uld say that 1PPS is one) although they have problems over distances longer than a few meters. There is a lot more detail on physical frequency distribution in Chapter 6. Radio Distribution Systems Quite early in the twentieth century, techniques to distribute time over radio signals were developed as engineers discovered that low frequency (LF) radio signals (30-300 k are ideal for broadcast- ing time signals. Today, many countries use LF and high frequency (HF) radio signals to offer time and frequency synchronization at a regional level. The original purpose for these clocks was to use them as a frequency reference, although this func- tion has been increasingly replaced by GNSS-based systems. Historically, these signals were used as a reference to synchronize the frequency of radio transmitters. The radio transmitters belonging to broadcasters needed to be calibrated to stop them drifting off frequency. There were also such things as radio-disciplined oscillators, much as GNSS-disciplined oscillators are used today. Besides frequency, the signals were also used as a reference for time intervals, but in many cases, the ability to broadcast actual time of day signals was added later (starting in the 1960s). Of course, even with these new functions, they continue to broadcast on a very strictly con- trolled frequency to allow their signals to be used as a frequency reference. As one example of many, in the United States, the National Institute of Standards and Techno- logy (NIST) operates a LF radio station (WWVB) from Fort Collins, Colorado at a frequency of 60 kHz. It broadcasts a time code that contains the year, day of the year, hour, minute, second, and val- ues that indicate the status of the daylight-saving time, leap years, and leap seconds. The other sys- tems around the world work similarly, another example being the DCF77 transmission on 77.5 kHz, covering much of Europe from Frankfurt, Germany and the MSF transmission on 60 kHz covering the United Kingdom. It was known that radio-based time signals a