INFRARED AND ULTRASONIC SYSTEMS 23
Borriello et al. measured the extent to which WALRUS’s beacons extended beyond room
boundaries. Figure 3.4 shows that even with the door open, the ultrasound beacons do not extend
much beyond the immediate vicinity of the doorway.
3.2 SUBROOM ACCURACY WITH ULTRASONIC TIME
The Cricket system, developed as part of the MIT Oxygen project, sought to enable subroom-level
location using ultrasound and to do it in a privacy preserving, decentralized fashion . In the
Cricket system, beacons are placed on ceilings or high on the walls in the environment and advertise
their identity using a combination of RF and ultrasound. Like WALRUS beacons, Cricket bea-
cons encode their identity using RF and send ultrasound pulses containing no digital data. Unlike
the systems we have described thus far, the Cricket designers wanted to be able to place multiple
Cricket beacons in a single room to allow Cricket listeners (shown in Figure 3.5) to ﬁgure out which
part of the room (which seat at the table, which work area, which bed, etc.) the user was occupying.
To do this, the Cricket system uses of the fact that relative to modern electronics, ultrasound travels
very slowly. (In room-temperature air, sound travels at only 0.34 m/ms.) On receiving the radio ID
from a beacon, Cricket listeners not only listen for an ultrasound pulse, but time how long it takes
to arrive. Because the radio signal travels almost instantaneously relative to the ultrasound, the delay
between start of the radio transmission and the observation of the ultrasound pulse is a close ap-
proximation of the time of ﬂight of the ultrasound which can, in turn, be converted into an estimate
of the distance the ultrasound traveled. By estimating and comparing the distances of the beacons
being observed, a Cricket listener can accurately predict which beacon is the closest. Thus, Crickets
can be used as a room-level location system, when beacons are sparsely deployed, and as a subroom-
FIGURE 3.5: MIT Cricket implementations .
24 LOCATION SYSTEMS
level system, when multiple beacons are placed in a room. Subsequent extensions to Cricket show
that with sufﬁcient beacon density, their system could determine location within several centimeters
and orientation to within 5° .
One of the major concerns with using time of ﬂight as a distance metric is multipath, or the
phenomenon that a signal may be received multiple times due to reﬂections of the signal in the
environment. The Cricket system mitigates errors due to multipath in two ways. First, the Cricket
designers observed that the ultrasound signals would commonly bend around the edge of obstacles
due to diffraction. As a result, they found it was uncommon to receive a pulse via an indirect path
without ﬁrst receiving it from the direct path. Thus, the Crickets could simply measure the time to
the ﬁrst ultrasonic pulse and ignore the subsequent reﬂections. Second, Crickets perform smooth-
ing by estimating the closest beacon to be the one with the lowest average estimated distance over a
window of samples. This smoothing allows the system to tolerate the occasional ranging measure-
ment error. In practice, these simple heuristics worked well: with a window size of 20 samples, a
listener 2 m from a pair of beacons 1.3 m apart was able to able to distinguish which beacon was
closer with better than 99% accuracy .
One of the most clever aspects of the Cricket system is the way it prevents listeners from
associating the infrared pulse from one beacon with the radio broadcast from another. Given that
the maximum effective ultrasonic range of the Crickets is 10 m, the Crickets beacons intentionally
FIGURE 3.6: The Cricket system fully overlaps each source’s radio transmissions (RF) with the source’s
ultrasonic pulses (US). This ensures that any ultrasonic pulses in ﬂight at the same time are ignored, as
the radio collision can be detected . © 2000, ACM, Inc. Reprinted by permission.