Calibration of a Terrestrial Laser Scanner - Institute of Geodesy and ...

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14 2. Components of Terrestrial Laser ScannercA2 2^f'(2 5)where / is the frequency of the sine wave Instead of determining the time At directly, compared to Equa¬tion (2 3), the travel time is replaced byAtA2^7(2 6)If the wave defining the maximum interval is fulfilling more than one period, a multiple N of the wave¬length A has to be added to the range, which is computed by the phase shift Adetermining the absolute range s is [Hmderlmg, 2004]Thus, the equation fors = UnA + A^2 V 2ttwith A =/(2 7)The resolution of the ranges not only dependson the determination of At but also on the determination ofA The ranges is moreprecise with higher frequencies or with more precise determinations of A Gen¬erally, the phase angle A can be solved from 1/4000 and up to 1/8000 of the wavelength, cf [Rueger, 1996]and [Kahmen, 1997] The commonway for determining longer ranges is to use more than one frequencyThe lowest frequency defines the maximum range and the higher frequencies are used for improving thecoarserange provided by the lowest frequency within the desired precision In practice, objectsat farerdistances than the period of the lowest frequency are shifted within the period of the lowest frequencyThis ambiguity is of importance and has to be taken into accountdistancereference object /reflectorFigure 2.7: Amplitude-modulated continuous wave phase difference principle according to [Zetsche, 1979]The range interval for laser scannersoperating with AMCW-method is limited up to nearly100 m sincethe intensity of the modulated signal, e g sine wave and square wave, is decreasing and the phase shiftA cannot be reliably detected However, electronic units allow fast determinations of the range, therebyincreasing the data acquisition rate Typical sampling frequencies of laser scanners operating with theAMCW-method reach values of 100 kHz and up to 700 kHzsurement can be increased within some millimetersThe distance accuracy for one single mea¬Unfortunately, each phase measurement represents asingle shot, which means no distinction can be made regarding first pulses and last pulses Especially atedges, the AMCW-method produces erroneous data that result in mixed pixels, cf Section 4 12 Figure 2 7shows schematicallythe AMCW-methodThe modulation is not only based on one frequencyquencies is also possible For example, the laser scanner 'ImagerThe modulation of two or more simultaneous fre¬5003' of Zoller+Frohlich is based on bimodulationusing two frequencies In Figure 2 8, the AMCW of the 'Imager 5003', representing the highfrequency signal 'hfs' and the low frequency signal Tfs' (close), cf Section 3 2, can be seen Based on thisfigure, the period of the 'hfs' is of about 180 ns and the period of the Tfs' is of about 22 ns and therefore, fitthe nominal values
2.1 Distance and Reflectance Measurement System 15Figure 2.8: Zoller+Frohhch laser scanner thethe amplitudemodulated continuous wavehigh frequency signal 'hfs' and the low frequency signal 'Ifs' (close) of2.1.5 Frequency-Modulated Continuous Wave (FMCW)The frequency-modulated continuous wave method (FMCW) is based on the modulation of a signal byvarying the frequency The emitted signal is modulated by a sine wave at varying frequencieswith the reflected frequency The mixed frequency is low-pass filtered and the generated signalto as the beat signal The range can be estimated by measuring the resulting beat frequencyand mixedis referredwhich followsa periodic waveform with a period Tr The distance to the object is proportional to the (maximum) beatfrequency B, l e the absolute difference in frequency A/ between the received signal and the emitted signal[Hancock, 1999]The FMCW technqiue is not widely-used in terrestrial laser scanners and is more related to radar systems,l e long range applications More information can be found in [Hmderlmg, 2004]Figure 2.9: Typical chirped waveforms and beat frequency for an FMCW laser fe(t) is the emitted frequency, andfr(t) is the received frequency B is the peak beat frequencyfor a measurement At is the delay in the reflected signaland Tr is the period of the frequency chirp (adaptedfrom [Hancock, 1999])
Page 1: DISS. ETH NO. 17036Calibration of a
Page 5 and 6: ZusammenfassungSeit einigen Jahren
Page 7 and 8: ContentsAbstractiiiZusammenfassungv
Page 9 and 10: Contentsix6.1.3 Results Ill6.2 Stat
Page 11 and 12: 1Introduction1.1 Terrestrial Laser
Page 13: 1.2 Motivation 3Precision [m10~1 "1
Page 16 and 17: 6 1. Introduction
Page 18 and 19: 8 2. Components of Terrestrial Lase
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64 3. Calibration of Terrestrial La
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j/-*'.-,•66 3. Calibration of Ter
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74 4. Static Laser Scanning4.1.2 Mi
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76 4. Static Laser ScanningOptical
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78 4. Static Laser Scanningmaximum
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80 4. Static Laser Scanningand gene
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82 4. Static Laser Scanning• Ther
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84 4. Static Laser Scanning4.3.3 NU
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86 4. Static Laser Scanning
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88 5. Kinematic Laser Scanningmirro
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90 5. Kinematic Laser ScanningThe a
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92 5. Kinematic Laser Scanninghas a
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'94 5. Kinematic Laser Scanningbeam
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''iI98 5. Kinematic Laser ScanningA
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102 5. Kinematic Laser Scanning5.3
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—1.104 5. Kinematic Laser Scannin
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propagationresults series Taylorter
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108 5. Kinematic Laser Scanningonly
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cf.aroundframe regardingsetupusinga
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112 6. Applications of Terrestrial
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acquisitiondescribingFinally, thatM
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132 7. Summaryrange, achievable by
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134 7. Summary
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136 A. Imaging System of Imager 500
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138 B. Technical Data of Imager 500
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u140 C. Adjustment of SphereA =dfi,
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142 C. Adjustment of Sphere
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144 D. Electronic Circuit for Deter
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aMulti-SensorPlatform,PhDGeodesy Sw
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Accuracy-AeinTPS1200 AG, -Version 1
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-GenauigkeitsbetrachtungenInvestiga
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152 BibliographyWunderlich, T. A. [
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List of Figures3.22 Residuals of th
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156 List of Figures
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158 List of Tables
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200220072002Curriculum VitaePersona
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