ROLSES Pathfinder Missions: Lunar Radio Astronomy

• ROLSES Pathfinder Missions are a series of lunar radio astronomy experiments leveraging novel deployable antennas and advanced calibration algorithms for plasma diagnostics and cosmic observations.
• They employ spiral-tube 'STACER' antennas and a 512-bin real-time WOLA spectrometer to accurately measure electron density profiles and characterize the radio noise spectrum on the Moon.
• These missions validate critical technologies for future lunar interferometric arrays by demonstrating robust data analysis, formation flying, and in situ spectral calibration techniques.

The ROLSES Pathfinder Missions refer to a sequence of lunar radio astronomy experiments deployed by NASA to establish the scientific and technological foundations for sensitive low-frequency radio observations from the surface of the Moon. These missions leverage novel deployable antenna technologies, adaptive radio spectrometry, and advanced calibration algorithms to analyze both the local plasma environment and the radio-frequency sky, as well as to address mission-specific formation flying concepts inherited from astrometry pathfinder studies. ROLSES serves as both a direct technology demonstration and a precursor to large interferometric arrays such as FARSIDE, targeting phenomena from solar plasma diagnostics to galactic synchrotron studies, and providing a roadmap for next-generation lunar radio science.

1. Historical and Scientific Context

ROLSES emerges from a lineage of precursor missions including PRISMA (the NEAT Pathfinder experiment) (Delpech et al., 2013), which established key formation flying and inertial pointing capabilities for astrometric satellites. While PRISMA’s context was primarily astrometry—precise alignment of multiple spacecraft to perform long-baseline measurements—ROLSES shifts focus toward radio astronomical investigation, but leverages similar principles of robust control, calibration, and system design.

ROLSES is distinguished as the first NASA radio telescope on the lunar surface, with ROLSES-1 deployed onboard the Odysseus lander during NASA’s Commercial Lunar Payload Services (CLPS) program (Hibbard et al., 12 Mar 2025). It marks the United States’ return of scientific radio payloads to the Moon after a gap of five decades. The mission is tightly coupled to a broader progression of lunar radio observatories, notably LuSEE and the envisioned FARSIDE array (Burns et al., 2021).

2. Mission Objectives and Measurement Approach

The primary objective of ROLSES is to demonstrate that high-fidelity, low-frequency radio measurements are feasible on the lunar surface. Scientific goals are bifurcated into plasma diagnostics—characterizing the photoelectron sheath above the lunar surface—and radio astronomy, encompassing both natural cosmic backgrounds and anthropogenic terrestrial signals (technosignatures).

Key measurements include:

• Electron density profile ($n_e$) and electron plasma frequency ($f_{pe}$) typically in the range $10\,\text{kHz} < f < 30\,\text{MHz}$, with $$f_{pe} = \frac{1}{2\pi}\sqrt{\frac{n_e\,e^2}{\varepsilon_0\,m_e}}$$ providing direct access to plasma parameters at vertical heights of 1–3 m.
• Continuous monitoring of the radio noise spectrum to characterize ground-based radio frequency interference (RFI), with a focus on the 0.01–30 MHz band.

ROLSES-1 demonstrated the capability to detect terrestrial technosignatures—shortwave transmissions modulated by Earth's ionospheric density fluctuations—through both spectral and waveform data, obtained via four stacer monopole antennas in a non-ideal deployment. Data were acquired via digital signal processor (DSP) spectra and raw waveform telemetry packets, facilitating cross-validation and precise spectral calibration (Hibbard et al., 12 Mar 2025).

3. Instrumentation and Technology

ROLSES deploys spiral-tube antennas ("STACERs"), each approximately 2.5 m in length and mounted at variable heights to probe the vertical structure of the lunar plasma (Burns et al., 2021). The front-end electronics are hardening for lunar surface operations, maintaining operational integrity at thermal extremes ($<$65 °C).

Spectral analysis is conducted via a 512-bin real-time digital Weighted Overlap and Add (WOLA) spectrometer, providing spectral resolutions of 1.76 kHz (low-frequency segment) and 58.01 kHz (high-frequency segment), with interleaved multi-antenna sampling completed every 4 seconds.

