RuggedCom RX1000, RX1100 manual GPS Cable compensation, Reference Clocks

RuggedRouter™ User Guide

Reference Clocks

The GPS provides the highest quality reference clock. It will always be used when it is available, but may require some time after boot before becoming acquired (or “GPS locked”). Typically, GPS lock is usually acquired within five minutes of boot. When GPS is the reference clock, IRIG-B timestamps are accurate to within ns.

If GPS has not yet locked and IEEE1588 is locked, the router will use IEEE1588 server as a reference clock. When IEEE1588 is synchronized, IRIG-B timestamps are accurate to within microsecond or sub microseconds.

If GPS and IEEE1588 have not yet locked, the router will use an NTP server or peer as a reference clock. NTP typically requires less than two minutes after boot to synchronize. When NTP is the reference clock, IRIG-B timestamps can be accurate to within ms.

Before NTP is able to synchronize, the router will use the local clock to obtain the time and will emit IRIG-B timestamps on a one second basis.

How The Router Selects A Reference Clock

The router can be configured to use the following as reference clocks:

∙GPS, IEEE1588, NTP and the local clock,

∙GPS, NTP and the local clock,

∙GPS and IEEE1588,

∙GPS

If the router is configured to use multiple reference clocks, it will start sending timestamps using the best ever locked reference clock (local clock is always locked). If better reference clock is locked later, the router will “step” (i.e. suddenly change) the time and use the new reference clock. If the current reference clock becomes unavailable, the router will keep running with its own high precision timing hardware. It will use this hardware until the last used reference clock is locked or a higher quality reference clock is available.

If the router is configured to use only GPS, no timestamps will be issued until GPS locks. If GPS fails, the router will keep running with its own high precision timing hardware. When GPS returns, the time will be stepped back to the GPS reference clock.

GPS Cable compensation

GPS signals received by the antenna will be delayed in time depending upon the type and length of the cable to the router. This delay will introduce inaccuracy in the calculated time and position.

The RuggedRouter provides a method to account for this delay. The table below gives some examples of the delay that can be expected for a given dielectric type. Please note that cable characteristics varies from one manufacturer to the other.

Dielectric Type	Time Delay in ns/m (ns/ft)
Solid Polyethylene	4.62 (1.54)
Foam Polyethylene (FE)	3.81 (1.27)
Foam Polystyrene (FS)	3.36 (1.12)
Air Space Polyethylene (ASP)	3.45-3.63 (1.15-1.21)
Solid Teflon (ST)	4.38 (1.46)
Air Space Teflon (AST)	3.39-3.60 (1.13-1.20)

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