# Method for Accuracy Assessment of the Length Measurement Unit of Laser Tracking Systems
## 1 Introduction
These papers constitute a valuable source of knowledge in studies on the accuracy of measuring devices used in large-scale metrology, in particular based on laser interferometry. In [10], [PERSON] et al. describe methods of accuracy assessment in systems used in large-scale metrology, while in [11], [PERSON] et al. present, e.g., a method of modeling the errors of the axis of rotation in a mirror tracking the measuring point. [PERSON] et al. in [12] describe the procedure for identifying kinematic errors in laser tracking systems. The procedure for identifying errors coming from the angle measurement system in a laser tracker is described in [13] by [PERSON] et al., while the procedure for determining the errors in angular encoders using the geometric model of the laser tracker system is described by [PERSON] et al. in [14]. In [15], [PERSON] et al. describe a method for correcting errors originating in a retroreflector tracking system. In [16], [PERSON] et al. describe a method for calibrating the laser tracker system using a reference interferometer. [PERSON] discusses laser interferometer calibration methods in [17]. The methods presented in this paper allow for testing different phenomena related to laser interferometer operation (laser frequency, counting system) and assessing the impact of various components on the accuracy of a measuring system. Based on the presented analysis of the state of art, it should be noted that, to date, the interferometers included in the laser trackers have not been tested separately. Accuracy tests of interferometers in laser tracking systems carried out to date are burdened with errors related to the research methodology, which the authors try to describe and estimate. Others describe the accuracy related to the whole system or to a complex system by giving its sum error. This article aims to develop a method of checking errors in laser interferometers used in laser tracking systems as one of the main sources of errors in such systems. The method allows for determining the share of errors coming from the interferometer independently of other systems, while minimizing the number of potential sources of errors in the test in relation to known methods. Moreover, all other methods of error determination of length measurement devices used in laser tracking systems that are based on comparison with reference interferometric systems use two or more separate retroreflectors, which brings additional errors caused by geometrical errors attributed to the system responsible for translating the retroreflectors. In the method presented in this paper, those errors do not occur. The experiments included in the developed method can be used to acquire input data for the interferometer error module of the virtual model of the laser tracker system. Implementing simulation methods is particularly justified in the case of large-scale metrology, both due to the time spent on measurements and the availability of appropriate standards. A very small number of institutions have standard lengths that allow for measuring distances of 3 m and more. The concept of the virtual laser tracker was described in [18] by [PERSON] et al., while [PERSON] et al. in [19] describe the methodology for estimating laser tracker uncertainty based on the Monte Carlo method. The authors of [20,21] describe simulation software that can be used to estimate uncertainty of large-scale measurements carried out with laser trackers using simulation software. The concept of the virtual laser tracker developed at the Cracow University of Technology includes three main modules responsible for simulating the operation of the device's main components: interferometer module, angular encoder module and environmental conditions module. Each module will contain information on the spread of results in the form of probability distributions.