Explosives Localisation and Pre-Identification Based on UHF Electromagnetic Waves

Abstract

There are several ways in which ultra-high frequency (UHF) radio waves can be used for explosive’ detection. Firstly, UHF radio waves have relatively high penetration ability, and can be used to quickly localize the “anomaly” in the soil. Secondly, they are sensitive to the electrical properties of the hidden object, which can be used to identify the nature of this object, i.e. to determine whether the object belongs to a certain class (for example: metal - not metal; dielectric with big dielectric permeability - dielectric with small dielectric permeability). The widely used pulsed radars are capable of fast localization of anomalies [(1994), (1976), (1997)] and give some hints to their classification on the basis of definition of geometrical parameters of these anomalies [(1996), (1998)], intensity of scattered radiation, and presence of natural resonances [(1997)]. Use of short pulses with duration Δt ∼ 1 ns with an equivalent band of frequencies Δf ∼ 1 GHz has its advantages and disadvantages. The main advantages of the pulsed radars are:

• They work at frequencies for which absorption in soil is low, and thus the pulse can penetrate rather deep.

• They allow one to have powerful short pulses while keeping power consumption low.

• There are a lot of standard ready-for-use methods for data analysis. The main disadvantages of using pulsed radiation are:

• Low spatial resolution ΔL ∼ cΔt ∼ 30cm.

• Need to account for signal distortion due to dispersion of the propagation medium.

• Pulses do not have well-defined boundaries due to transition processes.

• Strict requirements to the emitter and receiver: no multiple reflections, decoupling of emitter and receiver, stability.

• High cost of generator of nanosecond pulses. Systems with continuous radiation of variable frequencies can also be used for detecting anomalies [(1974)]. The main advantages of such systems compared to pulsed radars are:

• It is easy to provide a wide range of frequencies equivalent to the pulse length Δt ∼ 0.1 ns, hence spatial resolution ΔL ∼ cΔt ∼ 3 cm.