Satellite Time Transfer

Why do satellite signals work better than ground signals for time and frequency transfer? Path delay is easy to estimate and calibrate for timing

applications. The variation in path delay is small due to a clear,

unobstructed path between the receiver and transmitter. The coverage area is usually much larger. Interference due to weather and ground based noise is

usually less of a problem.

Ground-based signals

skywave

groundwave

line-of-sight

HF Radio Propagation (skywave)

LF Radio LF (low frequency) is the part of the spectrum from

30 to 300 kHz, also known as longwave. Used to send time codes via simple modulation

schemes. The carrier frequency is also used as a frequency reference.

Groundwave signals are more stable, and the delays are easier to estimate than the HF skywave signals.

Two LF signals widely used for time and frequency: LORAN-C (100 kHz) NIST Radio Station WWVB (60 kHz)

LF Radio Propagation (groundwave)

Disadvantages of LF Limited coverage area. Subject to diurnal phase shifts at sunrise and

sunset over long paths, skywave can interfere with groundwave.

When receiver is unlocked, cycle slips equal to the period of the carrier (16.67 s for WWVB, 10 µs for LORAN) are introduced in the data.

User must calibrate path delay for time transfer, and even then is limited by the cycle ambiguity. Cycle ambiguity is a much larger problem with WWVB than LORAN.

Line-of-Sight Radio Propagation

Line-of-sight signals (VHF/UHF) VHF (very high frequency) is defined as the spectrum from 30 to

300 MHz. UHF (ultra high frequency) is defined as the spectrum from 300 to 3000 MHz.

Generally speaking, signals transmitted from the ground in the VHF and UHF spectrums tend to be line-of-sight. In other words, they don’t bounce off the ionosphere or follow the curvature of the Earth, but instead are used for local transmissions with limited coverage area where there is a clear path between the transmitter and the receiver.

Line-of-sight signals are stable, but the coverage area is usually small.

Several line-of-sight signals have potential applications in T&F, including FM radio signals, television signals, and cellular phone and pager signals.

Satellite Signals The best signals for time transfer. Since the signals originate

high above the Earth, there is an clear path between the transmitter and receiver.

Coverage area can be worldwide with global navigation systems like GPS.

Small path delay changes occur as signal passes through ionosphere and troposphere, but these are measured in nanoseconds.

Satellite signals used for time and frequency include: GPS GLONASS (Russian version of GPS) Galileo (European GPS, coming in future years, first

satellite launched on December 28,2005)

Satellite Radio Propagation

Summary Table

Type of Signal Spectrum Coverage of single system

Stability Reliability of Reception

Skywave MF, HF Good Poor Poor

Groundwave VLF, LF Good Good Good

Line-of-sight VHF, UHF Poor Excellent Excellent

Satellite UHF Excellent Excellent Excellent

The GPS Infrastructure and Signal Format

24 satellite constellation

Semi-synchronous, circular orbits (~20,200 km/10,900 nautical miles altitude)

Six orbital planes, inclined at 55 degrees, four vehicles per plane

Orbital period is 11 hours, 58 minutes Spares can bring number of satellites

up to 32 – new satellites are launched as necessary, lately 2 or 3 per year

Designed to cover entire earth, with at least four satellites always in view

Cesium and/or rubidium oscillators are on board each satellite

GPS Satellite Constellation

Developed by the US Department of Defense Earlier satellite timing systems existed

Transit GOES Timation (first atomic frequency standards flown in

space) USAF 621B Program (PRN codes for ranging)

First prototype GPS satellite launched in 1978 First Block II (Operational) GPS satellite

launched 1989 Full Operational Capability declared in late

1993

GPS History

Assume the maximum acceptable error contribution from GPS satellite clocks is 1 meter Light travels 3 x 108 m/s, thus a one meter

requirement is equivalent to 3.3 ns ranging uncertainty

Clock error must be maintained below this level over 12 hour period (time between satellite uploads)

This requires a clock with < 1 part in 1013 stability, which can only be met by an atomic standard (3.3 x 10-9 s / 43200 s = 0.8 x 10-13)

Precise Timing is Fundamental to GPS

Atomic Clocks in Space

GPS has become the primary system for distributing accurate time and frequency globally

GPS satellites carry rubidium or cesium oscillators (or both) Precise frequency standard provides a reference for

generating the ranging signals transmitted by the satellites

Clocks on the satellites are steered by U. S. DoD ground stations to UTC as maintained by the US Naval Observatory (USNO) UTC(USNO) is usually within a few nanoseconds of UTC so

GPS provides a real-time link to the UTC time scale

Einstein would be proud of GPS, because it is a real world application for his theory of relativity. The oscillators onboard the GPS satellites are given a fixed frequency offset of -4.4645 x 10-10 to compensate for relativistic effects in the GPS satellite orbits.

Second-order Doppler shift – a clock moving in an inertial frame runs slower than a clock at rest.

