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ABOUT US
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Naval Oceanography One Pager
End of Year Graphic 2022
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Technical Director
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Naval Oceanographic Office
Fleet Numerical Meteorology & Oceanography Center
United States Naval Observatory
News from the Naval Observatory
Earth Orientation Department
Precise Time Department
The USNO Master Clock
The USNO Master Clock
Time Dissemination at the USNO
USNO Alternate Master Clock (AMC)
Cesium Atomic Clocks
Hydrogen Masers at the USNO
Rubidium Fountain Clocks
USNO Time Scales
International Time Scales and the BIPM
Definitions of Systems of Time
Global Positioning System
Global Positioning System Overview
USNO GPS Data Categories Explanation
CGGTTS Data Format
USNO GPS Time Transfer
Leap Seconds
GPS Information: SA, DGPS, Leap Seconds, etc.
GPS Week Number Rollover
GPS Timing Data and Information
USNO Format Explanation
USNO Computer Display Clocks
Two-Way Satellite Time Transfer (TWSTT)
Telephone Time
Network Time Protocol (NTP)
US Eastern Time Zone NTP Servers
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Senior Enlisted Advisor
Careers at the USNO
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United States Naval Observatory
Precise Time Department
The USNO Master Clock
The USNO Master Clock
Time Dissemination at the USNO
Home
Our Commands
United States Naval Observatory
Precise Time Department
The USNO Master Clock
The USNO Master Clock
Time Dissemination at the USNO
ABOUT US
Mission & Vision
History
Naval Oceanography One Pager
End of Year Graphic 2022
LEADERSHIP
Commander
Technical Director
Command Master Chief
All Leadership
OUR COMMANDS
Naval Oceanographic Office
Fleet Numerical Meteorology & Oceanography Center
United States Naval Observatory
News from the Naval Observatory
Earth Orientation Department
Precise Time Department
The USNO Master Clock
The USNO Master Clock
Time Dissemination at the USNO
USNO Alternate Master Clock (AMC)
Cesium Atomic Clocks
Hydrogen Masers at the USNO
Rubidium Fountain Clocks
USNO Time Scales
International Time Scales and the BIPM
Definitions of Systems of Time
Global Positioning System
Global Positioning System Overview
USNO GPS Data Categories Explanation
CGGTTS Data Format
USNO GPS Time Transfer
Leap Seconds
GPS Information: SA, DGPS, Leap Seconds, etc.
GPS Week Number Rollover
GPS Timing Data and Information
USNO Format Explanation
USNO Computer Display Clocks
Two-Way Satellite Time Transfer (TWSTT)
Telephone Time
Network Time Protocol (NTP)
US Eastern Time Zone NTP Servers
US Mountain Time Zone Servers
DoD Customer Servers
Astronomical Applications Department
Celestial Reference Frame Department
Senior Enlisted Advisor
Careers at the USNO
Naval Oceanography Operations Command
Fleet Weather Center - Norfolk
National Ice Center
Fleet Weather Center - San Diego
Joint Typhoon Warning Center
Naval Oceanography Anti-Submarine Warfare Center
Navy DoD Supercomputing Resource Center
PRESS ROOM
News Stories
Photo Gallery
Video Gallery
Public Use of Limitations
CONTACT US
Time Dissemination
The USNO Time-ball atop historic Building 1
Various timed systems are being kept within narrow tolerances of the USNO Master Clock. The
LORAN
chains covering North America have been within about 100 nanoseconds (ns) rms, whereas the overseas chains had larger tolerances, the largest in the case of the Mediterranean chain (1 microsecond).
The Navy Navigation Satellite System (NNSS, also known as
TRANSIT
) ceased broadcasting valid navigation messages and timing on December 31, 1996 after 32 years of service.
The
Global Positioning System (GPS)
, with the correction given in the navigation message (A0 and A1), is typically within 15 ns rms with
selective availability
removed. Including selective availability, observed with a single frequency receiver, the rms error has been about 70 ns, with a maximum error of 291 ns. 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
from the
Automated Data Service (ADS)
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 10 ns 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
National Institute of Standards and Technology (NIST)
in Boulder, Colorado, with the NRC in Ottawa, Canada, and with the
AMC
, the USNO Alternate Master Clock in Colorado Springs, Colorado. 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 3 ns. 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 4.5-meter 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.
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