OS/2 Shareware BBS: Educate/Educate.zip/cpro_16.zip/TIMEPRO.HLP

OS/2 Help File | 1997-01-08 | 84KB | 1,293 lines

ΓòÉΓòÉΓòÉ 1. Introduction ΓòÉΓòÉΓòÉ

Welcome to TimePro v 1.0!

Time Pro is a small utility that quickly sets your computer system clock to the
correct date and time as kept official clocks maintained by agencies of the U.
S. government. Using your PC's modem, Time Pro connects to computers either at
the U. S. Naval Observatory (USNO) or the U. S. National Institute of Standards
(NIST).

Click on a topic below to learn more about using Time Pro:

o   Main Window

o   Location Settings

o   Modem Settings

o   Host Settings

Click on a topic below to learn more that you probably care to know about time
and measurement.

o   History of USNO time keeping

o   Leap seconds

o   Leap years

o   Time and calendars

o   Time in the 21st Century

o   Time Dissemination

o   US Time Zones - legal definitions

o   Glossary of Measurement and Time terms


ΓòÉΓòÉΓòÉ 2. Results Window ΓòÉΓòÉΓòÉ

When Time Pro is first started, the current time and date set for your computer
is displayed in the Results Window for the moment that Time Pro is activated.

Time Pro is started by clicking on the menu selection labeled "Dial".

After connecting with one of the Host sites, the host computer will send a
special string of characters which indicate the current date and time in
Universal Coordinated Time-UTC (formally Greenwich Mean Time or GMT). The
string of characters (and any other messages) is displayed in the status area
below the menu bar.

At the end of that process, Time Pro records the time and date of your system
just before the new time and date from the Host site is applied. The algebraic
difference between the old time and date and the new time and date is
approximately the amount of time that your computer system clock differed from
official UTC time.

Keep in mind that there is a small lag time between the recording of the old
time and the time is takes for the new time to be transmitted from the Host
site and for the computer system clock to be reset.  All told, I estimate that
the computer clock will be set to UTC time to the nearest second.


ΓòÉΓòÉΓòÉ <hidden>  ΓòÉΓòÉΓòÉ

The format of the data string downloaded to Time Pro is:

MJD DOY HHMMSS UTC cr/lf * cr/lf

Where,

MJD        = Modified Julian Date (MJD)

DOY        = day of year (January 1st = 1)

HH         = hours in 24-hour notation (12 noon = 12)

MM         = minutes

SS         = seconds

UTC        = Universal Coordinated Time

cr/lf      = carriage return/line feed

*          = the on-time mark for the preceding time information, and is
           delayed by .0017 seconds (+/- .0004 sec.) from UTC (USNO).

The timing generator which produces this data stream is driven directly by the
Master Clock's reference signals without computer intervention.


ΓòÉΓòÉΓòÉ 3. Location Settings ΓòÉΓòÉΓòÉ

The Location settings page allows the user to make program settings that
correspond to their particular location.

These settings allow Time Pro to set the computer clock to the proper local
time using the GMT offset.

For North American users, just specifying a time zone such as Pacific and
whether or not daylight saving time is in place, e.g. Pacific Standard Time
(PST) or Pacific Daylight Time (PDT), will select the correct GMT offset. For
international users, GMT offset will have to be selected directly.  Most North
American location use negative GMT offsets (e.g. PST is -8, while EST is -5).

Push the save button on the bottom of the Location Settings page to save the
settings for future Time Pro sessions.


ΓòÉΓòÉΓòÉ 4. Modem Settings ΓòÉΓòÉΓòÉ

The modem settings page has several objects used to specify certain modem
hardware settings such as:

o   COM port

o   Modem speed

o   Default time-out

o   modem speaker on/off

o   modem speaker volume

These settings are straightforward, and the only setting required is the COM
port. You must choose the COM port which your modem uses, or Time Pro will not work.

The default settings will probably satisfy most users.  However, for others who
prefer to hear the modem connecting, or have soft modem speakers, they may wish
to experiment with some of the speaker settings.

Push the save button on the bottom of the Modem Settings page to save the
settings for future Time Pro sessions.


ΓòÉΓòÉΓòÉ 5. Host Settings ΓòÉΓòÉΓòÉ

The Host settings page is where the default Host site can be specified.
Currently, either the U. S. Naval Observatory or the National Institute of
Standards can be accessed through Time Pro.

The phone number can also be edited directly if these sites happen to be local
to your place of access.  In this case, for example, a person accessing the
NIST Host site from Boulder Colorado would not have to use the 1 or the 303
prefix because it is a local call, while someone calling NIST from Denver may
need just the 1 prefix.  Editing out the appropriate prefix and pressing the
save button would save the local number for future Time Pro sessions.

Push the save button on the bottom of the Location Settings page to save the
settings for future Time Pro sessions.


ΓòÉΓòÉΓòÉ 6. NIST ΓòÉΓòÉΓòÉ

The National Institute of Standards and Technology (NIST), it is an operating
unit of the Physics Laboratory of the National Institute of Standards and
Precision in Boulder, Colorado USA.

In addition to research and providing information to the scientific community
about time measurement, the NIST division is also responsible for coordinating
the information with other countries and for the development of frequency-based
standards of length. The latter task is an important function because of the
relation between the definition of the meter and the second.

If one needs to know what the official time is, the Time Division conveys the
correct time in a variety of mediums, including via HF and LF radio signals,
UHF satellite signals, the telephone, and the Internet: http://www.bldrdoc.gov/timefreq/index.html

You'll find out how to connect with each of these systems here. The Time and
Frequency Division also offers a number of its publications online.


ΓòÉΓòÉΓòÉ 7. USNO ΓòÉΓòÉΓòÉ

Located in Washington D.C., the U. S. Naval Observatory (USNO) is the official
timekeeper for the United States of America. Formerly known as the Directorate
of Time, the Time Service Department of the USNO operates a web site at: http://tycho.usno.navy.mil/time.html

The U. S. Naval Observatory (USNO) is charged with the responsibility for
precise time determination and  management of time dissemination. Modern
electronic systems, such as electronic navigation or communications systems,
depend increasingly on precise time and time interval (PTTI). Examples  would
be the ground-based LORAN-C navigation system and the satellite-based Global
Positioning System (GPS).

These systems are based on the travel time of the electromagnetic signals: an
accuracy of 10 nanoseconds (10 one billionths of a second) corresponds to a
position accuracy of 10 feet. In fast communications, time synchronization is
equally important.  All of these official systems are referenced to the USNO
master clock.

Thus, the USNO must maintain and continually improve its clock system so that
it can stay one step ahead of the demands made on its accuracy,  stability, and reliability.

The present USNO Master Clock is based on a system of dozens of independently
operating cesium atomic clocks and 5 to 10 hydrogen maser clocks. These clocks
are distributed over  20 environmentally controlled  clock  vaults, to insure
their stability.  By automatic intercomparison of all clocks every 100 seconds,
the USNO time scale can be computed which is not only reliable but also
extremely stable.  Its rate does not change  by  more  than about 100
picoseconds (0.0000000001 seconds) per day from day to day.


