Ultra-Sonic Waves, Human Following Robot

The problem with phase measurements is that the wavelength of 40 Khz sound is very small, so even
small angles off 90-deg [head-on to the target] cannot be determined correctly.

wavelength = v/f = (1100 ft/sec * 12"/ft)/40000 hz = 0.33", which is why the 40 Khz transducers are
the small size they are.

Send pulses of 40 kHz instead of continuous. Then you can measure the arrival times.

John is on to something here, except he means the "difference" of arrival times. The difference will be
zero when the target is head-on. Note also that, measuring difference of arrival times is a digital
measurement, and much easier to do than any sort of analog phase measurements.