WWVB Atomic Clock GUI Part 1

Enhanced WWVB Broadcast Format
John Lowe (john.lowe@nist.gov) Time and Frequency Services National
Institute of Standards and Technology
December 7, 2012
1. Introduction
The WWVB broadcast of the officialtime of the US government has
existed since 1965. Since then, NIST has upgraded the broadcast system
and modified the signal several times making the service more
accessible to the public, and resulting in large numbers of
radio-controlled clocks (RCCs). The most recent improvements included
a significant increase in broadcast power and an increase in the
modulation factor used for the amplitude-modulation, both of
whichserved to improve reception coverage for existing RCCs and
increase their reliability. Nevertheless, it has been realized that
these popular devices still often encounter difficulties in reception,
which depend on the geographical location, time of day, type of
structure, and interference sources that may be present in a given
environment. All of these factors determine what is called
thesignal-to-noise-and-interference-ratio (SNIR) that a receiver
experiences at a given instance. In order to address the reception
challenges and provide the public with a much improved system, NIST is
now introducing an enhanced communication protocol, to which phase
modulation was added, offering significantly improved performance in
new products that are designed according to the new protocol. The new
protocol maintains the amplitude-modulation (AM) and pulse-width
modulation (PWM) of the legacy protocol, the details of which have
been made available in NIST Special Publication250-67
(http://tf.nist.gov/general/pdf/1969.pdf) from 2005, where many
additional information about the WWVB station may be found. This
backward compatibility ensuresthat typical existing consumer-market
products, based on conventional envelope detection, are not impacted.
This means that although legacy receivers cannot benefit from these
improvements, their performance will not be degraded, either. However,
receivers designed to lock to the carrier＊s phase and perform coherent
detection, which are typically common only in more-professional
equipment, are impacted by the introduction of the phasemodulation
defined in the protocol. Based on the early notification provided by
NIST and trials that were performed throughout 2012, it is expected
that these receivers will either be modified or replaced. In the
transitional period, to extend into 2013, the phase modulation will be
disabled for 30 minutes twice a day, at noon and at midnight Mountain
Standard Time (MST), allowing carrier-locked based time-keeping
devices to resynchronize tothe broadcast in its legacy form (i.e.,
having only amplitude and pulse-width modulations). This document
specifies the data content, physical properties and scheduling
features of the phasemodulating (PM) time code that has been added to
the WWVB broadcast. It is intended to allow users to correctly
interpret the various components in the PM code.It should be noted
that there are differences between the informationmade available
through the PM codeand what has been available throughthe legacy
AM/PWM protocol. For example, while the time and date may be extracted
from both, the leap year indication is not duplicated in the PM code,
whereas the PM code contains a new field that provides advance
notification for daylight-saving time (DST) transitions. Additional
features, offering further enhancements to the user experience in
various SNIR conditions, will be described in a future public release
in 2013.
1
2. General Properties of the Phase Modulation (PM) Protocol
The signal properties of the new broadcast are designed to maintain
backwards compatibility with the common envelope detector-based
receivers that were designed to operate with the legacy AM/PWM WWVB
protocol. These receivers, found in many low-cost consumer market
products, are typically based on a crystal filter centered at60 kHz
and having a bandwidth narrower than 10 Hz, which is followed by a
non-linear envelope detection operation (rather than a coherent
detector, which is based on multiplication with a locally generated 60
kHz signal that is phase-locked to the modulated carrier). The PM
protocol was designed to allow for flexibility/scalability (i.e.,
optimized operation at a very wide range of SNR values), while also
making provisions for additional features and extensions. It is
anticipated that details for additional features will be published
before the end of the year. These features will allow faster and more
accurate synchronization, as well as address the problems of receivers
with particularly low SNR. 2.1. Definitionof the Phase Modulation The
PM format is based on antipodal binary phase shift keying (BPSK),
i.e., the two symbols are 180 apart. A ※0§ is represented by the
carrier＊s non-modulated phase, as with the phasemodulation turned off,
whereas a ※1§ is represented by an inverted carrier. The hourly 45
phase shift that had existed in the legacy broadcast for station
identification is eliminated, as station identification becomes
possible based on the many unique signatures in its new PM code,
detailed in the following sections, which distinguish it from other
broadcasts. 2.2. Physical Properties of the Modulating Baseband Signal
As can be seen in Figure 1, the baseband signal, which combines the
two-level legacy AM/PWM signal and the phase (sign) inversions, may
experience at least four different levels in a phase-modulated frame.