ΓòÉΓòÉΓòÉ 7.1. History of USNO Timekeeping ΓòÉΓòÉΓòÉ

In 1845, at the request of the Secretary of the Navy, the Observatory installed
a time ball atop the 9.6-inch telescope dome. The time ball was dropped every
day precisely at Noon, enabling the inhabitants of Washington to set their
timepieces.  Ships in the Potomac River could also set their clocks before
putting to sea. The Observatory's Time Service was initiated in 1865. A time
signal was transmitted via telegraph lines to the Navy Department, and also
activated the Washington fire bells at 0700, 1200, and 1800. This service was
later extended via Western Union telegraph lines to provide accurate time to
railroads across the nation. The Observatory participated in a program of
determining longitude by comparing local time with that telegraphed from a
clock at another fixed observatory, and thus exchanged time signals with other
observatories and with the Coast Survey field parties.

Beginning in 1934, the Observatory determined time with a photographic zenith
tube (PZT), a specialized instrument that points straight upward toward the
zenith and automatically photographs selected stars crossing the zenith. This
gave a measure of the Greenwich Mean Time (now called Universal Time), the
"time of day" based on the rotation of the Earth.  Improvements in clock
technology, including the Shortt free-pendulum clock and quartz crystal clocks,
soon proved conclusively that the Earth's rotation was not uniform, and a new
uniform time scale known as Ephemeris time came into use in 1956.

Defined by the orbital motion of the Earth about the Sun, in practice Ephemeris
time was determined by observations of the Moon, first undertaken with the dual
rate moon camera, invented by William Markowitz at the Naval Observatory in
1951. In 1984 the family of time scales known as dynamical time replaced
Ephemeris time as the time based on the motion of celestial bodies according to
the theory of gravitation, now taking relativistic effects into account. In the
meantime, the development of atomic clocks brought about the introduction of a
much more accessible time - the Atomic time scale based on the vibration (an
energy level transition) of the cesium atom.

 In 1958 the Naval Observatory and Britain's National Physical Laboratory
published the results of joint experiments that defined the relation between
Atomic time and Ephemeris time. (An interesting scientific and philosophical
question is whether the relationship between Atomic time and gravitational time
remains constant.) Since 1967 the international definition of the second has
been based on these joint experiments.  Atomic time is kept synchronized with
universal time by the addition or subtraction of a leap second whenever necessary.

Time dissemination has also been continuously improved.  In 1904 a naval radio
station transmitted the first radio time signals ever; they were derived from a
U.S. Naval Observatory clock.  This was the beginning of a system of radio
time, constantly improved and increasingly automated through the century, that
now spans the globe. The function of rating, repairing and disseminating
chronometers and other nautical instruments, a major and especially critical
effort during World War II , was transferred from the Observatory to the
Optical Section of the Norfolk Naval Shipyard in Portsmouth, Virginia in 1950.

The U.S. Naval Observatory continues to be the leading authority in the United
States for astronomical and timing data required for such purposes as
navigation at sea, on land, and in space, as well as for civil affairs and
legal matters.  Its current Mission Statement, promulgated in 1984 by the Chief
of Naval Operations, reads:

"To determine the positions and motions of celestial bodies, the motions of the
Earth, and precise time.  To provide the astronomical and timing data required
by the Navy and other components of the Department of Defense for navigation,
precise positioning, and command, control, and communications.  To make these
data available to other government agencies and to the general public.  To
conduct relevant research; and to perform such other functions or tasks as may
be directed by higher authority."

The U.S. Naval Observatory carries out its primary functions by making regular
observations of the Sun, Moon, planets, selected stars, and other celestial
bodies to determine their positions and motions; by deriving precise time
interval (frequency), both atomic and astronomical, and managing the
distribution of precise time by means of timed navigation and communication
transmissions and portable clocks; and by deriving, publishing, and
distributing the astronomical data required for accurate navigation,
operational support, and fundamental positional astronomy.  In addition, the
U.S. Naval Observatory conducts the research necessary to improve both the
accuracy and the methods of determining and providing astronomical and timing data.

By a Department of Defense directive, the U.S. Naval Observatory is charged
with maintaining the DoD reference standard for Precise Time and Time Interval
(PTTI). The Superintendent is designated as the DoD PTTI Manager. The U.S.
Naval Observatory has developed the world's most accurate atomic clock system.
Increasingly accurate and reliable time information is required in many aspects
of military operations.  Modern navigation systems depend on the availability
and synchronization of highly accurate clocks.  This holds for such
ground-based systems as LORAN-C as well as for the Department of Defense
satellite-based NAVSTAR Global Positioning System (GPS).  In the communications
and the intelligence fields, time synchronized activities are essential.

The U.S. Naval Observatory Master Clock is the time and frequency standard for
all of these systems.  Thus, that clock system must be at least one step ahead
of the demands made on its accuracy, and developments planned for the years
ahead must be anticipated and supported. The Master Clock system now
incorporates hydrogen masers, which in the short term are more stable than
cesium beam atomic clocks, and mercury ion frequency standards, which are more
stable in the long run. These represent the most advanced technologies
available to date.  Highly accurate portable atomic clocks have been
transported aboard aircraft in order to synchronize the time at Naval Bases and
other Department of Defense facilities around the world with the Master Clock.
Accurate time synchronization with the Master Clock is now beginning to be
carried out through the use of atomic clocks in satellites, such as the GPS
satellites, which will provide the primary means of time synchronization and
worldwide time distribution in the future.


ΓòÉΓòÉΓòÉ 8. Leap seconds ΓòÉΓòÉΓòÉ

Civil time is occasionally adjusted by one second increments to insure that the
difference between a uniform time scale defined by atomic clocks does not
differ from the Earth's rotational time by more than 0.9 seconds. Coordinated
Universal Time (UTC) an atomic time, is the basis for our civil time.

In 1956, following several years of work, two astronomers at the U.S. Naval
Observatory (USNO) and two astronomers at the National Physical Laboratory
(Teddington, England) determined the relationship between the frequency of the
cesium atom (the standard of time) and the rotation of the Earth at a
particular epoch.  As a result, they defined the second of atomic time as "the
length of time required for 9,192,631,770 cycles of radiation corresponding to
the transition between two hyperfine levels of the ground state of the cesium
133 atom at zero magnetic field".

The second thus defined was equivalent to the second defined by the fraction
1/31,556,925.9747 of the year 1900. The atomic second was set equal, then, to
an average second of Earth rotation time near the turn of the 20th century.

The Sub-bureau for Rapid Service and Predictions of the International Earth
Rotation Service (IERS), located at the USNO, monitors the Earth's rotation.
Part of its mission involves the determination of a time scale based on the
current rate of the rotation of the Earth.  UT1 is the non-uniform time based
on the Earth's rotation.

