Minimally Privacy-Violative System
for Locating Human by Ultrasonic Radar
Embedded on Ceiling∗
Yoshifumi Nishida
Digital Human Research Center
AIST & CREST, JST
2-41-6, Aomi, Koto-ku, Tokyo
135-0064, Japany.nishida@aist.go.jp

Shin’ichi Murakami
Mechanical Engineering Department
Tokyo University of Science
2641,Yamazaki, Noda-shi, Chiba
278-8510, Japans-murakami@aist.go.jp

Toshio Hori
Digital Human Research Center,
AIST & CREST, JST
2-41-6, Aomi, Koto-ku, Tokyo,
135-0064, Japant.hori@aist.go.jp

Hiroshi Mizoguchi
Mechanical Engineering Department
Tokyo University of Science
2641, Yamazaki, Noda-shi, Chiba,278-8510, Japanhm@rs.noda.tus.ac.jp

Abstract – Violating the privacy of individuals with systems that measure daily human activities is unacceptable inour society. In this paper, the authors propose a system using
ultrasonic radar embedded in the ceiling of a room to locate
human individuals with a minimum violation of privacy. It is
expected that this system will be used in the prevention of accidents among aged persons suffering from Alzheimer’s disease. The proposed system determines the three-dimensional
position of a person’s head by assuming that the human headis an object that will move at a relatively high vertical position within a living area. In this study, the method used
to locate the human head is analyzed theoretically from theperspective of acoustics. The feasibility of the proposed system is confirmed by experimental results using a system constructed by the authors.
Keywords: Minimum privacy violation sensor, Human location sensor, Ultrasonic radar

those patients who suffer from Alzheimer’s disease. In oursociety, however, systems that monitor daily human activity
are unacceptable if the privacy of the individuals is violated.
There are many problems in using privacy violative sensor
such as a camera[3]. This point is particularly important inprivate home and nursing home environments where longterm, daily monitoring of human activity occurs.
Collecting rich information on individuals leads to an increased possibility of privacy violation. A more rigorous
management of information is therefore required. This isexpensive, however, and more importantly, such a rigorous
management of information cannot be accomplished easily.
The solution is to minimize the possibility of privacy violation.
In this paper, ”minimum 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 to realize the target application.

1 Introduction
Applications based on the observation of human activities are increasingly realizable due to the availability of inexpensive sensing systems[1] and the recent development of
ubiquitous computing technologies[2]. There is a strong demand in 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∗ 0-7803-8566-7/04/$20.00 
c 2004 IEEE.

1549• 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 processing
layer, 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
a minimum 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.
The ultrasonic radar technology used in this study has
been applied widely in such fields such as robot collision
avoidance and medical diagnosis. It has not been applied,
however, in locating persons in their daily living spaces. In
this type of application, the area to be monitored and the distance from the radar system to the human subject will varydepending on the target area, and the amount of space available to install the ultrasonic radar system may be limited insome living spaces even when ceilings are used. To address
this problem the authors developed a simulation method, described in this paper, with which the developer of human locating systems can analyze and determine an optimal configuration of ultrasonic transmitters and receivers appropriate to
the target space.

2(a) arising(b) moving(c) sitting

Figure 1: Examples of detectable behaviors
If the positions of the i-th transmitter, j-th receiver andhead are Pti : (xti , yti , zti ), Pri : (xrj , yrj , zrj ) and P :(x, y, z), respectively, and the propagation distance is L , asshown in in Fig. 2, then the following equation of a spheroid
can be obtained.
Lti,rj = |Pti − P| + |Prj − P|(1)

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

Locating Humans by UltrasonicRadar Embedded in a Ceiling

2.1 Ultrasonic Radar Embedded in a Ceiling

2.2 Application as an accident prevention system
The authors believe this system can be applied as an accident prevention system for aged patients in nursing homes.
Some of the behaviors of aged individuals in and aroundtheir beds, for example, can be inferred from the detectionof head positions. Fig.1 shows some system-detectable behaviors that can be used to prevent accidents such as ”fallingfrom the bed to the floor,” ”sitting up in bed,” ”moving about
on the bed” and ”moving from the bed to another location.”

receiverPri:(xrj,yrj,zrj)

transmitter
Pti:(xti,yti,zti)

This paper proposes ultrasonic radar as a system for locating humans with minimal privacy violation. An ultrasonicradar system basically consists of several transmitters andreceivers embedded in a ceiling. It is assumed in the systemthat the human head is an object moving at a relatively highvertical position in a living area, and the position of the headcan be detected by emitting ultrasonic sounds and receiving
them back as they are reflected from the head.
This system is considered a minimum privacy violation
monitor because it merely detects the location of a humanand infers behavior about the subject from this information.
It cannot be easily used to reconstruct a high-resolution image.

x

L
y

z

head location
P:(x,y,z)

Figure 2: Principle used to locate human head by ultrasonicradar

2.4 Algorithm for estimating head position using redundant data
The following procedure for estimating the head positionis adopted in the experimental system described in Section 4

2.3 Principle of measurement

1. Emit ultrasounds with the i-th transmitter, and receive
ultrasounds with all of the receivers.

This section explains the principle used to measure andcalculate, that is, to locate, the position of a human head.

