Advanced Antenna Modelling with ITU-R S.1528-0: Optimizing NGSO Satellite Performance

Introduction

Recommendation ITU-R S.1528-0 establishes critical guidelines for modelling the radiation pattern of non-geostationary (NGSO) satellite antennas operating in the Fixed-Satellite Service (FSS) below 30 GHz. Recognized globally by the International Telecommunication Union (ITU) and the satellite industry, this framework serves as the foundation for:

• Network design, including coverage estimation and beam shaping,

• Spectrum coordination between co-frequency satellite systems to avoid or mitigate interference issues,

•  Compatibility studies with other radiocommunication services, such as terrestrial networks and radio astronomy.

With the 2027 World Radiocommunication Conference (WRC) expected to introduce new satellite allocations in the S-band (2-4 GHz), the precision of satellite antenna modelling has become more critical than ever. This is especially true for Agenda Item 1.13, which proposes enabling direct-to-cell communications from satellites, a ground-breaking capability that would allow seamless connectivity between satellite networks and terrestrial mobile devices. The success of this initiative hinges on effective cross-border coordination, as satellite beams often cover multiple jurisdictions simultaneously. In that regard, accurate modelling is essential to define protection criteria for terrestrial networks.

 The Limitations of Envelope-Based Models

Traditional antenna modelling in satellite communications have often relied on conservative envelope-based approaches, which by design tend to overestimate sidelobe levels. While these models simplify regulatory compliance, they introduce additional margins that can lead to:

• Spectrum underutilization, as excessive guard bands may be imposed unnecessarily,

• Reduced system efficiency, since worst-case assumptions limit optimization opportunities,

• Higher operational costs, as spectrum resources are allocated sub-optimally.

These limitations become particularly problematic in NGSO mega-constellations, where thousands of satellites must coexist without causing harmful interference to geostationary (GSO) networks or sensitive services like radio astronomy.

 A Realistic Alternative: Clause 1.4 of Recommendation ITU-R S.1528.0

To address these challenges, Clause 1.4 of Recommendation ITU-R S.1528-0 introduces a physics-based model using an elliptical and uniformly illuminated aperture. This approach offers several advantages:

1. Accurate sidelobe Modelling

•  Unlike envelope approximations, this method directly models sidelobe structures, providing a more realistic representation of antenna behaviour.

• This is particularly useful for interference analysis, as it avoids the overestimation of off-axis emissions.

2. Improved Spectrum Efficiency

• By replacing worst-case assumptions with actual radiation patterns, operators can optimize frequency reuse and minimize unnecessary guard bands.

•  This is crucial for NGSO constellations, where spectrum sharing with GSO networks is tightly regulated.

3.  Enhanced System Design Flexibility

•  Engineers can fin-tune beamforming strategies to maximize coverage while minimizing interference risks.

•  This is especially relevant for LEO constellations, where dynamic beam steering is essential for maintaining seamless connectivity.

To illustrate the benefits of this model, an example is provided of an NGSO satellite antenna operating at 1 GHz with a peak gain of 20 dBi. Clause 1.4 of Recommendation ITU-R S.1528-0 is applied to model an elliptical aperture (3.2 m transverse x 1.6 m radial), resulting in the antenna patterns shown below.

Illustrative example of an antenna radiation pattern based on Recommendation ITU-R S.1528-0 

The next figure displays the gain contour of this antenna when deployed on a satellite at 2,000 km altitude, positioned at 0° longitude and 30° latitude, and steered toward Bari, Italy. The main lobe coverage terminates near the Black Sea, while the first sidelobe extends as far as the Caspian Sea. This example highlights the complexity of cross-border coordination, as satellite signals can propagate well beyond national borders, reaching regions far from the intended service area.

Implementation and usage

At River Advisers, our in-house spectrum management tools incorporate Recommendation ITU-R S.1528-0 Clause 1.4 antenna pattern modelling to ensure regulatory compliance while optimizing computational performance. This capability is particularly crucial for running high-fidelity Monte Carlo simulations that accurately assess satellite constellation behaviour over extended periods, enabling statistically significant interference analysis and system performance validation.