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long distances, such as the front line tactical reconnaissance and distance
measurements from Earth to the Moon and from Earth's surface to artificial
satellites.
2. Method of Distance Measurement by Phase (Phase Difference Method)
We can directly measure the phase difference between the transmitted signal
and the echo to get the travel time. The measuring accuracy of such a method is
better than the millimeter level and its measurement range is within dozens of
kilometers. It is commonly used in precision distance measurement on the
ground.
3. Method of Distance Measurement by Interference (Interferometric Method)
This method adopts the physical principle of optical interference for precise
distance measurement with higher accuracy than that of distance measurement
by phase. Its precision generally reaches micrometer levels. It is normally used
for calibrating distance measuring instruments and for precise short-distance
measurement.
Classification of Electromagnetic Distance Measuring Instruments
EDM instruments can be classified into the following three categories according to
the band of their carrier waves:
1. Microwave EDM Instrument
The carrier wavelength ranges from 8 mm to10 cm in the microwave band.
2. Laser EDM Instrument
The carrier is usually red visible light of 0.6328
μ
m wavelength and it is
stimulated emission of radiation (i.e., laser emission).
3. Infrared EDM Instrument
The carrier wavelength usually ranges from 0.75 to 0.95
m and it is stimu-
lated emission of radiation (i.e., laser emission) or spontaneous radiation (fluo-
rescent light).
μ
Generally, electromagnetic distance measurements involve distance measure-
ments with radio waves and with light waves. Measuring distance using radio
waves refers to microwave distance measurement. Light wave distance measure-
ment includes two categories, one being visible light distance measurement and the
other infrared distance measurement.
Visible light wave distance measuring devices can be categorized into two types.
At the early stage of development, the instrument was based on spontaneous
emission with an incandescent lamp or mercury lamp as its light source. In later
models, a red laser (
m) is stimulated by emission of radiation and the
light source is generally a He-Ne-gas laser, as summarized below.
ʻ ¼
0.6328
μ
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