# Observing APOD with the AuScope VLBI Array
## 1 Introduction
[14] simulated VLBI observation, time-of-flight measurements using a time-encoded signal in the satellite transmission, and differential VLBI observations using angularly nearby quasars as calibration sources, and investigated their suitability for the estimation of frame tie parameters. However, the actual link between the satellite techniques GNSS, SLR, and DORIS and VLBI has not yet been established. The essential problem in this respect is the absence of real satellite observations with VLBI radio telescopes. While GNSS, SLR, and DORIS are operationally applied for observations of Earth satellites, geodetic VLBI is intrinsically different, with its roots being settled in astronomy. Present applications of VLBI satellite tracking can mainly be found in the field of planetary sciences, with VLBI being used for the navigation and positioning of spacecrafts (for example: [15; 16; 17]). The requirements on observation schemes and data processing essentially differ between VLBI observations being carried out for the purpose of interplanetary navigation and for establishing frame ties via Earth satellites. Hence, observation and processing routines established for planetary science cannot simply be adopted for use in geodetic VLBI. In the past few years, several groups within the geodetic community reported an effort on observing L-band GNSS satellite signals. Initial experiments were carried out to investigate basic signal chain characteristics and tracking capabilities mainly involving the European observatories in Wettzell (Germany), Onsala (Sweden), and Medicina (Italy) (described in, e.g., [18; 19; 20; 21]). In an extensive series of experiments with observations of GNSS satellites (L1 and L2 signals) on the Australian baseline Hobart-Ceduna the complete process chain--from scheduling, actual observations, correlation and fringe fitting, to the final analysis of the derived group delays--could be closed for the first time [22]. One main challenge in these experiments was the standard equipment of legacy geodetic VLBI antennas, being restricted to observations in the S- and X-bands and not covering the L-band frequency range. However, this may change in the near future with the advent of broadband antenna feeds and matching receivers following the VGOS design concepts [23]. VGOS-compatible antenna equipment is expected to cover the frequency domain from 2 to 14 GHz. This will enable more flexibility in the selection of observation targets, though the frequency range again does not cover the L-Band of GNSS satellites. Hence, scientists had to fall back on antenna equipment that is not operationally used for geodetic applications involving the risk of introducing unexpected biases and unknown systematic effects caused by the non-standard signal chain. Two dedicated co-location satellite missions were proposed by the geodetic community in recent years that would employ all four techniques used in the TRF combination: the Geodetic Reference Antenna in Space (GRASP [24]) and the European Geodetic Reference Antenna in Space (E-GRASP/Eratosthenes [25].) The GRASP satellite was planned to be equipped with a VLBI beacon emitting signals in the S- and X-bands that are compatible with frequency ranges of legacy and future VLBI antennas. The signals should have been encoded using Binary Offset Carrier (BOC) modulation to enable range measurements. The E-GRASP mission was proposed to carry a broadband noise beacon emitting signals in the S-, X- and Ku-bands. Since a GRASP-like satellite has not yet been realized, alternative observation targets must be employed for hands-on observation experiments. Actual VLBI satellite tracking experiments are considered valuable for future co-location satellite missions as they provide plenty of hands-on expertise, complementing the information derived from theoretical simulation studies.