coded groups (BPM, RWM), or a machine-readable digital code, which is all the LF stations send.
• The carrier itself — the part you use for frequency calibration, and accurate whether or not you can understand anything modulated onto it.

There is a manual amateur version of the same idea, and it is worth knowing: send QTR, get the time back followed by a long dash, and the dash breaks at the exact moment the second hand crosses the time given.

Using one to check a receiver, step by step

1. Pick a frequency you can hear. In the evening in India, try 10 MHz first; if it is empty, try 5 MHz, and by day try 15 MHz. Tune slowly — you are looking for a steady unmodulated carrier with ticks on it, not for speech.
2. Identify what you have found. Wait for the announcement. A male voice is WWV, a female voice WWVH, Morse identification on a half-hourly cycle is BPM. If you are on 9 996 kHz rather than 10 000, you have RWM.
3. Switch to CW or SSB. You need a beat note. On AM the carrier gives you nothing to measure against.
4. Tune for zero beat. Bring the pitch of the beat note down through a few hundred hertz until it falls to nothing and you feel rather than hear it. That point is your receiver's idea of exactly 10 000 kHz.
5. Read the dial and subtract. Suppose the dial reads 10 000.4 kHz at zero beat. Your receiver reads 400 Hz high at 10 MHz. As a fraction that is 400 ÷ 10 000 000, or 40 parts per million.
6. Scale the correction to the band you use. A fractional error stays the same fraction everywhere, so the error in hertz grows with frequency. That is what tells you where the true band edge falls on your dial.
7. Trim or record. If the receiver has a calibration adjustment, set it and repeat. If it has not, write the correction on a card and tape it inside the lid.

Steps 5 and 6 are the only arithmetic in this lesson, and they are worth writing down once:

So 400 Hz out at 10 000 000 Hz is 400 ÷ 10 000 000 × 1 000 000 = 40 ppm. Turn it round to predict the error elsewhere: at 7 MHz, 40 ppm is 40 × 7 = 280 Hz; at 14 MHz it is 560 Hz. Do this once when you get a radio and once a year afterwards, and you will never be the station transmitting outside the band while insisting the dial says otherwise.