At this ramp speed objects in the surrounding can be assumed to be quasi-stationary, i.e. relative movement of the objects can be ignored within a ramp period.
Explanation: (a) Tx signal sTx(t) and Rx signal sRx(t), (b) Delta signal sTx(t) - sRx(t); (c) phase of sTx(t) - sRx(t)
In Figure 1(a) an exemplary fast transmit signal sTx(t) and the received echo sRx(t) is depicted. The frequency difference signal of transmit minus receive signal (see Figure 1(b)) is generated by the down mixer unit. The delta frequency is directly proportional to the distance (range) to the objects that cause a reflection of the electro-magnetic wave. With former slow ramps the delta frequency was ambiguous, because it was comprised of the range component and the Doppler shift component, which was caused by the relative speed of the object. To resolve this ambiguity ramps with different directions and slew rates were used. Fast chirps do not have to cope with this ambiguity due to the quasi-stationary situation. However, a single chirp won’t allow the measurement of the relative speed. By analyzing multiple successive chirps the Doppler shift can be retrieved from the phase information (see Figure 1(c)). By doubling or even quadrupling the number of receive channels the azimuth (horizontal) angle of the surrounding objects can be determined, e.g. with a digital beam forming approach.