Comprehensive, global and timely specification of the Earth's atmosphere (particularly refractivity profiles of the troposphere and ionosphere) are required to ensure the effective operation, planning and management of many radio frequency systems. Although many ground-based techniques have been developed to measure atmospheric refractivity, Radio Occultation (RO) methods are being increasingly investigated. Radio occultation involves monitoring transmissions from Global Positioning System (GPS) satellites using receivers on Low Earth Orbiting (LEO) satellites. The geometry of an RO measurement is shown below.

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RO measurements provide the potential of measuring refractivity profiles in regions where ground-based sensors cannot easily be located, such as deep sea waters. Furthermore, constellations of RO satellite receivers can be used to provide a very large number of measurements with a global distribution. GPS/MET soundings with an 8-LEO constellation produce 4014 measurements in 24 hours.

Potential areas of exploitation for RO ionospheric specification include:

• Increase accuracy of ground and space based trans-ionospheric sensor systems
• Improved accuracy of single frequency navigation systems
• Improved HF communications
• Significant reduction in the errors for HF position finding systems

Click here for more information on data assimilation of ionospheric radio occultation measurements.