Symmetricom XLi IEEE 1588 manual Unlocked GPS PRI Local, Locked GPS PRI Local, 233182129

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Following power-up and initialization, the receiver requires at least four concurrent “good current” satellites to resolve its current position. In rare cases, when a pair of “good current” satellites are on intersecting paths, the receiver requires additional “good current” satellites or waits for the intersecting satellites to diverge before resolving the current position. Once resolved, the current position information is saved.

While resolving its current position, the GPS receiver also listens for the UTC leap-second offset periodically transmitted by GPS satellites along with GPS time and position information. Up to thirteen minutes may elapse from the time the receiver acquires its first “good current” satellite to the time it receives the UTC leap-second offset. Once received, the UTC leap-second offset is saved.

When the receiver has the UTC leap-second value, it starts providing valid time to the XLi system clock. When the system clock is locked to the GPS time reference and is operating within specifications, the system status is locked. The interval from initialization to system status lock is typically under twenty minutes, under nominal conditions. This transition is illustrated below.

Following initialization, the front panel display of an XLi with only one GPS receiver (GPS Status: Unlocked) would show the following:

UNLOCKED	* GPS PRI
LOCAL	365:16:01:05 1969

With the GPS receiver as a valid time reference, the following changes would take place:

•The asterisk (“*”) indicating the absence of a valid reference would disappear

•The system status would change to locked

The front panel status display would look like this:

LOCKED	GPS PRI
LOCAL	233:18:21:29 2004

Once the GPS receiver is a valid time reference, it requires at least one “good current” satellite to remain a valid time reference. If “good current” GPS satellites become temporarily unavailable, GPS status changes to unlocked and the XLi stops using the receiver as a valid time reference.

Typically, when a “good current” satellite becomes available again, GPS status locks and the receiver becomes a valid time reference almost immediately. Typically, the receiver does not need extra time to resolve its current position unless it is being used in a very mobile/dynamic environment such as an aircraft.

If the unit is powered-cycled, the receiver repeats the complete position and leap-second acquisition process before GPS status locks.

Note: GPS satellite visibility and signal strength affect the ability of the GPS receiver to lock and provide valid time to the XLi. Therefore, it is very important to select the best possible antenna site.

XLi IEEE 1588 Clock 111

997-01510-03, Rev. C, 12/12/2006