Runtime measurement

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Time-of- flight measurement is a method for indirect distance or speed measurement by measuring the time that a signal needs to travel through the measurement path .

Depending on the type of signal, devices that use transit time measurement are either referred to as

In the case of transit time measurements, essentially only time differences are determined. Therefore - in contrast to measurements on an absolute time scale (world time, atomic time, sidereal time, etc.) - they only need a relative time system , i.e. without a defined zero point.

Bat and dolphin as role models


A well-known model in nature is the bats' orientation system . These actively flying and globally distributed mammals are able to orientate themselves even in complete darkness. With the help of echolocation, you can locate the distance and direction of obstacles and prey from the reflection signals. Dolphins can even reproduce the “images” heard as echo signals in a certain situation and thus communicate to their fellow species about this situation.

This method of propagation time measurement for orientation can also man training phase . It can then be used for orientation for completely blind people.

In contrast to indirect localization, there is the localization of the direct signals from a sound source.

Technical implementation

Principle of the transit time measurement

Since electromagnetic waves travel at the speed of light , the transit times for short distances are extremely short. Time measurement is therefore carried out with special short-term counters or interval counters , and in the laboratory with an oscilloscope. The first applications were distance estimates by means of the difference in transit times of light and sound (lightning-thunder, cannon shots) and the determination of the speed of light.

In the case of distance measurements , the distance range goes from a few meters (hand laser for construction , etc.) to a few kilometers ( EDM for surveying and geosciences ) to millions of kilometers in astronomy and space travel .

The transit time measurement mainly uses:

  1. electrical signals and oscillators ( oscillating and control circuits ); most important applications in laboratory and time measurement , electrical engineering, computer operating systems, etc.)
  2. Sound or ultrasound , for example for depth measurements with an echo sounder
  3. Light waves and infrared for distance measurements, often in the form of laser beams
  4. Radio waves with wavelengths of a few millimeters or centimeters (for example radar and GPS ) up to meters
  5. Very short electrical impulses for fault location in cables.

Echo methods or reflectors are often used over longer distances ( 2, 3) in order to obtain sufficiently strong measurement signals. In the case of radio waves (4), active answering with transponders is also used. For reflected signals, the distance is calculated as , whereby the propagation speed depends on the refractive index of the medium (for light close to the ground about 1,0003) and the signal transit time is called.


The most important uses are:

See also

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