Since its storage, software and battery power reside on the iPod Nano, the Sport Kit's receiver is a little simpler than the transmitter. It plugs into the Nano's dock connection with a multi-pin connector -- these pins carry data and power back and forth between the two devices. The receiver itself has only a few components, including a processor, a receiver, an antenna and several resistors and capacitors.
The first time someone uses the Nike + iPod system, the sensor links to the receiver. The receiver learns to recognize that particular sensor's identification code. For this reason, multiple people can run together without their Nike + iPod sensors interfering with one another. If several people in one family use the same iPod but different sensors, the receiver can learn to recognize each sensor, and the Nano can store different settings for each user.
This system is flexible, and the 60-foot (18.2-meter) range of the transmitter makes it unlikely that the two parts of the system will lose contact with one another. However, the sensor's relatively long range has raised privacy concerns about the Nike+ system.
In November of 2006, members of the University of Washington's Computer Science and Engineering department released a paper detailing how the Nike + iPod Sport Kit could be used for nefarious purposes. The paper pointed out that the transmitter sends data regardless of whether the receiver is nearby or even plugged into an iPod. According to the paper's authors, this constant transmission could be used to invade a person's privacy.
The paper proposed several scenarios in which a Sport Kit could be used to track a person. The scenarios had a few technical requirements. In order to successfully track someone using the Sport Kit, a perpetrator must:
- Determine the identification number of the victim's sensor in order to track the right person.
- Conceal receivers and antennae, ensuring that the victim will pass within 60 feet (18.2 meters) of them
- Develop a method for retrieving and analyzing the sensor's data
Several media outlets picked up on the story, and some asserted that the Sport Kit was an easy-to-hack RFID system that threatened people's privacy. However:
- The Sport Kit is not technically an RFID system - its range is much greater than the range of an RFID tag, and it operates on a different radio frequency.
- Although the sensor transmits a unique identification code, it does not transmit a person's name or other identifying information.
- Deploying a surveillance system designed to track a person's Nike + iPod equipment would work only when the victim was wearing his running shoes with the sensor turned on. It would also require an extensive network of antennae.
- Easier location-tracking methods, such as GPS receivers, are readily available.
However, the Nike + Sport kit currently faces a different issue -- in February of 2007, electronics manufacturer PhatRat Technology sued Apple and Nike for patent infringement. PhatRat claims that the Sport Kit infringes on its wireless performance-tracking technology.