The Cricket Indoor Location System

Nissanka Bodhi Priyantha

and

Michel Goraczko

MIT Computer Science and Artificial Intelligence Lab

http://nms.lcs.mit.edu/cricket

Cricket

• Location for mobile applications– Resource discovery

– Human/Robot navigation/tracking

– Games

• Location-aware sensing– Routing

– Data dissemination

– Annotating data with location information

Resource Discovery

“Print map on a color printer,”and system sends data to thenearest available color printer

Virtual/Physical Games

Location-Aware Sensing• Ubiquitously deployed

networked sensors can enable new forms of monitoring and control– Tracking assets (RF-ID

tags)– Monitoring conditions– Traffic, facilities, physical

plant, leaks, smoke, fire…• Sensor streams usually need

to be annotated with location and time for them to be useful

• Goal: Cricket-enabled sensors

Temperature, light, sound sensors, buzzer

Temperature, light, sound sensors, buzzer

Functionality: Space, Position, Orientation

Room 7(N)

Room 7(S)

Co

rrid

or (x y z)

● Space: Rooms, parts of rooms● Position: Coordinates● Orientation: Direction vector

Cricket provides all three

Design Goals for Cricket

• Must operate well indoors• Must scale to large numbers of devices• Should not violate user location privacy• Must be easy to deploy and administer• Should have low energy consumption

Cricket Architecture

Beacon

Listener

Passive listeners + active beacons scales well, helps preserve user privacyDecentralized network of autonomous beacons

info = “a1”

info = “a2”

Estimate distancesto infer location

Obtain linear distance estimatesPick nearest to infer “space”Solve for device’s (x, y, z)Determine w.r.t. each beacon and deduce orientation vector

SPACE = NE43-510COORD = (146 272 0)

• A beacon transmits an RF and an ultrasonic signal simultaneously– RF carries location data, ultrasound is a narrow

pulse

• The listener measures the time gap between the receipt of RF and ultrasonic signals– Velocity of ultrasound << velocity of RF

Determining Distance

RF data(space name)

Beacon

Listener

Ultrasound(pulse)

Multiple Beacons Cause Complications

• Beacon transmissions are uncoordinated

• Ultrasonic pulses reflect off walls

These make the correlation problem hard and can lead to incorrect distance estimates

Solution: Distributed scheduling algorithms

Beacon A Beacon B

tRF B RF A US B US A

Incorrect distance

Listener

Solution

• Distributed beacon scheduling– Carrier-sense + randomized transmission (reduces

chances of concurrent beaconing)– Idle time between beacon chirps to allow ultrasound

to “die down”– Another possibility is distributed TDMA

• Listener inference algorithm– Processing distance samples to estimate location– Key problem is outlier rejection

Example Estimation Algorithm:Windowed MinMode

Distance(feet)

Frequency

A B

5 10

5

109Majority

6.47.2Mean (feet)

86Mode (feet)

86Actual distance (feet)

BA

Kalman Filter: For Mobile Listeners

● Min-mode based estimation is “slow”– Needs multiple samples from a beacon

– Poor performance with mobile listeners● Solution: Use a Kalman filter based estimator

– Kalman filter uses a “predict and correct” approach to filter outliers

Beacon Autoconfiguration

● For listeners to compute their position,beacons need to transmit their locations

● Problem: How do beacons determine their locations?– Manual configuration (e.g., counting ceiling tiles) is

possible but tedious● Goal: Scalable autoconfiguration

Beacon Autoconfiguration: For a Small Number of Beacons

● Place the listener under each beacon and collect distances to each beacons

● Calculate inter-beacon distances● Calculate beacon coordinates

– Selecet three initial beacons and assign coordiantes– Trangulate the others using these three

dA dB

dABA

B

dA dB

dABA

B

Beacon Autoconfiguration:Anchor Free Localization (AFL)

