Symmetricom TimeSource 3600 User Manual page 38

Chapter 1 Overview of the TimeSource 3600
Functional Description
Installations with full view to the sky see 4 satellites 23+ hours a day, seven days a
week. It is relatively easy to filter out multipath signals using multiple satellites and
simple voting schemes. In addition, rooftop antennas are mounted vertically and all
signals below the horizon are obvious multipath interference and can be masked
out of the system. With 4 satellites in view, the GPS timing receiver will provide an
error-free lock on its location through an automatic survey function.
With wall/window installations, voting schemes become less effective as the
number of satellites in view drop. Also, wall/window antennas are mounted
horizontally and are prone to seeing multipath signals reflected off nearby structures
and the ground. Wall/window installations also require the manual input of location
data, creating the potential for errors and the need to detect these errors.
The TimeSource has unique algorithms to account for, and defeat, the added
multipath complications and location data entry error possibilities of wall/window
antenna installations. Large amounts of multipath or major errors in location data
entry are easier to identify and are detected over a relatively short period of time.
These short-term errors are normally reported via the TimeSource tracking success
rate parameter.
Small amounts of multipath or minor errors in location data entry are difficult to
identify because they mimic a true signal or an expected satellite behavior pattern.
Single satellite reception over limited periods of time complicates the ability for the
internal TimeSource algorithms to filter out these ghost signals and longer periods
of time are needed to sort them out. TFOM tracks and reports these long-term
errors.
At time of installation, marginal or unacceptable TFOM readings can indicate the
need to adjust the antenna placement, the mask angle, and/or the location data.
Relatively small changes in the antenna placement can improve the ability of the
system to see satellites and therefore improve performance. The mask angle can
be adjusted to block out lower elevation portions of the sky if there is good visibility
at higher elevations, thereby reducing multipath. Correct location data is very helpful
in identifying and tracking satellites.
In addition, a clock may be stable for many weeks, months, or years but could
degrade because of changes in its environment. TFOM is useful in detecting these
infrequent subtle changes including:
Maturing foliage or seasonal foliage changes
Installation of new transmitters nearby (i.e. wireless base station)
New building construction
Variations in the day-to-day temperature of the CO
38 TimeSource 3600 User's Guide 097-72060-01 Revision J – June, 2007