Gravitational frequency shift – a clock at rest in a lower gravitational potential runs slower than a clock at rest in a higher gravitational potential.

Without this frequency offset, GPS satellite clocks would gain about 38 microseconds per day relative to clocks on the ground.

GPS receivers apply an additional correction of up to 23 ns (6 meters) to account for any eccentricity in the satellites orbit.

Relativistic Effects in GPS

Two L-band carrier frequencies L1 = 1575.42 MHz L2 = 1227.60 MHz

Two PRN Codes P(Y): Military Code

267 day repeat interval Encrypted – code sequence not published Available on L1 and L2

C/A: Coarse Acquisition (Civilian) Code 1 millisecond repeat interval Available to all users, but only on L1

Code modulated with Navigation Message Data Provides ephemeris data and clock corrections for the GPS

satellites Low data rate (50 bps)

GPS Signal Structure

13 satellites are Block IIA 12 satellite are Block IIR 5 are Block IIR-M and transmit the

new L2C carrier Two were launched in 2007 (PRNs

15 and 29)

Currently (January 2008) there are 30 GPS satellites in orbit All slots filled except PRNs 7 and 32 7 running off cesium oscillators 23 running off rubidium oscillators Oldest satellite is PRN 24, launched

in July 1991

Block II/IIA Vehicles

GPS Satellites

Block IIR/IIR-MBuilt by Lockheed MartinLaunched 1997 - 2007

GPS Monitor Station Network

Tahiti

Alaska

Austin, TX

St. Louis, MO

USNO

South Africa

Bahrain

United Kingdom

Australia

New Zealand

Korea

Ecuador

Argentina

GPS Monitor Stations NGA Site (11) NGA Test Site (2) USAF Site (5)

Hawaii

ColoradoSprings

Ascension Diego Garcia

Kwajalein

Five stations added in 2005, five more planned Monitor the GPS satellites for operational health Track the GPS satellites for orbit determination Upload satellite almanacs, ephemeris messages, and clock corrections

SPACE VEHICLEBroadcasts the SIS PRN codes, L-band carriers, and

50 Hz navigation message stored in memory

SPACE-TO-USER INTERFACE CONTROL-SPACE INTERFACE

MONITOR STATION Sends raw observations to

MCS

MASTER CONTROL STATION Checks for anomalies Computes SIS portion of URE Generates new orbit and clock predictions Builds new upload and sends to GA

GROUND ANTENNA Sends new upload to

SV

Corrections are uploaded to the clocks in space

GPS Signals

GPS Modulation

The carriers are pure sinusoids. Two binary codes are modulated onto them: the C/A (coarse/acquisition) code and the P (precise) code.

Binary biphase modulation (also known as binary phase shift keying [BPSK]) is the technique used to modulate the codes onto the carrier. There is a 180 degree carrier phase shift each time the code state changes.

The modulation requires a much wider frequency band than the minimum bandwidth required to transmit the information being sent. This is known as spread spectrum modulation. It allows lower power levels to be used.

“Spreads" the power spectrum of the transmitted data over a wide frequency band

Same principle is used for household cordless telephone (voice is the data)

Each satellite is assigned unique Pseudo-Random Noise (PRN) Code Allows Multiple Access – All GPS satellites transmit at

the same frequency but are identified by their PRN codes

Spread Spectrum Communication

The signal transmitted by the satellites is the product of the navigation data, a spread spectrum code, and the RF carrier (either L1 or L2).

In order to detect the GPS signal and recover the navigation data, the receiver must produce a replica of the PRN code to mix with the incoming signal. Thus, the firmware inside a GPS receiver has to be able to generate all 32 PRN codes and to match codes received over the air to the generated codes.

The measured phase offset between the incoming and replica PRN code is the GPS range measurement.

Spread Spectrum Communication - II

GPS Signal in Space

L1 Signal

L2 SignalP-CODE

20.46 MHz

1227.6 MHz

-166

P[dBW]

P[dBW]

20.46 MHz

1575.42 MHz

2.046 MHz

C/A-CODEP-CODE-160

-163f [Hz]

f [Hz]

Frequency Spectrum

CA/Code The C/A code stands for Coarse Acquisition It is available to anyone, worldwide, as part of the

Standard Positioning Service (SPS) of GPS The C/A code is on the L1 carrier Timing specification is 40 ns, 95% of time, averaged for 1

day over entire constellation Used by most commercially available GPS receivers,

including SIM network receivers

frequency

frequency

1. Data Message Spectrum

2. Data*Code Spectrum Signal is “Spread”

3. Data*Code*Carrier Spectrum

This is the transmitted signal

0 Hz

0 Hz

0 Hz 1.57542 GHz

50 Hz

1.023 MHz

1.023 MHz

GPS L1 signal (C/A code) in Frequency Domain

GPS L1 C/A Signal (Time-Domain)

1.023 Mbps

Carrier 1.57542 GHz

50 bps

Repeating 1023 Chip “Spreading Code” (20 per data message bit)

