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Making the most of errors

Most of us are familiar with using the Global Positioning System (GPS) whether it is in our cars or on our mobile phones. However, what most people don't realise is that the Met Office makes use of the same system for weather and climate observations.

The Global Positioning Systems (GPS) is one type of GNSS (Global Navigation Satellite System) which uses networks of ground sensors and a constellation of satellites for position, navigation and timing applications. Most people use GPS for position information, for example on sat navs, benefitting from measurements that are accurate to within a few metres.

Most of us would rarely if ever consider that when using GNSS there are lots of hugely complex factors to consider, including solar radiation, flexing of the Earth's surface and special relativity. What's more, GNSS signals are affected by the medium they encounter on their journey from satellite transmitter to ground-based receiver.

For instance, several different frequencies of GNSS are used to eliminate the effect of the ionosphere, part of Earth's upper atmosphere. Water vapour in the atmosphere also affects the satellite signal. When using GNSS for positioning, these different mediums are sources of errors. But, understanding and accurately estimating these error sources can actually be useful information for the meteorological community - and, in turn, can help the positioning community too.

How do we use GNSS?

Electromagnetic radiation (including GNSS) travels more slowly through water vapour than air. This leads to a delay in the arrival of the signal, using this signal delay plus surface observations of pressure and temperature it is possible to estimate the Integrated Water Vapour (IWV) of the atmosphere.

As Dr Jonathan Jones, Met Office Senior Scientist from Observations R&D describes: "IWV is a measure of atmospheric humidity, which is of value to numerical weather prediction models and in particular to 'nowcasting' (very short-term forecasts), and as such observations of humidity in real or near real-time help weather forecasters and is also useful for climate science."

Programmes and partnerships

To gather GNSS data, we have a resource sharing agreement with Ordnance Survey GB, which permits the Met Office access to a network of over 100 ground-based GNSS sensors. We are also part of the EUMETNET E-GVAP GPS water vapour project, ensuring access to a dense network European GNSS observing sites, which provides access to around 16 million observations per month.

COST (European Cooperation in Science and Technology) is the longest-running European framework supporting trans-national cooperation among researchers, engineers and scholars across Europe. Jonathan is Chair of the Management Committee of COST Action ES1206: Advanced GNSS tropospheric products for severe weather and climate. The project involves over 150 scientists from around 40 countries, and aims to improve existing, and develop new GNSS products, specifically for nowcasting of severe weather and for climate applications as well as improving GNSS accuracy through enhanced atmospheric modelling.

What next?

The Met Office has used GNSS in operational weather forecasting since 2007. We also use it to keep an eye on space weather - focussing on the ionosphere and measuring solar radiation and looking for abnormal activity.

"However, there is still scope to develop it the way we use it to provide even more benefits," says Jonathan. "For instance, it is possible to measure reflected signals to measure soil moisture and snow depth or even to use tomographic reconstruction to potentially develop real-time 3D humidity fields."