# Method for Accuracy Assessment of the Length Measurement Unit of Laser Tracking Systems
## 4 Discussion  
When analyzing the presented results, it should be noted that there is no significant difference in the nature of the error distribution between the errors obtained with active and inactive motors. This fact may suggest the correctness of the solutions used in the research methodology. However, the difference in values is clearly noticeable (Figures 3-5). Comparing the standard deviations (Figure 3) and the ranges (Figure 5), it can be seen that the activation of the motors caused greater values of both parameters, and particularly large values occur (with active motors) at the beginning of the measuring range. It is most likely caused by the beam tracking system, which has the greatest impact on the result at short distances. It is clearly visible that it does not occur in tests with inactive motors. The values of the mean errors (Figure 3) were increased in the case of a postive direction for active motors. Although the differences between the values are not great, an increase in the values can be observed in relation to the reference values. When trying to estimate the influence of active motors on the measurement accuracy based on the test, it can be assumed that the active LT motor system increases both the indication errors and the spread, and this value usually does not exceed 0.5 \(\upmu\)m. When analyzing the overall influence of the angular position measuring system, it should be noted that its influence has been eliminated with the motors off, because there is no rotation of the mirror positioning the LT measuring beam. On the other hand, in a situation where the LT motors are working, this influence is minimal due to the use of a very small operating range of the angle measurement system. Research on the influence of the angle measurement system basically differs from the research methodology presented above and offers direction for further work. The analysis of the hysteresis value also confirms the above theorem, as it is visible (Figure 4) that greater values were obtained during the tests conducted with active motors. The maximum hysteresis values in the case of active motors oscillate around \(\pm\)1 \(\upmu\)m. The mean absolute value of hysteresis for inactive motors is 0.36 \(\upmu\)m, while for active motors it increases to about 0.5 \(\upmu\)m.  
Results of conducted experiments can be utilized for simulating the uncertainty of the LT's intereferometer measurements. Comparing uncertainties estimated using experimentally obtained data and by means of simulation, it can be seen that the simulation method generally gave slightly smaller values then the second considered method. However, the results obtained with both methods indicate that the uncertainty of laser interferemeter measurements grows almost linear with an increase in measured length. One of the directions of further work on the presented method will be enlargment of the measured range that can be tested. This will require finding a new way to move the reflector, as the linear guide in the LCM practically does not allow tests to be carried out beyond 1.5 m. One potential option is to use a large CMM for this purpose, but it requires additional research and adjustment of the experimental setup. Conceptualization, M.G., A.G., P.G., S.M.S., V.D.M. ; methodology, M.G., W.H. ; software, M.G. ; validation, M.G., J.A.S., V.D.M. ; formal analysis, M.G., A.G., S.M.S., W.H. ; investigation, M.G. ; resources, M.G. and J.A.S. ; data curation, M.G., A.G., V.D.M., S.M.S. ; writing--original draft preparation, M.G. ; writing--review and editing, A.G., J.A.S., S.M.S., V.D.M. ; visualization, M.G., P.G., W.H. ; supervision, A.G., J.A.S., V.D.M. ; project administration, M.G. ; funding acquisition, M.G. All authors have read and agreed to the published version of the manuscript.