GPS Time Status

There are several states the navigation system will go through while acquiring GPS Time.

GPS Reference Time Status



Time validity is unknown


Time is set approximately


Time is approaching coarse precision


This time is valid to coarse precision


Time is coarse set and is being steered


Position is lost and the range bias cannot be calculated


Time is adjusting to fine precision


Time has fine precision


Time is fine set and is being steered by the backup system


Time is fine set and is being steered

The common sequence for GPS Time:

  • Unknown (initial state)

  • CoarseSteering (initial coarse time set)

  • FineSteering (normal operating state)

On startup and before any satellites are tracked, the navigation system can not possibly know the current time. Therefore, the receiver time starts counting at GPS reference week 0 and second 0.0. During this stage, the time status is set to Unknown.

After the first ephemeris is decoded, the navigation time status is set to CoarseSteering, with a resolution of ±10 milliseconds.

Once a position is known and range biases are being calculated, the internal clock model will begin modeling the range biases also known as the receiver clock offset. Modeling will continue until the model is a good estimation of the actual receiver clock behavior.

Once the navigation system has a good estimation, the time status is set to FineSteering and the receiver time is continuously updated (steered) to minimize the receiver range bias.

If a solution cannot be computed with the primary satellite system, it will attempt to use a backup system (if available). When the backup system is used and time is computed, the time status is set to FineBackupSteering. If the position is lost and the range bias cannot be calculated, the time status is degraded to FreeWheeling.

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