People Counter, No Camera Required

Apparently, researchers put a pair of wifi cards on either end of a space, and were able to count the number of people walking between them by measuring the energy of the signal bouncing off the people. They claim it is accurate up to nine simultaneous hits in an area 700 square feet large, in indoor and outdoor applications. I did not see a mention of how weather affects this kind of setup in outdoor applications.

So it's basically using WiFi as a radar system?

Well, different object detection systems bounce signals off objects and measure what gets reflected. Radar uses radio waves. Sonar uses sound waves. So this would be a third thing entirely.

This is a pretty good explanation of how radar works, and the different technical problems that will likely have to be worked out before a Wifi based detection and ranging system will be practical.

Radar uses radio waves. Sonar uses sound waves. So this would be a third thing entirely.

WiFi is still radio, no?

Well, yes, but at a much higher frequency than that reserved for WiFi and other ISM applications. This would therefore presumably use much less power and be more practical for civilian purposes such as security.

I also forgot to list LIDAR and ultrasound as other examples of remote sensing technologies.

Some commercial radar applications use higher than wifi frequencies it's true, but a good deal of it (longer range atc/weather) stuff is actually lower than 2.4Ghz.

Sometimes WiFi signals can actually confuse weather radar systems and vice versa, as discussed here.

Cool! Thanks for the links.

There is a small sensor company in Utah building sensors that use tomography in a similar way.

We profiled them a few years ago in our Miracle Alarm Sensor? (Xandem) note. They have a marketable product at this point, but it has not really caught on in any significant way.