Minimally Privacy-Violative Human Location Sensor
by Ultrasonic Radar Embedded on Ceiling
1Yoshifumi Nishida1,2 , Shin’ichi Murakami1,3 , Toshio Hori1,2 , Hiroshi Mizoguchi1,3Digital Human Research Center, AIST
2-41-6, Aomi, Koto, Tokyo, Japan.

2CREST, JST, Tokyo, Japan.

3

Tokyo University of Science
2641, Yamazaki, Noda-shi, Chiba, Japan.

E-mail: y.nishida@aist.go.jp
Abstract
A system observring human daily activities that may violate
privacy is unacceptable in our society. This paper proposesa minimally privacy-violative system for locating a personusing an ultrasonic radar embedded on a ceiling of a room.
One of presumable applications is a system for preventing
aged people suffering from Alzheimer’s disease from accidents. The proposed system can determine the three dimensional position of a person’s head by assuming that thehuman head is an object that will move at relatively highvertical position of a living area. The system doesn’t input
unnecessarily rich information such as images using a camera so that it can reduce the possibility for violating privacy.
The authors constructed an experimental system that consists of 18 ultrasonic transmitters and 32 receivers which areembedded on the ceiling of a room. Experimental results
conﬁrmed the feasibility of the proposed system.

violation, 2)unconstrained measurement of location for thehuman head, 3)easy calibration, and 4)position detection for
the ultrasonic tags, which are compounding this human activities observing system.
ULTRASONIC RADAR SYSTEM EMBEDDED IN A
CEILING
Minimally privacy-violative observation
In this paper, "minimal privacy violation" means that thepossibility of violating one’s privacy is minimized but noteliminated entirely. As such, the following design method isoffered as one solution.
•The developer clariﬁes the information necessary torealize the target application.
•The developer designs a system in which unnecessarilyrich information is not input and is not stored at the system layers, such as the sensor layer, signal process-inglayer, activity recognition layer and application layer.
•The resulting system should reduce the risk of information leaks, or should reduce the cost of an information
management system that avoids the risk of information
leaks.
This paper concerns a human location system that realizes aminimum of privacy violation in the sensor layer. The proposed system uses ultrasonic radar that is embedded in theceiling of a room. The system does not collect unnecessarilyrich information, such as the images taken by a camera, so
as to reduce the possibility of violating individual privacy.

Keywords
Minimally privacy-violative sensor, Human location sensor,
Ultrasonic radar
INTRODUCTION
Applications based on the observation of human activities areincreasingly realizable due to the availability of inexpensivesensing systems[1] and the recent development of ubiquitous computing technologies[2]. There is a strong demandin some ﬁelds for the observation of human activities. A
survey by the authors found that, in order to prevent serious
accidents, caretakers at nursing homes must observe thosepatients who suffer from Alzheimer’s disease.
In our society, however, systems that monitor daily humanactivity are unacceptable if the privacy of the individuals isviolated. There are many problems in using privacy violative
sensor such as a camera[3]. This point is particularly important in private home and nursing home environments wherelong-term, daily monitoring of human activity occurs.
On the other hand, measuring methods which involve devices or sensors attached directly to the human body are notsuitable, since there exist a possibility of discouraging the human’s natural behavior, not to mention the risk of the sensors
being detached or broken in the course of the experiment.
To solve these problems, a system for observation of human activities needs to satisfy two conditions of "minimallyprivacy-violative observation" and "unconstrained observation". The authors have developed an ultrasonic radar systemthat satisﬁes the demands for observing human activity.
In this paper we propose an ultrasonic radar based system
which can comply with the following: 1)minimal privacy

0-7803-8692-2/04/$20.00 ©2004 IEEE.

Unconstrained observation
A RFID and tagging system are the typical tools in observing
the human activities. These methods however request to
have a sensor attached to the human body, this sometimes
compromising the results of the conducted experiment. The
sensors may fall in the course of the experiment or theirplacement in the human body can discourage the naturalbehavior during the experiment’s subject performance.
To successfully address and solve this problem, the conditionof the unconstrained human behavior observation has to besatisﬁed. In this paper we propose an ultrasonic radar system
as one of the elegant solutions for this problem. In general theultrasonic radar system consists of several transmitters andreceivers embedded throughout the measuring environment.
In addition this system doesn’t use any sensors attached tothe subject’s body.
Principle of measurement in ultrasonic radar system433

Lti,rj = |Pti − P| + |Prj − P|

Probability of head existence [%]In the ultrasonic radar system developed by the authors, it isassumed that the human head is an object moving at a relatively high vertical position in a living area, and the positionof the head can be detected by emitting ultrasonic sounds andreceiving them back as they are reﬂected from the head.
This section explains the principle used to measure and calculate, that is, to locate, the position of a human head withunconstraint.
If the positions of the i-th transmitter, j-th receiver and headare Pti : (xti , yti , zti ), Pri : (xrj , yrj , zrj ) and P : (x, y, z),respectively, and the propagation distance is L , as shown in
Fig. 1, then the following equation of a spheroid can beobtained.

receiverPri:(xrj,yrj,zrj)

z20
100x [cm]Figure 2. Example probability distributions for humanhead detection

transmitter-controllers, four receiver-controllers, a network
device and a host computer. Component development forthis system was based on an ultrasonic location sensor system developed previously by the authors[5]. The transmittersand receivers are embedded in a ceiling. The frequency ofultrasounds emitted from the transmitters is 40 kHz. The
distance of ultrasound propagation is determined by measuring time-of-ﬂight, and the transmitters and receivers of thesystem are synchronized for this purpose.

