Mevo+’s phased array Doppler technology has four independent receivers picking up the tiny variations in the signals reflected from the spinning ball. These variations are measured and combined to provide the measured spin rate of the ball. When used indoors with limited ball flight distance, it becomes harder to measure the spin rate because the ball may execute only a small number of rotations before being stopped by a screen or net. Therefore reflective stickers are needed to enhance the signal variations due to ball spin, making measurement possible.