The Earth is constantly undergoing a deceleration caused by the braking action
of the tides.  Through the use of ancient observations of eclipses, it is
possible to determine the deceleration of the Earth to be roughly 1-3
milliseconds per day per century.  This is an effect which causes the Earth's
rotational time to slow with respect to the atomic clock time.  Since it has
been nearly 1 century since the defining epoch (i.e. the ninety year difference
between 1990 and 1900), the difference is roughly 2 milliseconds per day.
Other factors also affect the Earth, some in unpredictable ways, so that it is
necessary to monitor the Earth's rotation continuously.

In order to keep the cumulative difference in UT1-UTC less than 0.9 seconds, a
leap second is added to the atomic time to decrease the difference between the
two.  This leap second can be either positive or negative depending on the
Earth's rotation.  Since the first leap second in 1972, all leap seconds have
been positive.  This reflects the general slowing trend of the Earth due to
tidal braking.

 Confusion sometimes arises over the misconception that the regular insertion
of leap seconds every few years indicates that the Earth should stop rotating
within a few millennia.  The confusion arises because some mistake leap seconds
for a measure of the rate at which the Earth is slowing.  The one-second
increments are, however, indications of the accumulated difference in time
between the two systems.  As an example, the situation is similar to what would
happen if a person owned a watch that lost two seconds per day.  If it were set
to a perfect clock today, the watch would be found to be slow by two seconds
tomorrow.  At the end of a month, the watch will be roughly a minute in error
(thirty days of two second error accumulated each day).  The person would then
find it convenient to reset the watch by one minute to have the correct time again.

This scenario is analogous to that encountered with the leap second.  The
difference is that instead of setting the clock that is running slow, we choose
to set the clock that is keeping a uniform, precise time.  The reason for this
is that we can change the time on an atomic clock while it is not possible to
alter the Earth's rotational speed to match the atomic clocks! Currently the
Earth runs slow at roughly 2 milliseconds per day.  After 500 days, the
difference between the Earth rotation time and the atomic time would be one
second.  Instead of allowing this to happen, a leap second is inserted to bring
the two times closer together.

International Atomic Time (TAI) is the atomic time scale of the BIPM. its unit
interval is exactly one SI second at sea level.  The origin of TAI is such that
UT1-TAI is approximately 0 (zero) on January 1, 1958.  TAI is not adjusted for
leap seconds.  UTC is defined by the CCIR Recommendation 460-4 (1986).  It
differs from TAI by an integral number of seconds, in such a way that UT1-UTC
stays smaller than 0.9s in absolute value.

The decision to introduce a leap second in UTC is the responsibility of the
International Earth Rotation Service (IERS).  According to the CCIR
Recommendation, first preference is given to the opportunities at the end of
December and June, and second preference to those at the end of March and
September.  Since the system was introduced in 1972, only dates in June and
December have been used.

The first leap second was introduced on June 30, 1972.  Information on the most
recent leap second can be found at:

ftp://tycho.usno.navy.mil/pub/series/ser14.txt

Also, a historical list of leap seconds at:

ftp://maia.usno.navy.mil/ser7/tai-utc.dat

documents all leap seconds.

The OMEGA epoch is January 1, 1972 and OMEGA is synchronized to UTC. OMEGA is
NOT adjusted for leap seconds.

(Information excerpted from USNO).


ΓòÉΓòÉΓòÉ 9. Leap years ΓòÉΓòÉΓòÉ

According to the Gregorian calendar, which is the civil calendar in use today,
years evenly divisible by 4 are leap years, with the exception of centurial
years that are not evenly divisible by 400.  Therefore, the years 1700, 1800,
1900 and 2100 are not leap years, but 1600, 2000, and 2400 are leap years.

The Gregorian calendar year is intended to be of the same length as the cycle
of the seasons.  However, the cycle of the seasons, technically known as the
tropical year, is approximately 365.2422 days.  Since a calendar year consists
of an integral number of whole days, a calendar year cannot exactly match the
tropical year.  If the calendar year always consisted of 365 days, it would be
short of the tropical year by about 0.2422 days every year.  Over a century,
the calendar and the seasons would depart by about 24 days, so that the
beginning of spring in the northern hemisphere would shift from March 20 to
April 13.

To synchronize the calendar and tropical years, leap days are periodically
added to the calendar, forming leap years.  If a leap day is added every fourth
year, the average length of the calendar year is 365.25 days.  This was the
basis of the Julian calendar, introduced by Julius Caesar in 46 B.C.  In this
case the calendar year is longer than the tropical year by about 0.0078 days.
Over a century this difference accumulates to a little over three quarters of a
day.  From the time of Julius Caesar to the sixteenth century A.D., the
beginning of spring shifted from March 23 to March 11.

When Pope Gregory XIII instituted the Gregorian calendar in 1582, the calendar
was shifted to make the beginning of spring fall on March 21 and a new system
of leap days was introduced.  Instead of intercalating a leap day every fourth
year, 97 leap days would be introduced every 400 years, according to the rule
given above.  Thus, the average Gregorian calendar year is 365.2425 days in
length.  This agrees to within a half a minute of the length of the tropical
year.  It will take about 3300 years before the Gregorian calendar is as much
as one day out of step with the seasons.

(Information excerpted from Otavia Propper, USNO)


ΓòÉΓòÉΓòÉ 10. Time in the 21st Century ΓòÉΓòÉΓòÉ

Much has been discussed about the disruption caused to modern electronic data
storage and retrieval systems due to the way computers handle time. Supposedly,
1 second after midnight on December 31st 1999, the year digits on computer
clocks will reset to 00 to reflect the year 2000. Older software and BIOS that
are not designed to deal with this may have many problems such as the inability
to calculate correct times in spreadsheets.

A more interesting topic is when the actual new millennium will begin.  Without
giving much thought to this question, most people would say January 1, 2000.
However, the truth of the matter is that it begins on January 1, 2001.  Why is
this, you may ask? According to the USNO, years of the Gregorian calendar,
which is currently in use today, are counted from AD 1.  Thus, the 1st century
comprised the years AD 1 through AD 100.  The second century began with AD 101
and continued through AD 200.  By extrapolation we find that the 20th century
comprises the years AD 1901-2000.  Therefore, the 21st century will begin with
1 January 2001 and continue through 31 December 2100.

Similarly, the 1st millennium comprised the years AD 1-1000.  The 2nd
millennium comprises the years AD 1001-2000.  The 3rd millennium will begin
with AD 2001 and continue through AD 3000.


ΓòÉΓòÉΓòÉ 11. Time and Calenders ΓòÉΓòÉΓòÉ

Many initial epochs have been used for calendrical reckoning.  Frequently,
years were counted from the ascension of a ruler.  For a calendrical epoch to
be useful, however, it must be tied to a sequence of recorded historical
events.  This is illustrated by the adoption of the birth of Christ as the
initial epoch of the Julian and Gregorian calendars.  This epoch was
established by the 6th century scholar Dionysius Exiguus who was compiling a
table of dates of Easter.

