Feasibility study of GNSS as navigation system to reach the Moon

The Global Positioning System (GPS) Block II Replenishment (IIR) space vehicle (SV) has made up at least one-half of the GPS constellation since 2006. This consists of the 12 original ‘classic’ IIR SVs and the eight ‘modernized’ IIR-M SVs. As a stepping-stone toward the IIR-M modernization, Lockheed Martin developed and deployed an updated version of the satellite antenna panel for the L-band broadcast signal. This is the signal used by the worldwide GPS user population. This paper describes both antenna panel versions, their broadcast signal patterns, the performance observed in factory testing, and their on-orbit performance. This is the initial publication of these antenna panel patterns. Ground and on-orbit measurements of both versions of the antenna show that all specification requirements are exceeded. They also reflect the increased antenna gain for the new IIR antenna. The L1 signal shows an increase of 1 dB in received power at edge of Earth, and L2 shows an increase of 2 dB in received power. All users, both terrestrial and on-orbit, benefit from this enhanced power profile.

The GPS Block IIR and IIR-M Broadcast L-band Antenna Panel: Its Pattern and Performance

Use of GNSS for lunar missions and plans for lunar in-orbit development

Global Navigation Satellite Systems (GNSSs) were originally introduced to provide positioning and timing services for terrestrial Earth users. However, space users increasingly rely on GNSS for spacecraft navigation and other science applications at several different altitudes from the Earth surface, in Low Earth Orbit (LEO), Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO), and feasibility studies have proved that GNSS signals can even be tracked at Moon altitude. Despite this, space remains a challenging operational environment, particularly on the way from the Earth to the Moon, characterized by weaker signals with wider gain variability, larger dynamic ranges resulting in higher Doppler and Doppler rates and critically low satellite signal availability. Following our previous studies, this paper describes the proof of concept “WeakHEO” receiver; a GPS L1 C/A receiver we developed in our laboratory specifically for lunar missions. The paper also assesses the performance of the receiver in two representative portions of an Earth Moon Transfer Orbit (MTO). The receiver was connected to our GNSS Spirent simulator in order to collect real-time hardware-in-the-loop observations, and then processed by the navigation module. This demonstrates the feasibility, using current technology, of effectively exploiting GNSS signals for navigation in a MTO.

https://www.mdpi.com/1424-8220/16/3/347/pdf

Standalone GPS L1 C/A Receiver for Lunar Missions

Numerous applications, not only Earth-based, but also space-based, have strengthened the interest of the international scientific community in using Global Navigation Satellite Systems (GNSSs) as navigation systems for space missions that require good accuracy and low operating costs. Indeed, already successfully used in Low Earth Orbits (LEOs), GNSS-based navigation systems can maximise the autonomy of a spacecraft while reducing the burden and the costs of ground operations. That is why GNSS is also attractive for applications in higher Earth orbits up to the Moon, such as in Moon Transfer Orbits (MTOs). However, the higher the altitude the receiver is above the GNSS constellations, the poorer and the weaker are the relative geometry and the received signal powers, respectively, leading to a significant navigation accuracy reduction. In order to improve the achievable GNSS performance in MTOs, we consider in this paper an adaptive orbital filter that fuses the GNSS observations with an orbital forces model. Simulation results show a navigation accuracy significantly higher than that attainable individually by a standalone GNSS receiver or by means of a pure orbital propagation.

/pdf/gnss-based-orbital-filter-for-earth-moon-transfer-orbits-hl1f98c8cy.pdf

GNSS-based Orbital Filter for Earth Moon Transfer Orbits

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

J. V. Perelló

Papers

Weak GNSS Signal Navigation to the Moon