(1)

xL
y30

y [cm]

If Pti and Prj are known, then the head position P can becalculated from the three equations of a spheroid.

transmitterPti:(xti,yti,zti)

40

head location
P:(x,y,z)

receivertransmitter
1800
900
75

250

250 200

1200

250

250

345

220

Figure 1. Principle used to locate human head by ultrasonic radar

Algorithm for estimating head position using redundantdata
The following procedure for estimating the head position isadopted.
1. Emit ultrasounds with the i-th transmitter, and receive
ultrasounds with all of the receivers.
2. Select three receivers from among all of the receivers thatcan detect ultrasounds.
3. Solve for the head position using Eq. (1).
4. Repeat Steps 2 and 3 for all combinations of receivers thatcan detect ultrasounds.
5. Repeat Steps 1 through 4 for all transmitters.
6. Calculate the probability distribution for the existence ofa head in the target area. Figure 2 shows an example ofcalculated probability distribution.
7. Select the position with the highest probability as theoptimally estimated value for the head position.

measuring device on ceiling(Ultrasonic Radar)

Figure 3. Experimental system for locating a human headby ultrasonic radar

The upper part of Fig. 5 shows the measured trajectory of
the human head when the test subject moves as shown in thelower part of the ﬁgure. The ﬁgure shows that the system
can detect the positions of the head at a frequency of 1 Hz.

PROTOTYPE OF ULTRASONIC RADAR SYSTEM AND
EXPERIMENTAL RESULTS
Experimental system
The authors constructed an experimental system based on
ultrasonic radar. Figure 3 is an overview of the system. Figure 4 shows the conﬁguration of the ultrasonic radar. Thesystem is composed of 18 transmitters, 32 receivers, three

Evaluation of accuracy
The authors evaluated the accuracy of the constructed systemin terms of its ability to detect the location of a human head.
Figure 6 shows the results of the evaluation of the system,
where the blue points indicate true locations of the head, andthe red crosses indicate the measured locations of the head.
From the result of evaluation, we conﬁrmed that this system
could detect the position of a human head within an error of54 mm.
FUNCTIONS FOR ACTUAL USE
Simulation software

434

z

PtransPrcvy

Sound Pressure DistributionxPC

SYNC signal
data

RS232(9.6k to 921.6kbps)r

Collect data& SYNC generator

r1zhr2

PICPsRS485(921.6kbps,1km)

Measure time of flight

PrefPICPICPICPICTransmitterθRhdθ

Ultrasonic receiver& amplifier

Ultrasonic transmitter

φdSdφHead Model

Figure 7. Model of ultrasonic reﬂection from human head
and an example of simulation results
Figure 4. Conﬁguration of the constructed ultrasonicradar system

To robustly detect the position of a human head, receivers
must be arranged appropriately according to the environment
in which the ultrasonic radar is installed. The authors havedeveloped a method of simulating sound pressure on the ceiling by modeling, from an acoustic perspective, the emitted
ultrasounds and their reﬂections from a human head. This
allows the proper conﬁguration of ultrasonic transmitters andreceivers to be determined by analysis.
The left part of Fig. 7 deﬁnes variables used to describe ahuman head, a sound source and other points. We assume
here that the transmitter is a single sound source, and the human head is a hemisphere reﬂector. According to Rayleight’s
theory[4], the velocity potential from the reﬂector Φ̇R is expressed by
jkU0 e−jk(r1 +r2 )Φ̇R = −dS.
(2)
8π 2 r1 · r2

170160160z [cm]z[cm]location of a human head
170150150

1401401308013080y[cm]40 00100 150 20050x[cm]40 0y [cm]0100 150 20050x [cm]Hence, the sound pressure P˙R at Prcv becomeskρωU0 e−jk(r1 +r2 )˙˙PR = jωρΦR =dS,
8π 2 r1 · r2

Figure 5. Tracking the position of a human head

where,

head location
measured locationdS = Rh2 sin θ · dθ · dφ.

50z(cm)150-20020020150y (cm) 40

1005080 0(4)The right part of Fig. 7 shows an example of the analysis
results from the simulation. The conditions for this simulation are: 1) The higher part of the sound pressure is assumed
to be that part in which sound is available. 2)A transmitteris placed at the center of the ceiling. 3)A model of the human head is placed under the transmitter. By determining inadvance the sound pressure levels detected by the receivers
to be used, a developer can estimate which receivers can detect the reﬂected ultrasounds and thus conﬁgure the receivers
properly through analysis. According to this analysis, thelattice arrangement of receivers is adopted for the system.

