Symmetricom Time Server - Manual (Page 32)

2-18 TimeVault™ User’s Manual 6000-100Ch2.fm Rev. D
Chapter 2: Installation and Start-Up Power-Up
Satellite Acquisition
Satellite acquisition begins at power-up and continues until power-down. The length of
satellite acquisition time depends on many factors, some of which are described below.
When TimeVault performs initial acquisition of GPS satellite data, it ‘listens’ for all
available GPS signals. After three satellites are acquired, the receiver locks onto GPS as a
reference source, and delivers UTC time. This procedure takes from 3 to 20 minutes,
depending upon current satellite visibility. Longer aquisition times may be needed for
window mounted GPS antennas that have very poor visibility.
After initial acquisition, and the TimeVault has saved its current average position, and
recent ephemeris/almanac information, it can typically lock to GPS as a reference source
in 3 to 5 minutes. The TimeVault needs an intermittent fix on three satellelites several
times per day in order to maintain current position/ephemeris/almanac information.
Provided with current position/ephemeris/almanac information, the TimeVault can
maintain GPS as its reference source by tracking only one satellite. If the number of GPS
satellites being tracked drops to zero, the unit remains locked GPS as a reference source
for approximately 5 minutes while it attempts to re-acquire another GPS satellite.
Because the TimeVault is optimized for time functionality, accurate position readouts are
not available.
If the current position is unknown or in error by more than 1 km (0.62 miles), acquisition
typically requires from 3 to 20 additional minutes to locate current antenna position,
reacquire satellite almanac and ephemeris data, and deliver UTC time.
Window Antenna Satellite Tracking
Firmware version 8 and above enable the TimeVault to operate with window-mounted
GPS antennas. Because window mounted antennas have a restricted view of the sky; they
receive fewer GPS satellite signals than roof mounted antennas. In some situations, a
window-mounted antenna may provide only one intermittent GPS signal to lock onto as
individual GPS satellites pass through its field of view. Firmware version 8 enables
network time servers to use GPS as a reference source when intermittent GPS satellite
signals are available.
Table B.1: Summary of Firmware Version 8 Improvements
Version 7 and earlier Version 8 and after
Requires continuous lock on four satellite
signals to maintain GPS as the reference
source. Requires intermittent fix from three satel-
lite signals (several times a day). Will
hold lock on one satellite signal to main-
tain GPS as the reference source.
Operates with roof mounted antennas. Operates with window and roof mounted
antennas.