GPS Data Message20ms +1

-1

+1

-1

+1 -1 +1

How GPS provides position and time

GPS-based positioning is fundamentally based on: The precise measurement of time The constancy of the speed of light

GPS-based positioning uses the concept of trilateration GPS satellite positions are known Receiver position is not GPS-to-receiver range measurements are

used to compute position

GPS Positioning

Transmitter(location known)

Receiver(location

unknown)

Locus of points on which the receiver can be located

Measured Range

Positioning Example with 1 Transmitter

True Receiver Location

T1

T2

False Receiver Location

Positioning Example with 2 Transmitters

True Receiver Location

T1

T2

T3

Positioning Example with 3 Transmitters

The position solution involves solving for four unknowns:• Receiver position (x, y, z)• Receiver clock correction

Remember: Position accuracy of ~10 m implies knowledge of the receiver clock to within ~30 ns

Requires simultaneous measurements from four GPS satellites• The receiver makes a range measurement to the

GPS satellite by measuring the signal propagation delay

• The data message modulated on the GPS signals provides the precise location of the GPS satellite and corrections for the GPS satellite clock errors

GPS Positioning - II

Pseudo-Random Noise (PRN) Codes

Each GPS satellite transmits its own unique Pseudo-Random Noise (PRN) Code on L1 and L2

The C/A Code repeats every millisecond

The receiver generates replicas of the C/A code and uses code correlation to distinguish between different satellites

Pseudorange

t

GPS transmitted C(A)-code

Receiver replicated C(A)-code

Finding t for each GPS signal tracked is called “code correlation”

t is proportional to the GPS-to-receiver range Four pseudorange measurements can be used

to solve for receiver position

Ranging

--- [m]

SatellitePRN sequence

pseudo-range

pr

( x, y, z, t )

( xs, ys, zs, ts )

ReceiverPRN sequence

--- [s]

Receiver

222 )()()( zzyyxxR sss

)()()()(1 222 ttzzyyxxc

pr ssss

Although the primary purpose of GPS is to serve as a positioning and radionavigation system, the entire system relies on precise timing. After the receiver position (x, y, z) is solved for, the solution is stored.

Then, given the travel time of the signals (observed) and the exact time when the signal left the satellite (given), time from the clock on the satellite can be transferred to the receiver clock.

The measurement made by the GPS receiver reveals the difference between the satellite clock and the receiver clock by measuring the transit time of the signal:

Trk ttc

time of transmission,encoded in signal byGPS satellite clock (known precisely)

time of signal reception, (based on receiver clock,

can be significantly in error)

This measurement, when multiplied by the speed of light, produces not the true geometric range but rather the pseudorange, with deviations introduced by the lack of time synchronization between the satellite clock and the receiver clock, by delays introduced by the ionosphere and troposphere, and by multipath and receiver noise. The equation for the pseudorange is

p = ρ + c × (dt − dT ) + dion + dtrop + rn

where p is the pseudorange

c is the speed of light

ρ is the geometric range to the satellite

dt and dT are the time offsets of the satellite and receiver clocks with respect to GPS time

dion is the delay through the ionosphere (an estimate can be obtained from the GPS broadcast)

dtrop is the delay through the troposphere

rn represents the effects of receiver and antenna noise, including multipath.

Finding Position & Time

Two main factors determine accuracy of the position and time solution

UERE (User Equivalent Range Error) DOP (Dilution of Precision)

User Equivalent Range Error (UERE) The accuracy of the pseudo-range measurements

between a particular satellite and a particular user

UERE is the result of several factors the quality of broadcast signal in space, which

varies from satellite to satellite and time to time stability of particular satellite’s clock predictability of the satellite’s orbit

Year

URE Performance History

second accuracy limiting factor

depends on the geometry of satellites, as seen by the receiver

critical for determining accurate position and time

used in cooperation with the UERE to forecast navigation and time errors

Good (Low) DOPConditions:

Poor (High) DOPConditions:

Dilution of Precision (DOP)

The Future of GPS:New signals are being added to

the broadcasts

Enables higher civilian accuracy when combined with existing civil GPS signal (L1 C/A)

Overcomes some limitations of L1 C/A Allows receiver to measure and correct for ionospheric

delay Higher power reduces interference, speeds up signal

acquisition, enable miniaturization of receivers, may enable indoor use

Now broadcast by satellites launched since September 2005, available to entire constellation by about 2014

L2C codeL2C, a new civil GPS signal

Third Civil Signal (L5)

New signal structure for better accuracy Higher power than other GPS civil signals

Higher power (no less than -154.9 dBW) Wider bandwidth (1176.45 MHz +/- 10 MHz) Improves resistance to interference

Co-primary allocation with Aeronautical Radionavigation Services at WRC-2000 (1164-1215MHz)

Available to entire constellation by about 2016

L5 codeBegins with IIF satsFirst launch: 2008

Modernized L1 civil signal In addition to C/A code to ensure backward compatibility Increased robustness and accuracy for civil users Designed with international partners so that it can work with

other satellite navigation systems – will use same type of coding as Galileo

Begins with GPS Block III First launch: ~2013; 24 satellites: ~2021

L1C

Begins with GPS III satsFirst launch: ~ 2012

L1C

GPS Disciplined Oscillators

GPS Disciplined Oscillators (GPSDO)

Operated as standalone time and frequency standards, often as the primary standard for the lab. GPSDOs discipline a local oscillator (quartz or rubidium) to GPS and serve as a self-calibrating standard that will perform at a high level if the receiver hardware and the GPS constellation are functioning normally.

