Taking PNT to the Moon
Ricardo Verdeguer Moreno, Spirent Communications PLC
Date/Time: Friday, Sep. 20, 4:04 p.m.
As humanity continues with space exploration, navigation on the Moon emerges as the next milestone in our journey. Potential lunar applications and missions, ranging from scientific research to resource extraction, necessitate robust Positioning, Navigation and Timing (PNT) solutions. Whether relying on Earth-based GNSS signals or the promising Lunar Augmentation Navigation Service (LANS), the need for high-quality PNT solutions becomes critical to guarantee reliable performance within the lunar environment.
To successfully take reliable PNT to the Moon, developers of space receivers, shuttles and satellite constellations must adhere to a rigorous product development process, where each stage, from conceptualization to production, is followed by a comprehensive testing and validation phase. Within this context, this presentation focuses on the challenges facing PNT simulation to guarantee mission success.
The unique demands placed on GNSS technology when applied to lunar scenarios are covered, highlighting the need for precise dynamic models to account for the uneven gravitational pull from the Moon, perturbations from other celestial bodies, and differences in solar radiation pressure. Modelling the lunar orbital dynamics becomes essential to realistically replicate the vehicle and satellite trajectories in simulation software. Furthermore, new selenodetic reference and time systems must be implemented, incorporating an inertial reference frame for precise time resolution (including relativistic effects) and the numerical integration of the equations of motion; as well as a body-fixed reference frame for the expression of the broadcast navigation message to estimate position without implementing the rotations of the inertial frame.
In cislunar applications, accurately representing side lobe signals beyond the 23.5 degrees of SSV specification is vital. Simulation software must integrate realistic Tx antenna patterns, representative of the onboard GNSS SV antennas, and account for yaw attitude changes necessary for solar panel orientation. Moreover, signal propagation models together with impairments must be incorporated into simulation. Advanced Earth ionospheric models are required for calculating the Total Electron Count (TEC), considering potential double ionosphere crossings of Earth-based GNSS signals. In the context of Moon navigation, despite the extremely low TEC values of the lunar exosphere in the order of 0.01 TECU, variations of day and night gas concentrations shall be also taken into account (2×?10?^5 ?cm?^(-3) during the lunar night, and ?10?^4 ?cm?^(-3) during the lunar day). In addition, it's crucial to address the impact of obscuration and multipath effects, particularly on S-band PNT signals. Studies indicate that these effects can lead to significant doppler shifts of up to 2 kHz and signal delays of a few microseconds. To accurately simulate them, statistical multipath models and realistic 3D scenarios are necessary. Recent research suggests that the core and regolith electrical characteristics of the Moon resemble those of certain ceramic materials. Understanding these properties is key for exploring specific lunar regions, especially considering the extreme altitudes ranging from +8km to -9km. Notably, these extremes occur predominantly on the far side of the Moon.
Incorporating these insights into simulations is essential for comprehensive PNT solutions tailored to lunar exploration missions. All these aspects require a sophisticated GNSS simulation engine capable of reproducing these effects in a controlled laboratory environment for systematic and deterministic testing. Only employing these advanced simulation techniques, researchers and engineers will be able to evaluate the performance of GNSS receivers under diverse conditions, while ensuring adaptability to the evolving mission requirements of cislunar and lunar applications.
The challenges presented are turned into system requirements, which outline the need for a scalable and flexible platform, capable of simulating high-fidelity GNSS and LANS RF while providing future-proof capabilities to accommodate evolving needs. The insights shared aim to contribute to the ongoing efforts in advancing lunar exploration and exploitation by providing a robust foundation for PNT technologies in the challenging lunar environment.
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