The research described here reflects the investigations QinetiQ carried out into the performance limiting effects of clear-air turbulence on K_a band (26-40 GHz) radiowaves. Both atmospheric and platform generated turbulence effects are considered using measurement and simulation techniques for terrestrial and airborne applications. The ultimate aim fo this work is the formulation of turbulence mitigation techniques to improve system performance.

Measurement

QinetiQ has developed a five channel array for measuring temporal and spatial propagation impairments at 36 GHz. The instrument can be considered as consisting of two sections. Section 1 acts as a superheterodyne receiver that down-converts the millimetre wave signal to VHF using state-of-the-art monolithic microwave integrated circuits (MMICs) developed at QinetiQ. Section 2 performs the signal processing that includes hardware correlation for measuring amplitude and phase variations across the two dimensional array. The collected data is stored using an acquisition system that samples the complex (I and Q) data simultaneously from the hardware correlators.

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Functional block diagram of the five-channel array

The diagram below is an example of the received signal strength from the five element array during clear-air propagation impairments. The signal from the array, which was located 17 km from the transmitter across a near-horizontal link, varies from the benign condition mean level because of two different clear-air impairments:
1) deep fade (17.3 dB) resulting from tropspheric multipath;
2) scintillation event (8 dB) due to atmospheric turbulence.
The example illustrates the significance of clear-air impairments affecting millimetre wave systems, and therefore the need to develop mitigation techniques to maintain system performance.

Example of propagation impairments

Simulation

The simulation of a B3 fighter shown was generated using computational fluid dynamic models. It shows the mean value of refractive index (n-1) on the surface of the fighter as it travels at Mach 1.5 (482 m/s) at 15,000 ft and an angle-of-attack of 2°. The simulation indicates the platform generated turbulence intensity variations resulting from the aerodynamics of the aircraft.

A Monte Carlo parabolic equation (PE) can be used to simulate the effect of turbulence on a 36 GHz wavefront. In the plot shown opposite, the wavefront, travelling from left to right along the 50 m long by 25 m wide domain, encouters a 2 m wide turbulent layer centred along its length. As a result of the random variations in refractive index that is induced by the turbulent layer, the field intensity deviates from unity due to the constructive and destructive additions of the individual ray paths.