The German company Sensorberg offers Bluetooth Low Energy (BLE) beacons called “iBeacons” and a SDK for proximity detection.
WiFi & Bluetooth based
Ultrasound + RF based
RFID (active) based
- OpenBeacon (Open Source)
Blog entry on passive solutions. The industry term for this is RTLS (Real time location system): Passive RFID providing RTLS Real-or-hype
Shopkick uses the build in microphone of a smartphone, currently iOS or Android based, to triangulate its position via a network of ultrasonic beacons. Other then satellite based systems like GPS this allows a more precise location tracking especially in indoor situations where GPS is not or poorly available. Shopkick did a first installation in a Best Buy in San Francisco as part of a POS application.
See also this article here: technologie review article …