Arbiter Systems 1088A, Руководство по эксплуатации

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64 Timing, IRIG-B and Pulses
7.4.8 Cable Delays
Electromagnetic waves travel at the speed of light (C) in free space or vacuum and a fraction of that
speed through cabling. The speed of an electromagnetic wave in free space is given by Constant 7.6.
(7.6) C
≈
9 . 84
×
10
8
f eet/second
Since electromagnetic waves travel slower through any cable, cable manufacturers normally specify
cable with a velocity factor (VF), which is a percentage 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 GPS 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, 840 feet of RG-6
cable (with a velocity factor of 83%) would delay the timing 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.
7.4.9 Solutions
There are many solutions to providing an accurate timing signal to equipment 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 GPS 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.