2. Select three receivers from among all of the receivers
that can detect ultrasounds.

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3. Solve for the head position using Eq. (1).

z

Ptrans

4. Repeat Steps 2 and 3 for all combinations of receivers
that can detect ultrasounds.

Prcv

y
x

5. Repeat Steps 1 through 4 for all transmitters.

r

6. Calculate the probability distribution for the existenceof a head in the target area.

r1

zh

r2

7. Select the position with the highest probability as the
optimally estimated value for the head position.

Ps

3 Sensor Simulation for Adaptation of
the Environment

Pref

3.1 Analysis for receiver configurationθ

3.1.1 Theory of ultrasonic reflection from a humanhead
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
have developed a method of simulating sound pressure on
the ceiling by modeling, from an acoustic perspective, theemitted ultrasounds and their reflections from a human head.
This allows the proper configuration of ultrasonic transmitters and receivers to be determined by analysis. This section
describes a theory of ultrasonic reflection from the humanhead.
Figure3 defines variables used to describe a human head,a sound source and other points. We assume here that thetransmitter is a single sound source, and the human head is ahemisphere reflector.
In the absence of a reflector, the velocity potential Φ̇s at apoint Ps on the reflector is expressed byΦ̇s =U0 −jkr
e
,
4πr

(2)

where U0 , k and r denote the volume of the velocity ofthe sound source, the wave number of the sound wave andthe distance of Ps from the sound source, respectively.
The particle velocity V̇ref at Ps then becomesU0 (1 + jkr) −jkr∂ Φ̇sV˙s = −
=
e
.
∂r
∂r2

(3)

Rh
dθ
dφ

φ

Figure 3: Model of ultrasonic reflection from human head
˙s.
vibrating at particle velocity −V
According to Rayleight’s theory[4], when only the small
element dS situated at Pref vibrates at volume velocity V˙s ·dS , while all other elements do not vibrate, then the velocity
potential caused by the small element is expressed byΦ̇dS = −V̇ref e−jkr2
dS,
2πr2

(5)

where r2 denotes the distance from Pref of a point Prcv
on the ceiling. The velocity potential from the entire reflectorΦ̇R is obtained by integrating Φ̇dS .
Thus

If kr  1, we can then write
jkU0 −jkr
V˙s =
e
.
4πr

dS


Φ̇R =


Φ̇dS = −(4)

If the reflector is placed in this sound field and Ps is on thesurface of the reflector (i.e. Ps = Pref ), then particle velocity
becomes 0.
Now, assuming the reflector has a reflection efficiency of 1.0,
then the velocity potential generated by a reflected wave canbe considered equivalent to that generated by the reflector

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V̇s e−jkr2
dS,
2πr2

(6)

from which, by eliminating V˙s using Eq. (4), we obtainΦ̇R = −

jkU0 e−jk(r1 +r2 )dS.
8π 2 r1 · r2

Hence, the sound pressure P˙R at Pref becomes(7)

P˙R = jωρΦ˙R =kρωU0 e−jk(r1 +r2 )dS,
8π 2 r1 · r2

(8)where ω is the angular velocity and ρ is the density of air.
Note that r1 and r2 depend on the position of the element
dS. To prepare for numerical analysis, we must digitize Eq.
(8). Now, Pref is expressed by
Pref = (Rh sin θ cos φ, Rh sin θ sin φ, zc + Rh (1 − cos θ)) . (9)z=500mmz=900mmz=1300mm

z=600mmz=1000mmz=1400mm

dS can be replaced by
S = Rh2 sin θ · θ · φ.

(10)z=700mmz=1100mm

z=1500mm(11)

z=800mmz=1200mmz=1600mm

and Eq. (8) is digitized as follows.
2−jk(r1i +r2i )kρωU0 Rh
e
P˙r =
ΣΣ
2
8π
r1i · r2i

sin θ · θ · φTransmitter

Sound Pressure
Distribution

low

high

3.1.2 Simulation of ultrasound reflection from a humanheadSound pressure level
Figure 5: Results from a simulation of sound pressure distribution on ceiling3.1.3 Discussion on proper receiver configurationz

Figure 5 depicts the sound fields on the ceiling as theheight of the head z varies from 50 to 160 cm. This figure shows how sound pressure on the ceiling changes as thehead height changes.
By determining in advance the sound pressure levels detected by the receivers to be used, the developer can estimate which receivers can detect the reflected ultrasounds andthus configure the receivers properly through analysis. Figure 6(a) to (d) show examples in which the sound pressurelevel of each receiver on a ceiling is estimated for a ring arrangement as well as a lattice arrangement.
The lattice arrangement of receivers is adopted for the system described in Section 4.