● For large number of beacons● AFL is a two phase distributed algorithm for

node localization– Phase 1: Fold-free initialization'– Phase 2: Optimization

● AFL needs local inter-beacon distances– Cricket beacons cannot measure interbeacon

distances● Listener-assisted AFL

– Use a mobile listener to patch-together disconncted beacons

Cricket Hardware Platforms

• Two platforms– CF interface

– Rs232 interface

• Both platforms can be software configured as a Beacon or a Listener– Unused submodules can be powered down to save

energy

• Can be programmed using a standard Mica programming board (MIB-510CA)

Cricket Hardware

Cricket Block Diagram

u-controllerAtmega 128L

RF interfaceCC1000

Ultrasonic Tx

Ultrasonic Rx

Temperature Sensor

51-pin mote interface

• Ultrasonic transmitter

– 40kHz piezo transmitter

– Driven by a 12V square wave

• Ultrasonic receiver

– 40kHz piezo sensor

– Two amplifier stages followed by an envelope detector

– Second amp stage has software adjustable gain

Serial IDSerial IDHost InterfaceCF or RS 232

Cricket Block Diagram

• RF interface– CC1000 based – similar to Mica2– Additionally, there is a carrier threshold detector

• Mote connector – Can interface with a Mica sensor board or a Mica– Communication with Mica through RS232 or I2C

interface

• Temperature sensor– On-board temperature sensor– Used to offset ultrasonic velocity change with

temperature

Ultrasonic Ranging Characteristics

• Radiation pattern– Easy to block the signal at the transmitter, but

not at the receiver

Cricket Precision

● Distance estimation performance● Area coverage

– Error vs distance● Beacon density

Distance Estimation Performance

Coverage Area

● Ultrasound range is 10.5 meters● Radio range must be larger then ultrasound

0 50 100 150 200 250 300 350 400 4500

50

100

150

200

250

300

350

400

450

500

True Distance vs mesured distance

True distance

Mes

ured

dis

tanc

e

TinyOS architecture and implementation

● Application (CricketBeacon and CricketListener)

● Modification to the radio stack● Ultrasound activation● Compensation● Scheduling

Receive(30ms + 1-3ms)

Idle state(800-1200ms)

Send

If failed*

* The receive while is reset each time a beacon is heard

Cricket API

● Serial/CF port API ASCII Binary (not implemented yet)

● TinyOS API

module Cricket { command getDistance( int Beaconid) command BeaconRecord getClosestBeacon(int order) event beaconUpdate(BeaconRecord rec) event beaconChanged()}

example:

Cricket Serial APISerial

Beacon Listener Command Direction Parameters TypeParameter Description Read Write Read Write ASCII BIN Get Output Put Input ASCII BIN

ASCII/BIN Mode • • • • MD 0x01 <mode> <mode> String[1] Char

Node ID Unique node id • • ID 0x02 <id> N/A String[16] Long

Space Name User specified space id • • • • SN 0x03 <name> <name> String[12] String[12]

Distance • • DB 0x04 <distance units> <distance units> string[5],string[2] ushort,char

Duration • DR 0x05 <duration units> <duration units> string[5],string[2] ushort,char

Coordinates • • • • PC 0x06 <tag x y z units> <tag x y z units> String[1],3*string[6],string[2] Char,3*short,char

Sleep • • SL 0x07 <low high units> <low high units> 2*string[5],string[2] 2*ushort,char

Ultrasound lifetime • • • • UL 0x08 <lifetime units> <lifetime units> string[5],string[2] ushort,char

Temperature Temperature on the node • • TP 0x09 <temp units> <temp units> string[5],string[2] ushort,char

Battery Level • • BL 0x0A <level> <level> String[5] Short

Uptime Time since powered up • • UP 0x0B <units> <time units> string[5],string[2] ushort,char

Version Software version • • VR 0x0C None <version> String[5] String[5]

Filter Event filter • • FL 0x0D <filter expression> <filter expression> String[128] String[128]

User defined User defined • • DG 0xFF <user string> <variable> String[128] Variable

Communication protocol to use

Distance between the lis-tener and the beacon

Ultrasound time of flight (uncorrected)

Coordinates of the node (x y z)

Sleep range between beacon chirp

The ultrasound life be-fore the wave attenuate

Reading of the voltage level from the battery

Cricket Demo

● Ranging● 2D localisation demo● ASCII serial API demo