How and why drones need to be tested – The differences between testing in LOS and RIMP environments

Introduction: How and why do drones need to be tested?
Drones and UAV systems have rapidly evolved from niche technologies into a central part of modern industry, infrastructure, and security. Today, they are used in everything from industrial inspection and autonomous transportation systems to advanced defense applications. What all these applications have in common is a requirement that cannot fail: a stable and reliable wireless communication link.

Traditional measurement methods, where antennas and other radio components are tested separately – with the antenna often measured in anechoic chambers – provide valuable insights but do not capture how the complete system behaves during real operation. Once the antenna is integrated into the drone, its performance is influenced by the mechanical design, material selection, and interaction with other components. As a result, actual radio performance often differs from what was theoretically expected. This is where OTA testing (Over-The-Air) becomes essential. By testing the complete system in a controlled radio environment, it becomes possible to measure how communication actually performs under conditions that more closely resemble real-world operation.

At the same time, it is important to understand that drones do not operate in a single, uniform radio environment. Communication requirements vary depending on the application and surrounding environment. A drone operating in an urban environment is surrounded by reflections from buildings, vehicles, and infrastructure – a so-called RIMP environment (Rich Isotropic Multipath). A drone flying over open fields, conflict zones, or a high-altitude UAV, on the other hand, operates in a radio link that is effectively dominated by a direct signal path – a LOS environment (Line-of-Sight). These two environments place different demands on drone antenna performance and may expose different types of weaknesses in the system design. In practice, this means that both complex multipath environments and scenarios dominated by direct line-of-sight communication need to be analyzed. The following section introduces these two reference environments – RIMP and LOS – which are based on research from Chalmers University of Technology, led by Professor Per-Simon Kildal and further developed together with Jan Carlsson and colleagues. Together, they form the theoretical framework for this type of OTA testing and provide the foundation for this white paper.

Finally, it is worth noting that many modern drone applications, particularly within industry and defense, largely operate in environments where line-of-sight conditions dominate. This means that understanding how the antenna behaves across different directions and orientations is often critical to real-world system performance – and is therefore especially well captured through LOS-based testing in controlled test environments.

Two complementary test environments
Research conducted at Chalmers University of Technology by Professor Per-Simon Kildal and his colleagues (Carlsson, Carlberg & Kildal, LAPC 2012, as well as Kildal & Carlsson, 2013) established a theoretical framework for OTA testing based on two defined reference environments: RIMP (Rich Isotropic Multipath) and LOS (Line-of-Sight). In this context, LOS is used as the overarching concept, while RLOS (Random Line-of-Sight) describes the statistical model used to analyze variations related to random drone orientation.

These environments represent two extremes in how radio waves behave in reality. Although practical scenarios are often located somewhere between them, this distinction makes it possible to analyze and understand different aspects of wireless system performance in a structured manner.

In a RIMP environment, the signal propagates through many paths and reaches the receiver from multiple directions, which is typical for urban areas and complex installations. In a LOS environment, communication is instead dominated by a direct signal path, which is common in open environments or high-altitude operation.

The two environments highlight different characteristics of antenna and communication systems:

• RIMP provides insight into how the system performs in complex multipath environments
• RLOS demonstrates how antenna directional characteristics influence the communication link under line-of-sight conditions