These correspond to the legacy AM levels VH and VL, having the ratio
VH / VL 7, each of which may be multiplied by either a +1,
representing a ※0§ in the BPSK modulation, or -1 for phase reversal,
representing a ※1§ in the BPSK modulation. The phase transition
between each bit and the next one in the 1 bps (bit per second) PM
frame occurs 100 ms after the AM amplitude drop that indicates the end
of that second, as shown in Figure 1, illustrating anexample baseband
version of a transmitted symbol, where the information in the PM is
shown to transition from a ※0§ to ※1§, whilethe transmitted AM bit is
a ※1§. The baseband signal shown in this figure is multiplied by the
60kHz carrier in the transmitter, thereby resulting both in variations
in the carrier＊s amplitude and in sign reversals in it whenever the
baseband signal assumes negative values. Figures 2-5 illustrate the
modulated carrier for all of 4 combinations of 0/1 bit values for the
legacy and the PM frames (markers not shown).
amplitude VH ※0§ in BPSK (no phase inversion) ※1§ in AM/PWM (500ms at
low level and 500ms at high level) 100ms VL -VL 500ms t
beginning of second -VH
phase inversion
※1§ in BPSK (phase inverted)
Figure 1 每 The baseband signal when the bit transmitted both in
thelegacy protocol and in the PM is ※1§
2
1
normalized amplitude
0.5
0
-0.5
-1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
time [sec]
Figure 2 每 The modulated carrier for a ※0§ both in the legacy protocol
and in the PM
1
normalized amplitude
0.5
0
-0.5
-1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
time [sec]
Figure 3 每 The modulated carrier for a ※0§ in the legacy protocol anda
※1§ in the PM
1
normalized amplitude
0.5
0
-0.5
-1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
time [sec]
Figure 4 每 The modulated carrier for a ※1§ in the legacy protocol anda
※0§ in the PM
3
1
normalized amplitude
0.5
0
-0.5
-1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
time [sec]
Figure 5 每 The modulated carrier for a ※1§ both in the legacy protocol
and in the PM Since the lower-power portion of each bit, transmitted
with an amplitude of VL, is about 50 times weaker in power than the
high-power portion, it is recommended that the phase ofthe carrier for
each data bit be determined in the receiver based only on its
high-power interval.
3. Scheduling of Phase-Modulated Frames
The 60-second phase modulated time information frames coincide with
the legacy AM/PWM 60-second frames. However, while the legacy AM/PWM
modulation is present 100 % of the time, the normal phase modulation
on the carrier may be briefly interrupted for various purposes. During
the transitional period, to extend at least until the end of 2012, PM
interruptions for durations of 30 minutes will be scheduled to occur
daily at noon and at midnight MST, to allow for existing
carrierlocking based equipment to resynchronize or relock to the
legacy AM/PWM signal. Messages for various purposes, such as emergency
notices, may be incorporated into the broadcast and scheduled to
specific instances. A message frame, transmitted at the rate of 1 bps,
replaces a normal time frame. Therefore, it may not be used for time
acquisition in a phase-modulation based RCC. However, it may still be
used for timing correction (tracking), because it starts with a known
synchronizationword, as detailed in the next section. A message may
also exceed a single frame and span overmultiple frames. The
broadcasting of messages will be limited to a low duty cycle (e.g.,
below 10 % of the time), particularly during times assumed to be most
critical for reception in RCC devices.
4. Bit Allocation and Notation in 1 bit/s Time Frame
The PM bit allocation for a 60-bit 1bps frame dedicated to time
information is described in Table 1 against the bit allocation of the
legacy AM/PWM protocol. This section describes the contents and
significance of the various fields in the one-minute time frame, while
Section 5 and Table 9 describe the message frames, which may
infrequently replace time frames in the PM signal.
Note that bits in N-bit words are numbered from 0 for the
least-significant bit (LSB) to N-1 for the most significant bit (MSB),
with the MSB being transmitted first. For example, the 26-bit time
word (minute counter) is numbered from 0 (LSB) to 25 (MSB), with the
25th bit (MSB) being transmitted first.
