54 IEEE CIRCUITS AND SYSTEMS MAGAZINE FIRST QUARTER 2022 -40 -20 20 40 P1 Human Signature P2 Accelerating P3 Fading Out P4 -5 Stationary Clutter (a) -40 -20 20 40 Human Signature (e) -40 -20 20 Stationary Clutter 40 (i) (j) (k) (l) Stationary Clutter Stationary Clutter Stationary Clutter P1 Human Signature P2 Accelerating Accelerating (f) P3 Accelerating Signature Turning Back -10 -15 -20 -25 -30 Fading Out (g) P4 (h) -5 P1 Stationary Clutter P2 Stationary Clutter Stationary Clutter (b) P3 Stationary Clutter Stationary Clutter (c) P4 Stationary Clutter Stationary Clutter (d) -5 -10 -15 -20 -25 -30 -10 -15 -20 -25 -30 Figure 16. Different frames of ISAR images during fall incidents and an abrupt body movement. (a)-(d): falling toward the radar; (e)-(h): falling away from the radar; (i)-(l): an abrupt movement toward the radar [77] © 2016 IEEE. Doppler (Hz) Doppler (Hz) Doppler (Hz) (dB) (dB) (dB)