# What is the name for a constant-heading Kalman filter model for vehicle tracking?

When applying Kalman filtering to estimate the position of a car, there are several different vehicle dynamics models that you could use.

One of the simplest is "constant velocity" or CV, which uses the state $$\mathbf x = [x, y, v_x, v_y]$$

A more sophisticated model is "constant turn rate and velocity" or CTRV, which uses the state $$\mathbf x = [x, y, \theta, v, \omega]$$

My coworker has implemented an algorithm that's somewhere in between these two, and I'm wondering what to call it. It uses the state

$$\mathbf x = [x, y, \theta, v]$$ I would expect that this model is called "constant heading and velocity" or "constant bearing and velocity" or similar but I haven't found any papers describing it.

Is there a name in the literature for this model?