ALPS: An Ultrasonic Localization System

Patrick Lazik Niranjini Rajagopal Bruno Sinopoli Anthony Rowe

Carnegie Mellon University{plazik,niranjir,brunos,agr}@andrew.cmu.edu

ABSTRACTWe demonstrate ALPS, an ultrasonic zone-based andtime-of-flight localization system for mobile devices. ALPScapitalizes on the ability of smart-phones to detect au-dio above the human hearing range and does not requireany additional hardware. Synchronized transmitters de-ployed in the environment periodically send time multi-plexed ranging signals consisting of ultrasonic chirps.Each mobile device runs an application that recordsa snapshot of audio, determines the Time Of Arrival(TOA) of the captured ultrasonic signals and then usesout-of-band map data to compute a location. Blue-tooth Low Energy (BLE) hardware integrated into thetransmitters allows for the identification of the capturedultrasonic signals in order to map them to individualtransmitters and provides additional RSSI based loca-tion information. The confinement of ultrasound withinwalls is ideal for zone-based localization and the rel-atively slow propagation of sound allows for accurateTime Difference Of Arrival (TDOA) and TOA rangingin larger open spaces.

1. SYSTEM DESCRIPTIONALPS is an indoor ultrasonic ranging technique that

can be used to localize modern mobile devices like smart-phones and tablets without requiring any additional hard-ware on the receiver side. The method uses a commu-nication scheme in the audio bandwidth just above thehuman hearing frequency range where mobile devicesare still sensitive. ALPS uses a variant of the ultrasonicmodulation technique described in [2] where multipletransmitters can send time multiplexed ranging signals

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Figure 1: System Architecture

to any number of mobile receivers. The overall archi-tecture shown in Figure 1 shows the main componentswhich include the transmitters, the mobile device thatis being localized, along with the localization and map-ping backend. The transmitters are time synchronizedusing an 802.15.4 radio to enable high resolution TDOAranging given the geographical origins of the signals.The transmitters continuously broadcast their transmis-sion schedule via BLE to alllow every receiver to mapits captured ultrasound signals to the transmitters thatsent them. Once a receiver has successfully computedits location using TDOA ranging, it may synchronizeitself to the transmitting infrastructure as described in[1]. This allows TOA ranging to be performed, reducingthe amount of ultrasound signals that need to be cap-tured successfully. Additional information from inertialmeasurement sensors on the phone and BLE RSSI datafrom the transmitters is used to filter location estimates.

Figure 2 shows an ALPS transmitter. In our deploy-ment, the transmitters are mounted on tripod standsthat will be placed thoughout large rooms or in anysingle zone-based areas. The ultrasound transmitterfeatures an omnidirectional horn, which disperses theultrasound in a spherical pattern. The receivers canbe any mobile device sensitive to up to 24kHz of audio

Horn  

802.15.4  Antenna  

Ultrasonic  TransmiDer  

Piezo  Bullet  Driver  

Figure 2: ALPS Transmitter

bandwidth that runs our localization software. The userdownloads an application which includes a map con-structed during installation. As the user walks aroundthe room, the application tracks the user and providesthe current location. The application will be demon-strated on an iOS device.

2. REFERENCES[1] P. Lazik, N. Rajagopal, B. Sinopoli, and A. Rowe.

Ultrasonic time synchronization and ranging onsmartphones. In IEEE Real-Time ApplicationsSymposium (RTAS 2015). IEEE, 2015.

[2] P. Lazik and A. Rowe. Indoor pseudo-ranging ofmobile devices using ultrasonic chirps. InProceedings of the 10th ACM Conference onEmbedded Network Sensor Systems, pages 99–112.ACM, 2012.