# Observing APOD with the AuScope VLBI Array
## 1 Introduction  
The Global Geodetic Observing System (GGOS) has been established by the International Association of Geodesy (IAG) in order to satisfy the expected future requirements of science and society, which are facing increasing challenges on a changing planet. GGOS aims to establish a terrestrial reference frame with an accuracy on the level of 1 mm or better on a global scale [1]. This can only be achieved by a rigorous combination and integration of the different ground and space geodetic techniques. One of the most accurate realizations of a reference system aiming at this goal is the current International Terrestrial Reference Frame (ITRF2014) [2]. As a combination product, the ITRF makes use of the full history of observation time series of the four space geodetic techniques Very Long Baseline Interferometry (VLBI), Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). In addition, the ITRF determination fundamentally relies on differential coordinates--called local ties--connecting the reference points of the geodetic instruments that are typically located within distances of a few hundred meters at co-location sites (for a definition of local ties, we refer to [3]). These local tie vectors are usually determined by classical terrestrial measurements (angles plus distances andleveling) or by GPS measurements. Although local ties play a critical role in the ITRF combination by linking the contributing techniques (along with Earth rotation parameters), comparisons of the local ties and differential coordinates determined by various space geodetic techniques reveal significant discrepancies with residuals on the centimeter level in many cases. This issue was discussed in detail among others by [PERSON] et al. [4], [PERSON] et al. [2], [PERSON] et al. [5], and [PERSON] et al. [6]. As proposed, e.g., by [PERSON] et al. [7], an additional way to combine space geodetic techniques--complementary to local ties at co-located sites on the Earth's surface--is the co-location of geodetic instrumentation on board Earth orbiting satellites. Such so-called \"space ties\" (in accordance with the term local tie) would provide unprecedented possibilities to connect the coordinate frames of different space geodetic techniques, and they would help to reveal unknown technique-specific biases. Alternative approaches to establish frame ties are the co-location of VLBI with other techniques on the moon [8], and the application of so-called global ties [9], i.e., of commonly estimated parameters in different technique solutions of space geodesy. So far, there have been several successful attempts to combine the three satellite techniques GNSS, SLR, and DORIS via co-location on Earth-orbiting satellite platforms. [PERSON] et al. [6] demonstrated that it is feasible to combine GNSS and SLR by utilizing GPS and GLONASS satellites equipped with SLR retro-reflectors as co-location platforms in space. In a more recent study by [PERSON] et al. [10], satellites of other GNSS constellations (BeiDou and Galileo) were used to establish the SLR-to-GNSS frame connection. [PERSON] et al. [11] studied the combined analysis of GNSS, SLR, and DORIS on board the Jason-2 satellite with the goal to set a basis for the integration of a low Earth orbit (LEO) multi-technique satellite such as Jason-2 as space tie in the TRF determination. Contrary to that, space ties with VLBI have not been established so far and have solely been able to be studied in simulations: [PERSON] et al. [12] investigated general aspects such as suitable satellite orbits and antenna networks, finding that station position estimates at the level of a few millimeters in weekly solutions are achievable based on VLBI satellite observations only. Initial scheduling strategies for VLBI observations of the GNSS constellations were simulated and investigated by [PERSON] et al. [13], [PERSON] et al.