How can the measured radar doppler velocity be extracted?

[rs271]

          @whyekit-motional Thanks for your reply!

My main question is: How can the doppler velocity as measured by the radar sensor (i.e. the relative radial velocity in the radar frame of reference) be derived from those?

If you know the angle between the radar and the object you are measuring, you could resolve vx_comp and vy_comp to get the radial velocity

The radial component of (vx_comp, vy_comp) can't be the measured doppler velocity, because those are the compensated velocities; e.g., the doppler from stationary objects, when the ego vehicle is moving, are non-zero, whereas (vx_comp, vy_comp) is zero then.

It would be obvious to asume that instead, the doppler is the radial component of (vx, vy). But there are several problems with this assumption:

1. (vx, vy) don't always point into the sensor direction, as would have to be the case if they only contained the measured doppler velocity, converted into a purely radial vector. (In principle, it could be that the radar signal processing actually also estimates the tangential velocities, but that's clearly not the case, as can be seen from the plot below.)
2. Instead, for moving objects, (vx, vy) only has one nonzero cartesian component (namely the forward-direction in sensor coordinates), as can be seen from this example (a similar plot as in the tutorial, but using (vx, vy) instead of (vx_comp, vy_comp), for sample[200]; the ego vehicle is stopped at a crossing here):

One could then assume that vx (in sensor coordinates) is the doppler. For moving objects, this seems to work in general, as in this example (vx, vy for sample[20]):

Note that for the moving car in front, again (vx, vy) only have one nonzero cartesian component (in sensor direction). But for the detections from stationary objects, that's obviously not the case. Apparently, they have some additional velocity components induced by the ego motion, which doesn't make sense to me, as that should only be the case for (vx_comp, vy_comp), and implies that neither the radial component nor the vx component (in sensor coordinates) can be the doppler.

So it is still unclear how to obtain the measured doppler from the data.

Unfortunately, we do not have access to the algorithm that the radar uses internally for compensation - vx_comp and vy_comp are simply taken from the output of the radar

But could you perhaps provide some more detailed documentation on the meaning of (vx, vy), (vx_comp, vy_comp) that the radar algorithm outputs? Even if you don't have access to the algorith, I expect there should be a more detailed interface documentation?

[Note: I wanted to re-open this issue, because it's not resolved yet, but wasn't able because @whyekit-motional had closed it.]

Originally posted by @rs271 in https://github.com/nutonomy/nuscenes-devkit/issues/924#issuecomment-1569965977

[zsgj-Xxx]

I'm also curious about it. I have consulted the technical documentation of ARS408, and it states that ARS408 is not capable of providing the lateral (sideways) velocity. Therefore, I wonder how the lateral velocity (vy) is obtained in this case.

[whyekit-motional]

Duplicate of https://github.com/nutonomy/nuscenes-devkit/issues/924