Ross 4500DR-104-07, User Manual, Page: 139 / 161

SRG-4500 User Guide (v7.0) GPS Option • 139
Operation
This section provides additional information for operating the SRG-4500 while using an active GPS antenna.
For More Information on...
• GPS signals, refer to IS-GPS-200E .
• GLONASS signals, refer to ICD L1 L2 GLONASS Edition 5.1 2008 .
Locking the SRG-4500 to GPS
One of the best sources of reference for both frequency and time is the Global Positioning System (GPS).
The SRG-4500 uses a 32 channel GPS receiver. As the SRG-4500 is powered, the GPS receiver begins searching
for satellites from which it can get time, frequency and phase information. In a good reception area, as many as 12
satellites may be visible. The SRG-4500 requires a minimum of 3 satellites to be located and fully decoded before it
can start using the GPS data. The search for the first 3 satellites can take up to 3 minutes, but is often accomplished
in 2. If less than 3 satellites are available, the SRG-4500 will not lock to GPS.
Once the minimum of 3 satellites have been located and their GPS data decoded, the SRG-4500 averages the
number of counts from all of the detected satellites, and then continually adjusts the free-run oven frequency to
generate a condition where the SRG-4500 is locked in frequency and phase to the GPS receiver.
In marginal reception areas, locking/unlocking of the GPS receiver will be tolerated as the short term effect on the
free-running oven frequency of the SRG-4500 will be very small. When an unlocked condition is detected, the
SRG-4500 software will smoothly revert to the stored free-running oven frequency.
The number of satellites can be monitored on the GPS Report sub-menu available in the System Setup menu. Refer
to the section “ GPS Time Tab ” on page 111 for details.
Time Locking
Time lock is achieved by reading the GPS time from the receiver and calculating an appropriate timecode. GPS
locks the SRG-4500 central timing system.
Keep the following in mind:
• The timecode needs to be checked and updated each time the GPS receiver is detected.
• However, in marginal reception, the timecode update should be suppressed until it has been valid for a number of
seconds.
• Once the time-code has been set by the GPS receiver, the SRG-4500 should not need to recheck the GPS time
unless a crash lock has been forced by the phase locking arrangement.
Frequency Locking
Frequency lock can be established in a few seconds as the oven free-running frequency is within a few ppm of the
GPS frequency at all times. The first time the SRG-4500 is powered with a GPS receiver, the SRG-4500 waits for 3
or more satellites to be “fixed”, then the SRG-4500 acquires frequency lock. This first time may take up to 5
minutes from when the GPS lock was acquired.
Frequency lock is achieved when the short-term frequency error is less than 1ppm.
Phase Locking
Phase locking can be established a few minutes after frequency locking. Phase lock is achieved when the long-term
frequency error is less than 0.1ppm.