[LEAPSECS] Leap second to be introduced at midnight UTC December 31 this year

[Paul Hirose]

On 2016-07-25 2:37, Tony Finch wrote:
>
> As I understand it the problem with |DUT1| > 0.9s is to do with radio
> signal compatibility, e.g. MSF only allows |DUT1| <= 0.8s +/- 0.05s


One bit of radio trivia is the inconsistent DUT1 limits of the US NIST 
stations.

"UT1 information is broadcast on WWV during the first 16 seconds of 
every minute using doubled seconds pulses, or doubled ticks.  The amount 
of correction (in tenths of a second) can be determined by counting the 
number of doubled ticks.  If the ticks occur during the first eight 
seconds (1 to 8) of the minute, the sign of the correction is positive; 
if they occur during the second eight seconds (9 to 16), it is negative."

But, "UT1 corrections [encoded on the 100 Hz subcarrier] are transmitted 
on three bits: 56, 57, and 58.  This BCD set indicates the amount of 
correction when multiplied by 0.1.  The sign of the correction is found 
in bit 50; if that bit is high, or set at 1, the correction is added to 
UTC to find UT1.  If bit 50 is low, the correction is subtracted.  Since 
only three bits are used, the UT1 correction range broadcast by WWV/VH 
is from −0.7 to +0.7 s."

But, "UT1 correction information [on WWVB] is broadcast on two sets of 
bits.  Bits 36 through 38 indicate whether the correction will be 
positive or negative with respect to UTC.  If ones are transmitted on 
bits 36 and 38, the sign is positive; if a one is present on bit 37, the 
sign is negative.  The amount of correction (in tenths of a second) is 
transmitted on bits 40 through 43.  Since four bits are used, WWVB is 
capable of broadcasting a UT1 correction range from −0.9 to +0.9s."

But, "the UT1 time correction information that is available [on WWVB] in 
the legacy frame in the AM/PWM modulation is not provided in the PM 
signal." In other words, there's no DUT1 in the improved WWVB phase 
modulated signal that was superimposed on the original AM modulation a 
few years ago.


See "NIST Time and Frequency Radio Stations: WWV, WWVH, and WWVB" (NIST 
Special Publication 250-67)

http://www.nist.gov/manuscript-publication-search.cfm?pub_id=50031

The new WWVB phase modulation is not in the above document, but you can 
find a link on the WWVB page:

http://www.nist.gov/pml/div688/grp40/wwvb.cfm

Among the links at the bottom of that page is "WWVB Radio Controlled 
Clocks: Recommended Practices." It says this about leap second handling:

"In order to properly display the occurrence of a leap second, digital 
RCCs must be capable of displaying a value of 60 in the seconds’ field 
so that leap seconds can be indicated, for example 11:59:60 p.m.  Analog 
RCCs cannot display a minute containing 60 seconds [sic]; therefore the 
second hand must remain in the same position (pointed straight up) for 
two consecutive seconds to indicate that a leap second has elapsed."

Regarding the RCC acronym, NIST says, "We contend that use of the term 
“atomic clock” is technically incorrect and misleading to consumers, and 
its usage should be avoided.  Unless there is actually an atomic 
oscillator inside the RCC (such as a cesium or rubidium oscillator), we 
recommend that the term “radio controlled clock” be used to correctly 
describe the product."