Optics: measuring and testing – Range or remote distance finding – With photodetection
Reexamination Certificate
2002-04-16
2003-11-11
Tarcza, Thomas H. (Department: 3662)
Optics: measuring and testing
Range or remote distance finding
With photodetection
C356S004010, C356S320000, C356S302000
Reexamination Certificate
active
06646724
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a device for determining the influence of dispersion on a distance measurement by the principles of phase or pulse modulation according to the preamble of claim
1
, as well as a use of the device for correcting distance measurements for the influence of dispersion according to claim
1
and a geodetical instrument according to claim
12
.
In almost all electronic distance mesauring (EDM) devices, the influence of atmospheric parameters is added after the distance measurement properly speaking, in terms of a distance correction obtained in a calculation step. The pertinent atmospheric parameters are measured, not with the distance meter itself but with other, separate instruments such as thermometers, barometers, and hygrometers.
The rate of propagation of an optical pulse emitted in electro-optic distance measurements, or of a signal train modulated in whatever way, is determined by the group refractive index n. Here the refractive index and the group refractive index depend primarily on wavelength, temperature, atmospheric pressure, the gas mixture and humidity of the prevalent atmosphere, rather than being constant quantities.
A direct range reading D
0
as measured and displayed by the electronic distance meter (EDM) (the raw measurement) refers to a particular group refractive index n
0
. The true group index n=(T, p, RH, . . . ) can be calculated with the aid of the meteorological parameters of temperature T, atmospheric pressure p, and relative humidity RH. Using a so-called atmospheric correction
Δ
D
=
D
0
·
(
n
0
-
n
n
)
(
1
)
the true distance D can be determined:
D=D
0
+&Dgr;D.
(2)
Using this atmospheric “post processing” procedure one can attain distance measuring precisions attaining 1 ppm, but the raw distance D
0
that is read can easily deviate from the true value by 30 ppm or more when temperature T and atmospheric pressure p are not known or not representative over the full optical path.
When the distances are longer and then often cover a nonuniform topography, then it becomes doubtful that the effective group refractive indices can be determined reliably from meteorological data applicable at the two extreme points of the range. Attempts to determine such data along the way have not been successful so far.
In geodesy an economic distance meter is desirable which can automatically and rapidly correct for the influence of the atmospheric refractive index. At distances in the range between 100 m and several km this index has a decisive influence on the results of electro-optical distance measurements. This is true, both for electro-optical distance meters based on phase measurements and for distance meters based on travel time measurements. Correcting automatically for the influence of atmospheric refraction one could considerably reduce the time and instrumental requirements in precision measurements involving long distances.
One of the basic ideas is that of utilizing the spectroscopically wide-band dispersion, by measuring the distance with light or electromagnetic radiation having two different wavelengths. This two-color or multicolor procedure has been known since about 1975. When measuring the distance simultaneously with at least two different electromagnetic wavelengths, either optical or in the microwave range, and accounting for the known, spectroscopically wide-band dispersive behavior of the atmosphere, one can determine the most important atmospheric interference parameter(s), and thus substantially correct the distance value for the influence of the group refractive index, which as a rule is not precisely known.
Pertinent theories rely on the spectroscopically wide-band formulas of Edlen and of Barrel & Sears. (Ref. Rainer Joeckel, Manfred Stober: Elektronische Entfernungs-und Richtungsmessung [Electronic distance and direction measurements], Konrad Wittwer Publishers).
The resulting distance values from the two carrier wave-lengths are D
r
and D
b
, the corresponding refractive indices are n, und n
b
. The true distance is obtained from the following relation for distance correction:
D
=
D
r
-
(
D
b
-
D
r
)
·
(
n
r
-
1
n
b
-
n
r
)
.
(
3
)
The actual difficulty of this two-color method, which relies on the model of the spectrocopically wide-band formula, is related to the resolution and precision needed in determining the difference of distances (D
b
−D
r
).
The model parameter
Q
=
(
n
r
-
1
(
n
b
-
n
r
)
)
(
4
)
becomes smaller and more favorable the further apart the two carrier wavelengths. The model parameter is very large, and with it the error of distance correction, when the two carrier wavelengths are not sufficiently far apart.
The current limit of precision that can be attained in measurements of optical signals is known to be around 1%, the main factor influencing the signal measuring errors being the atmospheric turbulence. Since the influence of resolution is independent of the distance, this kind of two-color instrument will be potentially superior to single-color instruments at distances well beyond 2 km.
Known two-color instruments are for instance the Goran I from National Physical Laboratory (Teddington/UK) with &lgr;
b
=458nm and &lgr;
g
=514 nm and the large value of Q=57. For a distance error of 1 mm, the resolution needed is then 0.02 mm. This can only be realized, if at all, with very substantial input, hence this method has so far not become accepted. So far no commercial instruments are in use, and installations built up to now are very expensive and occupy the surface area of a lab bench.
From the patent document U.S. Pat. No. 5,233,176, a device using the two-color method is known which compensates the results for the influence of atmospheric effects by evaluating the departure of two laser beams having different wavelengths from corresponding reference beam pathways. Laser light is emitted in short pulses with two different carrier wave-lengths. From the dispersive shift of the two pathways from a straight line, the dispersive influence is deduced and the results corrected.
It is a considerable drawback of known devices with two or three carrier wavelengths that they utilize the small variations in wide-band optical dispersion resulting from two closely spaced carrier wavelengths. In doing so they follow the wide-band models of Barrel & Sears or the relations of Edlen. The main drawback is the small size of the quantity measured, which provides an inaccurate distance correction inferior in its quality to the classical atmospheric correction involving a determination of the meteorological parameters T, p, and RH. According to the models of Barrel & Sears or the relations of Edlen, the difference in group refraction of the atmosphere between red (e.g., 635 =m) and near infrared (e.g., 820 nm) is very small (about 5 ppm).
One thus can summarize the essential drawbacks of all hitherton existing instruments with two or three carrier wavelengths, as follows:
(i) Using carrier wavelengths having little separation leads to a very small difference in optical dispersion.
(ii) This leads to the need for an extremely high resolution which in the prior art cannot be realized under field conditions.
(iii) The alternative of using carrier wavelengths that are widely spaced, according to the prior art requires a high instrumental effort inasmuch as the requirements for controls, switching, etc. are different between the two wave-length regions.
(iv) The instruments to be realized when using carrier wavelengths that are widely spaced do not meet the requirements of surveying technology, especially so with respect to their weight and ruggedness.
SUMMARY OF THE INVENTION
It is a technical object of the present invention to provide a geodetic device, fit for field measurements of dispersion over a visible range of distances using a multi-color method requiring lower resolution for a given distance error or yielding a smaller distance error for a given, attainable re
Benz Paul
Hinderling Jürg
Andrea Brian
Leica Geosystems AG
Oliff & Berride PLC
Tarcza Thomas H.
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