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.