# ILRS Reference Point Determination Using Close Range Photogrammetry
###### Abstract  
A global geodetic reference system (GGRS) is realized by physical points on the Earth's surface and is referred to as a global geodetic reference frame (GGRF). The GGRF is derived by combining several space geodetic techniques, and the reference points of these techniques are the physical points of such a realization. Due to the weak physical connection between the space geodetic techniques, so-called local ties are introduced to the combination procedure. A local tie is the spatial vector defined between the reference points of two space geodetic techniques. It is derivable by local measurements at multitechnique stations, which operate more than one space geodetic technique. Local ties are a crucial component within the intertechnique combination; therefore, erroneous or outdated vectors affect the global results. In order to reach the ambitious accuracy goal of 1 mm for a global position, the global geodetic observing system (GGOS) aims for strategies to improve local ties, and, thus, the reference point determination procedures. In this contribution, close range photogrammetry is applied for the first time to determine the reference point of a laser telescope used for satellite laser ranging (SLR) at Geodetic Observatory Wettzell (GOW). A measurement campaign using various configurations was performed at the Satellite Observing System Wettzell (SOS-W) to evaluate the achievable accuracy and the measurement effort. The bias of the estimates were studied using an unscented transformation. Biases occur if nonlinear functions are replaced and are solved by linear substitute problems. Moreover, the influence of the chosen stochastic model onto the estimates is studied by means of various dispersion matrices of the observations. It is shown that the resulting standard deviations are two to three times overestimated if stochastic dependencies are neglected. close range photogrammetry; bundle adjustment; reference point determination; unscented transformation; stochastic model; satellite laser ranging; Geodtre 2021  
1 [PERSON]; [PERSON]; [PERSON]; [PERSON] ILRS Reference Point Determination Using Close Range Photogrammetry. _Appl. Sci._**2021**, _11_, 2785. [[https://doi.org/10](https://doi.org/10)]([https://doi.org/10](https://doi.org/10)). so-called local ties are introduced to the combination procedure [6]. A local tie is the spatial vector defined between the reference points of two space geodetic techniques. It can be derived by local measurements at multitechnique stations, which host at least two different space geodetic techniques. Local ties provide the crucial component to overcome the weak physical connection between the space geodetic techniques. The vectors integrate the space techniques to a uniform Earth observation sensor at multitechnique stations [7]. Local ties are identified as a critical component within the intertechnique combination [8]. For instance, [PERSON] et al. [9] evaluated the local ties used for the ITRF2014 and identified discrepancies of more than 5 mm for about 50 % of the local ties w.r.t. the global solution. Therefore, strategies must be developed to improve local ties and the reference point determination procedure to reach the 1 mm goal. The common procedure of local tie determination consists of three analysis steps: The determination of the reference points of the space geodetic techniques in a local reference frame, preparing the local ties and the related dispersion matrices, and the mandatory transformation of the local ties into the GGRF using, for instance, homologous points. The last two steps are independent of the measurement method used for reference point determination, and always have to be performed. Novel approaches are being developed in joint projects such as the current GeoMetre research project [10], to traceably transfer local ties to the global frame [11]. For that reason, we restrict ourselves to the important first analysis step in this investigation.