Geoscience Reference
In-Depth Information
Fig. 2.5 Principles of EDM
of the
electromagnetic wave and the meteorological elements of the atmosphere. The
relationship between n and the air temperature t, the barometric pressure p, and
the humidity e is expressed as (Duan 1996):
The value of the index of refraction n is dependent on the wavelength
ʻ
n
¼
f
ð
ʻ;
t
;
p
;
e
Þ:
ð
2
:
5
Þ
Knowing the wavelength of the electromagnetic wave and the temperature,
pressure, and humidity of the atmosphere, the value of the refractive index n can
be computed according to (2.5).
To sum up, the principle of EDM is to use instruments to measure directly or
indirectly the round-trip travel time t 2D of the electromagnetic wave along the
distance D and to measure the temperature t, the pressure p, and the humidity e at
the same time to compute the distance according to the above formula.
It is clear that we can directly measure the distance between two points at the
endpoint using the EDM method. Distances under any topographic conditions can
be measured provided that the measurement range is reached and no obstacles
interrupt the line of sight between the two points. Distances between mountains,
rivers, and even planets for instance, using satellite laser rangefinder can also be
measured directly, which can greatly reduce observation time.
Basic Methods of Electromagnetic Distance Measurement
There are three basic methods of EDM:
1. Method of Distance Measurement by Pulse (Pulse Method)
The distance D to the target is attainable if we directly measure the travel time
t between the transmitted pulse (dominant wave) and the reflected pulse (echo)
from the target. With this, the distance can be obtained with a measurement
performed only once, the measurement range varying from several kilometers to
hundreds of thousands of kilometers. The precision generally reaches centimeter
levels. Such a pulse method is chiefly used for measurements of low accuracy or
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