Radar utilizes the Doppler principle to measure what aspect of an object?

The correct choice is speed because the Doppler principle fundamentally relates to the change in frequency or wavelength of waves in relation to an observer who is moving relative to the wave source. In the context of radar technology, when a radar wave is transmitted towards a moving object (like a vehicle), the frequency of the reflected wave changes based on the object's speed. This shift is detected and calculated to determine the speed at which the object is traveling.

The other options, while relevant in certain contexts, do not apply to what the Doppler principle specifically measures. For example, shape and weight are not determined through the Doppler effect, as they relate to physical dimensions and mass, respectively. Distance can be measured by radar using the time taken for the radar signal to return, but that measurement does not directly involve the Doppler principle. Thus, the use of the Doppler principle in radar systems is specifically focused on measuring the speed of objects.