Code-independent navigation using Signals of Opportunity (SoOP) from non-cooperative low-Earth-orbit (LEO) satellites demands real-time knowledge of each transmitter’s state. In Frequency-of-Arrival (FoA) methods, Doppler changes rapidly because LEOs move at ~7–8 km/s; a few hundred milliseconds of latency or timestamp error can mis-map frequency to line-of-sight velocity and bias positioning. Short visibility windows (tens of seconds to a few minutes), frequent handovers across many satellites, and dense urban or aviation scenarios further compress the processing budget. Since non-cooperative LEOs do not broadcast navigation messages, the receiver must autonomously determine who is visible and propagate state as measurements arrive. Consequently, fast visibility extraction and timely ephemeris propagation are not conveniences; they are prerequisites for FoA observability and for scheduling resources (acquisition windows, handovers, antenna/beam steering, and per-satellite processing).
We present a simple, robust infrastructure that couples a GPU-accelerated visibility engine with a request-driven CPU ephemeris service. A vectorized SGP-4 implementation, validated to sub-millimeter agreement with MATLAB’s SGP-4 over representative horizons, runs on a low-cost NVIDIA Jetson Orin Nano. For merged TLE catalogs exceeding 8,000 LEO spacecraft, the GPU path delivers ~30× speedup over CPU baselines and completes global visibility checks in ≈1 min at sustained 100% GPU utilization; to demonstrate headroom, 50,000 TLE entries are processed in ≈3 min using the same workflow. A lightweight two-layer client–server design (stateless query handler over a pinned-memory ephemeris cache) returns per-satellite state vectors in ≈0.01 s worst-case on CPU while occupying <10% CPU. Under current traffic (~600 simultaneously visible LEOs over a 10-min and without elevation masking), per-request latency averages ≈0.003 s, meeting real-time FoA needs with zero operator interaction and scaling comfortably beyond six times today’s LEO counts. The end-to-end system has passed long-duration stability tests in the LASSENA laboratory; visibility lists and propagated ephemerides are numerically consistent with reference SGP-4 outputs. This architecture offers a practical, commodity-hardware path to autonomous, real-time SoOP navigation at constellation scale.

We present a reproducible workflow to synthesize and exploit non-cooperative LEO Signals of Opportunity (SoOP) in Skydel for Doppler-based navigation, and we validate performance against real-world data under matched satellite geometry. Using direct signal-generation method, each LEO spacecraft is represented as a moving emitter whose trajectory is driven by 1 Hz SGP-4 ephemerides; per-satellite custom baseband IQ is injected and up-converted by Skydel to the RF output. The Iridium-NEXT simplex waveform is emulated using whitening, DE-QPSK mapping with a 125-bit BPSK preamble for fast acquisition, dual-stage RRC pulse shaping, and sub-band translation to the 31.5 kHz channels (41.667 kHz spacing), with multi-channel summation and randomized duty-cycle bursts to mirror TDMA behavior. The same constellation set and site are used for simulation and field tests (L-band center ≈ 1.62627 GHz, sample rate 1 MHz); in replay, eight of nine links are reproduced (one below the elevation mask), yielding Doppler tracks within ±15 Hz (σ≈10 Hz) of predicted S-curves, while over-the-air captures exhibit 50–200 Hz variability attributable to oscillator instability, multipath, and propagation dynamics.
A common processing chain - including coarse acquisition, carrier/Doppler tracking with CFO drift compensation, outlier-robust pre-filtering, and an EKF-based Doppler-only geometry solver - operates identically on simulated and live data. Under identical conditions, Skydel-based navigation achieves ~12 m horizontal error, whereas the real-world experiment yields ~33 m. The gap is consistent with live-environment impairments (residual oscillator biases, local multipath, ephemeris/timing mismatches). Overall, Skydel-generated non-cooperative LEO SoOP faithfully reproduces the key observables and error modes required for design-space exploration and provides credible lower-bound accuracy predictions for resilient PNT when GNSS is degraded or denied.

Modern prosperity depends on the availability of energy, universal connectivity, and mobility, delivered by synchronized communication networks, precise positioning, navigation and timing (PNT), stable power grids, and autonomous operations. These capabilities increasingly rely on space-based assets, which provide global coverage and precision. Their rapidly growing criticality is enabled by affordability, itself driven by dramatic reductions in launch costs and economies of scale. Beyond cost, a less explicit but foundational pillar is asset coherency: the spatiotemporal synchronization of constellation elements so that economically relevant functions act as a coherent system.

For example, despite dynamic Doppler-shifts and variable propagation delays between different satellites, the efficiency of satellite communication payloads heavily depends on tight frequency allocations, time multiplexing, coordinated frequency and beam hopping and Multiple-Input Multiple-Output (MIMO) techniques. Improved synchronization reduces guard bands and guard times, raising spectral efficiency (bits/s/Hz) and user capacity. Present operational targets are in the order of tens of nanoseconds for network synchronization. More stringent requirements - nanosecond today and trending to picosecond - apply to payloads generating navigation signals for ground and near-ground users, where timing stability directly determines PNT accuracy. Earth-observation payloads using interferometric synthetic aperture radar (InSAR) and radio-frequency geolocation demand microwave signal phase alignment within a fraction of the carrier wavelength.

Obviously, high-fidelity frequency and time transfer across large baselines is at the heart of any critical infrastructure payload for satellite constellations. This paper presents a versatile on-board timing and synchronization concept centered on a spaceborne Global Navigation Satellite System (GNSS) receiver as the core of a payload electronics platform compliant with the SpaceVPX standard.

The FoX NavRIX (“PinPoint”) receiver delivers high-accuracy real-time navigation by exploiting the GALILEO High Accuracy Service in combination with a Kalman filter, leveraging heritage from multiple flight missions to achieve high reliability and availability in New Space constellation environments. The receiver supports simultaneous multi-constellation, multi-frequency GNSS, features low acquisition/tracking thresholds, provides multiple antenna inputs with optional dislocated low-noise amplifiers, and includes self-calibration for exceptional long-term stability. Integrated with a multi-technology oscillator, PinPoint forms a robust on-board timing subsystem disciplined to GALILEO time by default, but resilient via acceptance of alternative radiofrequency or optical synchronization references, reverting to a local clock ensemble under loss of synchronization connectivity. For particularly critical applications, support for the GALILEO Public Regulated Service (PRS) is available. Additional FoX platform elements - advanced Software Defined Radio (SDR) and Single Board Computer (SBC) modules - enable interference cancellation and the implementation of wideband transmit and receive functions.

Through worked examples spanning low-Earth to cis lunar orbits, the paper quantitatively evaluates GNSS receiver-centered payload performance for communication, navigation, earth-observation and surveillance use cases. A few examples for a LEO-PNT payload are provided below (downladable from https://mft.beyondgravity.com/download/ef2ef264-493c-4f75-95f4-015940b4bb08).