Measurement of large distances: New interferometric primary standard completed and tested in measurement campaigns

The traceability of measurement results of distances from a few hundred meters to a few kilometers with relative uncertainties much better than u = 1 ∙ 10 ^-6 (one millimeter per kilometer) is a major challenge. Compensating for the influence of the air refractive index on the propagation velocity of light limits the achievable accuracy. According to the classical approach, temperature, air pressure and relative humidity are measured along the entire measurement distance with appropriate sensors and the air refractive index is determined from this using empirical formulas. This implies a high measurement effort, especially for larger distances. Alternatively, methods are available with which the influence of the air refractive index can be compensated by means of light-based measurements parallel to the distance measurement in situ over the entire distance. PTB pursues the two-color method [1] using two widely separated light wavelengths.

This principle has already been successfully demonstrated by PTB in the past [2]. Within the framework of the European research project "Large-scale dimensional measurements for geodesy" (18SIB01, GeoMetre), PTB is currently developing a system with a range of up to five kilometers which is to function robustly over a longer period of time even away from stable laboratory conditions. Such longer distances are of great interest especially for the verification of GPS-based distance measurement methods. For this purpose, a special achromatic optic was designed and manufactured in cooperation with an industrial partner. When designing the mechanical system, PTB's scientific instrumentation department took great care to ensure thermal stability and lightweight construction. In order to achieve the ambitious accuracy target of the overall system, it was particularly important to maintain the tight tolerances in manufacturing. For this purpose, the manufacturing and assembly of the components was extensively monitored with a coordinate measuring machine [3]. For the light source, a laser stabilization system based on current FPGA technology (Xilinx Kintex-7 family) and transceiver unit was developed. In addition, a beam source consisting of two Nd:YAG solid-state lasers and eight acousto-optic modulator units was built, aiming for a high degree of robustness by using fiber-based components.

The system could be completed in July of this year. After successful test measurements in the laboratory and on PTB's 50 m interference comparator, it was successfully "released into the outside world". Measurement campaigns on PTB's 600 m baseline, the international reference baseline for high performance electronic distance meters in Nummela, Finland, and finally the SI-traceable calibration of important baselines of the reference network of the space-geodetic station in Metsähovi, Finland, were successfully carried out. In the process, the instrument was able to demonstrate the targeted resistance to external influences and its structural long-term stability.

Figure 1: Multi-wavelength interferometer TeleYAG-II developed for measurements under outdoor conditions. The small schematic picture shows the optical interferometer setup within the system (light beams are indicated in green).

This EMPIR project is co-funded by the European Union's Horizon 2020 research and innovation programme and the EMPIR Participating States.

References

[1] K. B. Earnshaw and J. C. Owens 1967 “A dual wavelength optical distance measuring instrument which corrects for air density” IEEE J. Quantum Electron. QE-3, 544–550

[2] K. Meiners-Hagen, T. Meyer, J. Mildner, F. Pollinger 2017 “SI-traceable absolute distance measurement over more than 800 meters with sub-nanometer interferometry by two-color inline refractivity compensation” Appl. Phys. Lett. 111 191104

[3] F. Pilarski, F. Schmaljohann, S. Weinrich, J. Huismann, D. Truong, T. Meyer, P. Köchert, R. Schödel, F. Pollinger 2021 “Design and manufacture of a reference interferometer for long-range distance Metrology” Proc. Euspen’s 21st International Conference & Exhibition, Virtual Conference, 7-10, June, 511 - 512