In recent years the use of high frequency GPR antennas such as the 2GHz horn antenna have become popular for railway trackbed evaluation due to their ability to help determine the extent of fouling within the ballast layer. Fouling of the ballast affects both its strength and drainage properties. The theory is based on the scattering behaviour of GPR signals in an inhomogeneous media, comprising evenly distributed particles and air voids. Whilst GPR has been successfully implemented as a tool for mapping ballast quality, the relation between the GPR scattering response and the physical properties of the ballast and fouling materials have not been examined and studied in detail up until now the method has been mainly empirical, based on measured correlations. This paper conducts an investigation of the GPR scattering response to the railway ballast aggregate layer by providing a comprehensive study of factors affecting the radar signal including; ballast grading (particle size distribution), particle shape, void size and the electromagnetic properties of the ballast (dielectric, conductivity, magnetic permeability). Sample analysis, numerical simulations, laboratory tests and real-world data collection have been conducted in this research. Detailed analysis and discussions are provided on factors contributing to the observed signal scattering and attenuation. This is the first in a series of papers looking at the influence of various factors on the response of GPR within ballasted trackbed. (8 pages)