# Observing APOD with the AuScope VLBI Array
## 2 Experiments
### The APOD-A Nano Satellite  
The Chinese Atmospheric density detection and Precise Orbit Determination (APOD) mission operated by the Beijing Aerospace Control Center (BACC, China) consists of a set of four CubeSats launched on 20 September 2015. It is mainly intended to carry out in-situ measurements of the  
Figure 2: The AuScope Very Long Baseline Interferometry (VLBI) array consists of the three antennas at Hobart (Hb, Tasmania), Katherine (Ke, Northern Territory), and Yarragadee (Yg, Western Australia) indicated by filled red dots. The continental wide baselines (dotdashed lines) with lengths from 2360 to 3432 km only allow for limited mutual visibilities of the APOD satellite which orbits the Earth on an altitude of only \(\sim\)470 km. Projected fields of view of individual sites are displayed by shaded red circles assuming local cut-off elevation angles of 5\({}^{\circ}\). The APOD ground tracks during Experiment a332 (27 November 2016; see Table 1) are represented by blue dots indicating successive satellite positions in a 30 s interval. The projected APOD positions in Scans 168 (10:41:04 to 10:42:30 UTC) and 169 (10:44:02 to 10:48:18 UTC) of Experiment 332a are indicated by lines in green and magenta, respectively. atmospheric density along with the derivation of density parameters via a precisely determined orbit. For details on the scientific tasks, the payload, and the Precise Orbit Determination (POD), we refer to [29; 30]. In this paper, we discuss observations of one of these satellites, named APOD-A nano, which was deployed in a near polar orbit with an inclination of about 97\({}^{\circ}\) and an initial altitude of about 470 km. Its size is only 391 \(\times\) 398 \(\times\) 398 mm, and it is equipped with a dual-frequency GNSS receiver (GPS and Beidou), an SLR retro-reflector, and a VLBI beacon. The VLBI beacon transmits signals in the S- and X-bands with narrow band carrier tones centered at \(f_{Carrys}=2262.01\) MHz and \(f_{Carrys}=8424.04\) MHz, respectively. Both carrier tones are symmetrically surrounded by four Differential One-way Ranging (DOR) tones designed according to recommendations of the Consultative Committee for Space Data Systems (CCSDS 401.0-B [31]). The DOR tone frequencies are calculated as \(f_{DOR_{12}}=f_{Carr}\pm f_{Carr}/440\) and \(f_{DOR_{3,4}}=f_{Carr}\pm f_{Carr}/2200\). All tones are listed in Table 2. The resulting frequency spans are 38.3 MHz in the X-band and 10.3 MHz in the S-band. The Effective Isotropic Radiated Power (EIRP) of the carrier tones is 4 dBm with the DOR tones being 12 dB weaker. Initially, the BACC performed the POD by analyzing GPS L1 and L2 carrier phase measurements and pseudorange data with their proprietary software. As described by [PERSON] et al. [29], the RMS of pseudorange residuals and carrier phase residuals were below 2 m and 2 cm, respectively. APOD-A was also observed by the International Laser Ranging Service (ILRS). The obtained SLR measurements were used for an independent validation of the orbit derived by GPS, by comparing SLR measurement ranges with ranges derived by GPS POD. The comparison showed good agreement with three-dimensional position deviations below 10 cm [29]. Unfortunately, the GNSS receiver partly failed in January 2016. Since then, GNSS raw-data records, i.e., pseudorange and carrier phase measurements, are no longer accessible by the ground segment. After failure, the orbit determination completely relies on three-dimensional satellite coordinates that are directly calculated by the on-board receiver and transmitted to the ground segment four times a day. As a result, the accuracy of the orbit, which is solely based on these coordinate data, dropped dramatically. For the duration of our VLBI experiments in November 2016, the orbit determination residuals were in the range of 10 to 20 m [29]. Although the APOD-A satellite was observed by the ILRS, SLR observations were only sparsely available and, therefore, could not reasonably contribute to the orbit determination.