# ILRS Reference Point Determination Using Close Range Photogrammetry
## 4 Conclusions  
Local tie vectors are one of the crucial components within the combination of space geodetic techniques for deriving a reliable reference frame. Such vectors are observable at multitechnique stations and are defined between the reference points of the hosted space geodetic techniques at the site. GGOS aims for a reference frame, which yields an accuracy of a global position of 1 mm. Current realizations do not meet these requirements and are worse by a factor of 5 to 10. One reason is the large discrepancy between the local ties and the global solution. To improve local ties and, thus, the reference points, suitable analysis procedures and measurement strategies must be developed. In this investigation, close range photogrammetry is used for the first time to determine the ILRS reference point at the Satellite Observing System Wettzell in a local frame. For this purpose, a measurement campaign consisting of seven experiments using three different configurations was carried out in September 2020. The surrounding of the SOS-W was equipped with several coded and uncoded markers. The data were acquired and preprocessed using the photogrammetric system Aicon DPA Industrial. However, the proprietary software package Aicon Studio 3D does not provide the fully populated dispersion of the estimated object points. To overcome this drawback, the in-house software package JAiCov was developed, which obtains the dispersion of the final bundle adjustment. For each experiment, the marker positions and the related dispersion were introduced to the reference point determination process. The variations of the resulting coordinate components of the reference points are in a range of about \(\pm 0.1\) mm w.r.t. the local datum. The axis offset varies of about \(\pm 25\) um and is insignificant. As shown in Figure 7, the benefit of a dense point cloud, as it was realized by configuration i, does not significantly improve the estimates in comparison to the reduced configurations ii and iii but increases the measurement effort significantly. Further investigations are needed to reduce the downtime of the telescope during the reference point determination. Procedures have to be developed to enable continues measurements during regular SLR experiments. In-process measurements allow for time series analysis to evaluate the repeatability and stability of the reference point. Both the bundle adjustment described in Section 2.1 and the reference point determination model presented in Section 2.2 are nonlinear but are solved by transformed linear substitute problems within the GMM. Since stochastic properties of a linear substitute problem cannot be passed to the underlying nonlinear problem, the estimates are generally biased. By applying the spherical simplex unscented transformation to the analysis process, second-order improved results were obtained in this investigation. However, due to the high accuracy of the used close range photogrammetric system as well as the realized  
Figure 9: Frequency of correlation coefficients for the observed positions, exemplified for DOY: 260. Whereas the histogram (**a**) contains the full set of correlation coefficients, in (**b**) component-wise intracorrelations \(\rho_{x,x_{i}}\), \(\rho_{y,y_{i}}\), and \(\rho_{z_{i}z_{i}}\) of an arbitrary position are depicted. Self-correlations are excluded in both figures. symmetric measurement configuration, the second-order effect becomes negligible. The first-order results derived by the GMM were proved to be proper. The stochastic model was identified as the most crucial part of the analysis process, because it influences the dispersion of the reference point and the axis offset. Introducing a simplified stochastic model, i.e., a scaled identity matrix, a diagonal variance matrix, or a block diagonal matrix, yields an overestimated dispersion of the parameters.