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Noncontact Ranging Sensors
Sensors that measure the actual distance to a target of interest with no direct physical contact are referred
to as noncontact ranging sensors. There are at least seven different types of ranging techniques employed
in various implementations of such distance measuring devices (Everett et al., 1992):
• Triangulation
• Time of flight (pulsed)
• Phase-shift measurement (CW)
• Frequency modulation (CW)
• Interferometry
• Swept focus
• Return signal intensity
Noncontact ranging sensors can be broadly classified as either active (radiating some form of energy into
the field of regard) or passive (relying on energy emitted by the various objects in the scene under
surveillance). The commonly used terms radar (radio direction and ranging), sonar (sound navigation
and ranging), and lidar (light direction and ranging) refer to active methodologies that can be based on
any of several of the above ranging techniques. For example, radar is usually implemented using time-
of-flight, phase-shift measurement, or frequency modulation. Sonar typically is based on time-of-flight
ranging, since the speed of sound is slow enough to be easily measured with fairly inexpensive electronics.
Lidar generally refers to laser-based schemes using time-of-flight or phase-shift measurement.
For any such active (reflective) sensors, effective detection range is dependent not only on emitted
power levels, but also the following target characteristics:

• Cross-sectional area—determines how much of the emitted energy strikes the target.
• Reflectivity—determines how much of the incident energy is reflected versus absorbed or passed
through.
• Directivity—determines how the reflected energy is redistributed (i.e., scattered versus focused).
Many noncontact sensors operate based on the physics of wave propagation. A wave is emitted at a reference
point, and the range is determined by measuring either the propagation time from reference to target, or
the decrease of intensity as the wave travels to the target and returns to the reference. Propagation time is
measured using time-of-flight or frequency modulation methods.

Ranging by Time-of-Flight (TOF)
Time-of-flight (TOF) is illustrated in Figs. 19.61 and 19.62. A gated wave (a burst of a few cycles) is
emitted, bounced back from the target, and detected at the receiver located near the emitter. The
emitter and receiver may physically be both one sensor. The receiver may also be mounted on the
target. The TOF is the time elapsed from the beginning of the burst to the beginning of the return signal.
The distance is defined as d = c ⋅ TOF/2 when emitter and receiver are at the same location, or d = c ⋅
TOF when the receiver is attached to the target. The accuracy is usually 1/4 of the wavelength when
detecting the return signal, as its magnitude reaches a threshold limit. Gain is automatically increased
with distance to maintain accuracy. Accuracy may be improved by detecting the maximum amplitude,
as shown in Fig.19.63. This makes detecting the time of arrival of the wave less dependent on the amplitude
of the signal. Ultrasonic, RF, or optical energy sources are typically employed; the relevant parameters

Emitter/Receiver Target
FIGURE 19.61 A wave is emitted and bounced from a
target object. The distance d is determined from the
speed of travel of the wave, c, and the time-of-flight, TOF d
as d = (1/2) · c · TOF.

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