Many radio and radar systems require a knowledge of the exact ray path of a radio wave as it travels through the ionosphere. The accuracy of these systems depends on both the accuracy of the ionospheric electron density model and the accuracy of the solutions to the equations used to trace through this model. Whilst a full numerical solution of the ray equations provides the most accurate result, the computation requires excessive computer processing time and this reduces the effectivness of real-time operational systems.

To overcome these difficulties QinetiQ developed the Segmented Method for Analytical Ray-Tracing (SMART), a fast and accurate analytic ray tracing technique.

HF ray paths through an ionospheric image

Function

The electron density distribution of the ionosphere is closely approximated by a series of quasi-parabolic functions for which analytic solutions to the ray equations are known. Segmenting the ionosphere and using the quasi-parabolic approach, ray tracing is performed up to 100 times faster than the conventional numerical techniques whilst retaining more than 95% of the accuracy.

Synthetic oblique ionograms have been produced by SMART ray tracing through ionospheric models defined by tomographic images. Comparisons with real oblique ionograms recorded over several UK paths have yielded very promising results for the combined SMART ray tracing and tomographic imaging technique.

Applications

In the HF (3-30MHz) range SMART applications include:

• over-the horizon radar (OTHR)
• single-station location direction finding
• frequency planning and broadcast coverage predictions
• jamming and interception vulnerability estimation

In the VHF/UHF (30-3000MHz) range applications include:

• tracking of satellites/orbiting debris/ballistic projectiles
• satellite geolocation of transmitters
• ionospheric corrections to (single-frequency) satellite navigation systems