5: Function Reference
The GPS receiver serves as a UTC time reference for the XL-GPS system clock. To be a
valid time reference, the receiver requires the following information:
• The GPS time (at least one “good current” GPS satellite)
• The current position of the receiver
• The UTC leap-second offset
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.
For additional information on GPS position information, see “F50 – GPS Receiver LLA/XYZ
Position” on page72.
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 current position, the UTC leap-second value, and the current
GPS time, it starts providing valid time to the XL-GPS 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 XL-GPS with a GPS receiver (GPS
Status: Unlocked) would show the following:
UNLOCKED * GPS
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
LOCAL 233:18:21:29 2005
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