Head Model
Figure 4: Example of simulation results

4 Experimental System

Using Eq. (11), we can create a simulation system forcalculating sound pressure distribution on a ceiling. Figure 4
shows an example of the analysis results from the simulation.
The conditions for this simulation are: 1) The higher part ofthe sound pressure is assumed to be that part in which soundis available. 2)A transmitter is placed at the center of theceiling. 3)A model of the human head is placed under thetransmitter.

4.1 Overview of experimental system
The authors constructed an experimental system basedon ultrasonic radar. Figure7 is an overview of the system.
Figure 8 shows the configuration of the ultrasonic radar.
The system comprises 18 transmitters, 32 receivers, threetransmitter-controllers, four receiver-controllers, a network
device and a host computer. Component development forthis system was based on an ultrasonic location sensor sys-1552

receivable point
unreceivable point

receivertransmitter
1800
900

Z=900

(a) ring arrangement(b) ring arrangement

250 200

250

75

1200

250

250

345

220

Z=600

measuring device on ceiling(Ultrasonic Radar)(c) lattice arrangement(d) lattice arrangement

high
low
sound pressure level

Figure 6: Proper arrangement of receivers
tem developed previously by the authors[5]. The transmitters and receivers are embedded in a ceiling. The frequency
of ultrasounds emitted from the transmitters is 40 kHz, andthe sampling frequency of the system is 1 Hz. The distanceof ultrasound propagation is determined by measuring timeof-flight, and the transmitters and receivers of the system aresynchronized for this purpose.

4.2 Evaluation of accuracy
Figure 9 shows examples of probability distributions forthe detection of a human head, that is, the probability thata human head exists at a particular location, as calculatedby the algorithm described in Section 2.4. The red areas indicate a high probability, and the blue areas indicate a low
probability.
The authors evaluated the accuracy of the constructed system in terms of its ability to detect the location of a humanhead under the following conditions:Figure 7: Experimental system for locating a human head by
ultrasonic radar
Pulse
SYNC
Data
UltrasoundTransmitter
Controller

ReceiverController

ReceiverController

Network
DeviceHost
PC

ReceiverController• A total of 50 measuring points were set at 15-cm intervals in the experimental area.
• The test subject stood at each of the measurement
points.
• The position of the test subject’s head was calculated ateach measuring point and compared with the true position.
Figure10 shows the results of the evaluation, where theblue points indicate true locations of the head, and the redcrosses indicate the measured locations of the head. From
this figure we can see that the system could detect the position of a human head within an error of 5 cm.

Receiver100d

Transmitter
40kHz

Figure 8: Configuration of the constructed ultrasonic radarsystem

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Figure 11: Tracking the position of a human head

Figure 9: Example probability distributions for human headdetection
head location
measured location
50

space and target application, we presented a theory on the reflection of ultrasounds from the human head. The authors
constructed an experimental system using ultrasonic radar
technology comprising 18 transmitters and 32 receivers embedded in a ceiling. The experimental results show that theconstructed system can detect a human head within 5 cm oferror and can track a human head at a frequency of 1 Hz.

z(cm)Acknowledgment

100

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

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References

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60

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x (cm)[1] T. Hori,”Overview of Digital Human Modeling,” in Proceedings of 2000 IEEE/RSJ International Conference onIntelligent Robotics and Systems(IROS2000), Workshop
Tutorial Note, pp. 1-14, 2000

Figure 10: Evaluation of human head location accuracy
The upper part of Fig. 11 shows the measured trajectoryof the human head when the test subject moves as shown in
the lower part of the figure. The figure shows that the system
can detect the positions of the head at a frequency of 1 Hz.

5 Conclusions
In 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 ceiling ofa room as one example of a human location sensor that exhibits minimum privacy violation. The system does not input
unnecessarily rich information, such as the images taken bya camera, so that the possibility of privacy violation is reduced. In order to analyze an optimal configuration of ultrasonic receivers and transmitters in accordance with the target[2] U. Hansmann, L. Merk, M.S. Nicklous, T. Stober, Pervasive Computing Handbook. The Mobile World, SpringerVerlag Telos, 2001
[3] R. Hunter, World Without Secrets: Business, Crime andPrivacy in the Age of Ubiquitous Computing, John Wiley& Sons, Inc., 2002
[4] J.W.S Rayleigh, The Theory of Sound, Vol. 2, Dover
Publications (New York), (original) 1877, (reprint) 1945
[5] Y. Nishida, H. Aizawa, T. Hori, N.H. Hoffman, T.
Kanade, M. Kakikura, ”3D Ultrasonic Tagging System
for Observing Human Activity, ” in Proceedings of IEEE
International Conference on Intelligent Robots and Systems (IROS2003), pp.785-791, 2003

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