Masterclock GPS 500, Руководство пользователя

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GPS500 User Manual – Rev E – 2020.04.14 2484 W Clay St, St Charles, MO 63301
10
Lightning Arrestor
In-line lightning arrestors are mounted on a low impedance
ground between the antenna and the point where the cable
enters the building. They require no additional power or wiring
except the ground lead.
Cable Delay
GPS position as well as precise UTC time is determined at the
point the GPS signals are received at the antenna. Since the
antenna is typically attached to the GPS timing receiver via a
cable, signal propagation delays through the cable cause the
time calculated by the receiver to be slightly behind UTC. In
GPS clocks with nanosecond and microsecond accuracies this is
a critical factor. In products such as network time servers cable
delay is not important because time transfer over IP networks
degrades the time to the millisecond level.
Cable delay is a function of the cable type. RG-59 cable for
example typically delays the signal 1.24 ns/ft. For 50 feet of
cable (15 m) the delay would be 62 nanoseconds. Cable delay is
removed by advancing the antenna signal inside the GPS
receiver. In this example, advancing the 62
nanoseconds removes all cable latency. Solutions such as the
GPS Down/up converter also introduce signal latency but this
latency can also be removed by adjusting the signal. All
precision GPS timing receivers with nanosecond or
microsecond timing accuracy have the ability to compensate
for cable delay.
Advanced Planning
It is time well spent to estimate in advance the cable length
from the GPS antenna to the receiver for any planned
installation. Cable lengths that are too short or too long
can each introduce problems. In some cases adding an inline
amplifier and some extra cable may be a quick and economical
solution. In other instances retrofitting for a GPS down/up
converter may be necessary which will require installing a
different cable type. Keep in mind that some extra cable coiled
in a ceiling leftover from over estimating the cable length is not
necessarily negative. Provided you know the length of the total
cable, the cable delay can be accounted for and the timing
accuracy maintained.