JAESat is designed for a high degree of on-board autonomy. However, the operations of JAESat will be conducted via a ground station located at the QUT in Brisbane, Australia.

The core of the testing environment is the Welnavigat GPS signal simulator. The signal simulator is capable of simulating the entire GPS satellite constellation and transmitting RF signals on six channels. The GPS development platform (Architect Development Kit) is directly connected to the GPS signal generator and controlled/monitored from a notebook computer. An important feature in this context is the option to import scenario fi les, generated by the user.

The results shown here are related to the expected performance of the SPARx in space, the position, velocity and time solution, the capability of Orbit Determination (OD) and also the performance for the Attitude Determination (AD) based on GPS measurements. Test and simulations have been conducted with and without Hard Ware in the loop.

The main objective of these tests was to identify the GPS signal acquisition and tracking performance of SPARx. Results of tests conducted for JAESat including the GPS signal simulator and SPARx are given in Figure 7. In most cases SPARx generates a 3D position solution for more than 80% of the time per orbit. In this context it is also important to understand that the GPS signal simulator only provides a total of six simulated channels. This means that these tests can be seen as a kind of worst case scenarios.

Another test objectives was also the testing of the time synchronisation performance and the generation of a Hard Ware Pulse Per Second (PPS) output. SPARx performance for a 3D absolute position solution in space is currently better than 20 m (1 Sigma).

Simulations for orbit and attitude determination based on GPS

Simulations for the Orbit -and Attitude Determination have been conducted. The results are presented in Table 2 and Figure 8. For the attitude simulations, the orientation of the antenna array was anti nadir. The baseline length was 36 cm for baseline AB, AC and 51 cm for baseline AD. The multipath error was assumed to be 3mm on the carrier Single Difference (SD). The Orbit Determination concept is based on GPS
position solutions, used as observations in a batch Least Square process (Enderle et. al. 2003). A total data arc of 20 min. was used with a position
solution every 10 sec and an assumed position error of 25 m (1 Sigma) for each x, y and z component of the position vector. The results in Table
2 show that a satellite on-board 3D position accuracy better than 10 m can be achieved with the proposed Orbit Determination concept.

The results for the JAESat Attitude Determination (AD) based on GPS, presented in Figure 8 are highlighting that GPS based AD would already by sufficient to comply with the JAESat attitude accuracy requirements. In Figure 8, it can also be clearly seen that the number of visible GPS satellites for JAESat lies between 4 SV and 12 SV with an average of 8 SV. This means a substantial higher number of visible GPS satellites for the JAESat as have been used for simulations with the GPS signal simulator.

This work was carried out at CRCSS/ QUT with the support of many partners. Special thanks to Kayser- Threde GmbH for the contribution of the Star Sensor, Intrinsyc for the CerfBoard, Aerospace Concepts and Cooperative Research Centre for Satellite System for the constant support and Solar Cells. Finally, I would like to mention all the QUT Aerospace Avionics Students and their valuable contributions to the JAESat project.

Enderle W., QUT - JAESat Project Documentation, JAESat – GPS - PO-0001, 2002

Enderle W., JAESat Mission Concept Description, QUT Project Documentation, 2004

Enderle W, Yanming F, Zhou N, Orbit Determination of FedSat based on GPS Receiver

Position Solutions – First Results, Proceeding SatNav2003, Melbourne, Australia, 22–25 July 2003

Enderle W. Roberts P., GPS Receiver Development for Space Applications at Queensland

University of Technology (QUT), Proceeding of the 10th Australian International Aerospace

Congress and 14th National Space Engineering Symposium, Brisbane, Australia, 29 July – 1 August 2003

Bruggemann T, Enderle W. Time Synchronization Capability of SPARx – the Queensland

University of Technology (QUT) GPS Receivers, The 2004 International Symposium on GNSS/GPS, Sydney, Australia, 6–8 December 2004

MITEL Semiconductor, DS4605, GPS Architect 12 Channel GPS Development System,1997