In early 2015, the Navigation Support Office of the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) began a collaboration. At its core, the ESA-JAXA collaboration is designed to cross-validate Japan’s Quasi-Zenith Satellite System (QZSS) Precise Orbit Determination (POD) results and share expertise to improve the POD accuracy of QZSS.
The cross-validation of the QZSS POD performance was implemented by jointly analyzing QZSS observations and validating the POD results of the QZSS satellites. As a result of this joint activity, ESA and JAXA have significantly improved the robustness and accuracy of their respective POD products. This collaborative approach not only ensures the continuous improvement of QZSS force modeling and precise orbit determination performance but also demonstrates the effectiveness of international cooperation in advancing the field of space navigation, especially as the benefits of GNSS interoperability become very evident.
An important milestone in this collaboration was ESA’s role in supporting the In-Orbit Testing (IOT) activities for QZS-1R towards the end of 2021. The successful execution of these tests demonstrated the practical results of the ESA-JAXA partnership and further solidified the commitment of both agencies to enhance their capabilities for QZSS POD and associated products.
The benefits of this collaboration extend beyond the agencies to the entire scientific community. Notable achievements include the revision of metadata for the QZSS constellation, such as the optical properties of the QZS-1 solar arrays, which have been refined and improved through shared expertise, while simultaneously releasing the satellite mass and attitude mode history in a machine-readable file format for easy access and adoption by the users.
To evaluate the spacecraft models and metadata for QZS-1R prior to their public release, ESA and JAXA conducted several comparative tests. Since both organizations use different software packages for satellite POD — ESA uses NAPEOS (Dow, Springer 2009, Enderle et al., 2019 and 2022) and JAXA uses MADOCA (Kawate et al., 2023) — their results can be considered as largely independent. One comparison involved the Solar Radiation Pressure (SRP) model results produced by both organizations. FIGURE 1 shows the accelerations in satellite-Sun frame computed by ESA’s SRP model. The comparison of the computed SRP accelerations in different reference frames, spacecraft-fixed and inertial, showed excellent agreement with differences of less than 0.1 nm/s².
In addition, pseudo-range and carrier phase dual-frequency measurement data from 200 tracking stations of the International GNSS Service (IGS) network were used to generate precise QZS-1R satellite orbits and clock offsets on a day-to-day basis over a 12-month period spanning from January to December 2022. Comparison between ESA and JAXA solutions yielded a root-mean-square (RMS) agreement of 8.6 centimeters (orbit) and 0.21 nanoseconds (clock), respectively. Analysis of Satellite Laser Ranging (SLR) data from seven stations of the International Laser Ranging Service (ILRS) suggests a radial RMS accuracy of the generated orbital trajectories of about 4 cm. Without applying the analytical models for SRP and other non-gravitational perturbation forces, such as antenna thrust (AT), the RMS accuracy decreases by a factor of five (FIGURE 2).
In conclusion, the ESA-JAXA collaboration on Japanese Quasi-Zenith Satellite System POD has been a resounding success. Through this continuous and mutual support, performance cross-validation and knowledge sharing, significant improvements related to modeling and subsequently to POD accuracy could be achieved for ESA as well as for JAXA. Additionally, the global scientific community benefitted from this ESA/JAXA collaboration via improved QZSS POD products and validated metadata.
Figure 1 and 2 courtesy of the authors
