Dr.S.Pal

Distinguished Scientist/Program Director, Satellite Navigation Program

Chairman, GAGAN/IRNSS PMB

ISRO Satellite CentreBangalore

spal@isac.gov.in

AN EPOCH In English “TIME” is used to specify

an instant (time of day) or as a measure of time interval

In Sanskrit Time is termed “KAAL”

Did the concept of time had any meaning before the birth of the universe?

Universe perhaps existed forever or on the theory that it was set in motion at some time `To’ in such a manner as to look as though it had existed forever.

In 1929 EDWIN HUBBLE threw a spanner in the ring by suggesting that universe is expanding.

Hubble’s observations suggested big bang. It is just possible that time started with “BIG BANG”

Aristotle & Newton’s concept of “ABSOLUTE TIME” gets changed by theory of relativity

The only reason for time is so everything does not happen at once” – Albert Einstein

We do not know what is happening at the moment farther away in the universe: the light that we see from distant galaxies left them millions of years ago…. Thus, when we look at the universe, we seeing it as it was in the past.

- Stephen Hawking

3500 BC Egyptian Obelisks & Sundials 2000-1500 BC

Mayan Calendar

400 BC-1600 ADAztec Calendar

1094 Sung Su’s Chinese Water Clock Perfected

1900-1600 BC Stonehenge

1583Galileo Discovers Pendulum constancy

1656Huygens pendulum clock

1727 - 1734Jantar MantarRaja Jai Singh II

1918Quartz Oscillator

Developed

1948-49Lyons Develops first atomic clock

1955Essen and Parry start keeping time with cesium atomic clock

1960HP 105B Quartz frequency standard

1964HP 5071 A Primary Reference Standard

1978First GPS Block ISatellite

1978First GPS Block ISatellite1991

HP 5071 A Primary Reference Standard

1997- FutureNAVSTAR Block 2R, Next generation of Satellites

1736Harrison H1 Chronometer tested at sea

NEVER REPEATS AND ALWAYS MOVES

FORWARD We think of time as moving

upward, progress We define the progress by

timelines We define the process of life by

clocks In linear time all units are the

same Linear time not driven by

subjective experience Success a function of never

missing a unit of time

Yesterday

Today

Tomorrow

Linear Time

Training We are taught to be on time, to worry about time

lost We are penalized when we miss a time or waste

time We teach time management We learn from the railroad and the assembly line Through agriculture, seasons, weather cycles History repeats itself Emphasis on birth, anniversaries, death and rebirth Through agriculture, seasons, weather cycles History repeats itself Emphasis on birth, anniversaries, death and rebirth

Always repeats One day is like every other

day Each year is like every

other year Life is about matching

behaviour to cycles Progress less important

than adaptation Competition to meet

demands of cycle Knowledge used to

understand the cycles

Measurement of time is against a device or phenomena has regular interval events (frequency) to take place e.g. oscillators or some astronomical events.

A frequency standard whose frequency corresponds to the spotted definition of the second, with its specified accuracy achieved without external calibration of the device.

Note: the second is defined as follows:“The duration of 9,192,631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom-133”

PRIMARY CLOCKPRIMARY CLOCKA time standard which operates without external calibration

PROPER TIME PROPER TIME The local time, as indicated by an ideal clock, in a relativistic sense

Secondary frequency standardA frequency standard which requires

external calibration Standard frequency and time signal service

A radio communication service for scientific, technical and other purposes, providing the transmission of specified frequencies, time signals or both of stated high precision, intended for general reception

Allan Variance or Allan DeviationThe standard method of characterizing the frequency stability of oscillators in the time domain, bothshort and long term.

Atomic Time Scale (TA)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. Earlier time scales were based on the rotational rate of the earth.

ClockA device for maintaining and displaying time.

Coordinated Universal Time (UTC)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. A UTC clock has the same rate as a Temps Atomic 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 leap seconds to ensure approximate agreement with UT1.

Frequency Drift 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

DUT1The approximate time difference between UT1 and UTC, expressed to the nearest 0.1 s. DUT1 = UT1 + or - UTC. 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 0.1 s.

Epoch

The beginning of an era (or event) or the reference date for a system of measurements.

Frequency

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

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

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

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

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

An oscillator such as a rubidium, cesium, or hydrogen maser whose output is used as a frequency.

Greenwich Mean Time (GMT)

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.

International Atomic Time (TAI)

An atomic time scale based on 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

Leap secondAn 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 slightly more often than once per year.

SecondThe 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.

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. 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.

SynchronizationThe 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).

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

Terrestrial Time (TT)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. 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 intervalThe duration between two instants read on the same time scale.

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

Time standardA continuously operated device used for the realization of a time scale in accordance with the definition of the second and with an appropriately chosen origin.

