Radio-controlled-clock (RCC) devices that rely on time signal broadcasts have become widely used in recent years. A radio-controlled-clock (RCC) is a timekeeping device that provides the user with accurate timing information that is derived from a received signal, which is broadcast from a central location, to allow multiple users to be aligned or synchronized in time. Colloquially, these are often referred to as “atomic clocks” due to the nature of the source used to derive the timing at the broadcasting side. In the United States, the National Institute of Standards and Technology (NIST) provides such broadcast in the form of a low-frequency (60 kHz) digitally-modulated signal that is transmitted at high power from radio station WWVB in Fort Collins, Colo. The information encoded in this broadcast includes the official time of the United States.
Similar services operating at low frequencies exist in other regions worldwide, including Europe and Japan. Many consumer-market products exist, including watches, alarm clocks and wall clocks, that are capable of receiving one or more of these broadcasts and which can display the correct time to within approximately one or two seconds of accuracy. While the broadcast may be active continuously, a typical radio-controlled clock may be set to receive the broadcast only once a day. Such reception, if successful (depending on the condition of the wireless link and potential interference), is typically used to reset the timekeeping device, such that if it were set incorrectly or has drifted away from the correct time, it will be set in accordance to the time communicated through the broadcast signal.
Reception of the time signal, however, is being challenged by a growing number of sources of electromagnetic interference. In particular, the on-frequency interference from the MSF radio station in the United Kingdom has been identified as a particularly challenging jammer for receivers on the East Coast.
There is thus a need for an improved protocol for time signal broadcasts, such as that provided by WWVB in the United States and radio stations in other countries, that attempts to cost-effectively address the reception challenges. Such a new protocol should preserve existing amplitude modulation properties of the transmitted signal, in order to maintain backwards compatibility and not impact existing devices.