Short term stability limited by the local oscillator, long term accuracy and stability provided by the GPS signal

Produce on-time pulse (1 pps). Produce standard frequency outputs (such as 1, 5, and 10 MHz). Some units produce frequencies used in telecommunications

(1.544 MHz or 2.048 MHz, for example).

GPSDO (steered, no synthesizer) In this design, the local oscillator phase is continuously compared to GPS. A

PLL is then used to “pull” the local oscillator into phase with GPS. The PLL is usually (but not always) software based. Performance depends on the response time of the PLL and the quality of the local oscillator.

GPSDO (unsteered, synthesizer) In this design, the local oscillator is used as the time base for a frequency synthesizer (called a

translator in the diagram). The phase of the synthesizer is continuously compared to GPS and the frequency offset is measured. A correction is sent to the synthesizer to compensate for the frequency offset and eliminate the phase difference, but no corrections are applied to the local oscillator.

GPS Antennas Small and inexpensive, higher

gain units (> 35 dB typical) generally used for timing to drive long cables.

These antennas are normally active, with internal amplifiers powered by 5 V dc from the antenna cable.

Most bring the 1575 MHz L1 carrier straight to the receiver, without any down conversion.

Omnidirectional, need to have clear sky view on all sides for best results.

GPSDOs are easy to install and use

The antenna needs to be mounted on a roof, with a clear view of the sky on all sides. Make sure that the antenna has enough gain to drive the antenna cable. A combination of high-gain antenna, low-loss cable, and inline-amplifiers might be needed for some long cable runs.

Survey position. You can enter the position, but most receivers can do a self survey of their antenna position. This might take up to 24 hours. The GPSDO always uses the same coordinates after the survey, because the antenna is not moving.

Getting Time from a GPSDO

What is GPS Time? Controlled by the United States Naval Observatory (USNO), but not

exactly the same thing as UTC(USNO). GPS time differs from UTC by the number of leap seconds that have

occurred since the origination of the GPS time scale (January 6, 1980); this value is equal to 14 s as of January 2008, it will increase each time there is a leap second. The navigation message contains a leap second correction, however, and GPS receivers automatically correct the time-of-day solution.

It also differs from UTC(USNO) by a small number of nanoseconds that continuously changes. The current difference between the UTC(USNO) estimate and GPS time is also in the navigation message, and this correction is applied to the 1 pps signal.

After the leap second and UTC(USNO) corrections are applied, GPS time as broadcast is nearly always within 30 nanoseconds of UTC, UTC(NIST), and UTC(USNO). This makes it the best estimate of UTC being broadcast anywhere, and it is available free of change to anyone, worldwide.

The frequency offset between the UTC(USNO) estimate and UTC(NIST) is very small, typically a few parts in 1015 or less when measured over a one month interval.

Comparison of UTC(NIST), UTC(USNO) and GPS Time Scales

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-5

0

5

10

53610 53620 53630 53640 53650 53660 53670 53680 53690 53700 53710 53720 53730 53740

September - December 2005 (MJD 53614 to 53735)

Nan

ose

con

ds

UTC(USNO) - GPSUTC(NIST) - GPSUTC(USNO) - UTC(NIST)

No more SA, a Great Time Source Gets Even Better

GPS performance improved dramatically after May 2, 2000, when the Selective Availability (SA) program was deactivated, removing the intentionally introduced jitter from the signal

SA had been previously implemented by the United States government for reasons of national defense.

GPS before and after SA Deactivation

-70

-50

-30

-10

10

30

50

70

51663 51664 51665 51666 51667 51668

MJD

na

no

se

co

nd

s10 minute averages of all satellites in view

Factors that limit the uncertainty of GPS time received by users

Errors in the antenna coordinates, particularly errors in elevation

Poor estimates of cable, antenna, and receiver delays Multipath Ionospheric and Tropospheric corrections Changing delays in hardware and cables due to

temperature and other environmental factors All of these factors make it difficult to validate time received

from GPS to within better than about 50 ns. The typical uncertainty limit is about 100 ns, and nearly all receivers will provide time within 1 microsecond with any attention to the details.

