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Time Scales and Time Zones

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Time Scales and Time Zones Jim Ingleson NYISO ingleson_at_nyiso.com 2006 FDA Conference GA Tech May 1 2, 2006 13:47:04.563 UTC If you were presented with the above ... – PowerPoint PPT presentation

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Title: Time Scales and Time Zones


1
Time Scales and Time Zones
  • Jim Ingleson
  • NYISO
  • ingleson_at_nyiso.com
  • 2006 FDA Conference
  • GA Tech May 1 2, 2006

2
134704.563 UTC
  • If you were presented with the above time, please
    consider what it implies to you (now).

3
Time Scales
  • Many Time Scales
  • Recognized Standard Time Scales
  • Consider Town Hall Time
  • Stability
  • Availability
  • Rules
  • Correction to solar observation (astronomical),
    correction by monitoring another time scale
    (dependent).
  • Steering vs Correction

4
Town Hall Time Scale
  • Someone could synchronize a watch to THT and
    consider that they are carrying town hall time,
    accurate enough for their purpose.
  • If all aspects of this time scale were handled
    carefully, it is likely that most people in town
    would use this as their standard.
  • Where would you go to pick up town hall time?
  • Give an example of a time scale which is
    dependent on THT.

5
Time Scales in General
  • Times scales, particularly widely recognized time
    scales, include a good clock or clocks, various
    rules, methods of distribution. Notice the
    important distinction between time scale and
    clock.
  • Many time scales are astronomical such as UT,
    GMT (both no longer used), UT0, UT1, and UT2.

6
An Atomic Time Scale
  • In 1967 the second became an SI unit.
  • Our standard time scale is now based on precise
    measurement of this defined second.

7
The TAI UTC Time Scales
  • TAI is based on the absolute second as measured
    using cesium clocks in many world laboratories
    and coordinated by BIPM (France).
  • UTC is also maintained by these laboratories.
    Leap seconds are added to UTC when necessary to
    keep the difference between UTC and UT1 to less
    than 0.9 s. 23 leap seconds have been added thus
    far. When necessary, leap seconds are always
    added at the end of December or June. UTC is
    sometimes called a paper time scale.
  • Observing a leap second 58, 59, 60, 0, 1, 2,

8
Conversion Factors
  • Remember that an extra second occurred at the end
    of the year 2005 and certainly will occur again.
  • The last minute of 2005 had 61 seconds.
  • The last hour of 2005 had 3601 seconds.
  • The last day of 2005 had 86401 seconds.
  • 2005 (not a leap year) had 31536001 seconds.
  • By the way, 2000, 2004, and 2008 are years which
    contain a leap day. 3 of 4 century years are not.

9
No clock is perfect
  • Many Cesium clocks in laboratories around the
    world very carefully coordinated with each other
    are not perfect, either.
  • UTC(NIST) a a particular realization of UTC.
    NIST is one of the participants, or of the 50
    (80) laboratories.

10
UTC(BIPM)
80 Others
UTC(NIST)
(Many other means of disseminating UTC.)
(20 ns)
UTC(USNO)
11
UTC(USNO)
(20 ns)
GPS System
Now 14 seconds ahead of UTC
GPS Receiver
(Corrections Applied)
12
GPS Receiver
What comes out of a GPS receiver is very nearly
UTC. Uncertainty can be on order of 1
microsecond. For our purposes, we consider it to
be UTC, but it is not truly UTC.
13
UTC Time Scale
  • UTC is really the only game in town. Probably
    no other time scale is worthy of consideration,
    except for TAI.
  • Now, consider that UTC is often presented raw
    with no offset, but it can also be presented with
    whatever offset is desired, EDT for example is 4
    hours behind raw or zero offset.

14
Time Zones
15
Offset Letter Also Called -8 U
Uniform Pacific Standard Time (PST) -7 T
Tango Mountain Standard Time (MST) -6 S
Sierra Central Standard Time (CST) -5 R
Romeo Eastern Standard Time (EST) -4 Q
Quebec Atlantic Standard Time (AST) -3 P
Papa Greenland Standard Time (GST) -2 O
Oscar Greenland Eastern Standard Time (VTZ) -1 N
November Azores Time (AT) 0 Z Zulu Western
Europe Time (WET)
Unfortunately, the last time zone is also called
by the names of several time scales, i.e. GMT and
UT (no longer used), and UTC, which is currently
our main worldwide standard time scale.
16
Zulu Time Zone
  • To change EDT to Zulu, add 4 hours.
  • To change EST to Zulu, add 5 hours.
  • Example At 0830 EDT today, it is of course
    0730 EST. The Zulu time at the same hour is
    1230, 30 minutes after noon.
  • Some ways to get mentally used to Zulu time.

17
What shall we call the time zone with no offset?
  • Z, Zed, or Zulu?
  • WET?
  • UT?
  • GMT?
  • UTC?
  • UTC Time Zone?
  • UTZ?

