The document discusses various methods of GPS time transfer using fiber optics, including one-way, common view, and carrier phase techniques. It highlights the technical parameters, benefits, and performance of these methods, such as improved time and frequency accuracy. Additionally, it notes future developments in GPS technology aimed at enhancing stability and accuracy through advanced clock systems.

1. Telecomunicacions per Fibra Òptica 1
GPS Time TransferGPS Time Transfer
Yatish Bathla(Yatishbathla@gmail.com)
Bruno
Curs 2011-12

2. Telecomunicacions per Fibra Òptica
TypesTypes
A. One Way : Uses the signals
obtained from a GPS
receiver as the reference for
a calibration
B. Common View : Compare
two clocks or oscillators
located in different places.
C. Carrier Phase : This
technique uses both the L1
and L2 carrier frequencies
instead of the codes
transmitted by the satellites
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3. Telecomunicacions per Fibra Òptica
Type A: One WayType A: One Way
• This mode is also called
“Direct Access”.
• User Obtain satellite time by
locking onto given
broadcasting GPS Composite
clock
• Obtain GPS time by correcting
satellite clock offsets in sub-
frame 1 of navigation message
• Finally obtain UTC(GPS) by
applying GPS-UTC(USNO)
correction in sub-frame 4 of
the Navigation message.
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4. Telecomunicacions per Fibra Òptica
Type A: One WayType A: One Way
1) USNO currently uses
receivers (L1 and L2
frequency), capable to track
P(Y) Code and not C/A code
2) In future MCS will able to
apply corrections for known
observables in MCS files,
which are the ERDs
3) MCS send these refined
tracks into a two state kalman
filter
4) No need of additional
Processor
5) More Error and time delay
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5. Telecomunicacions per Fibra Òptica
Type A: One Way ExampleType A: One Way Example
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6. Telecomunicacions per Fibra Òptica
Common View GPS Time TransferCommon View GPS Time Transfer
• two different observers, separated
by large distances, observe the
same GPS satellite at the same
instant of time.
• data must be exchanged and
shared between the users.
• The final measurement result is
(Clock A - Clock B) - ( SA - SB).Ԏ Ԏ
• two types of GPS common-view
measurements:
Single - channel and multi-channel
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7. Telecomunicacions per Fibra Òptica
Common View GPS Time TransferCommon View GPS Time Transfer
• Single Channel Common View requires a GPS receiver that can
read a tracking schedule.
• Multiple Channel Common View does not use a schedule. The
receiver simply records data from all satellites in view.
• Lesser time and frequency uncertainty than one way mode.
• Satellite clock errors are eliminated and Ephemeris errors are
minimized.
• No discontinuities can be seen in the data
• Measurement results are not known until after the data is
exchanged and processed
• Whole processing depends upon geometry between two users
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8. Telecomunicacions per Fibra Òptica
Common View GPS Time TransferCommon View GPS Time Transfer
• T e c h n i c a l p a r a m e t e r s
• TTS-2 fully conforms to CCTF recommendations for multi-channel GPS
time transfer
• technology common-view method.
• Tracks simultaneously up to eight GPS satellites.
• Single-frequency (L1, 1575.42 MHz).
• C/A code (1.023 MHz chip rate).
• Code carrier aided tracking.
• Time interval resolution of the counter: 500 ps.
• External reference frequency 10 MHz, female BNC connector.
• Local 1 PPS input connector: female BNC.
• Antenna type: active GPS antenna (with temperature stabilisation option).
• Antenna cable length: as ordered, up to 50 m.
• Time transfer data format according to the Technical Directives for
Standardisation of
• GPS Time Receiver Software (specified in Rapport BIPM-93/6, October
1993).
• Power requirements AC 230 V ± 10%, 45 to 60 Hz or AC 110 V ± 10%, 55
to 65 Hz.
• The receiver before shipment to a user is differentially calibrated with the
uncertainty
• of a few nanoseconds with respect to the AOS time receiver calibrated by
the BIPM
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9. Telecomunicacions per Fibra Òptica
Carrier Phase GPS Time TransferCarrier Phase GPS Time Transfer
• it is also possible to use the
carrier itself for both position
determination and for time
transfer
• frequency of the carrier is about
1000 times higher than the
frequency of the C/A code,
carrier-phase methods have
much greater resolution
• The collected data are analyzed
and processed using precise
satellite orbit information and
detailed models of the
ionosphere and troposphere
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10. Telecomunicacions per Fibra Òptica
Carrier Phase GPS Time TransferCarrier Phase GPS Time Transfer
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Performance
● 40 Channels Continuous/Simultaneous
● Data type: L1/L2 Code and Carrier,
GPS,
GLONASS, EGNOS, WAAS
● Data output format: CGGTTS and
RINEX
● Time interval resolution of the internal
counter 100 ps
● 0.5 V trigger level
● Time transfer precision about 1 ns;
Time transfer accuracy about 2 ns
Interfaces
● 100BASE-T Ethernet port
● USB port
● 210GB free for data storage (standard
version)
● The system is working under LINUX
providing multitasking and better
Integration with networks
● External reference frequency input:
10 MHz
● Local 1 PPS input connector: female
BNC
● Antenna cable length: as ordered, up to
50 m; TNC connector
● Power requirements AC 220 V ― 10%,
45 to 60 Hz or AC110 V }10%, 55 to
65 Hz.

11. Telecomunicacions per Fibra Òptica
GPS Feature(Selective Availability)GPS Feature(Selective Availability)
• To limit GPS position and velocity
accuracy available to non military
users, the Department of Defense
established a policy of Selective
Availability (SA).
• time varying biases into the GPS
signals to intentionally degrade
performance
• resulting time error can be as much
as 340 nanoseconds (95 percent).
• During both the Gulf War and the
incursion into Haiti, the DoD
directed that SA be turned off,
increasing the accuracy.
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Future DevelopmentFuture Development
• Improvement to the GPS must take place along with
improvements of PTTI field
• New generation of clocks will be developed soon for more stability
and accuracy
• There are also new techniques for time transfer over large
distances
• small ensemble of cesium clocks, clock averaging software and
independent means for comparing the clocks with those of others
Master stations
• determination of the frequency and phase offset of the monitor
station clocks from the kalman filter process used at the Master
control stations
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