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Rockwell Automation Publication 1783-UM007I-EN-P - December 2017 151
Configure Switch Features Chapter 3
GNSS Signaling
There are two stages in the process for the GNSS receiver to acquire satellites
and provide timing signals to the host system:
•
Self-survey mode—On reset, the GNSS receiver comes up in Self-survey
mode and attempts to lock on to a minimum of four different satellites
to obtain a 3-D fix on its current position. It computes nearly 2000
different positions for these satellites, which takes about 35 minutes.
Also during this stage, the GNSS receiver is able to generate accurate
timing signals and achieve normal (locked to GPS) state. Because the
timing signal obtained during Self-survey mode can be off by
20 seconds, the software collects PPS data only during Over-determined
(OD) Clock mode.
After the self-survey process is complete, the results are saved to the
internal memory of the GNSS receiver, which speeds up the transition
to OD mode the next time the self-survey process runs. You can
manually restart the self-survey process by using the command-line
interface (CLI). After the self-survey process completes again, the
software updates the results in the internal memory of the GNSS
receiver.
•
Over-determined (OD) Clock mode—The device transitions to OD
mode when self-survey process is complete and the position information
is stored in memory on the switch. In OD mode, the GNSS receiver
outputs timing information based on satellite positions obtained during
Self-survey mode.
The GNSS receiver remains in OD mode until there is a reason to leave it, such
as the following reasons:
•
Detection of a position relocation of the antenna of more than 100 m,
which triggers an automatic restart of the self-survey process.
•
Manual restart of the self-survey process via the CLI.
After the GNSS receiver locks on to a satellite system, it sends a 10 ms-wide
PPS pulse and the current time and date according to the satellite system to the
time service.