A new smart phone system under development can locate you even when you're inside a building, which GPS can't do. While the concept sounds powerful, it's not yet clear how practical it might be. The set-up, called SurroundSense, sends information from cameras and microphones--gear in the latest smart phones--to a server that produces a fingerprint of the location. "You can't tell much from any of the measurements individually, but when combined, the optical  , acoustic   and motion information creates a unique fingerprint of the space," said Ionut Constandache, a student in computer science. The system distinguishes between two indoor spaces in this manner: It knows that a bar is likely to be dark and noisy and people move little, sit a lot, whereas a department store will be brightly lit, contain vibrant colors, and will see a lot of people moving up and down passages. The more the system is used, the more it learns about specific places. More work is needed before the set-up would be ready for prime time, however. For now, it can't work if a person's cellphone is in a handbag or otherwise unable to employ the camera. Finally it would be best served if paired with phones that were worn on the wrist or hung around the neck. Batteries, too, are a problem. Students on the research team went out across Durham, North Carolina, with their cellphones collecting data in different types of businesses. The students mirrored the actions of the selected customers so that they would not prejudice the measurements. "We went to 51 different stores and found that SurroundSense achieved an average accuracy   of about 87 percent," Constandache said. "As the system collects and analyzes more and more information about a particular site, the fingerprint becomes more precise," said Romit Roy Choudhury, professor of computer engineering. "Not only is the _ different at different locations, but also can be different at different time at the same location." The main idea of the passage is about  _  .
comparing GPS with a smart cellphone the development of cellphones a cellphone knowing where you are what kind of future cellphones face
Question: A new smart phone system under development can locate you even when you're inside a building, which GPS can't do. While the concept sounds powerful, it's not yet clear how practical it might be. The set-up, called SurroundSense, sends information from cameras and microphones--gear in the latest smart phones--to a server that produces a fingerprint of the location. "You can't tell much from any of the measurements individually, but when combined, the optical  , acoustic   and motion information creates a unique fingerprint of the space," said Ionut Constandache, a student in computer science. The system distinguishes between two indoor spaces in this manner: It knows that a bar is likely to be dark and noisy and people move little, sit a lot, whereas a department store will be brightly lit, contain vibrant colors, and will see a lot of people moving up and down passages. The more the system is used, the more it learns about specific places. More work is needed before the set-up would be ready for prime time, however. For now, it can't work if a person's cellphone is in a handbag or otherwise unable to employ the camera. Finally it would be best served if paired with phones that were worn on the wrist or hung around the neck. Batteries, too, are a problem. Students on the research team went out across Durham, North Carolina, with their cellphones collecting data in different types of businesses. The students mirrored the actions of the selected customers so that they would not prejudice the measurements. "We went to 51 different stores and found that SurroundSense achieved an average accuracy   of about 87 percent," Constandache said. "As the system collects and analyzes more and more information about a particular site, the fingerprint becomes more precise," said Romit Roy Choudhury, professor of computer engineering. "Not only is the _ different at different locations, but also can be different at different time at the same location." The main idea of the passage is about  _  .
A. comparing GPS with a smart cellphone
B. the development of cellphones
C. a cellphone knowing where you are
D. what kind of future cellphones face
Answer: