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

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