# GNSS Antennas

It is extremely important to measure the offsets from the IMU’s center of navigation to the GNSS antenna L1 phase center. These measurements will be your IMU to GNSS antenna offsets.

1\. Locate the GNSS antenna L1 Phase center and the IMU center of navigation. The location of the antenna L1 phase center can be found in the antenna’s datasheet. If using the Rover GPS Antenna supplied by Phoenix LiDAR Systems, refer to the drawing below for the location of the L1 Phase center.  while the IMU center of navigation is displayed on the IMU/enclosure label or NavBox enclosure as shown in the example pictures below. 

{% tabs %}
{% tab title="Air NavBox IMU Center of Navigation" %}
![IMU center of navigation on Air NavBox enclosure ](/files/-MCOl8jbMoFNHOOUhqmM)
{% endtab %}

{% tab title="Air NavBox IMU Center of Navigation" %}
![IMU center of navigation on Air NavBox enclosure ](/files/-MCOlH18V_E0HoL_XsWH)
{% endtab %}

{% tab title="Scout NavBox IMU Center of Navigation" %}
![IMU Center of Navigation on Scout NavBox enclosure](/files/-MCOmYfFZE--DR11ml4_)
{% endtab %}
{% endtabs %}

![Rover (Novatel Pinwheel) GNSS antenna L1 Phase center](/files/-M37Dp49XaZdC5Q3K6T9)

2\. Measure the distance from the IMU center of navigation to the GNSS antenna L1 phase center (not the other way around) along each axis of the IMU and record the measurements.

{% hint style="info" %}
If the measurement is in the positive direction of the IMU axis, the recorded value will be positive; if the measurement is in the opposite direction of the axis, the recorded value will be negative.
{% endhint %}

3\. Open the *Orientation & Offsets* settings in SpatialExplorer located in *Settings* → *Rover* → *Navigation System*.

4\. Update the “Offset IMU to Antenna 1” parameters with the recorded measurements obtained in step 2 for the X, Y, and Z axes. The entered values must be in meters. If your measurements are in centimeters or millimeters, you must convert them to meters.

![Offset IMU to Antenna settings in SpatialExplorer](/files/-LVp5TmUEIqRKcULD2x-)

5\. If the system has a second antenna, repeat the procedure for the second antenna. Ensure that you are measuring the offsets to the correct antenna (antenna 1 is the antenna connected to GPS1, antenna 2 is connected to GPS2).

6\. Save the current profile settings in SpatialExplorer.

7\. In order for the updated IMU to GNSS antenna offsets to take effect, you must restart the rover. Just click the "Shutdown Rover" button in SpatialExplorer or use the webinterface. You can also press the button on the enclosure to powercycle the system.


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