Uncertainty due to Antenna Coordinates GPS computes dimensions in Earth-Centered, Earth-Fixed

X, Y, and Z coordinates. Position in XYZ is converted in the receiver to geodetic

latitude, longitude, and elevation. Errors in the X,Y,Z coordinates translate to timing errors as

large as 3 nanoseconds per meter, depending upon the satellite’s position in the sky.

GPS excels at finding horizontal position (latitude/longitude) Most receivers can quickly survey latitude/longitude to

within 10 meters, and to < 1 meter after several hours of averaging.

GPS is weak at determining vertical position (elevation) GPS provides distance from the center of the earth and

then by using the radius of a model of the Earth’s surface, provides elevation. There is nearly always a bias in the elevation.

The vertical position error is usually several times larger than the horizontal position error.

A 10 meter altitude error (timing error of up to 30 nanoseconds) is not uncommon, even if the receiver surveys the antenna coordinates by averaging position fixes for several hours or more.

Position error in multiple 24-hour surveys of known GPS antenna coordinates

3.5

4

4.5

5

5.5

6

6.5

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Antenna surveys (one per day)

Po

siti

on

Err

or

(met

ers)

Total Position Error

Vertical Position Error

Average position error of TMAS survey was 5.37 m, with nearly all of this error (5.30 m) in the vertical position

Uncertainty due to Antenna/Cable Delays Most GPSDOs allow you to enter a delay constant

for the antenna cable. The antenna cable delay be easily measured with a time interval counter, or estimated fairly accurately using data from the cable vendor. However:

The antenna delay is very difficult to measure unless the entire system (receiver, antenna, and cable) is calibrated by NIST or another timing laboratory as a unit.

Some GPS receivers advance the on-time pulse to at least partially compensate for the cable delay. Thus, the delay entered into the receiver should actually be less than the cable delay.

Other problems arise to impedance mismatches, connectors, etc.

For all of the above reasons, it is unlikely that the cable delay can be estimated to better than 10 nanoseconds, unless the manufacturer provides guidance or instructions.

Uncertainty due to Multipath Multipath is caused by GPS

signals being reflected from surfaces near the antenna. These signals can then either interfere with, or be mistaken for, the signals that follow the straight line path from the satellite.

If the antenna has a clear, unobstructed view of the sky, the uncertainty due to multipath is usually very small (a few nanoseconds or less), but some uncertainty due to multipath is nearly impossible to avoid and detect.

Uncertainty due to Environment Although not really of concern for most

applications, the receiver, antenna, and cable delays can change over the course of time, sometimes by as much as several nanoseconds. This is usually due to temperature.

Receivers probably have the most sensitivity to temperature, but they are normally located in a laboratory with a relatively small temperature range, so this is usually not a problem.

The antenna and cable are outdoors, and the temperature range can be very large over the course of a year. Sometimes temperature compensated antennas are used (see photo), but that is usually not practical. If possible, however, use a high quality antenna cable with a low temperature coefficient.

Uncertainty due to ionospheric conditions The ionosphere is the part of the atmosphere

extending from about 70 to 500 km above the earth. The troposphere is the lower layers of atmosphere, where clouds form.

When radio signals from the satellites pass through the ionosphere and troposphere, their path is bent slightly, changing the delay. The delay changes are largest for the satellites at low elevation angles.

GPS broadcasts a ionospheric and a tropospheric correction, which most receivers automatically apply (or which can be turned on by the user). This reduces the effect.

These corrections are called modeled ionospheric corrections, or MDIO

For the very best results, the ionospheric conditions are measured at a receiving location on the ground by a dual-frequency GPS receiver (one that receives both L1 and L2). These measurements are used to generate local corrections that are used in place of the broadcast corrections. In some cases, this can reduce the uncertainty by a few nanoseconds.

These corrections are called measured ionosphere corrections, or MSIO.

Interference

GPS Signals are easy to interfere with, or “jam” due to their low power levels

Under certain condition, the amplifier in a GPS antenna will oscillate in the GPS frequency bands, resulting in interference to adjacent GPS equipment

The owner may not notice the antenna is in oscillation Locating the interference is difficult due to the

intermittent nature of the oscillation

Interference from a GPS Antenna

Antenna oscillated around 1.579GHz (about -90dBm) in low temperature, affecting all

the GPS antennas in view

GPSDO Time Uncertainty The time uncertainty relative to UTC usually less than 1 s for nearly

any receiver. An uncertainty of less than 100 ns can usually be achieved if the

receiver and antenna delays are calibrated. It is difficult to prove an uncertainty of < 50 ns even if the GPSDO

has been calibrated. Time stability is very good, often just a few nanoseconds at 1 day, as

measured using TDEV.

Getting Frequency from a GPSDO

Getting Frequency from GPSDOs Antenna survey doesn’t matter as much for frequency.

Altitude error has no real effect on frequency. Lat/Lon errors will cause bigger short-term variations,

but average frequency over long intervals won’t change substantially.

