# ILRS Reference Point Determination Using Close Range Photogrammetry
## 3 Satellite Observing System Wettzell
### Measurements and Configurations  
The measurement campaign for the evaluation of the potential of close range photogrammetry for reference point determination was carried out at GOW in September 2020. The investigations focus on the reference point determination of the Satellite Observing System Wettzell in a local frame. The SOS-W is the modern SLR laser telescope at GOW. It operates with a two-color laser for daytime and nighttime conditions to mainly reduce atmospheric refraction uncertainties [43]. The telescope is enclosed by a turnable protecting dome, comparable to a sphere with a diameter of about 5 m and the telescope in its center, cf. Figure 3. A detailed description and the specifications of the laser telescope are given by [PERSON] et al. [44]. Hexagon's Aicon DPA Industrial measurement system is chosen for data acquisition and analysis, cf. Figure 4. The handheld digital camera-based photogrammetry system consists of the C1 camera, i.e., a Canon EOS 5D equipped with a 28 mm Aicon metric wide-angle lens that is protected by a robust and extremely stiff IP51-rated camera case, and the analysis software Aicon Studio 3D. The maximum permissible error (MPE) of a measured length, which is located between two signalized points, is specified by \(15\,\upmu\mathrm{m}+15\,\upmu\mathrm{m}\,\mathrm{m}^{-1}\). The typical standard deviation of a position-based measurement obtained by a bundle adjustment, i.e., \(\bar{\sigma}_{\mathrm{3D}}=\sqrt{\bar{\sigma}_{X}^{2}+\bar{\sigma}_{Y}^{2}+ \bar{\sigma}_{Z}^{2}}\), is stated by \(2\,\upmu\mathrm{m}+5\,\upmu\mathrm{m}^{-1}\)[45]. Investigations on the geometric stability of the camera can be found in [PERSON] et al. [46]. In this campaign, uncoded and 14-bit coded circular black-and-white markers are used. About 175 coded markers at the dome wall establish the photogrammetric reference frame. The frame is temporarily enlarged for the vertical component by further 50 coded markers beyond the wall of the dome. The scale information for the photogrammetric reference frame results from three calibrated scale bars, which are traced to the SI meter. The scale bars are located surrounding the telescope and contribute to all three coordinate components. Figure 4: Hexagon’s Aicon DPA Industrial measurement system consisting of the C1 camera (**d**) with a 28 mm metric lens, a coordinate-cross (**f**) for initial datum definition during the measurement process, scale bars (**a**) to trace to the SI meter, circular coded (**b**) and uncoded (**e**) photogrammetric markers, and the software package Aicon Studio 3D (**c**) for data analysis. Figure 3: Satellite Observing System Wettzell with turnable protecting dome at GOW. Nine interoperable drift nests (DN) are also mounted at the dome wall. They magnetically support reference markers for photogrammetric systems and corner cube reflectors for multilateration coordinate measurement systems [47] and for polar measurement systems such as laser trackers [26]. This interoperability of measurement systems is essential to combine different measurement techniques in a consistent reference frame, e.g., the station survey network. The drift nests can be embedded in the station survey network and ensure the integration of the results of the photogrammetric measurements. Thus, different measuring systems can be used for the reference point determination of each space geodetic technique and for the station survey network itself. [PERSON] and [PERSON] [48] evaluate the optimal position of mounted markers at the telescope structure by simulations and recommend marker positions close to the elevation axis. For that reason, \(m=12\) uncoded photogrammetric markers are attached to the turnable part of the telescope. The maximum distance between the photogrammetric markers and the elevation axis is about 1 m, and all markers are allocated at one side of the telescope balancing aspects of visibility, accessibility, and spatial coverage. Once the telescope is rotated to a different position, the markers are photogrammetrically observed.