10060(3)x (cm)

Figure 6. Evaluation of human head location accuracy

435

Integration with ultrasonic tagging system
We have developed a 3D ultrasonic tagging system which isable to track the three-dimensional motion of tagged objects
within a error of 2 to 8 cm in real time [5]. The system uses atechnology similar to the ultrasonic radar system, that is, thethree dimensional positions of the tagged objects are calculated based on time-of-ﬂight data. By integrating with the 3D
ultrasonic tagging system, we can obtrain more advantages:
1) The system can detect not only a position of human head
unconstrainedly but also positions of multiple tagged objects
rubostly. Application of this function is described later. 2)
A user can quickly calibrated positions of transmitters andreceivers attached on the ceiling using a calibration method
developed by the authors[6].
Figure 8 shows the conﬁguration of the integrated system.
The ultrasonic tags are synchronized and controlled via.
wireless communication. The receivers are utilized for sampling ultrasonic waves that emited from the transmitters embedded on the ceiling as well as the transmitters of the ultrasonic tags.

ultrasonic radar

ultrasonic tag

exhibits minimum privacy violation. The system does not input unnecessarily rich information, such as the images taken
by a camera, so that the possibility of privacy violation isreduced. We constructed its experimental system composedof 18 transmitters and 32 receivers. The experimental results
show that the constructed system can detect a human headwithin 5 cm of error and can track a human head at a frequency of 1 Hz. This paper also shows the ultrasonic radarsystem integrated with a ultrasonic location system that wasalso developed by the authors. The integrated system satisﬁes
the following demands: 1)minimal privacy violation, 2)unconstrained measurement of location for the human head,3)easy calibration, and 4)position detection for positions oftagged objects.

PC
Collect data& SYNC generator
PIC

Measure time of flight

RS485(921.6kbps,1km)

Radio transmitter
PIC

PIC

PIC

PIC

VHF 314.9MHz
wireless RS232

ACKNOWLEDGEMENTS
This study was funded in part by a Grant-in-Aid from Secom
Science and Technology Foundations, and a Grant-in-Aid aspart of the Special Coordination Fund for Promoting Scienceand Technology from the Ministry of Education, Culture,
Sports, Science and Technology of Japan.

SendS
YN
C&ID

Ultrasonic receiver& amplifier

Ultrasonic transmitter

Ultrasonic 3D tag
Radio Ultrasonicreceiver transmitter
40kHz
PIC

tag controller
for calibration

Battery(b) detection of a locationof the wheerchairby tagging system

Figure 9. Application in nursing home

RS232(9.6k to 921.6kbps)

SYNC signaldata(a) observation for a person’s
behavior on a bed
by radar sytem

calibration device

REFERENCES[1] Hori T. Overview of Digital Human Modeling
Proceedings of 2000 IEEE/RSJ International
Conference on Intelligent Robotics andSystems(IROS2000), Workshop Tutorial Note pp. 1-14,
2000[2] Hansmann U., Merk L., Nicklous M.S. andStober T., Pervasive Computing Handbook. The
Mobile World Springer-Verlag Telos, 2001
[3] Hunter R. World Without Secrets: Business, Crimeand Privacy in the Age of Ubiquitous Computing John
Wiley & Sons, Inc., 2002
[4] Rayleigh J.W.S., The Theory of Sound Vol. 2,
Dover Publications (New York), (original) 1877,(reprint) 1945
[5] Nishida Y., Aizawa H., Hori T., Hoffman
N.H., Kanade T. and Kakikura M. 3D
Ultrasonic Tagging System for Observing HumanActivity Proceedings of IEEE International Conferenceon Intelligent Robots and Systems (IROS2003)
pp.785-791, 2003
[6] Nishitani A., Nishida Y., Hori T. andMizoguchi H. Portable 3D Ultrasonic Tag System
Based on a Quick Calibration Method Proceedings of2004 IEEE International Conference on Systems, Manand Cybernetics (SMC2004) 2004 (in press)

This part is used onlywhen calibration is carried outFigure 8. Conﬁguration of integrated systemPresumable application
One of presumable applications by the integrated system isan observation of patients in nursing home and a hospital.
A survey conducted by the authors, a patient never wants
to have a special device even if the sensor can contribute to
reducing the possibility of accident. However it is acceptablethat caretakers attached a device to a patient’s wheelchair.
The integrated system can cope with such a case, namely,
it can monitor the position of the patient on a bed using
the ultrasonic radar function and monitor the position of thewheelchair using the ultrasonic tagging function as shown inFig. 9. The system as shown in this ﬁgure would work as acomponent of a system for supporting accident prevention.
The authors are developing such a system in cooperation witha nursing home.
CONCLUSIONSIn this paper, we described the concept of minimum privacy violation, in which the possibility of privacy violation
is minimized but cannot be eliminated completely. We proposed an ultrasonic radar system embedded in the ceilingof a room as one example of a human location sensor that436