Quality of local oscillator matters, particularly when signal is lost and receiver goes into “holdover” mode. The lowest priced units use TCXOs or even simpler

crystal oscillators Better units use OCXOs The best units for holdover use rubidiums

Some GPSDOs are optimized for short-term stability, others for long-term stability. Others are good for timing, but not so good for frequency.

OCXO with and without GPS

GPSDO “optimized” for short-term stability

Test of GPSDOs vs. UTC(NIST)

NIST tested four commercial GPSDOs in the spring and summer of 2005 against the UTC(NIST) time scale.

Two had rubidium time bases, two had OCXO time bases.

We measured the 1 pps and 10 MHz outputs at the same time with two different measurement systems.

Results showed very different behavior amongst different GPSDOs.

Test of GPSDOs vs. UTC(NIST)

10 MHz Phase of GPSDOs vs. UTC(NIST)

10 MHz Stability of GPSDOs vs. UTC(NIST)

Common-View GPS

Common-View Time Transfer

Common View GPS Two users, A and B, compare their clock to the same satellite at the same time.

Two data sets are recorded (one at each site):

Clock A - GPS Clock B - GPS

Data is exchanged. Subtract A from B to get difference between clocks.

Common errors cancel.

Used for international computation of UTC and by SIM Network

All-in-view GPS

A B

Receivers at remote stationary locations track all the satellites in view

Each receiver makes the all-in-view measurements, (REFstation_i – GPS): time difference between a local reference clock and the received composite timing signal from all the satellites being tracked

The all-in-view measurements from two receivers are differenced to obtain the time and frequency difference of two remote clocks

Works when no satellites are in common-view

Performance is about the same as common-view for short Performance is about the same as common-view for short baselines (2500 km or less), better than common-view for baselines (2500 km or less), better than common-view for long baselines (5000 km or longer)long baselines (5000 km or longer)

Keys to Successful Common-View Measurements

Same type of receiver (manufacturer and model) should be used at each site

All antenna, cable, and receiver delays must be calibrated and used as delay constants

The antenna at each site should be surveyed with the least amount of uncertainty possible, ideally less than 1 meter for both 2D and 3D position. A good survey of altitude is especially important.

The ionospheric, tropospheric, and multipath delays should be nearly equivalent at each site. For the absolute best results, corrections for these delays can be generated at each site.

The relative delays at both sites should be as close to zero as possible.

Common-View Common-Clock Comparisons, 10-minute averagesType 1 to Type 1 Rx (top), Type 2 to Type 2 Rx (middle), Type 1 to Type 2 Rx (bottom)

52275 52280 52285 52290 52295 52300

Modified Julian Date (MJD)

Te

ns

of

Na

no

se

co

nd

sCommon-View works best if the same model of

GPS receiver is used at both sites

Impact of SA on GPS Time and Frequency Transfer

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40

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80

100

120

140

160

180

200

220

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340

51662 51663 51664 51665 51666 51667 51668 51669 51670 51671 51672

MJD, days

Tim

e di

ffere

nce

(ns)

REFa - GPS (difference is offset for demonstration purpose)

REFb - GPS (difference is offset for demonstration purpose)

Common-view difference (REFa - REFb)

Sample Common-View Track

TT

Sample Common-View Track (A-B)

SIM Receiver

Calibrations

SIM systems are calibrated at NIST prior to shipment. Calibrations are performed using the common-view, common-clock method. The SIM laboratory installs the same antenna cable and antenna that were used during the calibration.

Calibrations last for 10 days. The time deviation (Type A uncertainty) of the calibration is less than 0.2 ns after one day of averaging. The combined uncertainty is estimated at 4 ns, because a variety of factors can introduce a systematic offset.

Type BUncertainty Component

Explanation Estimated Uncertainty

Calibration of SIM unit at NIST

Absolute accuracy of delay calibration is limited to about 4 ns.

4 ns

Environmental variations

Receiver delays can change due to temperature or voltage fluctuations from antenna cables or power supplies.

3 ns

Antenna Coordinates Error

Assumes that antenna position (x, y, z) is known to within 1 m.

3 ns

Propagation delay changes due to multipath

Multipath is caused by GPS signals being reflected from surfaces near the antenna.

2 ns

Propagation delay changes due to ionospheric conditions

The SIM system uses the ionospheric corrections broadcast by the satellites, and does not apply measured ionospheric delay corrections. This uncertainty represents the typical difference between the modeled and measured correction.

2 ns

Cable delay measurements made by the SIM laboratory.

Usually done with a time interval counter and is subject to small errors.

0.5 ns

Resolution Uncertainty Software limits the resolution of entered delay values to 0.1 ns.

0.05 ns

U bU akc

U 22

SIM Network Uncertainty Analysis

Uncertainties are expressed using a method complaint with the ISO GUM standard.

We use the time deviation (TDEV) at an averaging time of 1 day as our Type A uncertainty (1.5 ns in this example).

Type B uncertainties are summarized in the table.

