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Note: This blog is based on a presentation I gave as part of a recent Inside GNSS webinar: Lunar Navigation Services: A Global View from the Next Frontier. You can watch the full webinar here.
When Firefly Aerospace’s Blue Ghost lander touches down on the Moon’s surface later this year, it will have a pioneering piece of equipment on board.
During its 12-day stay on the Moon, the Lunar GNSS Receiver Experiment (LuGRE), developed by NASA and the Italian Space Agency, will record the faint signals it receives from GPS, Galileo, and other global navigation satellite services (GNSS).
The aim is to understand the extent to which terrestrial satellite systems can be used for navigation on the Moon, and to provide data to assist developers of new lunar navigation services.
These new services will support an exciting new phase of lunar exploration that is just getting underway. More than 80 national space agencies have stated an intention to operate or support lunar missions in the coming years, with over a dozen missions already planned, according to NASA.
Simulation will play a key role in lunar PNT testing
All of those missions will require positioning, navigation and timing (PNT) capabilities, of which GNSS and a new Lunar Augmented Navigation Service (LANS) will form key components. Developers of PNT equipment will need to be sure it will work as planned in the lunar environment. But with extensive field testing out of the question, the bulk of the testing must take place here on Earth.
The only way that can be done effectively is with simulation. As well as being cost-effective, simulation has the huge advantage of making scenarios controllable and repeatable, so iterative tests can be carried out in identical conditions with no unintended interference. It also allows developers to use signals that aren’t yet live, and to model corner cases and rare error states.
Six key considerations for realistic lunar PNT lab testing
Conditions on the Moon and in lunar orbit are very different from conditions on Earth and in Earth orbit. Developers will need to be able to reproduce specific lunar conditions in the lab using appropriate simulation equipment. Key considerations include:
Lunar orbital dynamics modelling: The gravitational effect of the Moon on a GNSS satellite orbiting around the Earth is negligible. But for new PNT satellites orbiting the Moon, the effect will be much more pronounced. In generating lunar orbit trajectories, the gravitational pull of the Moon (and the Sun) must be taken into account in the simulation models.
New reference systems: With so many lunar missions planned, interoperability will be key. That will require the production of new selenodetic reference frames to express trajectories in space and time, equivalent to the ECF and ECI geodetic reference frames we have today for terrestrial navigation. These new reference frames should be embedded in the simulation, which will also need the flexibility to incorporate future changes and additions.
Realistic TX antenna pattern modelling: Antenna patterns of lunar PNT satellites will be different from those of terrestrial GNSS satellites. Even when the terrestrial GNSS constellations are used, lunar receivers will rely on the side-lobe signals which overspill the Earth. The yaw of each space vehicle (SV) will need to be considered, as will power levels at the receiver antenna. All of these novel antenna patterns should to be accurately represented in the signal simulator.
Signal propagation: GNSS simulators typically allow for scintillation caused by electrons as the signal passes through the ionosphere. But lunar missions will rely on ‘spillover’ signals from GNSS satellites on the other side of Earth, meaning the signal may pass twice through the ionosphere before reaching the receiver, increasing the total electron count (TEC). By contrast, signals from navigation satellites orbiting the Moon will encounter much less interference due to the lack of atmosphere. Simulators must be able to accurately and realistically model these varied propagation effects.
Multipath: On Earth, multipath interference occurs when satellite signals are reflected or delayed by buildings and other tall structures, elongating the time taken to reach the receiver and causing erroneous measurements if not mitigated. On the Moon, the rocky regolith layer has a similar effect, and multipath and obscuration may also be exacerbated by the Moon’s extreme topography, with summits of up to 8km high and craters of up to 9km deep. GNSS simulators for lunar applications will need to be able to model these effects as realistically as possible—first with statistical models, but in the future with 3D modelling of specific lunar locations based on survey work.
Libration: The use of Earth-based GNSS navigation signals for lunar navigation will also be affected by libration. Factors like the rotation of the Earth and the velocity of the Moon relative to the Earth mean that waveforms from terrestrial GNSS systems will reach slightly different parts of the moon at different times. The simulator must be able to model these effects to provide accurate insights into the real-world performance of lunar PNT equipment.
With so many new parameters to take into account, developers of lunar PNT services and receivers will need to choose their simulation equipment carefully.
Choosing the right simulation platform for lunar PNT development
With so many new parameters to take into account, developers of lunar PNT services and receivers need to choose their simulation equipment carefully.
In addition to the considerations I’ve set out above, the equipment should also to be able to simulate the Augmented Forward Signal (AFS) that will be at the heart of the forthcoming LANS PNT system-of-systems in orbit around the Moon.
High hardware update rates will be required to realistically model highly dynamic vehicle trajectories. The ability to generate non-SIS RF signals using IQ files will be a key requirement for many, allowing developers to add custom signals into the simulation environment. In addition, a remote interface will be vital for integrating the simulator into existing setups or interfacing with other lab equipment, as well as for controlling the unit remotely.
Lastly, as we’re only at the start of this very exciting phase of expanded lunar activity and exploration, the simulator platform will also need to be flexible enough to accommodate the demands of tomorrow’s applications as well as today’s.
Supporting space-based navigation for over 35 years
Spirent has been the trusted signal generation partner of space agencies and commercial space developers for over 35 years. Our simulation platforms offer all of the capabilities outlined in this blog and are already supporting organizations around the world to develop the next generation of lunar PNT services.
Learn more about our test and measurement solutions for lunar PNT, visit spirent.com/solutions/pnt-space-industry or download our brochure Enabling Lunar Navigation.