The readout electronics and data reduction pipeline address quantization noise, analog system noise, antenna deployment systematics, and use laboratory sweep calibrations to convert raw counts to calibrated voltage time series. Final power spectral densities ($S_{DSP}[f]$) are computed as $$S_{DSP}[f] = \frac{2}{f_s}\frac{V_{DSP}^2[f]}{W_{ss}}$$.

4. Data Analysis and Calibration Algorithms

The data analysis protocol for ROLSES incorporates multiple statistical methodologies:

• Sigma-clipping and time-frequency averaging to isolate astrophysical signals from transient and instrumental noise.
• Jack-knife resampling for variance estimation under low data volume.
• Bayesian posterior sampling via nested sampling algorithms (PolyChord) for multidimensional parameter inference, including the separation of noise and galactic background contributions.

Physical models for the galactic synchrotron spectrum employ templates of the form $B_\nu = B_0\,\nu^{-0.76}\exp(3.28\,\nu^{-0.64})$, fitted to PSD data after noise subtraction and calibration. Antenna effective parameters (product $\Gamma\ell_{eff}$) are extracted by singular value decomposition (SVD), with best-fit results in agreement with monopole theory ($L/4$ for a 2.5 m antenna).

Periodograms reveal temporal modulations in terrestrial technosignature amplitudes (fluctuations on minute timescales, 5–10 dB), consistent with scintillation by Earth’s ionosphere (Hibbard et al., 12 Mar 2025).

5. Implications for Pathfinder and Future Missions

ROLSES directly informs the design and operational strategies of next-generation lunar radio telescopes:

• It validates mission concepts and hardware solutions for deployable antennas and robust spectral calibration under lunar surface constraints.
• As a pathfinder, it establishes the technological and operational reference for FARSIDE (a 10-km scale interferometric array of 128 antenna pairs) and the risk-reduction PRIME array.
• Planned upgrades for ROLSES-2 include polarization measurement (full Stokes parameters), onboard reference calibration sources, improved EMI shielding, and more advanced antenna architectures. These are intended to facilitate measurements of the low-frequency sky, solar bursts, planetary emissions, and potentially the cosmological 21-cm signal from the Dark Ages (Hibbard et al., 12 Mar 2025).

The dual role in plasma diagnostics and radio astronomy, coupled with the capacity to detect both natural galactic backgrounds and anthropogenic terrestrial emissions, make ROLSES the keystone for future lunar radio science (Burns et al., 2021).

6. Connections to Astrometric Formation Flying

The operational strategy and mission design of ROLSES are underpinned by heritage formation-flying technologies validated by the PRISMA NEAT Pathfinder experiment (Delpech et al., 2013). Principles borrowed include:

• Translating inertial target specification into orbital frame trajectories using relations $R = d\cdot u_x$, $h = d\cdot u_y$ for geometric configuration.
• Employing feed-forward compensated LQR control laws ($u = -[K]X + [M]X_a + G$) for trajectory maintenance and environmental disturbance mitigation.
• Lessons in propellant management and attitude control (star tracker blinding mitigation, thruster impulse optimization) are applicable for anticipated lunar deployments where mass and energy constraints are severe.

The demonstrated centimeter-level control in inertial pointing, verified through multi-day orbital rehearsals, supports the feasibility and reliability of high-precision formation maneuvers underpinning lunar science payloads, including radio arrays (Delpech et al., 2013).

7. Scientific Contributions and Prospects

ROLSES-1, despite operational challenges and limited data, achieved the detection of both terrestrial technosignatures and the galactic synchrotron background, verifying instrument performance in the demanding lunar environment. The results substantiate the capability of lunar-based radio telescopes to open previously inaccessible windows onto the low-frequency Universe, including but not limited to heliophysics, cosmic dawn, reionization, and the characterization of planetary magnetospheres (Hibbard et al., 12 Mar 2025).

The rigorous data calibration, adaptive spectral analysis, and statistical validation frameworks developed for ROLSES-1 are directly applicable for upcoming missions, informing payload design, observational protocols, and post-processing pipelines. The mission sequence advances the state of lunar radio science, providing a critical validation step ahead of full-scale lunar interferometer deployment.

In aggregate, the ROLSES Pathfinder Missions represent a systematic progression toward operational lunar radio astronomy and allied plasma diagnostics, integrating technological innovations and methodological rigor, while creating a foundation for breakthroughs in astrophysics and planetary science.