NIAC project report: Solar system-scale VLBI to dramatically improve cosmological distance measurements

We investigate the feasibility and scientific potential of the Cosmic Positioning System (CPS), a space mission concept enabling purely geometric distance measurements to sources at hundreds of megaparsecs by directly detecting electromagnetic wavefront curvature. CPS consists of a constellation of radio antennas distributed across the outer Solar System, operating on baselines of tens of astronomical units. By precisely timing the arrival of repeating fast radio bursts (FRBs), CPS infers source distances via trilateration – analogous to global navigation satellite systems such as GPS but on cosmological scales. We show that CPS distance measurements could result in sub-percent constraints on the Hubble constant with even a handful of detections, whereas we predict that 10-100 FRB sources are likely visible. We evaluate dominant sources of uncertainty – wavefront timing precision, interstellar refractive delays, spacecraft positional knowledge, and onboard clock stability – finding these controllable at required levels using near-term technologies. Our nominal design employs five spacecraft with 8 m deployable antennas, 3-6 GHz receivers with sub-30 K system temperatures, and space-qualified atomic clocks similar to those on GPS satellites, supported by a ground network for ranging calibration and FRB alerts. Beyond cosmic expansion, CPS may enable frontier measurements in astrophysics and fundamental physics, including constraints on small-scale dark matter structure, microhertz gravitational waves (bridging pulsar timing arrays and LISA), and the outer Solar System mass distribution. The most significant viability issue concerns FRB properties at several-GHz frequencies; we recommend observational campaigns to characterize repeating FRBs in this band.

💡 Research Summary

The NIAC Phase I report presents the Cosmic Positioning System (CPS), a bold mission concept that would place five radio‑telescopes throughout the outer Solar System and use them as a gigantic interferometer with baselines of tens of astronomical units. By measuring the curvature of the wavefront from distant, millisecond‑duration fast radio bursts (FRBs), CPS can determine the absolute distance to each source through pure geometry, analogous to the trilateration used by GPS but on cosmological scales.

The authors derive a simple scaling law for the fractional distance error: σd/d ≈ 2 % × (40 AU/x)² × (200 Mpc/d) × (5 GHz/ν) × (σt · ν), where x is the typical spacecraft separation, ν the observing frequency, and σt the timing precision. With baselines of 30–100 AU, observing at 3–6 GHz, and achieving timing precision of 0.1–1 ns (i.e., σt ≈ ν⁻¹), a single FRB at 200 Mpc can be measured to better than 1 % distance accuracy. Repeating FRBs allow multiple measurements, so a handful of sources would already constrain the Hubble constant H₀ to sub‑percent precision, directly addressing the current ≈7σ tension between distance‑ladder and CMB determinations.

Technically, the baseline design consists of five spacecraft each equipped with an 8–9 m lightweight deployable high‑gain antenna, 3–6 GHz receivers with system temperatures of 20–30 K, and space‑qualified atomic clocks comparable to those on GPS satellites (stability ≈10⁻¹⁰ s over hours). Inter‑spacecraft ranging (laser or microwave) would deliver centimeter‑level relative position knowledge, while ground stations (3–4 × 8 m dishes) provide clock synchronization, FRB alerts, and occasional data downlink. On‑board storage of ~10 TB would buffer burst data for later transmission; the required downlink time is a few hours per month via the Deep Space Network. Power would be supplied by radio‑isotope thermoelectric generators, and the mass/power envelope is argued to be compatible with existing outer‑Solar‑System mission classes.

Four dominant uncertainty sources are examined: (1) wavefront timing noise (receiver jitter, ADC quantization, digital pipeline delays) – mitigated by modern high‑speed digitizers with sub‑10 ps jitter; (2) plasma dispersion and refractive delays in the interstellar and interplanetary medium – these scale as ν⁻² and become negligible above ≈3 GHz; (3) spacecraft position knowledge – centimeter‑level ranging combined with precise orbit determination yields the required c/ν ≈ 6 cm accuracy at 5 GHz; (4) clock stability – space‑qualified rubidium or cesium clocks, thermally regulated, can meet the 0.1 ns requirement.

Beyond cosmology, CPS would enable three high‑impact secondary science programs: (i) probing dark‑matter clumpiness on ∼100 AU scales via differential gravitational time delays between the spacecraft; (ii) detecting micro‑hertz gravitational waves (10⁻⁷–10⁻⁴ Hz), filling the sensitivity gap between pulsar‑timing arrays and LISA; (iii) mapping the mass distribution of the Kuiper Belt and searching for unseen planetary bodies (e.g., the hypothesized Planet 9) through precise astrometric perturbations of the spacecraft.

The most serious viability risk is the unknown behavior of repeating FRBs at the targeted 3–6 GHz band. Existing surveys are concentrated below 2 GHz; extrapolations suggest reduced flux and lower repetition rates at higher frequencies, which could limit the number of usable events. The authors therefore recommend dedicated high‑frequency FRB monitoring campaigns, both ground‑based (e.g., with the VLA, ATCA, or upcoming ngVLA) and space‑based, to characterize spectra, burst rates, and scattering properties.

In summary, the report demonstrates that CPS is technically feasible with near‑term hardware, that it can achieve the required timing and positional accuracies, and that it would provide a fundamentally new, ladder‑free distance measurement method capable of delivering sub‑percent H₀ constraints. If the high‑frequency FRB population proves sufficient, CPS could simultaneously open new windows on dark‑matter microstructure, ultra‑low‑frequency gravitational waves, and the mass architecture of the outer Solar System, making it a compelling candidate for a flagship cosmology mission in the next decade.