Symmetricom XL-GPS User Manual
Page 115
997-01530-01, Rev. C-25, Jan 2008
105
2
5
1
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
•
GPS Acquisition State - (Dynamic Mode, Stop Site Survey, Stop TRAIM, Start Site Survey, Start
TRAIM, Survey Position, Position Hold, TRAIM Active)
Operation
Each of F119’s information, status, and configuration items are explained below. Because F119 and the
GPS receiver are important elements of the XL-GPS Time and Frequency System, this section explains
interactions and behavior of the F119, the GPS receiver, and other system functions in some detail.
Part Number, Software Version, and FPGA Number
This information may be useful for identifying the GPS receiver to Symmetricom technical support.
GPS Status (Locked or Unlocked)
During normal operation, “Locked” means the receiver has its current position and the current GPS time.
“Unlocked” means the receiver doesn’t have its current position yet, or that no “good current” satellites
are available to provide the current GPS time. For additional information on “good current” GPS
satellites,
see “F60 – GPS Receiver Satellite List” on page 56
.
Note:
GPS status can be “Locked” before the GPS receiver is a valid time reference. This is explained
below.
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
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.