Now, I can see wifi signals

This is why most WiFi devices have two antennas and a diversity receiver.

Spread-spectrum systems are resistant to Rayleigh fading, because multipath nulls are frequency-specific; the two paths have to be half a wavelength different in length to get cancellation. So it seems surprising at first to see this with WiFi, which is a spread-spectrum system.

There's a reason for that. Cancellation becomes less of a problem as you get further from a spread spectrum transmitter, because the phase difference is related to the product of the frequency change times the number of wavelengths along the path. (This really needs pictures.) Once you're far enough away that the spreading moves the nulls at least a half wavelength, the effect is that a fixed percentage of data is lost, and error correction deals with that. In this demo, transmitter and receiver are very close, so the nulls are strong.

Because terrific work causes us to think of additional questions, I'd add: The only thing I found missing is that there is more than one polarization. So to accurately map the field, you'd need to perform multiple scans, with the receiving antenna rotated 90 degrees for each one.

For reconstruction there should be some ideal interpolation, maybe a 3d sync filter?

Another random interesting fact: if you capture not only the intensity but also phase of only a 2D slice, and have some environmental information (i.e. know how the signal propagates in your room), you can reconstruct the full 3D intensity.

http://www.itsnicethat.com/articles/luis-hernan-digital-ethe...

Anyways, reminds me of this: https://vimeo.com/20412632

The author had a nice summary here: http://www.reddit.com/r/dataisbeautiful/comments/2vuvhf/prec...

http://gfycat.com/ImportantSafeBlackmamba

It's always nice to see multiple approaches to the same problem.

Imagine all these things, wifi, FM/AM Radio, iBeacons, cell towers etc.. It must be so noisy in the radio spectrum..

[1] http://en.wikipedia.org/wiki/Geordi_La_Forge