Universal Time (UT) FamilyUniversal 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. 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.

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 ecliptic 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.

Since Jan 1, 1998, the maintenance of International Atomic Time (TAI) and of Coordinated Universal Time, UTC ( with the exception of decisions and announcements concerning leap seconds of UTC) has been the responsibility of the international bureau of weights & measures (BIPM) under the authority of the International Committee Of Weights & Measures (CIPM). The dates of leap seconds of UTC are decided and announced by the International Earth Rotation & Reference System Service (IIERS) which is responsible for the determination of earth rotation parameters and the maintenance of the related celestial and terrestrial reference systems.

International Atomic Time (TAI) and Coordinated Universal Time (UTC) are obtained from a combination of data from some 300 atomic clocks kept by about 65 laboratories spread world wide. The data are regularly reported to BIPM by about 55 timing centres which maintain a local UTC. The data are in the form of time difference (UTC(IC) – clock) taken at 5 day intervals at oh. UTC.

An interactive algorithm process a Free Atomic Time Scale (EAL defined as a weighted average of clock readings. The processing is carried out & subsequently treats one month block of date. The weighting procedure and clock frequency predictions are chosen so that EAL is optimised for along term stability.

UTC Labs Network

340

550

830

1110

1320

The ground reference time for IRNSS satellite system, is provided by IRNSS Network Time (IRNWT) Facility. This has the following functions:

Providing the Navigation Timekeeping to support the navigation mission for Orbit Determination and Time Synchronization (OD&TS) services of the IRNSS constellation.

Generating the Metrological Timekeeping to steer IRNWT towards International Atomic Time (TAI) and also provide the IRNSS-UTC timing determination service to the user.

IRNWT shall also provide time offset between IRNWT and GPS time for the use of common user community

IRNWT shall be the reference time scale used in IRNSS system. This shall be continuous time scale generated from ground atomic clocks ensemble at IRNSS Timing Facility.

The IRNWT shall be steered to International Atomic Time (TAI) such that the difference between TAI‑IRNWT shall be within 20 ns (1 σ) at any time (target is 10 ns) measured over 30 days cycle.

The difference between IRNWT-UTC is maintained with an uncertainty of 20 ns (1 σ), to obtain UTC through IRNWT (with leap seconds corrections) by end user.

IRNWT shall provide the stability viz.,         a) The short term stability: 5.0 x 10-14 over a day      b) The long term stability : 1.5 x 10-13 over 30 days

IRNWT shall establish a clock model for calibrating all ground clocks of the system. Frequency offset and frequency drift of ground clocks is corrected using time keeping system using the ensemble algorithms. IRNWT frequency stability shall be optimized on short

term (=1 day) IRNWT time stability shall be optimized to medium/long

term (=30 days) IRNWT frequency offset (normalized to TAI): <5.5 x 10-

14

(1 day) Interface with NPL(Delhi), PTB (Germany), Bureau

International des Pods et Measures (BIPM) by exchange of all relevant clock data

Two way Satellite Time and Frequency Transfer and GNSS Common View techniques for the time transfer and clock synchronization for the remote clocks at IRIMS, IRCDR stations and to international time laboratories.

Cs 1

Cs 2

Cs 3

Cs N

Cesium

----

AHM 1

AHM 2

AHM 3

Active H-

Masers

Time Signal

Multiplexer

Time Interval Counter

Multi Channel Phase

Comparator

Switch

Matrix

Micro Phase

Stepper

Pulse and RF Distribution

Amplifier

Processing

Subsystem

1 PPS

1 PPS

10 MHz

10 MHz 10 MHz

RS 232

TWSTFT

1 PPS1 PPS

IRIG B Code Generator

GPS CV

UTC Labs

1 PPS 10 MHz

1 PPS

10 MHz

Processing System

• IRNWT computation• IRNWT steering execution• System clocks characterization• Spacecraft clock drift estimation

1 PPS 1 PPS

10 MHz

TWSTFT

UTC Labs

1 PPS 10 MHz

GPS / GLONASS/

Galileo Common

View Receiver.

AHM 1

AHM 2

IRNSS Timing Centre

AHM

H-Maser of ISRO

1 PPS

10 MHz

Cs 1 Cs 2

Cs 1 Cs 2

Cs 1 Cs 2

IRIMS Stations

10 MHz

1 PPSIRIMS-1

IRIMS-2

IRIMS-n

TW

STFT

or

OFC

Lin

ks

10 MHz

Cs 1

Cs 2

Cs 3

Cs 4

------------

10 MHz

1 PPS

10 MHz

1 PPS

Cs 1 Cs 2

Cs 1 Cs 2

Cs 1 Cs 2

CDMA Ranging

Stations

10 MHz

1 PPSCDMA-1

CDMA-2

CDMA-n

TW

STFT

or

OFC

Lin

ks

10 MHz

------------

10 MHz

1 PPS

10 MHz

1 PPS

Active Hydrogen Masers are used to provide stable signals for Frequency and time and measurement and the differences of time between events. This will be the Master Clock.

