Simon Collings - Rugby MSF

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Rugby MSF

Introduction

There are now many radio controlled clocks on the market in Europe which either use the MSF standard frequency reansmission from Rugby in England on 60 kHz or the DCF77 signal from Mainflingen, Germany on 77.5 kHz. In the United States, the transmission on 60 kHz from WWVB in Fort Collins, Colorado is used. Here in Europe, there are also signals from Russia (RTZ on 50 kHz from Irkutsk) and Switzerland (HBG in Nyon on 75 kHz), but these are less widely used.

In this article, I will explain a little about the UK based MSF signal which has a useful coverage of about 1000 km from its location in the Midlands of England and has been in operation since 1950.

The MSF transmitter is maintained by the National Physical Laboratory using a Caesium beam time standard and run by BT Radio Engineering Services on contract. The frequency is exactly 60kHz (itself a standard accurate to 2 parts in 10¹²) and conveys the time using pulse width modulation of this carier to represent binary coded decimal (BCD) digits. There is a fast code (which is sent in the first second of every minute but not normally used in commercial products) and a slow code which is sent at a nominal rate of one bit per second completing a cycle every minute. Certain seconds in the slow code carry two bits as parity for error detection and other functions. The time sent by the slow code during each minute is valid at the end of the minute.

Pulse Width Modulation

The begining of each minute is marked by 500 ms with the carrier nominally off (although the fast code sequence is transmitted in this period). The other 59 seconds of the minute always begin with at least 100 ms of no carrier and have at least 700 ms of carrier on. All keying edges are transmitted with an accuracy of +/- 1ms. When a "leap second" has to be inserted or deleted (this is needed from time to time to maintain the calendar) there will be either 60 or 58 additional second periods in the minute.

The slow code is sent using seconds 1 to 59 as 100 or 200 ms interruptions of the carrier on the leading edge of each second to convey binary "0" or "1". This is illustrated below:

Second marker with pulse width modulation showing 1st level data transmission only

Most of the interruptions of the carrier are used to convey only one bit of data (1st level data ) but some seconds carry two bits so that the interruption in the carrier is extended to convery 2nd level data bits. The maximum length of interruption with second level data bits present is 300 ms if "11" is being sent. Second level data bits are used for a number of purposes. The 2nd level bits in seconds 54-57 correspond to parity bits for a selection of the data fields sent during the current minute so that elementary error detection can be done. There is also a bit indicating if daylight saving is in operation (the BST bit in second 58). Second level bits are used to send negative values of the offset between GMT (standard British time, also known as UT1) and UTC (Coordinated Universal Time) in the DUT1 code (positve values are encoded using 1st level bits only).

Slow Code

During each minute, the slow code sends in sequence the DUT1 code (16 bits), year (8 bits), the month (5 bits), day of month (6 bits), day of week (3 bits), hour (6 bits), minute (7 bits), the framing pattern "01111110" and the BST bit with odd parity bits for the year, month & day of month combined, day of week, hour & minute combined. The DUT1 code is the difference between GMT and UTC in the range +/- 800 ms expressed in steps of 100ms (seconds 1-8 for positive offsets, and seconds 9-16 for negative offsets). This is illustrated in summary in the diagrams and table below:

Table 1: MSF minute cycle

Diagram 1: MSF minute cycle

The DUT1 code is sent during seconds 1 to 16 using both 1st and 2nd level bits as follows:

Table 2: DUT1 code sequence

The parity bits in seconds 54-57 are sent in the order year, combined month & day of month, day of week and combined hour & minte. In each case, the parity bit is set so that the total number of binary "1"s in the corresponding data field plus the parity bit itself is odd.

Second 58 contains the BST bit ("1" when British summer time is active), however, sixty one minutes before BST come into effect, the 2nd level bit in second 53 is set then cleared as the change occurs.

The parity and BST bits are summarised in table 3:

Table 3: Parity and BST bits

The framing pattern at the end of each minute in the 1st level data is a sequence of 8 bits having a unique value that can never occur elesewhere in the code (ie it is not a valid BCD sequence).