LEO-PNT constellations are under definition and soon major system enabling technologies will be demonstrated in-orbit. Their objective is to offer additional services and improve existing GNSS systems on different aspects, such as accuracy, convergence time, robustness and resilience. The ‘multiple layer’ concept is based on an additional fleet of LEO satellites emitting navigation signals to ground users. A joint ground receiver can process these new signals by themselves or together with existing GNSS signals for enhanced navigation performances. The core of a LEO-PNT concept is the orbit determination and time synchronization (ODTS) sub-system of the constellation.
Current system designs assume that the LEO-PNT navigation signals’ content (i.e. Navigation Messages) is directly generated on-board based on observations taken by a GNSS Rx and processed by an ODTS filter. GNSS code and carrier-phase observations together with real time GNSS broadcast ephemeris are fed to the ODTS. Thanks to the high accuracy of Galileo Navigation Messages and the availability of HAS E6B corrections, decimeter-level precise orbits and below-nanosecond accurate clocks can be estimated [1, 2] in-orbit and used to generate LEO Navigation Messages, ready to be broadcasted to ground users. Access to a GNSS reference time scale is directly guaranteed by the processing of these same inputs [2]. Synchronization to one or the other reference time is then a direct consequence of the internal processing of the ODTS.
The aim of this paper is to assess the achievable ODTS performance, focusing mainly on the accuracy of the LEO Navigation Messages broadcasted to final users. Orbit and clock errors are both assessed. The time synchronization process is explored in depth, focusing on the different instrumental biases that intervene on-board and how to overcome these potential sources of errors. Moreover, we shall investigate the benefits of an on-board steering architecture with respect to letting the on-board oscillator in free running. The advantage of aligning the ODTS clock estimation to GST, an external and more stable timescale, and improving the long-term stability of the on-board clock is discussed and defended with real closed-loop (Oscillator - GNSS Rx – ODTS – Oscillator) simulations.

The future deployment of Positioning, Navigation, and Timing (PNT) systems in low Earth orbit (LEO), also known as 'LEO-PNT', is a key milestone in the technological evolution of Global Navigation Satellite Systems (GNSS). However, a crucial prerequisite for precise point positioning (PPP) user solutions is the real-time provision of high-accuracy satellite corrections, including orbits and clocks, as well as transmitter hardware delays. The latter are fundamental for enabling Integer Ambiguity Resolution (IAR), which allows users to fully exploit the millimeter-level carrier-phase measurement noise.

The availability of GNSS receivers on board the LEO-PNT satellites makes it possible to estimate (and predict) orbital products in real time, with 3D RMS errors generally of 1–2 decimeters. However, the on-board estimation of clocks and transmitter biases most likely requires signals to be observed from a network of ground stations, also due to their potentially reduced temporal stability. This poses new challenges due to the lower orbital altitude foreseen, e.g. 600–1200 km, for the next-generation LEO-PNT systems, leading to passes over ground stations shorter than 15–20 minutes.

In this contribution, we focus on a regional network estimation of LEO-PNT satellite corrections based on a few network sizes (e.g., from tens to hundreds of kilometers). We consider an uncombined and undifferenced approach for PPP Real-Time Kinematic (PPP-RTK) network processing, where satellite clocks and hardware delays are computed via a Kalman filter. Then, we investigate three scenarios in which satellite orbits are assumed to be I-known (i.e. no orbital errors), II-estimated (i.e. cm-level errors), III-predicted (i.e. dm-level errors). Note that satellite orbital errors in along-/cross-track components now play a major role in determining the user’s signal-in-space ranging errors (SISRE), unlike what is observed from GNSS in medium Earth orbit (MEO).

The presence of orbital errors affects the PPP-RTK users, as well as the PPP-RTK network processing, thereby contaminating the satellite corrections estimated via ground station receivers. Based on end-to-end simulations, this work analyses the numerical performance when computing these corrections, along with the impact on user positioning in terms of accuracy and convergence time. In both the network and the user processing, phase ambiguities are resolved using the Least-squares AMBiguity Decorrelation Adjustment (LAMBDA) 4.0 toolbox, whereas different IAR strategies are adopted for the two cases.

