# GPS Time Status

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

| GPS Reference Time Status | Description                                                |
| ------------------------- | ---------------------------------------------------------- |
| **Unknown**               | Time validity is unknown                                   |
| **Approximate**           | Time is set approximately                                  |
| **CoarseAdjusting**       | Time is approaching coarse precision                       |
| **Coarse**                | This time is valid to coarse precision                     |
| **CoarseSteering**        | Time is coarse set and is being steered                    |
| **FreeWheeling**          | Position is lost and the range bias cannot be calculated   |
| **FineAdjusting**         | Time is adjusting to fine precision                        |
| **Fine**                  | Time has fine precision                                    |
| **FineBackupSteering**    | Time is fine set and is being steered by the backup system |
| **FineSteering**          | 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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