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IRIG-B Primer
Since electromagnetic waves travel slower through any cable, cable manufacturers normally
specify cable with a velocity factor (VF), which is a fraction of the speed of light in free space,
and characteristic of the specific cable. The Velocity Factor for the RG-6 cabling used by Arbiter
Systems for GNSS antenna connections, is about 83% of C. Most transmission lines have velocity
factors in the range of 65% to 97%. Using these values you can determine the actual time delay in
your cable distribution system and compare it to your required accuracy. As an example, it would
take 840 feet of RG-6 cable (with a velocity factor of 83%) to delay the signal by one microsecond.
For IRIG-B timing applications, these delays may not be important, compared to other criteria.
Otherwise, you would be forced to compensate for the time delay using another method, such as
advancing the timing output or placing another master clock at the remote site.
Calculating the signal delay in a transmission line with a velocity factor of 83% is as shown in
Equation C.4:
(C.4)
T
=
λ
1
CKv
Where:
T = Cable delay, in nanoseconds;
λ
= Cable length, in meters;
C = Speed of light (3
×
10
8
meters per second);
Kv = Nominal velocity of propagation (e.g. 0.83).
C.2.7
Solutions
There are many solutions to providing an accurate timing signal in distant locations. However, the
most satisfying solution may not be to string cabling for hundreds of meters. The costs associated
with installing and maintaining cabling over a wide area may be unsatisfactory. Since the GNSS
is so pervasive, it may prove to be less costly to install another clock at a distant location, which
would also improve accuracy and provide redundancy. Before installing cabling over a wide area,
be sure to first examine all the possibilities.