4
Table 1 - Allocation of bits in one-minute time frame (legacy protocol
in black, PM in red)
Second 0 1 2 3 Legacy AM/PWM Marker min_40 min_20 min_10 Phase
sync_T[12] sync_T[11] sync_T[10] sync_T[9] Second 10 Legacy AM/PWM 0
Phase sync_T[2]Second 20 Legacy AM/PWM 0 Phase time[24] Second 30
Legacy AM/PWM day_8 Phase time[15] 11 0 sync_T[1] 21 0 time[23] 31
day_4 time[14] 4 0 sync_T[8] 5 min_8 sync_T[7] 6 min_4 sync_T[6]7
min_2 sync_T[5] 8 min_1 sync_T[4] 9 Marker sync_T[3] 19 Marker time[0]
29 Marker R 39 Marker R 49 Marker notice 59 Marker 0
12 13 14 15 16 17 18 hour_20 hour_10 0 hour_8 hour_4 hour_2 hour_1
sync_T[0] time_par[4] time_par[3] time_par[2] time_par[1]time_par[0]
time[25] 22 day_200 time[22] 32 day_2 time[13] 23 day_100 time[21] 33
day_1 time[12] 24 0 time[20] 34 0 time[11] 44 0 time[2] 25 day_80
time[19] 35 0 time[10] 45 year_80time[1] 26 day_40 time[18] 36
UT1_S[2] time[9] 46 year_40 time[0] 27 day_20 time[17] 37 UT1_S[1]
time[8] 47 year_20 dst_ls[4] 28 day_10 time[16] 38 UT1_S[0] time[7] 48
year_10 dst_ls[3]
Second 40 41 42 43 Legacy AM/PWM UT1_C_0.8 UT1_C_0.4 UT1_C_0.2
UT1_C_0.1 Phase time[6] time[5] time[4] time[3] Second 50 Legacy
AM/PWM year_8 Phase dst_ls[2] 51 year_4 dst_ls[1]
52 53 54 55 56 57 58 year_2 year_1 0 LYI LSW DST[1] DST[0] dst_ls[0]
dst_next[5] dst_next[4] dst_next[3] dst_next[2] dst_next[1]
dst_next[0]
- The significance of each of the fields in the legacy AM protocol is
given in previous NIST publications (referenced in the Introduction
section) and is not repeated here. -Bits 29 and 39, designated ※R§,
are reserved for future use and have not been assigned at this time. -
Bits 4, 14, 24, 34, 44 and 54 in the legacy AM system were
historically reserved for future use, but are now set permanently to
0.
4.1. Listing of the Fields in a Time Frame Table 2 lists the six
differentfields in a time frame, which add up to 60 s in duration.
Since the duration of the high power level in markers, as defined by
the legacy protocol, is only 200 ms (compared to 500 ms and 800 ms for
the ※1§ and ※0§ symbols respectively), none of the time information
bits rely on a marker. The purpose of each field is described in the
subsequent subsections. Table 2 - List of fields in a one-minute time
frame
purpose of field 1 2 3 4 5 6 synchronization word (may include last
bit of previous frame) time word (includes 5 parity bits and repeated
LSB) daylight saving time (DST) state and leap second notification
advance notice for nextDST transition (or message word) NIST notice
indication reserved bits (coincide with markers in AM/PWM) bits
allocated 0-12, 59 13-28, 30-38, 40-46 47-48, 50-52 53-58 49 29, 39
total: total number of bits (and seconds in duration) 14 32 5 6 1 2 60
5
4.2. Synchronization Word The known 13-bit synchronization
word,{sync[12], sync[11],＃ sync[0]}, populating bits 0-12, is used for
the purpose of timing (marks the beginning of a minute) and conveys no
information. The last bit in every frame (bit 59, which is a marker in
the AM frame), is always 0, and may be considered an extension to the
sync word, extending it to a total of 14 seconds (i.e., the last bit
from a previous frame may be appended to the first 13 bits in the next
frame). One of two synchronization words may be used,denoted sync_T
and sync_M, for time frames and message frames respectively, as
specified in Table 3. Table 3 - 13-bit synchronization words in time
and message frames bit # sync_T sync_M 12 0 1 11 0 110 1 0 9 1 1 8 1 0
7 0 0 6 1 0 5 11 4 0 1 3 1 1 2 0 0 1 0 1 0 0 0
4.3. Time Word The time word, fromwhich the year (excluding century),
the date, and the hour and minutes may be extracted, is represented
bya 26-bit minute counter that is reset at the turn of the century,
i.e., the current value in it should be considered to represent the
number of minutes that have elapsed since this century began at 00:00
UTC on January 1st 2000. For example, as the time turns from21:29:59
UTC to 21:30:00 UTC onJuly 28th, 2016, the minute counter will be
incremented from 8717609 to 8717610 (decimal). The binary
representation of 8717610 will appear within the 60 s frame that
starts at that instance(i.e., the 21:30:00 UTC instance isannounced
after it has already occurred). This timing of the frame contents with
respect to the beginning of the minute frame (the ※on time§ mark at
the beginning of the first second of the minute) is identical to the
convention defined in the legacy protocol.


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