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which degrades the accuracy of the calculated t
ime and position. The time delay
is dependent on the type of dielectric material in the cable and ranges from 1 to 2
ns/ft. RuggedCom provides a method to account for this delay through the web
management interface by entering the time delay into the cable compensation
box under PTP General Configuration The table below gives some examples of
the delay that can be expected based on the dielectric type.
Dielectric Type
Time Delay
(ns/ft)
Propagation
Velocity
(% of c)
Solid Polyethylene (PE) 1.54 65.9
Foam Polyethylene (FE) 1.27 80.0
Foam Polystyrene (FS) 1.12 91.0
Air Space Polyethylene (ASP) 1.15-1.21 84-88
Solid Teflon (ST) 1.46 69.4
Air Space Teflon (AST) 1.13-1.20 85-90
Table 14: Coaxial Cable Delay
2.9.2.3 Lightning Considerations
Although it is not possible to protect the antenna from a direct lighting strike, the
antenna and connected components can be protected from secondary effects
through installation location and protection devices.
Install the antenna at least 15 meters away from and lower than any structures
that attract lightning. GPS antenna damage is usually not the result of a direct
lightning strike, but due to high currents induced by the effects of a lightning
strike on a nearby structure. RuggedCom also recommends installing lightning
arrestors in the antenna line to protect the receiver and connected devices. If a
lightning arrestor is installed, it is important to ensure that it has a low impedance
path to the ground.
2.9.2.4 Line Amplification and Filtering
Although an active antenna has gain, depending on the length of the coaxial
cable used it may not be enough in which case a line amplifier will be required as
well.
Most active antennas include filters; however, if there is a high potential for
electromagnetic interference, such as from the near field of a radio transmitter,
though the antenna system, additional antenna line filtering may be necessary.
2.9.3 IRIG-B Outputs
The PTP card provides IRIG-B outputs in both AM (Amplitude Modulated) and
PWM (Pulse Width Modulated) formats. The IRIG-B123 signal format is
supported on the AM OUT port, and the IRIG-B003 signal format is supported on
the TTL OUT port. Enabling and disabling the outputs, and selecting between
PWM and PPS on the TTL OUT port, is done through software.
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RuggedCom
®
RuggedSwitch
®
RSG2288 Installation Guide rev103