Why does the Doppler Effect detect only radial velocity?

Answer 1

This answer assumes that the reader has a fundantal knowledge of what Doppler effect and Doppler radar are. To be effective, Doppler radar, which is an application of the Doppler effect, must be used with the object moving right at or directly away from the Doppler source. It is possible to use Doppler radar on a target moving at a slant, but the angle of convergence or divergence would have to be known and calculations would have to be made to get a correct answer as to that object's velocity. In an situation where it is used to determine radial velocity, it would be tricky to get it to work in this application. Radial velocity is the movement of an object in a circle or an arc of a circle. If the radial velocity of an object was being was investigated, one could use Doppler radar. But there are some issues. Understand that the object is moving in an arc or a circle, like a ball on the end of a string that is being whirled about. The observer would probably want to stand outside the circumference of the circle. Then a "line of sight" would have to be set up to catch the tangent of the circle so the speed of the object could be sampled in an instant when it was moving directly at or away from the Doppler source. This is possible, but challenging. The radius would have to be measured (if it was not know) and a calculation would have to be made. With the radius of the path of travel measured, and the instantaneous speed discovered via the Doppler radar, a calculation would be rather simple. Note that the radial velocity found would be an instantaneous radial velocity. It the radar unit could not be used to sample an instant when the object was moving right at or directly away from the Doppler source, this would make the calculation really, really difficult. It is still mathematically possible to find an object's radial velocity by measuring the speed of an object that is not moving directly at or away from the source, but it's really, really hard. The angle of the objects path of travel with reference to the direction to the Doppler source would have to be known at the instant the radar imaged its speed. Some calculations could then be made. But it's tough. Certainly if the object under investigation is moving laterally with reference to the radar source (moving directly perpendicular to the line of sight from the radar unit), the source will not be able to see it as it is moving at a right angle to the radar. That means no Doppler effect with which the electronics can work to determine speed. Let's wrap up on this important note. When any object moves in anything but a straight line, its velocity is changing - even if its speed remains constant. That's because velocity is speed with a direction vector tacked on to it. An object moving on a radial path is constantly changing velocity, even if it's just changing direction and moving at a constant speed.

Answer 2

The Doppler effect only detects radial velocity because it only works for measuring velocities that are 'towards' or 'away.' These are the only motions that can compress or extend a wavelength.