Decoding Intelligence, Joerg Arnu and Clark

Along with fellow base-watcher Joerg Arnu, Clark began mapping the sensors, using a handheld frequency counter to sniff out their tell-tale radio transmissions, Arnu said in an interview last year. Together they exhumed as many as 40 of the boxes, noted their unique three-digit codes, then reburied and tested them, said Arnu.

Taken from:
http://www.theregister.co.uk/2005/01/28/area_51_charges_dropped

The precedent events are next:

"The pair found that, at close range, they could use a handheld frequency counter to pick up the wireless signals given off by the devices as a car passes. Over the following month and half, Clark and Arnu engaged in a kind of geocaching game with the Men in Black, systematically sniffing out the road sensors with the frequency counter, exhuming them, and opening them up. They discovered that each device was coded with three-digit identifier that could be read off an internal dial, allowing Arnu to make a list that correlated each unit's I.D. number with its GPS coordinates, creating a virtual map of a portion of the surveillance network surrounding the Groom Lake facility...

Based on their survey, Clark and Arnu have estimated that there are between 75 and 100 sensors...The reaction from the government was immediate, according to Arnu: the road sensors were fitted with a new feature aimed at better eluding detection. Now the transmitters would wait a minute or two before broadcasting an alarm, so that desert wardrivers are out of range before the transmission takes place -- at least, using relatively insensitive detection equipment like a frequency counter...
FBI and Air Force agents raided Clark's trailer home in Rachel, and carted off his computer, photographs and records... they charged Clark with a single count of interfering with a communications system used for the national defense. On March 12th, 2003 Clark allegedly obstructed, hindered and delayed "a signal from a mini intrusion device" located outside "the Nevada Test and Training Range"...

Clark's adventures near the most famously secret patch of real estate in the world appear to have pulled him beneath the very cloak of secrecy he poked and scratched at for so many years. He has, in a sense, become a part of Area 51.

Taken from:
http://www.securityfocus.com/news/8768