18
Terminology Problem
  • Unfortunately, the Zulu time zone is also called
    by the names of several time scales, i.e. GMT and
    UT (no longer used), and UTC, which is currently
    our main worldwide standard time scale.
  • This is not a problem from the point of view of
    the NIST staff. When NIST announces a time in
    UTC, that time is a correct representation of
    both UTC time scale and UTC time zone.

19
UTC
  • Although UTC is definitely not a Time Zone, when
    the letters UTC are presented immediately after
    a time, I suggest that we understand that to mean
    UTC is presented RAW or straight, or in other
    words, there is no time zone offset applied.

20
Daylight Saving Time
  • The (USA) Uniform Time Act of 1966 does not
    require that DST be observed, but if DST is
    observed, it must the done uniformly. For
    example, DST is not observed in the Eastern Time
    Zone part of Indiana.
  • Ammended in 1986.
  • Energy Policy Act of 2005.

21
Daylight Saving Time
  • Spring Forward date will change
  • 2006 - First Sunday in April
  • 2007 - Second Sunday in March
  • Fall Back date will change
  • 2006 Last Sunday in October
  • 2007 First Sunday in November
  • Could this policy be changed in the future?

22
Daylight Saving Time
  • Potential confusion at time of shift.
  • Additional potential for confusion when the rules
    are changed.
  • Not suitable for serious logging and scientific
    timekeeping.
  • Use of 24 hour Zulu time makes clear that no DST
    shift is observed.

23
Many GPS Clocks are Programmed with the old dates!
  • What are the possible solutions to this problem?
  • Number 1 possible solution is to change the clock
    IRIG-B output stream to Zulu time zone, a.k.a.
    UTZ time zone.
  • An example of the directions for a particular
    example clock follow.

24
- Press the SETUP key repeatedly until the
display says "Set IRIG Time Data?" - Press ENTER
key. The display will say "Select UTC or
LCL." - Use the DOWN key to change to "IRIG DATA
TIME UTC" - Press ENTER key to confirm choice
and move on. The display will now say "IRIG-B
IEEE 1344? - 1344 OFF" - Exit the setup mode by
pressing some other key such as TIME.
25
Solution Number 1
  • You can do this one anytime between now and March
    11, 2007.
  • Above is the date of the first change with the
    new date, which you clock is not programmed for.
  • Never again worry about changeovers.
  • Not necessary to make any physical change to the
    clock itself.

26
Solution Number 2
  • The clock manufacturers instructions and advice
    should be followed, but this information is
    presented as an example.
  • Sometime after October 29, 2006, visit the clock
    and .

27
Convert Clock from Pre-programmed to
User-Programmed
  • You will need to wait until after Fall 2006
    changeover to standard.
  • Clock case must be opened and EPROM changed.
  • Considerable front panel programming will be
    required in most cases.
  • The following is an example taken from a
    prominent manufacturers website as an example.
    (Edited only to remove name and model numbers.)

28
The tentative plan is to change the DST
selections to OFF, ON, and AUTO. AUTO will become
a manually configurable date that is similar to
the existing AUTO-CUSTOM feature. We will ship
clocks with AUTO configured for the existing DST
settings. After October 2006, you will have to
configure the new DST settings. This
configuration will only have to be completed one
time unless DST changes again. This solution will
involve a firmware ROM replacement. Some older
GPS clocks may also require a receiver upgrade.
It may be best to wait until after October 29,
2006 to upgrade the GPS clocks that only require
the firmware replacement. That way the chip
change and new DST settings can be completed in
one trip to the substation. All clocks have two
DST work-arounds available. The first is to
manually change the local offset and the second
is to manually turn the DST feature on/off.
29
WAIT!
Just say NO.
30
Begin now to convert to UTZ!
  • No delay is necessary
  • Any future confusion from DST eliminated
  • We will never achieve full conversion to UTZ.

31
Please consider now what is implied by the
following times
  • 134704.563 Z
  • 134704.563 UTZ
  • 134704.563 GMT, or UT
  • 134704.563 UTC
  • 094704.563 EDT
  • 2005 December 31 235960.563

32
Conclusions
  • When UTC appears after a time, dont assume it
    implies synchronization to the UTC time scale.
  • In fact, any letters after an expression of time
    should be taken as a time zone designation.
  • We intend to report all times closely
    synchronized to UTC time, accurate enough for the
    purpose.
  • UTC is most conveniently available in the output
    of a GPS clock.
  • Keep leap seconds in the back of your mind.
  • We suggest the terms GPS Time or GPS Time
    Scale not be used.

33
Conclusions
  • UTC is a poor time zone designation because it
    is the name of our principal worldwide time
    scale, however many do use it, and we have to
    accept it.
  • GMT, UT, are sometimes used they are the name of
    old time scales, no longer maintained, and as
    such are better than UTC, less likely to cause
    confusion.
  • We suggest that Z and UTZ or Universal Time
    Zone are good time zone terminology.
  • Switch to UTZ now!

34
Time Scales and Time Zones
  • Jim Ingleson
  • NYISO
  • ingleson_at_nyiso.com
  • 2006 FDA Conference
  • GA Tech May 1 2, 2006
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