Dionysius followed previous precedent by extending an existing table to cover
the 19-year period 228-247, reckoned from the beginning of the reign of Emperor
Diocletian. However, he did not want his Easter table "to perpetuate the memory
of an impious persecutor of the Church, but preferred to count and denote the
years from the Incarnation of our Lord Jesus Christ." To accomplish this he
designated the years of his table Anni Domini Nostri Jesu Christi 532-550.

Thus, Dionysius' Anno Domini 532 is equivalent to Anno Diocletiani 248, so that
a correspondence was established between the new Christian Era and an existing
system associated with historical records.  What Dionysius did not do is
establish an accurate date for the birth of Christ.  While scholars generally
believe that Christ was born a few years before AD 1, the records are too
sketchy to allow a definitive dating.

Given an initial epoch, one must consider how to record preceding dates.  Today
it is obvious that a year designated 1 would be preceded by year 0, which would
be preceded by year -1, etc.  But since the concept of negative numbers did not
come into use in Europe until the 16th century, and was initially only of
interest to mathematicians, its application to chronological problems was
delayed for two more centuries.  Instead, years were counted from a succession
of initial epochs.  Even as Dionysius' practice of dating from the Incarnation
became common in ecclesiastical writings of the middle ages, traditional dating
practices continued for civil purposes.

In the 16th century Joseph Justus Scaliger tried to resolve the patchwork of
historical eras by placing everything on a single system.  Not being ready to
deal with negative year counts, he sought an initial epoch in advance of any
historical record.  His approach was numerological and utilized three
calendrical cycles: the 28-year solar cycle, the 19-year cycle of Golden
Numbers, and the 15-year indiction cycle.

The solar cycle is the period after which week days and calendar dates repeat
in the Julian calendar.  The cycle of Golden Numbers is the period after which
moon phases repeat (approximately) on the same calendar dates.  The indiction
cycle was a Roman tax cycle of unknown origin.  Therefore, Scaliger could
characterize a year by the combination of numbers (S,G,I), where S runs from 1
through 28, G from 1 through 19, and I from 1 through 15.  Scaliger first
stated that a given combination would recur after 7980 (= 28 x 19 x 15) years.
He called this a Julian cycle because it was based on the Julian calendar.
Scaliger knew that the year of Christ's birth (as determined by Dionysius
Exiguus) was characterized by the number 9 of the solar cycle, by Golden Number
1, and by number 3 of the indiction cycle, or (9,1,3).  Then Scaliger chose as
this initial epoch the year characterized by (1,1,1) and determined that
(9,1,3) was year 4713 of his chronological era.  Scaliger's initial epoch was
later to be adopted as the initial epoch for the Julian Day numbers.

We would say that Scaliger's initial epoch was 4713 BC or -4712.  In the
historical system of dating, AD 1 is preceded by 1 BC; there is no year 0.  In
the astronomical system, AD 1 is designated +1; this is preceded by year 0,
which is preceded by year -1.  The historical system was introduced in the 16th
century.  However, the astronomical system was not introduced until the 18th century.

(Excerpted from USNO information).


ΓòÉΓòÉΓòÉ 12. Time Dissemination ΓòÉΓòÉΓòÉ

Various timed systems are being kept within narrow tolerances of the USNO
Master Clock.  The chains covering North America have been within about 100ns
(rms), whereas the overseas chains had larger tolerances, the largest in the
case of the Mediterranean chain (1 micros). The Transit navigational satellite
system has been within 100us of UTC (USNO).  Most of the timing users will find
it advantageous to switch now to the GPS as a worldwide source of precision time.

The Global Positioning System (GPS) with the correction given in the navigation
message (A0 and A1), is typically within 15ns rms with selective availability
removed.  Including selective availability, observed with a single frequency
receiver, the rms error has been about 70ns, with a maximum error of 291ns.
These measurements include all available satellites with a 13-minute
observation per pass.  By obtaining the small residual difference between UTC
(USNO,MC) and UTC (BIPM) from the Automated Data Service of the USNO, a near
real time access to UTC is, therefore, possible via the GPS at the level of
accuracy given above.  By averaging over all available satellite passes per
day, a fixed station with a cesium frequency standard can increase this
precision to below 10ns with appropriate filtering.  The obtainable accuracy
will usually be limited by the stability and calibration of the local
antenna-receiver delays.

For highest accuracy, the USNO has extended the use of its two-way satellite
time transfer  instrumentation.  Regular time transfers have been continued
with the NIST in Boulder, Colorado, with the NRC in Ottawa, Canada, and with
the USNO station in Richmond, Florida. Since 1992, experiments have also been
conducted with the Technical University in Graz, Austria, and with OCA in
Grasse, France.  An additional high precision time reference station has been
established on the island of Oahu, Hawaii, and initial two-way time transfers
have been started with that station.  Some problems with the spread-spectrum
modems have limited the obtained precision of these measurements to about 3ns.
The mobile Earth station has been used to make relative delay calibration
between USNO and several other sites.

The instrumentation at the USNO consists currently of two 4.5m VERTEX antennas,
the mobile Earth station, one VSAT, and a new "Fly-Away" small terminal that is
used for the quick calibration of remote stations because this terminal can be
easily transported by air and assembled by one person in a few hours.


ΓòÉΓòÉΓòÉ 13. US Time zones - legal definitions ΓòÉΓòÉΓòÉ

15 USC Sec. 260

UNITED STATES CODE

TITLE 15 - COMMERCE AND TRADE

CHAPTER 6 - WEIGHTS AND MEASURES AND STANDARD TIME

SUBCHAPTER IX - STANDARD TIME

260. Congressional declaration of policy; adoption and observance of uniform
standard of time; authority of Secretary of Transportation.

It is the policy of the United States to promote the adoption and  observance
of uniform time within the standard time zones prescribed by sections 261 to
264 of this title, as modified by  section 265 of this title. To this end the
Secretary of  Transportation is authorized and directed to foster and promote
widespread and uniform adoption and observance of the same standard  of time
within and throughout each such standard time zone.

260a. Advancement of time or changeover dates

(a) Duration of period; State exemption

During the period commencing at 2 o'clock antemeridian on the first Sunday of
April of each year and ending at 2 o'clock antemeridian on the last Sunday of
October of each year, the standard time of each zone established by sections
261 to 264 of this title, as modified by section 265 of this title, shall be
advanced one hour and such time as so advanced shall for the  purposes of such
sections 261 to 264, as so modified, be the standard time of such zone during
such period; however, (1) any State that lies entirely within one time zone may
by law exempt  itself from the provisions of this subsection providing for the
advancement of time, but only if that law provides that the entire State
(including all political subdivisions thereof) shall observe  the standard time
otherwise applicable during that period, and (2) any State with parts thereof
in more than one time zone may by law exempt either the entire State as
provided in (1) or may exempt the entire area of the State lying within any
time zone.

(b) State laws superseded

It is hereby declared that it is the express intent of Congress  by this
section to supersede any and all laws of the States or  political subdivisions
thereof insofar as they may now or hereafter  provide for advances in time or
changeover dates different from  those specified in this section.

