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Originally Posted by ErikJ
Something I haven't seen made mention of yet is multipath interference*.
* multipath interference - in short, when you recieve a signal that has been reflected off a few objects before you receive it.
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Multipath is a degradation of signal due to a reflected signal arriving with the direct signal at a receiver. When the reflected siganl is far enough apart that the reflected signal is 180 degrees out of phase and near the same level as the direct signal, the two cancel out. Obviouosly at the frequency we are interested in (the IR frequency not the modulation 40kHz) there can be several objects on the field that will produce reflections that are 1/2 wavelength or multiples of 1/2 wavelength giving the out of phase condition.