Combined standard uncertainty (k = 2) is < 15 nanoseconds for time, and < 1 10-13 for frequency after 1 day of averaging.

ns 3.132525.4225.22 c

U

UTC(CNM) - UTC(NIST)

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54

24

9

54

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54

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9

54

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4

54

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9

54

27

4

54

27

9

54

28

4

54

28

9

54

29

4

54

29

9

54

30

4

54

30

9

54

31

4

Modified Julian Dates (June/July 2007)

Nan

ose

con

ds

SIM Network Data obtained in real-time

Post processed Circular-T data

CGGTTS Format for submission to BIPM

BIPM Circular T (www.bipm.org) Published monthly, it contains the official results of international time comparisons.

Five labs in the SIM network have their standards listed on the Circular-T. The Circular-T numbers are post processed and obtained with completely independent receiving equipment.

The real-time numbers obtained through the SIM network are in good agreement with the Circular-T numbers, well within our stated uncertainties. This helps validate our results.

GPS All-in-view

Primary method for most national timing centers in the world to contribute clock data to the computation of International Atomic Time (TAI) and Coordinated Universal Time (UTC)

PTB in Germany is the pivot laboratory

Coordinated by the BIPM (Bureau International des Poids et Mesures located near Paris, France)

About 60 laboratories contribute by submitting data in the CGGTTS format*

* Consultative GPS and GLONASS Time Transfer Sub-committee

Multi-channel Common-view Track Schedule Starting at 0:00 (UTC) on the reference date (October 1, 1997), the

24 hours of a day are divided into 90 16-minute intervals.

The first 89 intervals are used for common-view. Start time of each 16-minute interval is shifted 4 minutes earlier everyday. The 90th

interval is reserved for handling the 4-minute start time update.

The 13-minute common-view measurement starts 2 minutes after the beginning of the 16-minute interval.

The multi-channel common-view track schedule contains the single channel common-view track schedule as a subset.

2

lock up data processingmeasurement

t1 3 4 89

90

1 2

0:00 0:16 0:32 0:48 1:04 23:28 23:44 23:56 0:12 0:28

Day 1 Day 2

13 12

The CGGTTS Common-view Data FormatGPS RCVR: NBS10V9809MJD= 51658 YR=00 MONTH=04 DAY=24 HMS=14:47:20 (UT)GGTTS GPS DATA FORMAT VERSION = 01REV DATE = 2000-04-03RCVR = NBS10....................CH = 01IMS = 99999LAB = NISTX = -1288398.27 mY = -4721698.10 mZ = +4078625.68 mFRAME = ITRF....COMMENTS = NO COMMENTS..............INT DLY = 53.0 nsCAB DLY = 0199.9 nsREF DLY = 0066.7 nsREF = UTCNISTCKSUM = 74

 PRN CL MJD STTIME TRKL ELV AZTH REFSV SRSV REFGPS SRGPS DSG IOE MDTR SMDT MDIO SMDI CK hhmmss s .1dg .1dg .1ns .1ps/s .1ns .1ps/s .1ns .1ns.1ps/s.1ns.1ps/s 3 08 51655 105800 780 380 760 -1058301 -1131 -571 -1098 415 163 107 +2 76 +0 02 8 32 51655 111400 780 319 2933 -7071115 -3061 -246 -3082 290 074 125 -20 85 -9 34 13 28 51655 113000 780 415 3083 +6965884 -30 -94 -241 625 019 100 -12 71 -7 FB 3 74 51655 114600 780 296 530 -1058331 +929 -503 +962 470 163 133 +19 92 +24 17 31 08 51655 121800 780 498 706 -7572 -400 -197 -390 470 180 87 +4 99 +14 DD 13 32 51655 123400 780 569 2693 +6966345 +171 -440 -40 424 011 79 +0 90 +9 F0 18 68 51655 125000 780 279 1829 -341335 +18 -132 +22 698 182 141 +35 152 +44 16 31 74 51655 132200 780 283 472 -7436 +2669 -73 +2678 441 206 139 +29 190 +36 24

BIPM-Compatible Common-view Receivers

A single channel GPS receiver records a maximum of 48 common-view tracks each day according to a schedule. The receiver starts tracking the satellite 2 minutes before the beginning of each 13-minute measurement. Receiver models include:

AOA TTR-6 (might be discontinued)

A multi-channel receiver continuously tracks all the satellites in view. For each satellite tracked, the receiver groups the measurements into the 13-minute interval according to the multi-channel schedule. Receiver models include:

AOS TTS-2 AOS TTS-3 (dual frequency) NPL TimeTrace Novatel (dual frequency) PolaRx2eTR (dual frequency) SIM system with conversion software (possibly)

GPS Performance Comparison

Technique Timing Uncertainty, 24 h, 2

Frequency Uncertainty, 24 h, 2

One-Way

< 100 ns (with good coordinates and calibration of GPSDO receiver/cable delays, but claims below 50 ns are hard to prove.)

