# A question about VIO reference system.

Source: https://forum.modalai.com/topic/2745/a-question-about-vio-reference-system
Category: VOXL SDK and Software (https://forum.modalai.com/category/47/voxl-sdk-and-software)
Tags: mpa
Posted: 2023-10-12 09:06:39 UTC by Caio Licínio Vasconcelos Pantarotto
Replies: 1 · Views: 568

## Caio Licínio Vasconcelos Pantarotto · 2023-10-12 09:06:39 UTC

There is one thing on the VIO reference system that made me confused... We have T_imu_wrt_vio which is "Translation of the IMU with respect to VIO frame in meters", and we have T_cam_wrt_imu which is "Location of the optical center of the camera with respect to the IMU." I just want to clarify... 

1) I would think that T_cam_wrt_imu (Also the rotation) is a fixed number (since the vector between the camera and the imu should be constant in the fixed frame of the drone).. and when I measured it on the Starling indeed is something like (0.026   0.007   0.024) most of the time.. but the T and R (0 45 90) changes slightly during movement.. is this because the IMU itself moves internally a bit ? 

2) is the T_imu_wrt_vio the XYZ global position of the IMU that is given by the VIO algorithm ? So it should be interpreted as the position in space of the drone... Sorry the sentence "with respect to VIO frame" confused me a bit...

## Reply by Moderator (ModalAI staff) · 2023-10-18 18:49:46 UTC

@Caio-Licínio-Vasconcelos-Pantarotto the VIO algorithm takes the starting extrinsics and refines based on movement of the system. So, it's normal to see small updates to the value