# HERAS: A Modular Matlab Tool Using Physical Optics for the Analysis of Reflector Antennas
## 1 Introduction
The co-design of the antenna with the payload becomes critical to optimise the performance of the satellite. In this context, estimating the multi-beam coverage characteristics becomes a step towards the desired system performance evaluation, which in turn is iteratively performed to optimise not only the antenna architecture but also the resource allocation and interference mitigation strategies. This calls for iterative estimations of the coverage characteristics by reflector antennas with several thousands of beams. The high computational demands associated with the aforementioned optimisation drives the need for efficient methodologies to obtain key characteristics of multiple beam coverage associated with the antenna subsystem. In order to accelerate the coverage estimation, system studies have so far relied on simple models for the antenna patterns based on closed form expressions [14], which are accurate enough for DRA architectures but do not sufficiently capture the beam degradation due to optical aberrations in reflector-based solutions, particularly for beams towards the edge of coverage. A more accurate evaluation of the antenna performance in trade-off with the computational effort is critical. Traditional methods for the analysis of reflector antennas usually rely on numerical integrations, which are the main contributor to the total computation time. Physical optics (PO) is the preferred technique to predict the scattered field of electrically large structures such as reflector antennas, radar cross section of targets, etc. [15]. GRASP from TICRA [16] is the reference commercial tool for the analysis and design of reflector antennas, which uses a combination of PO and MoM techniques in order to analyse electrically smaller and larger parts with great accuracy [17]. Other electromagnetic simulation software are available for the analysis of large structures [18]: CST (integral equation or asympsotic solver) [19], Ansys HFSS (PO Solver as part of the integral equation solver) [20], and FEKO (MoM/PO hybrid method) [21]. These tools focus primarily on accurately predicting the measured results and are addressed to the antenna development teams. Despite major advances in accelerated computations, they still require a PO integral per beam across the coverage region. When it comes to satcom payload design and system engineering, trade-offs in the accuracy of the antenna patterns can be accepted on the basis of significant acceleration in the computation of the entire multi-beam coverage. In this paper, we present HERAS (Heriot-Watt reflector antenna solver), a tool for educational and research purposes that comes from the need for a flexible tool for the computation of far-field characteristics of a reflector antenna. HERAS was designed and developed to provide adaptability to system engineering requirements. HERAS is also of interest to antenna engineers working on the design of reflector antenna systems and was, in fact, originally developed to enable the analysis of antenna systems using reflecting polarisers [9; 22; 23; 24; 25] at a time when this functionality was not readily available in commercial tools. A description of the tool followed by some projects where the tool has been of contribution are presented in the paper.