TWSTT: What It Is and How It Is Performed
The U.S. Naval Observatory (USNO) maintains precise time and the nation's Master
Clock with the goal of providing the most precise and accurate time to remote
users. USNO has been developing the two-way time
transfer method (TWSTT) since the first communications satellites,
Telstar and Relay-II, were put into operation in the very early 1960s. The
recent development of low-cost, portable, very small aperture terminals
(VSATs) and a new generation of time transfer modems have allowed the two-way
time transfer method to be put into regular use at USNO.
Some of our base-station antennas (click on to enlarge)
The ultimate goals of the two-way project have been to improve the
USNO Master Clock
(USNO (MC)) and its operational utility to military and civilian remote users,
and to improve time comparisons with other timing centers. The
USNO (MC) is improved because two-way transfer allows inclusion into USNO's
operational time scale remote clocks via the most precise and accurate timing
links possible. The operational utility to military users is increased because
high-precision military users are able to make direct comparisons of remote
clocks with the USNO (MC) via independent means from other military operational
timing systems, such as the
Global Positioning System
(GPS). The two-way timing links with national timing centers will
improve the long-term stability of the
International Atomic Time (TAI), which is the
NATO timing reference.
Our mobile Earth station antenna; click on to display (USNO Image - JPG - 223Kb)
What is two-way time transfer? In its simplest form, two-way time transfer
between two timing standards may be accomplished by having each of two time
standards send a one-pulse-per-second (1pps) signal to the other time standard
over a communications circuit. It is a point-to-point communications link.
The communications circuit used is not important and may be made through any
wide-band circuits, such as coaxial cable, fiber-optic cable, microwave
transmission, television, laser light transmission, or communications
satellites, to name a few. The transmission medium introduces delays, but
this delay must be nearly reciprocal, i.e. the delay is the same in both
directions, thus cancelling out. Each lab measures the time interval
between the transmission of its local 1pps and the time it receives the
remotely generated 1pps signal, typically using a time interval counter (see
Figure 1 for a simple diagram of the procedure). The true time offsets of the
two time standards can be measured very precisely (<0.2 nanosecond) and
accurately (~1.0 nanosecond). By taking data over a period of time, the
long-term behavior, i.e. frequency changes, rates, jumps, drifts, etc.,
that will affect the accuracy and stability may be characterized. The
operational usefulness of the clocks are improved as well as the confidence
in related decision making. The day-to-day stability of two-way time transfers
can nearly reach the performance of the best reference clocks.
Figure 1. A simple diagram of the two-way timing method as applied between
two timing centers. The communications link is a Ku-band link using a
commercial satellite (courtesy of Bill Powell).
The two-way timing system employed at the USNO uses geostationary
communications satellites for the communications circuit. Currently,
commercial Ku-band satellites and X-band satellite links are possible.
The time transfer modem being used is a very important piece of equipment
for TWSTT to succeed. The modem codes/decodes the spread-spectrum signals of
the 1-pps that is transmitted and received over the satellite communications
circuit. The modem also is used to exchange the data collected during the
run at each location, allowing near-real-time data processing to take place.
The U.S. Naval Observatory, in Washington, DC, has a wide range of steerable and
fixed antennas, VSATs, and related equipment to fulfill any users requirement
to transfer high precision time.
The U.S. Naval Observatory's
Alternate Master Clock
(AMC), at Schriever AFB, Colorado has a 1.8-meter Ku-band VSAT and a 3.7-m Ku-band
base-station and a AOA-TWT-100 time transfer modem, which are used to fulfill
its backup timing source function.
Currently, USNO is conducting time transfer experiments with users the world