Climbing into his Volvo, outfitted with a Matrics antenna and a Motorola reader he'd bought on eBay for $190, Chris Paget cruised the streets of San Francisco with this objective: To read the identity cards of strangers, wirelessly, without ever leaving his car. It took him 20 minutes to strike hacker's gold. Zipping past Fisherman's Wharf, his scanner detected, then downloaded to his laptop, the unique serial numbers of two pedestrians' electronic U.S. passport cards embedded with radio frequency identification, or RFID, tags. Within an hour, he'd skimmed" the identifiers of four more of the new, microchipped PASS cards from a distance of 20 feet. Paget's February experiment demonstrated something privacy advocates had feared for years: That RFID, coupled with other technologies, could make people trackable without their knowledge or consent. He filmed his drive-by heist, and soon his video went viral on the Web, intensifying a debate over a push by government, federal and state, to put tracking technologies in identity documents and over their potential to erode privacy. With advances in tracking technologies coming at an ever-faster rate, critics say, it won't be long before governments could be able to identify and track anyone in real time, 24-7, from a cafe in Paris to the shores of California. The key to getting such a system to work, these opponents say, is making sure everyone carries an RFID tag linked to a biometric data file. On June 1, it became mandatory for Americans entering the United States by land or sea from Canada, Mexico, Bermuda and the Caribbean to present identity documents embedded with RFID tags, though conventional passports remain valid until they expire.