By: Bob Potter

Given that LEO, MEO and GEO each have their own unique characteristics which offers distinct advantages, the ability to switch between orbits provides users in both commercial applications and the defence sector with much needed flexibility, resilience and security. LEO, for example, with its close proximity to Earth, offers low latency and high resolution, while MEO provides wider coverage, simpler network connectivity and higher data rates than LEO, and GEO, with a higher altitude compared to LEO and MEO, delivers a broad footprint but with higher latency, but with flexible network topology, simple operation and potentially lower cost per bit. Given these different characteristics and the advantages that each orbit brings, it’s hardly surprising then that we’re in the midst of an industry-wide shift towards a multi-orbit environment. A key component of this shift is phased array flat panel antennas (FPAs), which offer capabilities that traditional parabolic antennas cannot match.

FPAs contain tens, hundreds or even thousands of tiny antenna elements that can be controlled by precise amplitude and phase shifts. This design enables FPAs to track and switch between fast-moving satellites as they pass overhead, making them highly useful in LEO as well as in multi-orbit networks. Their compact form factor also makes them ideal for applications where space is limited or for COTM.

However, because of the inherent difference between LEO and GEO satellites’ positions, moving from those different orbits without loss of signal strength remains a challenge. The amount of beam steering required to switch from one orbit to the other requires the antenna to steer away from its central position. This causes a number of technical difficulties.

Impact of scan angle on gain

Central to these challenges is antenna gain, the measurement of how effectively an antenna concentrates RF energy towards a satellite. Unlike a parabolic reflector antenna, FPA performance changes with pointing or beam steering, known as scan. An electronically steered array generally performs most favourably when operating at broadside, where the full aperture can contribute to the beam. As the beam moves away from broadside, the net result is a less effective aperture, as the combining of the individual elements can become less effective, resulting in scan loss.

This means that a satellite can remain clearly within the antenna’s field of view while the quality of the RF link changes. A loss of several dB of useful gain at a particular scan angle can have a direct impact on link margin, with the consequences depending on factors including transmit power, receiver sensitivity and atmospheric conditions.

Scan-angle performance is particularly important for a multi-orbit FPA antenna. Users need to consider not just how much gain an antenna can achieve under ideal broadside conditions, but also how it performs across the range of angles required to operate between different orbits.