How Does Nike + Sensor works for ipod?

Over the last few years, warnings about the health risks associated with obesity have become increasingly dire. At the same time, computers, game consoles and other electronic devices have become increasingly present in people's homes. So it's not surprising that many new gadgets, from the Nintendo Wii to the ExerStation console controller, combine technology with fitness. The Nike sensor + iPod Sport Kit is similar. Essentially, it combines a portable music player with a pedometer -- two devices that runners have used for years. But the Sport Kit is considerably more advanced than an ordinary pedometer. It uses circuitry, radio waves and software to track and report on a person's workout. The Kit has two components -- a sensor and a receiver, both of which are about an inch (2.5 centimeters) long. The sensor fits into a small space under the insole of a Nike+ shoe. The receiver plugs into an iPod Nano. The Nano is not included, but it is required for the system to work. It provides battery power for the receiver and a user interface for the workout software. Runners use their Nano's click wheel to control the software, which is accessible through the "Nike sensor + iPod" menu. The workout software lets people: Create workout playlists See how far and how fast they've run as well as how many calories they've burned View statistics about past workouts Set workout goals All iPod Nanos shipped after July 13, 2006 come with the workout software already installed. Older Nanos can automatically download the software using the iPod Update feature in iTunes. In addition to providing power and a user interface, the Nano tells runners how the workout is progressing. A computerized voice describes how far they've run, how quickly and how far away the destination is. The Nano's flash drive also provides storage space for workout data. When synched, the Nano transfers that data to a PC or Mac. The computer's iTunes software can automatically upload the data to a Nike+ account. At the Nike Website, runners can view workout statistics and send challenges to other runners. People can also use the site's Map It feature to map and share their routes.

A site called RUNNER+ dissected the the Nike+ sensor to figure out how it works. The Nike+ sensor consists of a accelerometer that measures the acceleration of the runner's foot; the accelerometer is made of piezoelectric material and produces an electrical current that feeds into a local processor chip whenever the material changes shape from impact and compression. The processor converts the acceleration to velocity and distance and sends the data to a nearby receiver on the iPod or iPhone through a small wireless RFID transmitter and antenna in the sensor.

To avoid interference among multiple sensors and receivers, each sensor uses a unique identifier in its transmissions. As pointed out in the Wired article in Markus Tressl's answer, because the RFID transmitter broadcasts the unique ID to all Nike sensor receivers within a 60 feet, malicious attackers could set up receivers to detect and record who is near by, therefore raising privacy implications.