The RDS Time Code for FM Radio. 

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... time displaye d on your cellular telephone should be correct, at least when the phone is first turned on and has just synchronized to the signal. Cellular signals originate from Code Division Multiple Access (CDMA) base stations that operate in compliance with Telecommunications Industry Association (TIA/EIA) Standard IS-95 [26]. All base stations contain GPS receivers, and by definition, IS-95 time is GPS time. The base stations act as GPS repeaters by retransmitting the GPS time they receive from the satellites. There are two types of IS-95 CDMA systems, distinguished by their frequency bands. The original analog cellular system, called the Advanced Mobile Phone System (AMPS), transmits base station signals (forward link signals) using frequencies from 869 to 894 MHz. The newer and more common Personal Communications Systems (PCS) band sends forward link signals in the 1930 to 1990 MHz range. Since the time and frequency information on either band is receive- only (no reverse link signals are sent), it is not necessary to subscribe to a telephone service to receive time signals. Therefore, a few radio controlled cellular clock products have appeared on the market. These clocks should work anywhere a cell phone would work, and actually in some places where a cell phone will not work, since the forward link signal from the base station usually travels farther than the reverse link signal from a cell phone [27]. FM receivers can obtain time-of-day information from the Radio Data System (RDS) broadcast by many FM radio stations on a 57 kHz subcarrier. RDS is used to identify stations and radio programs, and automatically synchronizes the clocks on some car radios, clock radios, and communication receivers that already receive FM broadcasts. An estimated 15% of the approximately 5000 FM radio stations in the United States now utilize RDS. Every minute, the RDS sends a time code (Figure 18) containing the Modified Julian Date (MJD), the UTC hour and minute, and the local time zone offset [28]. RDS is also used for auto clock synchronization on some clock radios and communication receivers. Like cell p hones, the time displayed on pagers is indirectly synchronized to GPS. Several wristwatches now sold in the United States automatically synchronize using the FLEX paging protocol developed by Motorola, and other paging protocols have also been used for clock synchronization. The FLEX time code is modulated on to a forward link frequency near 931 MHz. As with the cellular phone systems described earlier, it is not necessary to subscribe to a paging service to use the time code. Wristwatches that utilize the Smart Personal Objects Technology (SPOT) developed by Microsoft and SCA Data Systems are expected to appear in late 2003. These devices will receive information such as stock prices, weather data, and sports scores. They also will receive a time code that will synchronize the watch, including a correction for the local time zone. The SPOT technology is similar to the Radio Data System described earlier; information is sent from existing FM radio stations on a 67 kHz subcarrier leased by Microsoft. The watches are initially expected to work in 100 large metropolitan areas covering all 50 states, plus in 13 metropolitan areas in Canada. In today’s worl d, most of us already either knowingly or unknowingly obtain the time from radio controlled clocks. Due to recent advances in technology, we appear to be moving quickly towards the day when nearly every clock display will be synchronized with an uncertainty of less than 1 second, and when the “exact time” is available to everyone. 1 . A. V. Simcock, “Sir Howard Grubb’s proposals for radio control of clocks and watches”, Radio Time , vol. 4, issue 10, Autumn 1992, pp. 18-22. 2 . Sir H. Grubb, “Proposal for the Utilisation of the ‘Marconi’ System of Wireless Telegraphy for the Control of Public and Other Clocks”, Scientific Proceedings for the Royal Dublin Society , vol. X, part I, no. 7, 1899, pp. ...