Time Figure Of Merit - Symmetricom TimeSource 3100 User Manual

Chapter 1 Overview of the TimeSource 3100
Functional Description

Time Figure of Merit

Time Figure of Merit (TFOM) is a moving 24 hour measurement reported in
nanosecond (ns) against an ideal model. TFOM has a frequency component used
to measure GPS wander caused by multipath and a time loop component used to
measure long term oscillator wander.
The TFOM alarm threshold is set to 500 ns and is not user configurable. A TFOM
below 500 ns indicates a stable clock well within PRS output performance
specifications. A TFOM between 500 ns and 800 ns is an early warning of a clock
becoming unstable and in danger of going out of PRS specification. When the
TFOM exceeds 800 ns, the clock is no longer meeting the stratum 1 MTIE mask.
TFOM is most useful for installations where the antenna has a limited view to the
sky. This is defined as any installation where fewer than 4 satellites are in view for
greater than 1 hour per day on average. This is typical for installations where the
antenna is mounted in a window or on an outside wall of a building, but can also
include rooftop installations with partial sky view blockage. TFOM's primary use is to
help troubleshoot multipath issues associated with antenna placement and incorrect
latitude, longitude, and/or altitude (location) data entry.
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.
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