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

3.2 Distance Measurement System 41The first one has to be proven by making periodic distance calibrations, called scale factor, and are causedbythe oscillator. Thesecond one can have different reasons that are caused by the environment such astemperature changes inside and/or outside the laser scanner.Variations in frequency results in variations in the distance measurements. However, it is not possible todistinguish between variations in distances and variations in the frequency because several parametersinterfere with each other and lead to distance variations, e.g.conditions, properties of the target (such as colour, material, roughness), angleadditive constant, scale factor, atmosphericof incidence.Thus, thedetermination of frequency variations has to be done directly by using an electronic unit for frequencymeasurements, cf. Section 3.1.4.The important frequency for the resolution of the distance measurement system is the modulated frequencyof the fine channel component, i.e. high frequency signal 'hfs'. Thus, the investigationis focused on thismodulation frequency. In the experimental setup, the laser beam is aligned towards the diode, the signal ofthe frequency is detected and the real frequency is derived, cf. Figure 3.6. The investigation was performedwithin an observation period of nearly three hours with a samplinginterval of one minute. One assumesa time-dependent and temperature-dependent behaviour. Thus, the temperatureinside the laser scanneris recorded additionally. The temperature of the environment is constant because the investigation wascarried out in an air-conditioned laboratory.transmitter temperaturedifference to nominal frequency (« 45 MHz)Figure 3.20:Development of the frequency of high frequency signal 'hfs' and the internal temperature of the laserscanner.Figure 3.20 shows the time variations in both the modulation frequency 'hfs' and the internal temperatureof the laser scanner, i.e. transmitter temperature. First, the deviations of the 'hfs' to the reference frequencycan be seen. Second, the 'hfs' reaches a constant frequency after nearly two hours in operation and showsa deviation to the nominal frequency of nearly 50 Hz. Third, the behaviour of the internal temperature ofthe laser scanner correlates to the frequency behaviour: an increasing temperature leads to a decreasingdeviation between real and nominal frequency, that means the real frequency is also increasing becausethe difference to the nominal frequency is calculated by subtracting the real frequencyfrom the nominalfrequency, and vice versa. The nominal frequency is defined by /hfs=44.781250 MHz.Based on the given nominal frequency /hfs and the real frequency / a scale factor rrif for the correction ofa distance measurement caused by the difference of the real frequency to the nominal frequencycan bederived with [Joeckel and Stober, 1991]-/hfs /rrif 1 = '(3.6)/Applying this formula to all acquired real frequencies of the modulation frequency'hfs' one can achieve amaximum scale factor of 3.2 ppm. The constant frequency after two hours shows a value of —1.2 ppm. Con¬sidering the working range of the laser scanner of 50 m, the differences of the modulation frequency result