RuggedCom RuggedRouter RX1100 - Manual (Page 270)

32. Configuring IRIGB And IEEE1588
Revision 1.14.3 270 RX1000/RX1100™
32.1.2.2. Reference Clocks
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.
32.1.2.3. 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.
32.1.3. 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)