Inconsistency in SITL JSON interface example

[Tuxliri]

ardupilot/libraries/SITL/examples/JSON/C++/minimal.cpp

Lines 71 to 76 in 4088ab9

	ap.SendState(timestamp,
	0, 0, 0, // gyro
	0, 0, -9.81, // accel
	0, 0, 0, // position
	0, 0, 0, // attitude
	0, 0, 0); // velocity

In the example above the IMU is supposed to have a FRD (x-axis pointing forward, y-axis pointing right and z-axis pointing down) orientation. In this case the expected measured acceleration in the line above should be 9.81 rather than -9.81. Is this correct or did I understand this incorrectly?

[Tuxliri]

Okay I have figured this out. When the drone is on a surface the accelerometer of the imu senses the counteracting force of the flat surface pushing up against gravity, thus an upward acceleration of 9.81 m/s^2. In the FRD body reference frame (z-axis pointing down) this is an acceleration of -9.81 m/s^2.

[Tuxliri]

Added a clarification in the code.