Varaibility in solar radiation levels and geomagnetic activity cause fluctuations in the ionosphere that affect radio communications, navigational positioning, satellite altimetry, HF radar and direction finding systems. Recent developments in near-real time ionospheric specification techniques may considerably improve the operational efficiency and accuracy of these systems. In addition, the accurate specification of the present and recent state of the ionosphere is essential for accurate forecasting of the future state of the ionosphere. Tomography is a relatively new technique for determining the electron density structure of the ionosphere.

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Ground-satellite paths

Function

Ionospheric tomography provides a high-resolution image of the electron density distribution in the vertical, meridional plane using only a small number of satellite receivers on the ground. Polar orbiting satellites of the Naval Ionospheric Measuring System (NIMS) transmit phase-locked radio carrier waves at 150 MHz and 400 MHz and these signals are measured at five ground stations along the length of the UK. The total electron content (TEC) along each ground-satellite path is calculated. A computerised algorithm then reconstructs an image of the ionospheric electron distribution from the TEC measurements.

Applications

The tomographic technique is particularly useful in regions where the ionosphere is highly structured (e.g. in the mid-latitude trough, auroral and equatorial zones).

In the HF (3-30MHz) range 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