Most existing GPS devices must have clear satellite views in order to work. If the view is obstructed by metal or other materials, the GPS signal isn't detectable. So, that limits the effectiveness, if the device is placed inside an ATM or if a stolen ATM is put into an enclosed van. Some say the answer to that problem may be a technology called "assisted GPS." Rather than sending a signal directly to satellites, assisted GPS uses existing cellular networks to send signals to cellular towers or bases, which then transmit the signal to a satellite. That's similar to the type of technology being used in tracking devices installed by ATM manufacturer Wincor Nixdorf International. The company's current tracking system, which is available as an option on all Wincor ATMs, uses mobile communications technology. "The location system ensures that, at the very moment the ATM is removed from the bank, an SMS (short message service) text message is sent automatically to the tracking center, triggering a tracking and pinpointing process," Wincor says on its Web site. "The new tracking system means that, should an ATM be stolen, the route it takes can be detected and traced immediately. The emergency services can be directed accordingly and are able to re-locate the stolen object quickly, secure the cash contained in the ATM, and track down the perpetrator(s).