A systematic error has been eliminated from a measurement technique for analysing the physical properties of the Earth's atmosphere using signals from GPS satellites – thanks to an Austrian Science Fund FWF project. As part of this project, the radio occultation technique, which is based on phase shifts in GPS signals, was systematically tested for error sources. A significant error was found through a day-night comparison of measurement data recorded over a ten-year period. The findings have now been published along with a correction proposal. This will make it possible to attain greater accuracy using this method, which is set to become the future gold standard of sensing techniques used in climate research.
First it worked for Mars, and then for other planets – for the Earth, though, it was longer in coming: radio occultation (RO). This technique provides information about the properties of the atmosphere. It is based on the phase shift of radio signals caused by the fractive index of an atmosphere. Just as water causes the path of light to bend, the atmosphere affects a radio signal – the corresponding effect is quantifiable and depends on the properties of the atmosphere. Thanks to numerous GPS satellites, a comprehensive measurement system is available for the Earth. However, prior to its optimal use for climate research, rigorous error analysis had to be carried out – and this is precisely what was done at the University of Graz, Austria.
Project leader Prof. Ulrich Foelsche from the Wegener Center for Climate and Global Change explains the significance of his study: "Although the climate is largely dictated by the free atmosphere, we still do not know enough about its development. RO provides a completely new way of collecting continuous, long-term and highly accurate data on density, pressure, temperature and humidity. However, before it can be optimally used for climate research, there are questions to be answered about the existence of systematic errors. And this is what we are doing."
The scientists recently succeeded in identifying a key influence that affects the measurement data, and can be traced back to solar activity. To understand this effect, it is necessary to take into account that GPS satellites orbit at an altitude of 20,000 km. For RO their signals are received by satellites located closer to the Earth – in the process they travel through both the upper, ionised atmosphere and the lower, neutral atmosphere. For climate research, the data from the lower, neutral atmosphere are of greatest relevance. In point of fact however, the signal is already influenced by ionised particles in the upper atmosphere – an effect that must be corrected during the data processing.
Dark side of solar activity
The recently published findings of Prof. Foelsche's group show that this correction is not as simple a matter as was previously assumed. It was known that the signal refraction in the ionised atmosphere is greater during the day than at night. However, the evaluation of ten years' data from two satellite missions (COSMIC, CHAMP) revealed that the extent of the day-night differences varies. The variations are caused by the prevailing solar activity. In phases of higher solar activity, the ionisation in the upper atmosphere increases more during the day than in phases of lower activity – and this has varying effects on the refraction of the radio signal.
Having identified these variations, the Graz-based team developed a formula that will enable the correction of the measurement values to be improved in future – and which has already proven effective in the context of model calculations. In addition to the prevailing solar activity, the formula also takes into account the latitude of the Earth at which the measurement is taken – a factor that also affects the extent of the ionisation.
All in all, with its calculations, this FWF project has carried out urgently necessary groundwork. RO enables the collection of comprehensive volumes of data on the state of the Earth's atmosphere with hitherto unattained accuracy – for this reason alone, the critical analysis of possible error sources is crucial.
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Danzer, J. et al. Systematic residual ionospheric errors in radio occultation data and a potential way to minimize them, Atmos. Meas. Tech. Discuss., 6, 1979 - 2008, 2013.