Operation of GNSS Module on the Routers

The GNSS module has the following stages of acquiring and providing timing signals to the Cisco router:

• Self-survey mode - When the router is reset, the GNSS module comes up in self-survey mode. It tries to lock on to a minimum of four different satellites and computes approximately 2000 different positions of the satellites to obtain a 3-D location (Latitude, Longitude, and Height) of its current position. This operation takes about 35 to 40 minutes. During this stage, the module is also able to generate accurate timing signals and achieve a Normal or Phase-locked state.

  When GNSS moves into Normal state, you can start using the 1PPS, 10 MHz, and ToD inputs from GNSS. The quality of the signal in self-survey mode with Normal state is considered good enough to lock to GNSS.

• Over determined clock mode - The router switches to over determined (OD) mode when the self-survey mode is complete, and the position information is stored in non-volatile memory on the router. In this mode, the module only processes the timing information based on satellite positions captured in self-survey mode.

The router saves the tracking data, which is retained even when the router is reloaded.

The GNSS module stays in OD mode until one of the following conditions occur:

• A position relocation of the antenna of more than 100 meters is detected. This detection causes an automatic restart of the self-survey mode.

• A manual restart of the self-survey mode or when the stored reference position is deleted.

• A worst-case recovery option after a jamming-detection condition that cannot be resolved with other methods.

You can configure the GNSS module to automatically track any satellite or configure it to explicitly use a specific constellation. However, the module uses configured satellites in only the OD mode.

Note	GLONASS and BeiDou satellites cannot be enabled simultaneously.

When the router is reloaded, it always comes up in OD mode unless:

• the router is reloaded when the self-survey mode is in progress.

• the physical location of the router is changed to more than 100 meters from its pre-reloaded condition.

When the system restarts the GNSS self-survey by using the default gnss slot R0/R1 command in config mode, the 10MHz, 1PPS, and ToD signals are not changed and remain up.

Operation of GNSS Module on the Routers

The GNSS module has the following stages of acquiring and providing timing signals to the Cisco router:

• Self-survey mode - Self-survey is the procedure that is performed prior to using time mode. It determines the position of a stationary receiver by building a weighted mean of all valid 3D position solutions.

  To stop self-survey mode, the following two requirements must be met:

  • The minimum observation time defines a minimum amount of observation time regardless of the actual number of valid fixes that are used for position calculation. Reasonable values range from one day for high accuracy position calculation to a few minutes for coarse position determination.

  • The required 3D position standard deviation defines a limit on the spread of positions that contribute to the calculated mean. As the position error translates into a time error when using time mode, you should carefully evaluate the time accuracy requirements and choose an appropriate value.

  Self-survey ends when both requirements are met. After self-survey is completed successfully, the receiver automatically enters time-fix mode.

  Recommendations

  • Self-survey must run for at least twenty-four hours because it is important to cover a full constellation run and a day-night cycle.

  • CNO or signal strength for achieving time-fix during self-survey is 35dB.

• Time-fix mode - In order to use the time-fix mode, the exact position of the receiver must be known. The exact position of the receiver is determined by using self-survey. Errors in the fixed position translate into time errors depending on the satellite constellation.

  There is no minimum and maximum value (Time) in the self-survey process and the receiver makes the best effort to achieve the time-fix mode.

  For achieving the time-fix mode, a minimum of four satellites are required and it may take several days to get into the time-fix mode.

  If the position is already known, the module can achieve time-fix mode even if there is only one satellite received.

You can configure the GNSS module to automatically track any satellite or configure it to explicitly use a specific constellation.

When the router is reloaded, it always comes up in the time-fix mode unless the router is reloaded when the self-survey mode is in progress.