# Navigation System Alignment

The rover requires an initial attitude estimate to start the navigation filter. This is called system alignment. Upon startup, the rover has no position, velocity or attitude information. When the rover is first powered up, the following sequence of events occurs:

1. The first satellites are tracked and coarse time is solved.
2. Enough satellites are tracked to compute a position.
3. Receiver “fine time” is solved, meaning the time on board the receiver is accurate enough to begin timing IMU measurements.
4. Raw IMU measurements begin to be timed by the receiver and are available to the INS filter. The INS Status field changes from Inactive to DeterminingOrientation during this period.
5. During DeterminingOrientation, the INS filter determines which IMU axis is pointing upwards with respect to gravity.
6. Once the inertial alignment routine begins the INS Status field will change to Aligning. For rovers with dual antenna systems, alignment is complete once an RTK position is solved between both antennas.
7. The rover will transition to navigation mode when alignment is complete, indicating that the heading has been determined with sufficient certainty. The INS Status field will change to AlignmentComplete.
8. The solution is then refined using updates from GNSS. Once the system is operating within specifications and after some vehicle movement, the INS Status field changes to SolutionGood. This indicates that the estimated azimuth standard deviation is below the preset value. If it increases above the preset value, the status changes to HighVariance.

| INS Status                 | Description                                                                                                                                                                                                                                                                                                               |
| -------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **Inactive**               | INS is inactive. The alignment routine has not started.                                                                                                                                                                                                                                                                   |
| **DeterminingOrientation** | INS is determining which IMU axis is aligned with gravity.                                                                                                                                                                                                                                                                |
| **Aligning**               | INS is in alignment mode.                                                                                                                                                                                                                                                                                                 |
| **AlignmentComplete**      | The INS filter is in navigation mode, however not enough vehicle dynamics have been experienced for the system to be within specifications.                                                                                                                                                                               |
| **SolutionGood**           | The INS filter is in navigation mode and the INS solution is good.                                                                                                                                                                                                                                                        |
| **HighVariance**           | The INS solution is still being computed but the azimuth solution uncertainty has exceeded the threshold. The default threshold is IMU dependent. The solution is still valid but you should monitor the solution uncertainties. You may encounter this state during times when the GNSS, used to aid the INS, is absent. |
| **WaitingInitialPos**      | The INS filter has determined the IMU orientation and is awaiting an initial position estimate to begin the alignment process.                                                                                                                                                                                            |


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