# Re-Tweeted IMU Messages (IMU Raw Data)

If the DVL is set to re-tweet (i.e., forward, relay, pass-thru) IMU raw data messages, each time the DVL interrogates the IMU it will output an IMU raw data message. The IMU is interrogated at the DVL ping rate, which varies from 5 Hz to 20 Hz depending on many factors.

The IMU raw data message consists of the string “IMU:” followed by eleven floating point numbers in ASCII format, followed by a line terminator (cr/lf). The units of the ten floating point numbers may vary between the baseline IMU and the enhanced IMU.  See the table below:

<table data-header-hidden><thead><tr><th width="150"></th><th width="207.10709715219636"></th><th width="156"></th><th></th></tr></thead><tbody><tr><td><strong>Field</strong></td><td><strong>Name</strong></td><td><strong>Units (Baseline IMU)</strong></td><td><strong>Units (Enhanced IMU)</strong></td></tr><tr><td>0</td><td>IDENTIFIER</td><td>“IMU:”</td><td>“IMU:”</td></tr><tr><td>1</td><td>Quaternion W (“real” or “r”)</td><td>-1.0 to 1.0</td><td>-1.0 to 1.0</td></tr><tr><td>2</td><td>Quaternion X ( “i”)</td><td>-1.0 to 1.0</td><td>-1.0 to 1.0</td></tr><tr><td>3</td><td>Quaternion Y ( “j”)</td><td>-1.0 to 1.0</td><td>-1.0 to 1.0</td></tr><tr><td>4</td><td>Quaternion Z ( “k”)</td><td>-1.0 to 1.0</td><td>-1.0 to 1.0</td></tr><tr><td>5</td><td>Linear Acceleration X</td><td>m/s/s</td><td>m/s/s</td></tr><tr><td>6</td><td>Linear Acceleration Y</td><td>m/s/s</td><td>m/s/s</td></tr><tr><td>7</td><td>Linear Acceleration Z</td><td>m/s/s</td><td>m/s/s</td></tr><tr><td>8</td><td>Angular Rate about X axis</td><td>degrees/s</td><td>radians/s</td></tr><tr><td>9</td><td>Angular Rate about Y axis</td><td>degrees/s</td><td>radians/s</td></tr><tr><td>10</td><td>Angular Rate about Z axis</td><td>degrees/s</td><td>radians/s</td></tr><tr><td>11</td><td>Delta time since last IMU msg</td><td>seconds</td><td>seconds</td></tr></tbody></table>

{% hint style="info" %}
“Linear Acceleration” = Acceleration with gravity vector subtracted, aka “Free Acceleration”
{% endhint %}

{% hint style="info" %}
Note: These are raw messages. The quaternion values relate the sensor head coordinate system to the real world (magnetic) coordinate system, including magnetic declination if supplied. The accelerations and gyro rates remain aligned with the sensor frame. The user can verify these experimentally. Cerulean does not represent that IMU orientations will be remain consistent between revisions or manufacturing lots.
{% endhint %}

When the IMU message is sent, the quaternion has already been transformed into the sensor head coordinate frame using the transformations above.   &#x20;


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