# don't understand the specified imu frame in robot_localization package

The document of robot_localization link text says:

Adherence to specifications: As with odometry, be sure your data adheres to REP-103 and the sensor_msgs/Imu specification. Double-check the signs of your data, and make sure the frame_id values are correct.

Acceleration: Be careful with acceleration data. The state estimation nodes in robot_localization assume that an IMU that is placed in its neutral right-side-up position on a flat surface will:

Measure +9.81 meters per second squared for the Z axis.

If the sensor is rolled +90 degrees (left side up), the acceleration should be +9.81 meters per second squared for the Y axis.

- If the sensor is pitched +90 degrees (front side down), it should read -9.81 meters per second squared for the X axis

So, my question is:

if the imu body frame follows the REP-103, which means it follows **x-forward y-left z-up**, is the measurement supposed to be -9.81 for z-axis when the imu is placed on a flat surface?

if the imu sensor is rolled +90 degrees (left side up), the y-axis should be up. In this situation, is the measurement supposed to be -9.81 for y-axis?

If the sensor is pitched +90 degrees (front side down), the x-axis should be down. In this situation, is the measurement supposed to be +9.81 for x-axis?