@modalab Hello, can you help me to resolve this problem?

@Chad-Sweet , No matter the tf setting in the urdf file is, the ros topic outputs are shown in the followings

1. rostopic echo mavros/imu/data (in base_link frame): x is backward, y is right, z is down
2. rostopic echo downward/imu (in imu frame): x is right, y is backward, z is down

@Chad-Sweet
I am using customized urdf for snap_vio which has different numbers (positions and orientations) of sdf_pro.urdf.

Is there any urdf file for VOXL?

@Chad-Sweet
Update.

I have output /downward/vio/odometry with some error messages

[ INFO] [1648150781.892213161]: [VISLAM] Got uav1/imu to uav1/dfc transform; Initializing VISLAM
[ INFO] [1648150781.892816862]: [VISLAM] (from tf) tbc:
x: 0.2
y: 0
z: -0.025

[ INFO] [1648150781.893485304]: [VISLAM] (from tf) ombc:
X: 0
Y: 2.356
Z: 0

MachineVision is licensed as community user
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AR ERROR: arFileOpen(): Failed to open file: /data/.ros/vislam/Configuration.SF.xml
FASTCV: fcvAvailableHardware Linux
mempool cur block size 307200, new block size 307200
AR ERROR: arFileOpen(): Failed to open file: /data/.ros/na

Error:   TF_NAN_INPUT: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of a nan value in the transform (0.000000 0.000000 0.000000) (-nan -nan -nan nan)
         at line 244 in ~/geometry2/tf2/src/buffer_core.cpp
Error:   TF_DENORMALIZED_QUATERNION: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of an invalid quaternion in the transform (-nan -nan -nan nan)
         at line 257 in ~/geometry2/tf2/src/buffer_core.cpp
[ERROR] [1648150782.073463584]: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of a nan value in the transform (0.000000 0.000000 0.000000) (-nan -nan -nan nan)
[ERROR] [1648150782.077556213]: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of an invalid quaternion in the transform (-nan -nan -nan nan)
[ERROR] [1648150782.094956631]: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of a nan value in the transform (0.000000 0.000000 0.000000) (-nan -nan -nan nan)
[ERROR] [1648150782.095484499]: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of an invalid quaternion in the transform (-nan -nan -nan nan)
Error:   TF_NAN_INPUT: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of a nan value in the transform (0.000000 0.000000 0.000000) (-nan -nan -nan nan)
         at line 244 in ~/tf2/src/buffer_core.cpp
Error:   TF_DENORMALIZED_QUATERNION: Ignoring transform for child_frame_id "grav" from authority "unknown_publisher" because of an invalid quaternion in the transform (-nan -nan -nan nan)
         at line 257 in ~/geometry2/tf2/src/buffer_core.cpp

and repeated error message of

[ERROR] [1648150919.088496536]: [VISLAM] ERROR CODE = 2048, Stamp: 1648150919.022946523

I am wondering how urdf should be set. Should there be any rotations of IMU or camera in urdf file?
urdf file I used has some rotations in y-axis for IMU and cameras.

The running file is similar with this.

<!-- camera nodelet -->
<!-- VIO nodelet -->
<!-- fisheye camera info spoofer -->
<!-- imu nodelet -->
<!-- cpa nodelet -->

@Chad-Sweet
I had a look the link you shared: snap_vio
But I think it is not working correctly with the voxl module I have.
Currently, I am running ROS noetic on the voxl module, and try to run the VIO with downward camer.
By using snap_camera_ros (I edited it a little bit to use the downward camera), I can see the downward camera is correctly set up for snap_vio. Also, I can see snap_imu is working correctly, so I can see the output of imu (angular_velocity and linear_acceleration).

But I cannot see any output of /downward/vio/odometry.

Could you please help me to run snap_vio with the voxl module?
What should I check first?