Weak GNSS Signal Navigation to the Moon

TLDR: In this article, the authors present the main challenges of a GNSS receiver in different phases of a mission to the moon including Moon Transfer Orbit and Low Lunar Orbit, including expected signal strengths, DOP and number of visible satellites.

Abstract: INTRODUCTION Reception of weak GNSS signals in challenging environments using techniques such as Assisted GNSS has become a reality and today such techniques are considered important enablers of GNSS receivers in mobile devices [1]. Furthermore, the use of GPS and Galileo signals for indoor navigation is also receiving increased attention in recent studies which push the limits of minimum signal to noise ratios [2]. Space users have also seen remarkable achievements, where GPS signal reception has been confirmed and reported in GEO orbits [3] and seen as enabler for increased spacecraft autonomy in GEO orbits [4]. The study of weak GNSS signal reception techniques for lunar missions appears as a logical sequential step which is further supported by the growing international efforts for lunar exploration, which typically involve radiometric range and Doppler measurements from the earth to perform orbit determination. CONTEXT AND MOTIVATION The first missions flying to the Moon after the Apollo era were the NASA's Clementine (launched in 1994) and Lunar Prospector (launched in 1998). Both missions searched for lunar polar ice deposits and explored unknown areas thus producing more detailed surface maps. The interest in Lunar exploration missions then started to spread out in the international community and SMART-1, the first European mission to the Moon, was launched in 2003. Since then, the European Space Agency has fostered investigation on Lunar Exploration Missions. Lunar Lander is an example of such mission [5], which has reached CDR in 2012 for which DEIMOS has been actively involved as co-prime for the G&C subsystem during this descent phase and prime for the HDA subsystem. ESA’s interest in moon exploration continues with the recent agreement for the provision of a service module for NASA Orion uncrewed spacecraft Exploration Mission-1 (EM-1) in 2017. Additionally, DEIMOS has lead the GNSSGEO feasibility study for ESA, for the use of GNSS in high orbits (GEO, GTO, HEO) for autonomous orbit determination. In this study, a GNSS receiver simulator and receiver design has been developed including detailed design of signal processing algorithms. Orbit Determination (OD) in GEO and large portions of HEO orbits down to 18 dB-Hz was as shown as an alternative to traditional radiometric based OD, with potential cost savings and simplifications in the mission. SIGNIFICANCE OF WORK In the frame of an ESA study, the Lunar GNSS project investigates the use of weak GNSS signals from existing GPS and future Galileo towards future lunar exploration missions for real-time position, navigation and timing information. More specifically, GNSS signals could be used if receivers are complemented with advanced processing signal and filtering techniques, allowing acquisition and tracking down to very weak signal to noise ratios. The paper presents the main challenges of a GNSS receiver in different phases of a mission to the moon including Moon Transfer Orbit and Low Lunar Orbit, including expected signal strengths, DOP and number of visible satellites. Factors such as GNSS antenna radiation patterns (transmitter and receiver), frequency, constellations, spacecraft attitude, integration times and TT&C link characteristics are considered. Requirements for the Lunar Lander mission based on conventional radiometric measurements from the earth as well as onboard sensors and then presented. An overview of selected high sensitivity techniques which take benefit of Galileo signals and modernized GPS to cope with the challenges is then presented followed by the description of an orbital filter complementing GNSS measurements in closely-coupled fashion. The paper also describes a dedicated test platform which allows demonstrating the main functional and performance capabilities for weak signal navigation. Such platform includes both the capability to use realistic IF data and to execute high fidelity simulations of the signal processing and navigation functions of an AGGA4 based chipset (ESA’s future generation GNSS receiver). A first batch of simulation results for a selected trajectory of the Lunar Lander mission is presented, highlighting the achievable navigation performance throughout the trajectory. In conclusion, this paper contributes to the understanding of how GPS and Galileo could be used for orbit determination in lunar missions, which may have an impact on the use of ground segment assets (ground station use times, operations manpower and equipment, etc.). REFERENCES [1] Mulassano P., Dovis F. “Assisted Global Navigation Satellite Systems: An Enabling Technology for High Demanding Location-Based Services”. In: Location-Based Services Handbook / Ahson S. A., Ilyas M. CRC Press, Boca Raton, FL (USA), pp. 279-298. ISBN 9781420071962, 2011 [2] Vecchione et al., “DINGPOS, A GNSS-based Multi-sensor Demonstrator for Indoor Navigation”, IEEE/ION PLANS 2010, Myrtle Beach, USA, April 2010 [3] Moreau M. C. et al, “Results from the GPS Flight Experiment on the High Earth Orbit AMSAT OSCAR-40 Spacecraft”, ION GNSS, Sept. 2002 [4] Lorga J.F.M et al., ”Autonomous orbit determination for future GEO and HEO missions”. In: 5th ESA Workshop on Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing (NAVITEC), 2010, Noordwijk (NL), 8-10 Dec. 2010. [5] Richard Fisackerley, Alain Pradier: “The ESA Lunar Lander Mission”, AIAA, 2011