Overall, common LEO-PNT constellation designs lead to satellites generally visible at low elevations, which further reduces the duration of passes. At the same time, a shorter tracking period is partly compensated by the rapid variation of geometry in LEO, allowing for a faster convergence on the user side. In this research, we consider a representative 30-satellite constellation in six orbital planes with 55° inclination, inspired by CentiSpace (Layer 1) system configuration, yielding around 1-2 LEO satellites in view. They are assumed to be tracked on L-band, e.g. L1 and L5, while three different orbital altitude regimes (at 600 km, 900 km, and 1200 km) are considered as a basis for comparison, ultimately to highlight the different impact on both network processing and user performance.

Xona is deploying Pulsar, a near 260-satellite LEO constellation offering dual L-band navigation services, X1 and X5, near L1 and L5 respectively. Designed for interoperability, Pulsar provides centimetre-level accuracy, resilience, and authentication, while maintaining a format that existing GNSS receivers can support through a firmware update.

With the launch of the Pulsar-0 IOV in June 2025, Pulsar as a service is becoming a reality. Xona has worked for years with established industry receiver companies to prepare pulsar-enabled units, which are now showing promising results.

This presentation highlights the milestone achievements made by Pulsar-0 to date, boasting ranging authentication, 42mm accuracy, indoor signal reception and jamming resilience, all carried out with live-sky signals, through user-centric demonstrations.

The satellite sector has seen significant growth and diversification in recent years, driven by advancements in satellite technology, miniaturization, and the increasing demand for satellite-based services. Satellites, orbiting between 200 to 2,000 kilometers above Earth, offer advantages like low latency and high resolution for Earth observation, communications, and positioning.
Spaceopal and the German Aerospace Center (DLR) have joined forces for technology transfer to create the HAUT-S, a commercial space qualified payload for real-time onboard precise orbit determination (OD) and time synchronization (TS) for Low Earth Orbit (LEO), low Medium Earth Orbit (low-MEO), CubeSat satellites and sounding rockets trajectories (>100km). HAUT-S reduces latency for onboard real-time precise obit information that enables advanced mission concepts such as LEO-PNT, only possible if LEO satellites know their orbits and time very precisely.
HAUT-S brings to the market innovative features that make it a capable and resilient component. It integrates Orbit Determination (OD) and Time Synchronization (TS) into a single building block, achieving real-time, onboard accuracy of one decimeter in 3D when using Galileo HAS Initial Service corrections. HAUT-S is optionally equipped with a Chip Scale Atomic Clock (CSAC) fit for space applications directly integrated on the main electronics board to deliver a time synchronized 10 MHz and 1 PPS output signal for other subsystems for the navigation signal generation in a LEO-PNT satellite.

The design incorporates fault tolerance and redundancy to ensure reliability and includes interference detection and mitigation, as well as antispoofing capabilities through Galileo Open Service Navigation Message Authentication. It offers multiple outputs: observables, orbit data, PPS, 10 MHz signals, etc. The HAUT-S supports configurable orbit determination setups (constellations, signals, corrections, force models), interfaces with Optical Time Transfer and Ranging data inputs, and features upgradable software along with spare hardware processing capacity for future enhancements.
The HAUT-S product plan has completed where the primary focus was consolidating requirements, developing software and hardware and manufacturing units. The philosophy chosen for the design of the HAUT-S is the rigorous use of COTS EEE parts, especially space qualified or with flight heritage. The performance verification campaign includes orbit accuracy tests through Hardware-in-the-Loop (HiL) tests using GNSS RF signal simulator, designed to confirm that HAUT-S meets its performance specifications across representative orbital configurations with varying inclinations and altitudes, inspired by real satellite missions. These tests are executed both in laboratory and duringa thermal vacuum cycling test, exposing the system to vacuum and repeated transitions between high and low operational temperature extremes. The structural integrity of the HAUT-S is verified through vibration tests, with test margins defined according to requirements of a general launch vehicle.
This paper outlines the development and qualification of the HAUT-S, an Orbit Determination and Time Synchronization building block for LEO-PNT satellites, highlighting the engineering approach, performance achievements, and lessons learned on the path to space readiness.