(c) Violations; enforcement

For any violation of the provisions of this section the Secretary  of
Transportation or his duly authorized agent may apply to the  district court of
the United States for the district in which such violation occurs for the
enforcement of this section; and such  court shall have jurisdiction to enforce
obedience thereto by writ of injunction or by other process, mandatory or
otherwise,  restraining against further violations of this section and
enjoining obedience thereto.

261. Zones for standard time; interstate or foreign commerce

For the purpose of establishing the standard time of the United  States, the
territory of the United States shall be divided into  eight zones in the manner
provided in this section. Except as  provided in section 260a(a) of this title,
the standard time of the  first zone shall be based on the mean solar time of
the sixtieth  degree of longitude west from Greenwich; that of the second zone
on  the seventy-fifth degree; that of the third zone on the ninetieth  degree;
that of the fourth zone on the one hundred and fifth  degree; that of the fifth
zone on the one hundred and twentieth  degree; that of the sixth zone on the
one hundred and thirty-fifth  degree; that of the seventh zone on the one
hundred and fiftieth  degree; and that of the eighth zone on the one hundred
and  sixty-fifth degree. The limits of each zone shall be defined by an  order
of the Secretary of Transportation, having regard for the  convenience of
commerce and the existing junction points and  division points of common
carriers engaged in interstate or foreign  commerce, and any such order may be
modified from time to time. As  used in sections 261 to 264 of this title, the
term ''interstate or  foreign commerce'' means commerce between a State, the
District of  Columbia, the Commonwealth of Puerto Rico, or any possession of
the  United States and any place outside thereof.

262. Duty to observe standard time of zones

Within the respective zones created under the authority of sections 261 to 264
of this title the standard time of the zone shall insofar as practicable (as
determined by the Secretary of Transportation) govern the movement of all
common carriers engaged  in interstate or foreign commerce. In all statutes,
orders, rules, and regulations relating to the time of performance of any act
by any officer or department of the United States, whether in the legislative,
executive, or judicial branches of the Government, or  relating to the time
within which any rights shall accrue or determine, or within which any act
shall or shall not be performed by any person subject to the jurisdiction of
the United States, it shall be understood and intended that the time shall
insofar as  practicable (as determined by the Secretary of Transportation) be
the United States standard time of the zone within which the act is to be performed.

263. Designation of zone standard times

The standard time of the first zone shall be known and designated as Atlantic
standard time; that of the second zone shall be known and designated as eastern
standard time; that of the third zone shall be known and designated as central
standard time; that of the  fourth zone shall be known and designated as
mountain standard time; that of the fifth zone shall be known and designated as
Pacific standard time; that of the sixth zone shall be known and designated as
Alaska standard time; that of the seventh zone shall  be known and designated
as Hawaii-Aleutian standard time; and that  of the eighth zone shall be known
and designated as Samoa standard time.

264. Part of Idaho in third zone

In the division of territory, and in the definition of the limits of each zone,
as provided in sections 261 to 264 of this title, so much of the State of Idaho
as lies south of the Salmon River, traversing the State from east to west near
forty-five degree thirty minutes latitude, shall be embraced in the third zone:
Provided, That common carriers within such portion of the State of Idaho may
conduct their operations on Pacific time.

265. Transfer of certain territory to standard central-time zone

The Panhandle and Plains sections of Texas and Oklahoma are  transferred to and
placed within the United States standard  central-time zone. The Secretary of
Transportation is authorized and directed to  issue an order placing the
western boundary line of the United  States standard central-time zone insofar
as the same affect Texas and Oklahoma as follows:

Beginning at a point where such western boundary time zone line  crosses the
State boundary line between Kansas and Oklahoma; thence westerly along said
State boundary line to the northwest corner of  the State of Oklahoma; thence
in a southerly direction along the west State boundary line of Oklahoma and the
west State boundary line of Texas to the southeastern corner of the State of
New Mexico; thence in a westerly direction along the State boundary line
between the States of Texas and New Mexico to the Rio Grande River; thence down
the Rio Grande River as the boundary line between the United States and Mexico:
Provided, That the Chicago, Rock Island and Gulf Railway Company and the
Chicago, Rock Island and Pacific Railway Company may use Tucumcari, New Mexico,
as the point at which they change from central to mountain time and vice versa;
the Colorado Southern and Fort Worth and Denver City Railway Companies may use
Sixela, New Mexico, as such changing point; the Atchison, Topeka and Santa Fe
Railway Company and other branches of the Santa Fe system may use Clovis, New
Mexico, as such changing point, and those railways running into or through El
Paso may use El Paso as such point: Provided further, That this section shall
not, except as herein provided, interfere with the adjustment of  time zones as
established by the Secretary of Transportation.

267. ''State'' defined

As used in this Act, the term ''State'' includes the District of  Columbia, the
Commonwealth of Puerto Rico, or any possession of the  United States.


ΓòÉΓòÉΓòÉ 14. Glossary of measurement terminology ΓòÉΓòÉΓòÉ

GLOSSARY OF FREQUENCY AND TIMING TERMS

(Excerpted from NIST, Boulder Colorado, USA) and USNO, Washington, D.C. USA

 Accuracy:The degree of conformity of a measured or calculated value to its
           definition or with respect to a standard reference (see uncertainty).

 Aging:   The systematic change in frequency over time because of internal
           changes in the oscillator. For example, a 100-kHz quartz oscillator
           may age until its frequency becomes 100.01 kHz (see drift). NOTE:
           Aging is the frequency change with time when factors external to the
           oscillator such as environment and power supply are kept constant.

 Allan Variance:Also called Allan Deviation, is the standard method of
           characterizing the frequency stability of oscillators in the time
           domain, both short and long term.

 Atomic Time (TA) scale: A time scale based on atomic or molecular resonance
phenomena. Elapsed time is measured by counting cycles of a frequency locked to
an atomic or molecular transition. Other scales use mechanical reference
devices such as quartz crystals or are based on the rotation rate of the earth.
The unit of duration for Atomic Time is the Systeme International (SI) second
which is defined as the duration of 9,192,631,770 cycles of radiation
corresponding to the transition between two hyperfine levels of the ground
state of cesium 133.

 Barycentric Coordinated Time: Barycentric Coordinated Time (TCB) is a
           coordinate time having
           its spatial origin at the solar system barycenter. TCB differs from
           TDB in rate.
           The two are related by: TCB - TDB =Lb * (JD -2443144.5) * 86400
           seconds, with
           Lb = 1.550505e-08.

 Barycentric Dynamical Time: Barycentric Dynamical time (TDB) is the
           independent argument of ephemerides and dynamical theories that are
           referred to the solar system barycenter. TDB varies from TT only by
           periodic variations.

 Binary coded decimal: A numbering system which uses decimal digits encoded in
a binary representation.

 Binary number system: A numbering system which has 2 as its base and uses two
symbols, usually denoted by 0 and 1.

 BIPM: Bureau International des Poids et Mesures (International Bureau of
Weights and Measures located in France).

 Bit: An abbreviation for a binary coded digit of which a word or subword is composed.

 Bit transition time: The time required for a bit in the time code or subword
to change from one logic level to the next such as a logical 0 to a logical 1
or vice versa.