< 1 x 10-12

(Nearly all GPSDOS will can do this, some reach 1 x 10-13 )

Common-View

15 ns (with SIM Network)

< 1 x 10-13

(with SIM Network)

Other Satellite Navigation Systems

India and Australia among other nations developing GNSS augmentations

GLONASS (Russia) Galileo (European Union) QZSS (Japan) COMPASS (China)

GLONASS: GLObal NAvigation Satellite System

Operated by the Coordinational Scientific Information Center of the Russian Federation Ministry of Defense

First satellite launched in 1982, constellation still not fully populated today

Russian Government has renewed commitment to replenish and modernize the GLONASS constellation

Completed system would have 24 satellites in 3 orbital planes ascending nodes 120 degrees apart 8 satellites equally spaced in each plane

Two frequency bands L1 = 1602 + n*0.5625 MHz L2 = 1246 + n*0.4375 MHz Where n is frequency channel number (n=0,1,2,…)

Circular 19,100 km orbit inclination angle of 64.8 degrees

Cesium clocks on board satellites

GLONASS Constellation

Key Differences between GLONASS and GPS

Signal Structure: GPS: CDMA (different satellites transmit different PRN

codes on the same carrier frequencies) GLONASS: FDMA (different satellites transmit the same

PRN codes on different carrier frequencies) Period of Satellite Orbit:

GPS – 11:58 (same satellite can be observed at the same position, with same velocity every sidereal day -- 23:56)

GLONASS – 11:15 (satellite of the next slot of the same plane can be observed at about the same position every sidereal day)

PRN Code for Civilian User: GPS (L1) C/A code, chipping rate: 1.023MHz (L1) C/A code, chipping rate: 511KHz

GLONASS Status: January 2008 13 operational satellites

Plane I : 4 satellites operating Plane II: 4 satellites operating Plane III: 5 satellites operating

Four satellites were launched in 2007 Russian Space Agency Information Analytical Centre:

http://www.glonass-ianc.rsa.ru

Performance of GLONASS Time

GLONASS time, UTC(SU) is not nearly as accurate as GPS.

GLONASS time is canceled in the common-view method and GLONASS CV is an accepted BIPM time transfer method.

Difference between UTC and GPS/GLONASS Time( Source of data: BIPM Circular T )

-200

-150

-100

-50

0

50

100

150

200

250

53370 53420 53470 53520 53570 53620 53670 53720 53770 53820

MJD (days)

Dif

fere

nce

(n

s)

UTC - GPS (w/ leap-second correction)

UTC - GLONASS (w/ 3-hr offset correction)

European Global Navigation Satellite System Galileo is a joint initiative of the European Commission (EC) and

the European Space Agency (ESA). Completed system will have 30 satellites Only 3 orbital planes compared to 6 for GPS Will offer a basic service for free (open service), but will charge

user fees for premium services. T First signal in space transmitted by GIOVE-A satellite which

became operational in January 2006 Should be operational around 2013-2014

GALILEO

Constellation Configuration

altitude ~23616 kmSMA 29993.707 km

inclination 56 degrees

• period 14 hours 4 min• ground track repeat about 10 days

GGALILEOALILEO DATADATA

27 + 3 satellites in three Medium Earth Orbits (MEO)

Walker 27/3/1Constellation

Galileo System Design Galileo System Time (GST):

Shall be a continuous coordinate time scale steered towards the International Atomic Time (TAI) with an offset of less then 33 ns.

Offset between GST and the GPS system time is monitored and broadcast to users, but may also be estimated in the receiver.

Each Spacecraft will have 4 onboard clocks 2 Rubidium Vapour 2 Passive Hydrogen Maser

Galileo Frequency Structure

GALILEO Bands (Navigation)

GALILEO SAR Downlink

GPS Bands (Current & modernized)

E5a/L5 E5b E6 L1E2 E1

1164

MH

z

1214

MH

z

1260

MH

z

1300

MH

z

(*) 1

176.

45 M

Hz

1278

.75

MH

z

1544

MH

z15

45 M

Hz

1559

MH

z

1587

MH

z

1591

MH

z

1563

MH

z

1575

.42

MH

z

1215

MH

z

1237

MH

z

(*) 1

207.

140

MH

z

SARL2

RNSS Bands RNSS Bands

ARNS Bands ARNS Bands

GALILEO Bands (Navigation)

GALILEO SAR Downlink

GPS Bands (Current & modernized)

E5a/L5 E5b E6 L1E2 E1

1164

MH

z

1214

MH

z

1260

MH

z

1300

MH

z

(*) 1

176.

45 M

Hz

1278

.75

MH

z

1544

MH

z15

45 M

Hz

1559

MH

z

1587

MH

z

1591

MH

z

1563

MH

z

1575

.42

MH

z

1215

MH

z

1237

MH

z

(*) 1

207.

140

MH

z

SARL2

RNSS Bands RNSS Bands

ARNS Bands ARNS Bands