Use standard messages for AHRS

[chapulina]

• Part of Make more use of standard ROS messages #6

I didn't find a standard representation for data coming from an AHRS sensor, so here's a first draft for how we could use some messages from common_interfaces.

I'm not sure about the frame of reference for the IMU values, it would be good to document them.

Another thing to consider is to break this message apart and publish each standard message on a separate topic. The advantage of that is to have immediate access to all the existing tooling around the standard messages, like visualizing IMU data on RViz, plotting lat/lon to a map, and so on.

Let me know what you think, @andermi !

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This is the mapping from the previous fields to the new ones:

Before	After
header	header (no change}
roll_angle, pitch_angle yaw_angle_true	sensor_msgs/Imu: imu.orientation (this is a quaternion, not RPY)
x_rate_corrected	sensor_msgs/Imu: imu.angular_velocity.x (stays as rad/s)
y_rate_corrected	sensor_msgs/Imu: imu.angular_velocity.y (stays as rad/s)
z_rate_corrected	sensor_msgs/Imu: imu.angular_velocity.z (stays as rad/s)
x_accel	sensor_msgs/Imu: imu.linear_acceleration,x (the message documentation recommends m/s^2 instead of g)
y_accel	sensor_msgs/Imu: imu.linear_acceleration.y (the message documentation recommends m/s^2 instead of g)
z_accel	sensor_msgs/Imu: imu.linear_acceleration.z (the message documentation recommends m/s^2 instead of g)
n_velocity	geometry_msgs/Vector3: ned_velocity.x
e_velocity	geometry_msgs/Vector3: ned_velocity.y
d_velocity	geometry_msgs/Vector3: ned_velocity.z
latitude	sensor_msgs/NavSatFix: gps.latitude
longitude	sensor_msgs/NavSatFix: gps.longitude
altitude	sensor_msgs/NavSatFix: gps.altitude
x_rate_temp	sensor_msgs/Temperature: x_rate_temp.temperature (still in Celsius)

[andermi]

publish each standard message on a separate topic

I agree with this as long as those messages would each have headers.

Where should we put documentation that those messages/topics exist? Should our ICD exist in buoy_entrypoint?

[chapulina]

I agree with this as long as those messages would each have headers.

Sounds good 👍🏽 So we could do something like:

- type: `sensor_msgs/Imu`
  topic: `/xb_record/imu`

- type: `geometry_msgs/Vector3Stamped`
  topic: `/xb_record/ned_velocity`

- type: `sensor_msgs/NavSatFix`
  topic: `/xb_record/gps`

- type: `sensor_msgs/Temperature`
  topic: `/xb_record/x_rate_temp`

Where should we put documentation that those messages/topics exist? Should our ICD exist in buoy_entrypoint?

Buoy entrypoint sounds like a good place to me.

[chapulina]

As discussed earlier, we'll keep the existing aggregated messages being published in a single topic, but within these messages, make as much use of standard ROS messages as possible.

In the future, we may publish separate standard messages in their own topics alongside the aggregated ones, so users have a choice to subscribe to one or the other.