 Calibration: The process of identifying and measuring time or frequency
errors, offsets, or deviations of clock/oscillator relative to an established
and accepted time or reference frequency standard such as UTC - National
Institute of Standards and Technology (NIST) or UTC - Bureau International des
Poids et Mesures (BIPM).

 Clock:   A device for maintaining and displaying time.

 Clock time difference: The difference between the readings of two clocks at
the same instant. NOTE: To avoid confusion in sign, algebraic quantities should
be given, applying the following convention. At time T of a reference time
scale, let a denote the reading of the time scale A, and b the reading of the
time scale B. The time scale difference is expressed by A-B = a-b at the
instant T. The same convention applies to the case where A and B are clocks.
There is no universally accepted convention for the significance of the sign.
If A-B is measured electrically, a positive value usually implies that a given
tick from A arrives before the same tick in B, whereas, the reverse is usually
true if A and B are calendar dates.

 Coherence of phase:Phase coherence exists if two periodical signals of
           frequency M and N resume the same phase difference after M cycles of
           the first and N cycles of the second, where M/N is a rational number.

 Coordinated clock: A clock synchronized within stated limits to a spatially
separated reference clock.

 Coordinated time scale: A time scale synchronized within stated limits to a
reference time scale.

 Coordinated Universal Time: Also called Universal Time Coordinated (UTC), UTC
is a coordinated time scale, maintained by the Bureau International des Poids
et Mesures (BIPM), which forms the basis of a coordinated dissemination of
standard frequencies and time signals. NOTE: A UTC clock has the same rate as a
Temps Atomique International (TAI) clock or international atomic time clock but
differs by an integral number of seconds called leap seconds. The UTC scale is
adjusted by the insertion or deletion of seconds (positive or negative leap
seconds) to ensure approximate agreement with UT1 (also known as the Julian Date).

 Date:    A unique instant defined in a specified time scale. NOTE: The date
           can be conventionally expressed in years, months, days, hours,
           minutes, seconds, and fractions. Also, Julian Date (JD) and Modified
           Julian Date (MJD) are useful dating measures (see Julian Date and
           Modified Julian Date).

 Delta T: Delta T is the difference between Earth rotational time (UT1) and
           dynamical
           time (TDT).  Predicted values of TDT - are provided by the Earth
           Orientation Department.

 Disciplined oscillator:An oscillator with a servo loop that has its phase and
           frequency locked to an external reference signal.

 Drift (frequency): The linear (first-order) component of a systematic change
in frequency of an oscillator over time. Drift is due to aging plus changes in
the environment and other factors external to the oscillator (see aging).

 DUT1:    The approximate time difference between UT1 and UTC, expressed to
           the nearest 0.1s. DUT1 = UT1 + or - UTC. NOTE: DUT1 may be regarded
           as a correction to be added to UTC to obtain a better approximation
           to UT1. The values of DUT1 are given by the International Earth
           Rotation Service (IERS) in integral multiples of 0.1s.

 Dynamical Time:  Dynamical Time replaced ephemeris time as the independent
argument in dynamical theories and ephemerides. Its unit of duration is based
on the orbital motions of the Earth, Moon, and planets.

 Ephemeris Time (ET): An astronomical time scale based on the orbital motion
of the earth around the sun (see Terrestrial Time).

 Epoch: Epoch signifies the beginning of an era (or event) or the reference
date of a system of measurements.

 Error: The difference of a measured value from its known true or correct
value (or sometimes from its predicted value).

 Frequency: The rate at which a periodic phenomenon occurs over time.

 Frequency analysis techniques:Analysis techniques in the frequency domain,
           where signals are separated into their frequency components and the
           power at each frequency is displayed.

 Frequency deviation: The difference between frequency values of the same
signal at two different times or the difference between the instantaneous
signal frequency and the average signal frequency.

 Frequency difference: Difference between the frequencies of two different signals.

 Frequency drift: See drift and aging.

 Frequency offset: The frequency difference between the realized value and a
reference frequency value. Offset is often not referenced to the nominal. For
example, during irradiation testing the offset is referenced to the frequency
before irradiation.

 Frequency shift: Change in frequency from a standard reference.

 Frequency stability: Statistical estimate of the frequency fluctuations of a
signal over a given time interval.

           o   Long term stability  usually involves measurement averages
               beyond 100s.

           o   Short term stability  usually involves measurement averages from
               a few tenths of a second to 100s.

Note:  Generally, there is a distinction between systematic effects such as
frequency drift and stochastic frequency fluctuations. Special variances have
been developed for the characterization of these fluctuations. Systematic
instabilities may be caused by radiation, pressure, temperature, and humidity.
Random or stochastic instabilities are typically characterized in the time
domain or frequency domain. They are typically dependent on the measurement
system bandwidth or on the sample time or integration time.

 Frequency standard: A precise frequency generator such as a rubidium, cesium,
or hydrogen maser whose output is used as a frequency.

           o   Primary frequency standard: A standard whose frequency
               corresponds to the adopted definition of the second with its
               specified accuracy achieved without external calibration of the
               device. Currently, only the cesium frequency standard is defined
               as a primary standard. Rubidium gas cells, hydrogen masers, and
               other types of atomic standards are not, by definition,
               considered primary standards.

           o   Secondary frequency standard: A frequency standard which
               requires external calibration. For example, a crystal oscillator
               might be considered a secondary frequency standard.

 Geocentric Coordinate Time (TCG): TCG is a coordinate time having its spatial
origin at the center of mass of the Earth. TCG differs from TT as: TCG - TT =
Lg * (JD -2443144.5) * 86400 seconds, with Lg = 6.969291E-10.

 Global Positioning System (GPS): GPS is a highly accurate, global satellite
navigation system based on a constellation of 24 satellites orbiting the earth
at a very high altitude. In addition to navigation, the system also provides
very precise time.

           GPS signals {broadcast signals of GPS and their functions are as follows}:

                      L1 - 1575.42 MHz
                      - Primary navigation signal
                      - C /A and P codes and navigation data

                      L2 - 1227.6 MHz
                      - second frequency provides higher accuracy ionospheric delay calibration
                      - P code and navigation data

                      L3 - 1381.05 MHz
                      - global burst detector
                      - SBand command channel

 GPS C/A code: The standard GPS code known as the coarse/acquisition code or
"civilian code". The code is a series of 1023 pseudorandom binary byphase
modulations on the carrier and has a chip rate (bit transition time) of 1.023
MHz (often called "Standard Positioning Service").

 GPS Pcode: This is called the precise code or "protected code"; and is a
series of pseudorandom, binary byphase modulations on the carrier and has a
chip rate of 10.23 MHz. The P code repeats about every 267 days. Each 1 week
segment of the code is unique to a particular GPS satellite and is reset each
week (on Saturday).

 Differential GPS: The precise measurement of the difference in the positions
of two receivers tracking the same GPS signal. One of the receivers may be a
stationary reference point (precise benchmark) for position and the other could
be a roving receiver for determining the position of a remote location.

