Increasing and decreasing chirp signals give same frequency spectrum in fourier domain. Then how wud one find out what it is?.. its' the phase of the signal. The phases are just opposite. See the matlab figure generated u get to know. This file is for beginners who wants to know what role FFT and phase play in chirp.
Yella (2020). Chirp - distinguishing whether it is increaing or decreasing (https://www.mathworks.com/matlabcentral/fileexchange/31746-chirp-distinguishing-whether-it-is-increaing-or-decreasing), MATLAB Central File Exchange. Retrieved .