Non-Terrestrial Networks (NTN) are an emerging key component for future positioning, navigation, and timing (PNT) architectures, by assisting or even augmenting legacy Global Navigation Satellite Systems (GNSS). As the 3rd Generation Partnership Project (3GPP) is taking an active role in the standardization process of NTN, it has risen a growing necessity for simulation tools which allow to analyze satellite geometries, antenna characteristics, and signal properties in accordance with the 3GPP NTN recommendations. This work presents the first public release of a configurable NTN simulation framework developed in MATLAB, and designed to support further research in satellite-based positioning and satellite-communication-assisted navigation. The simulator provides a flexible environment for constructing and analyzing arbitrary satellite constellation architectures, spanning customizable Walker-Delta and Walker-Star orbital patterns. Each satellite is assumed to be equipped with a planar multi-beam antenna array whose geometry layout, in-orbit-orientation, and beam pattern follow the definitions provided in the relevant 3GPP NTN technical specifications and reports. For instance, we have validated use cases defined within 3GPP, such as 600 km, 1200 km, and 8000 km of orbital altitude, anticipating IRIS2-like (Infrastructure for Resilience, Interconnectivity and Security by Satellite) architectures. The simulation tool implements the complete transformation chain from the antenna’s local UV-plane representation to its projection onto the Earth’s surface, in order to generate realistic multi-beam coverage footprints and evaluate the antenna beams’ on-the-ground geometric distortion as a function of off-nadir angle and satellite altitude. For any defined user equipment (UE), the simulator aims to provide a set of primary observables essential for NTN-based PNT research, such as satellite position and velocity, satellite azimuth and elevation from the UE perspective, satellite-to-UE visibility and access, geometric range and rate-of-range between satellite and UE, and the beam identifier corresponding to the satellite antenna footprint which encompasses the UE. A link-budget module tailored for outdoor conditions outputs the resulting carrier-to-noise density ratio (C/N₀), enabling performance analyses which couple geometric factors with signal quality. The proposed framework aims to provide the research community with an accessible, flexible and customizable simulation environment to accelerate positioning algorithm development, performance assessment, and integration of NTN assets into future positioning systems.

The Kepler satellite navigation system uses Optical Inter-Satellite Links (OISL) to synchronize Ultra Stable Oscillators (USO) that serve as a time reference. The OISLs are additionally used to measure inter-satellite ranges for orbit determination and to exchange data within the constellation. The Kepler system shall be rather autonomous, which requires that most of the processing be performed on-board. The movement of the satellites in the earth’s gravity potential requires relativistic adjustments. The presentation focuses on this aspect as well as on the extraction of ranges and time offsets from measured optical pseudoranges.

GPS and all other GNSSs use Atomic Frequency Standards (AFS). They tick regularly in the rest-system of the clocks. In GNSS, the difference of time evolution on the satellites and on ground is corrected by shifting the frequency of the AFSs and by adjusting times in the receivers. Kepler uses Ultra Stable Oscillators (USO) and has the freedom to map the complete correction into an on-board frequency shift. Terrestrial Time (TT) is thus brought to the satellite at the price of USOs running at a different frequency for every satellite. The resulting universal time scale simplifies processes, such as the measurement of pseudoranges or the computation of system time. Kepler satellites can also integrate AFSs with the necessity of mapping their time scales. The latter capability is important, since Kepler satellites are meant to replace Galileo satellites one-by-one. Because of the universal use of TT, Kepler receivers do no longer apply clock corrections (not for clock parameters, nor for relativity). The continued operation of fielded Galileo receivers is supported by adjusting clock parameters in the navigation message and thus canceling undue corrections.

The optical pseudoranges between two satellites depend on the inter-satellite range and on the time offset. Although synchronization is aimed at, it is never realized perfectly. Tiny offsets are the basis for adjusting the USOs to track Kepler System Time (KST). KST itself needs to be estimated on every satellite individually which is achieved by a Kalman filter with a covariance renormalization, as proposed by Greenhall. The latter algorithm runs on time differences which are fortunately all represented in TT (on all satellites and on the ground). This ensures that all satellites estimate a nearly identical KST, with the availability of measurements being the only source of differences.

Running the KST-estimation on the satellites requires that time offsets be derived from the optical pseudoranges. Fortunately, the highly complex overall estimation process can be simplified. The assumption that some velocities are known, leads to an expression for every inter-satellite range and clock offset in terms of the pseudoranges measured in both direction on a given link. Luckily, the expression for the range depends only weakly on velocities. As a consequence, orbit determination can be performed independently and can then be used to enable the estimation of the clock offset. The complex process is thus reduced to two much simpler sub-processes.

The Optical Synchronized Time and Ranging (OpSTAR) mission, part of the European Space Agency’s “FutureNAV” activities, aims at in-orbit validation and verification of optical links integrating highly accurate synchronization and ranging functions. This key optical technology is an enabler for next-generation Global Satellite Navigation Systems (GNSSs), and their evolution as part of global multi-layer Position, Navigation and Timing (PNT) “system-of-systems”.
The cornerstone of the OpSTAR mission is the in-orbit testing of optical inter-satellite and satellite-to-ground links to autonomously synchronize remote clocks, obtain accurate ranging measurements to enhance satellite orbit determination, and exchange measurement and mission data with low latency and high throughput across all mission elements.
To achieve this objective, two satellites, equipped with multiple optical terminals, are supported by several Optical Ground Stations (OGSs) providing an optical interface between space and ground segments. In order to ensure compatibility across all optical terminals realizing the links, open specifications are produced to ensure interoperability, transparency, and long-term compatibility.
A subset of OGSs interfaces directly with a purposefully-built system synchronization testbed supporting validation of novel approaches to system synchronization enabled by optical observables. Specifically, a distributed space and ground synchronization system is in focus, in which all satellites and ground elements contribute to the definition of a common system time, to which all assets are kept synchronized with unprecedented accuracy.
A system simulator, built drawing from the capabilities under verification in the OpSTAR mission, is used to extrapolate system and end-user performance for GNSSs integrating optical links in their system and processing architectures. An OpSTAR-dedicated (optical) ground PNT test range addresses system exploitation by implementing a number of actual end-user cases, focusing on L-band, optical and hybrid (joint L-band and optical) users. The PNT test range enables the validation of diverse use cases under realistic conditions, assessing the potential of hybrid (optical-radiofrequency) signal processing, and system performance from the user perspective.
The OpSTAR mission is operationally supported by a ground control segment overseeing the operations of both spacecraft, from launch to decommissioning, as well as the operational interface between space and ground segments via traditional radiofrequency links, handling part of the mission data flow (mission commands, telemetry, download and dissemination of mission-related data).
A dedicated orbit and time reference system provides the necessary capability to accurately estimate and predict both spacecraft orbits and onboard clocks, in order to support the execution of real-time operations (scheduling, pointing for establishing optical links, generation and distribution of a geodetic reference and a system time, etc.) and all verification activities (e.g. ranging and time-synchronization verifications).
The OpSTAR mission overarching objective is to prove optical technologies for PNT system operators and end-users, quantifying the overall benefits offered. To this purpose, the OpSTAR mission is a testbed for validating architectures integrating optical links in next-generation GNSSs for more accurate, autonomous, robust and resilient global position, navigation and timing services.
This work presents the mission overall OpSTAR mission architecture from a technical standpoint, focusing on experimental elements and their coordinated operations for achieving the mission objectives.