 GPS common view: A technique which involves two separated receivers, whose
positions are accurately known, tracking the same GPS satellite for precise
time determination. Most satellite, atmospheric and ionospheric errors in GPS
are reduced using this technique.

 Greenwich Mean Time (GMT): GMT is a 24-hour system based on mean Solar time
plus 12 hours at Greenwich, England. Greenwich Mean Time can be considered
approximately equivalent to Coordinated Universal Time (UTC), which is
broadcast from all standard time and frequency radio stations. However, GMT is
now obsolete and has been replaced by UTC.

 Identification bit (ID): A bit with a fixed state (logic level) used for time
code identification and other information.

 Inhibit/read bit: A bit generated with the time code which can be used to
prohibit a user from reading the code during the time code update.

 Instant: A specific time.

 International Atomic Time (TAI): The acronym TAI is derived from its French
name, Temps Atomique International. TAI is an atomic time scale based on
statistical data from a worldwide set of atomic clocks. It is the
internationally agreed upon time reference conforming to the definition of the
second, the fundamental unit of atomic time in the International System of
Units (SI). It is defined as the duration of 9 192 631 770 periods of the
radiation corresponding to the transition between two hyperfine levels of the
ground state of the cesium - 133 atom. The TAI is maintained by the Bureau
International des Poids et Mesures (BIPM) in France. Although TAI was
officially introduced in January 1972, it has been available since July 1955.
Its epoch was set so that TAI was in approximate agreement with UT1 on 1
January 1958 (see second).

 Julian Day: Obtained by counting days from the starting point of noon on 1
January 4713 B.C. (Julian Day zero). One way of telling what day it is with the
least possible ambiguity.

Note:  The Julian Date is conventionally referred to UT1, but may be used in
 other contexts, if so stated.

           o   Julian Date (JD): The Julian Day number followed by the fraction
               of the day elapsed since the preceding noon (1200 UT). {
               Example:  The date 1900 January (1) 0.5 day UT corresponds to JD
               = 2 415 020}.

           o   Julian Day Number (JDN): The number of a specific day from a
               continuous day count having an initial origin of 1200 UT on 1
               January 4713 BC, the start of Julian day zero. { Example: The
               day extending from 1900 January (1) 0.5 day UT to 1900 January
               1.5 days UT has the number 2 415 020}.

           o   Modified Julian Day (MJD): Equal to the Julian day. Shifted so
               its origin occurs at midnight on 17 November 1858. The MJD
               differs from the Julian date by exactly 2 400 000.5 days.

           o   Modified Julian Date (MJD): Julian date less 2 400000.5

           o   Truncated Julian Day (TJD): The JDN 2 440 000.5 occurred on 24
               May 1968 and defines the origin of the TJD time scale used in
               the PB5 time code. NOTE: The TJD is used by the scientific
               community for recording astronomical and historical events and
               for archival data storage and is useful in the space sciences
               area. The TJD has an epoch of 24 May 1969 with a repetition
               period (recycle time) of 10000 days (27.379 years) and recycled
               on 9 October 1995. The TJD is currently equal to MJD minus
               50000. TJD = MJD truncated to four digits.

 Leap second: An intentional time step of one second used to adjust UTC to
ensure approximate agreement with UT1. An inserted second is called a positive
leap second, and an omitted second is called a negative leap second. A positive
leap second is presently needed about once per year. Click here for more
information on leap seconds.

 Nominal value: The ratio of a value to a reference value. NOTE: In a device
that realizes a physical quantity, it is the specified value of such a
quantity. It is an ideal value and free from tolerance.

 Normalized frequency difference: The ratio between the actual frequency (f1)
minus the nominal frequency (f2) over the nominal frequency.

 Offset: The difference between the realized value and a reference value.

 On time: The state of any bit (in a time code) that is coincident with the
Standard Time Reference (U.S. National Institute of Standards and Technology).

 Parity bit: A bit derived from and generated with the bits in the time code
word or subword to facilitate error detection and correction.

 Phase: A measure of a fraction of the period of a repetitive phenomenon,
measured with respect to some distinguishable feature of the phenomenon itself.
In the standard frequency and time signal service, phase time differences such
as time differences between two identified phases of the same phenomenon or of
two different phenomena are mainly considered.

           o   Phase jump: A sudden phase change in a signal.

           o   Phase shift: An intentional change in phase from a reference.

           o   Phase deviation: The difference of the phase from a reference.

           o   Phase signature: A deliberate phase offset for the purpose of
               signal identification. For example, NIST's radio station WWVB
               broadcast is deliberately phase shifted at 10 minutes after the
               hour, so a person knows that WWVB is being tracked and not some
               other signal.

 Precision: The degree of mutual agreement among a series of individual
measurements. Precision is often, but not necessarily, expressed by the
standard deviation of the measurements.

 Proper time: The local time, as indicated by an ideal clock, in a
relativistic sense. NOTE: Proper time is distinguished from a coordinated time
which involves theory and computations. If a time scale is realized according
to the proper time concept, it is called a proper time scale.  {Examples (a)
for proper time: the second is defined in the proper time of the cesium atom;
(b) for proper time scale: a time scale is produced in a laboratory and not
transmitted outside the laboratory}.

 Reproducibility: With respect to a set of independent devices of the same
design, it is the ability of these devices to produce the same value. With
respect to a single device, it is the ability to produce the same value and to
put it into operation repeatedly without adjustments.

 Resettability: The ability of a device to produce the same value when
specified parameters are independently adjusted under stated conditions of use.

 Resolution: The degree to which a measurement can be determined is called the
resolution of the measurement. The smallest significant difference that can be
measured with a given instrument. For example, a measurement made with a time
interval counter might have a resolution of 10 ns.

 Resolution of a time code: The smallest increment of time or least
significant bit which can be defined by a time code word or subword.

 Second: The basic unit of time or time interval in the International System
of Units (SI) which is equal to 9 192 631 770 periods of radiation
corresponding to the transition between the two hyperfine levels of the ground
state of cesium-133 as defined at the 1967 Conference Generale des Poids et Mesures.

 Sidereal time: The measure of time defined by the apparent diurnal motion of
the vernal equinox; hence, a measure of the rotation of the Earth with respect
to the reference frame that is related to the stars rather than the sun. In
other words, the Sidereal Time unit of duration is the period of the Earth's
rotation with respect to a point nearly fixed with respect to the stars, and is
equal to the hour angle of the vernal equinox.

           Two types of sidereal time are used in astronomy: mean sidereal time
           and apparent sidereal time. One sidereal day is equal to about 23
           hours, 56 minutes, and 4.090 seconds of mean solar time. Also,
           366.2422 mean sidereal days equal 365.2422 mean solar days.

 Standard frequency: A frequency with a known relationship to a reference
frequency standard. NOTE: The term standard frequency is often used for a
signal whose source is from a reference standard frequency.

 Standard frequency station: Also called a time-signal station, this is a
station which provides a standard frequency or time signal emissions such as
NIST's radio station WWV.