The Cesium Atomic Clocks are used in an ensemble to generate the virtual paper clock and to build stable time measurements by characterizing the clocks and identifying and removing the noise components.

The performance of all clocks measured at the regular intervals by using suitable algorithms and time transfer techniques with other time labs in India and abroad.

The measured offset between the Master clock and virtual paper clock will be applied to the Phase Micro Stepper(PMS)/Auxiliary Output Generator(AOG) system at regular intervals to get highly stable time (IRNWT time).

The system shall have two active hydrogen masers (AHM) on two independent chains in the ‘Maser Room’ (MR) in a Precision controlled environment. This will provide the short term stability.

The system shall also contain four cesium atomic clocks (two per chain) in the Maser room. Each clock shall be backed up with battery and two steerable cleanup oscillators with reference generators. This will provide the long term stability.

All the clock signals are fed to the Multi-channel Phase Comparator(MPC) and the output of the MPC is fed to ensemble algorithm. The Clock ensemble provides the combined advantage of AHM’s better short-term stability and Cesium clock’s better long-term stability

Two-Way Satellite Time and Frequency Transfer (TWSTFT) and GNSS Common View (GCV) equipment shall be used for the purpose of estimating the offset between IRNWT and TAI using time transfer method.

The measured offsets generated from the steering algorithm provides the paper clock. This shall be used for steering the selected Maser clock thro’ Auxiliary Output Generator(AOG) to provide the IRNWT time.

Data collected from all the clocks are stored and maintained at IRNWT facility in a data base using data storage and management system.

The characterization of clocks will be carried out using the data in archive and also data collected from the various other time labs in India and abroad using Two Way Time transfer and GPS common view techniques.

Suitable backup procedures will be followed using the secondary media like tapes, DVD/optical discs.. etc for maintenance of the history data base.

Clock data will be preserved for the period of three years for any analysis purpose.

Time also will be distributed using the IEEE-1588 protocol on LAN which distance limited and requires special interface at the receiving end.

1 PPS, 5 MHz, 10MHz clocks will also be available in the BNC connector for distribution in IRIG-B time code for the distribution thro’ NTP.

IRNWT time generated will be distributed thro’ two numbers of NTP servers for all the computer systems at Navigation Center(INC) on the TCP/IP LAN.

Measured offsets from the paper clock & time transfer method, shall be used for steering the Master clock to provide the IRNWT time.

GNSS Common View receivers will be used to characterize the atomic clocks in collaboration with NPL, Delhi in India and PTB,Germany and other time labs in the world through suitable working arrangement. This will give the minimum resolution of 3 to 7 ns measurement accuracy.

Two Way Time Transfer techniques will be used for improving the accuracy for 1 to 3 ns with PTB, Germany and NPL, UK to steer towards the International Atomic Time.

Traveling calibrated atomic clocks will also used for the synchronizing misbehaved (restored after the repair) atomic clocks at CDMA/IRIM stations.

GNSS receivers are also used for the time offset estimation of the IRNWT time with UTC.

Generation of IRNSS System Time Estimation of IRNSS Clock Corrections

in terms : Satellite Clock Bias afo

Satellite Clock Drift af1

Satellite Clock Drift rate af2

The clock corrections are estimated in the Navigation Control Centre and uplinked to the IRNSS Satellites. The user receives the clock correction parameters and computes the postiion accurately. These parameters are updated at regular intervals of time (once in half an hour)

•The fundamental measurement namely pseudo range in the GNSS receiver works by accurately timing how long it takes for the signals to travel from the satellites' antennas to the receiver's antenna and converting the time delays to ranges using the speed of light

•Clock Jumps: result in a jump in pseudorange. Consequently, large jumps will affect the navigation solution

•Frequency drift: affects the pseudo range more significantly, since the drift gets integrated with time and manifests in large error in the pseudo range (see the slope change after the clock jump)

Clock jumps

Clock performance of GIOVE A

For on-board clock, the effect of clock jump associated with the change in clock drift (similar to GIOVE-A) will result into a clock error value reaching as high as ~300 m over a day. Consider the values similar to GIOVE-A clock,

Frequency clock jump = 2.0E-12Frequency drift = 1.0E-11 / dayClock Error Build-up over two hours

= [2.0E-12 * 7200 + 1.0E-11 * 7200] * 3.0e^8 m

= 26 m (Approx)

It can be noted that the clock error build-up is primarily due to clock-drift and the contribution of clock jump is not significant. If the clock error build is observed, then it can be corrected immediately and communicated to the user.