 Standard-frequency emission: An emission which disseminates a standard
frequency at regular intervals with a specified frequency accuracy.

           NOTE: In Recommendation 460, the Consultative International du Radio
           (CCIR) recommends a normalized frequency deviation of less than
           1E-10. The CCIR is now known as the International Telecommunications
           Union-Radio (ITU-R).

 Standard frequency satellite service: A radio communication service using
earth satellites for the same purpose as those of the terrestrial standard
frequency service.

 Standard-time-signal emission: A broadcast which disseminates a sequence of
time signals at regular intervals with a specified accuracy, for example,
NIST's radio station WWV. NOTE: In Recommendation 460, the ITU-R recommends
standard time signals to be emitted within 1 ms with reference to UTC and to
contain DUT1 information in a specified code.

 Subword: A subdivision of the time code word containing only one type of time
unit such as days, milliseconds, or microseconds.

 Synchronization: The process of measuring the difference in time of two time
scales such as the output signals generated by two clocks. In the context of
timing, synchronization means to bring two clocks or data streams into phase so
that their difference is 0 (see time scales in synchronism).

 Syntonization: Relative adjustment of two frequency sources with the purpose
of canceling their frequency difference but not necessarily their phase difference.

 Stratum clocks: Accuracy requirements placed on clocks in four strata.
Accuracy of stratum clocks refers to clock performance when the clock receives
no input reference.

 Terrestrial Time: Terrestrial Time (TT) or sometimes Terrestrial Dynamical
Time (TDT) is the new 1991 International Astronomical Union replacement for
what was once called Ephemeris Time. On 1 January 1997, TT = TAI + 32.184
seconds, and the length of the second is chosen so that it agrees with the
International Second (SI) on the geoid (i.e. the unit of duration 86400 SI
seconds on the geoid). The TT scale differs from the old Ephemeris Time in its
conceptual definition. Practically, however, it is realized by means of
International Atomic Time (TAI).

 Time code: A system of symbols (digital or analog) used for identifying
specific instants of time. An information format used to convey time
information. Time is used to specify time of day or a measure of time interval.

 Time-code word: A specific set of time code symbols which identify one
specific time. A time code word may be subdivided into subwords.

 Time comparison: The determination of a difference between two time scales.

 Time interval: The duration between two instants read on the same time scale.

 Time marker: A reference signal enabling the assignment of dates on a time scale.

 Time reference: The basic repetition rate chosen as the common time reference
for all instrumentation (usually 1 pulse per second (pps)).

 Time scale: A system of unambiguous ordering of events. A time scale is meant
to be stable and homogeneous.

 Time-scale difference: The difference between the readings of two time scales
at the same instant (see clock time difference).

 Time scales in synchronism: Two time scales are in synchronism when they
assign the same date to an instant. NOTE: If the time scales are produced in
spatially separated locations, the propagation time of transmitted time signals
and relativistic effects, including the reference coordinate frame, are to be
taken into account.

 Time-scale reading: The value read on a time scale at a specific instant.
NOTE: The reading of a time scale should be qualified by giving the time scale
a name.

 Time-scale unit: The defining basic time interval in a time scale. NOTE: This
unit is different from the realized time scale unit.

 Time signal satellite service: A radio communication service using Earth
satellites for the same purpose as those of the time signal service.

 Time standard: Device used for the realization of the time unit. Continuously
operating device used for the realization of a time scale in accordance with
the definition of the second and with an appropriately chosen origin.

 Time step: A discontinuity in a time scale at some instant. NOTE: A step is
positive (+) if the time scale reading is increased and negative (-) if the
reading is decreased at that instant.

 Uncertainty: The limits of the confidence interval of a measured or
calculated quantity. NOTE: The probability of the confidence limits should be
specified, preferably as one standard deviation.

 Universal Time (UT) Family: Universal Time (UT) is the general designation of
time scales based on the rotation of the Earth. In applications in which a
precision of a few tenths of a second cannot be tolerated, it is necessary to
specify the form of UT such as UT1 which is directly related to polar motion
and is proportional to the rotation of the Earth in space. The UT1 is further
corrected empirically for annual and semiannual variations in the rotation rate
of the earth to obtain UT2. Coordinated Universal Time (UTC) differs from TAI
by an integral number of seconds. UTC is kept within 0.9 seconds of UT1 by the
introduction
of one-second steps to UTC, the leap second usually being a positive step.

           o   Universal Time is the mean solar time of the prime meridian plus
               12 hours, determined by measuring the angular position of the
               Earth about its axis. The UT is sometimes designated GMT, but
               this designation should be avoided. Communicators use the
               designation (Z) or (Zulu). Timekeepers should use UTC of the
               national standard, for example, UTC (USNO) rather than GMT.

           o   Mean Solar Time is simply apparent solar time corrected for the
               effects of orbital eccentricity and the tilt of the Earth's axis
               relative to the elliptic plane; that is, corrected by the
               equation of time which is defined as the hour angle of the true
               Sun minus the hour angle of the mean Sun.

           o   UTO- UT0 measures UT with respect to the observer's meridian
               (position on earth) which varies because of polar motion. In
               other words, UTO is the rotational timescale of a particular
               place of observation. It is observed as the diurnal motion of
               stars or extraterrestrial radio sources

           o   UT1- UT1 is computed by correcting UT0 for the effect of polar
               motion on the longitude of the observing site. It varies from
               uniformity because of the
               irregularities in the Earth's rotation.



ΓòÉΓòÉΓòÉ 15. Product Information ΓòÉΓòÉΓòÉ

Time Pro is a member of the Conversion Pro utilities, and a product of:

NEGC Software
2522 North Proctor Street
Suite 135
Tacoma, WA  98406
Technical Support: [e-mail]

Conversion Pro Utilities Registration Fee: $19.95 (US). Conversion Pro for OS/2
is now marketed exclusively by BMT Micro.

Time Pro, Conversion Pro, and the Conversion Pro utilities are Copyright (C)
1995, 1997, by J. F. Kaminsky. All Rights Reserved.


ΓòÉΓòÉΓòÉ 16. Registration ΓòÉΓòÉΓòÉ

BMT Micro is now handling the marketing of Conversion Pro. To order Conversion
Pro, contact BMT Micro at the following address:

BMT Micro
P.O. Box 15016
Wilmington, NC  28408
Phone orders: 1-800-414-4268
Fax orders: 1-800-346-1672 (24 hours/7 days)
internet: http://www.bmtmicro.com

BMT Micro accepts the following credit cards:

o   Visa

o   Mastercard

o   Discover

o   American Express

o   Diners Club

o   Carte Blanche

Cashiers check, personal check and Eurochecks in DM are welcome. Please read
the BMTOrder.txt file located in the Conversion Pro application directory. For
ordering and other questions, visit BMT Micro at the following sites:

o   Via AOL: bmtmicro

o   Via MSN: bmtmicro

o   Via Compuserve: 74031,307

o   Via Prodigy: HNGP66D

o   World Wide Web: http://www.bmtmicro.com

o   [e-mail]

o   e-mail: [e-mail]