OSNet-Tiree

OS Net data helps predict weather

13 February 2014

Mark Greaves, Geodetic Analyst at the Ordnance Survey, looks at how OS Net, Ordnance Survey's network of permanent, high accuracy GNSS (Global Navigation Satellite System) receivers, helps predict the weather.

OS Net is Ordnance Survey's network of permanent, high accuracy GNSS (Global Navigation Satellite System) receivers (see Tiree station above). A GNSS is a satellite system that is used to pinpoint the geographic location of a user's receiver anywhere in the world. For OS Net, its day to day operation is to supply a stream of real time GNSS data covering the whole country. The data streams enable the correction of GNSS errors to be computed in real time and, when transmitted to our surveyors, allow them to coordinate new map features to an accuracy of just a few centimetres using RTK (Real Time Kinematic) GNSS.

OS Net data is not all about positioning. Another lesser known use is that it helps the Met Office predict the weather. Met Office scientist Dr Jonathan Jones explains...

"Often, rapid changes in the Integrated Water Vapour (IWV) content in our atmosphere are associated with high humidity conditions caused by extreme weather events, such as thunderstorms. Water vapour is one of the most important parts of the atmosphere as moisture and latent heat are primarily transmitted in this phase. The speed at which IWV can be calculated is of critical importance to short term weather forecasting, and trying to predict extreme weather events. Using GNSS data, IWV can be calculated in almost real time. Also, in addition to short-term forecasting, water vapour is one of the most important greenhouse gases, and as such, accurate monitoring of water vapour is of great importance to climatological research.

The signal between a GNSS satellite and a ground-based GNSS receiver are subject to several delays as they travel through the atmosphere. The delays which occur in the lowest part of the atmosphere, called the troposphere, are related to the density of dry air as well as the water vapour content along the signal path. The signal delay from each individual satellite is known as the slant delay, but as there are too many errors in the slant delay itself, all slant signals are mapped up into a vertical (or zenith) direction and this is known as the Zenith Total Delay (ZTD). ZTD gives a measure for the integrated tropospheric condition and when taking into account surface pressure and temperature, we can convert a portion of the ZTD into an estimate of the water vapour above the receiver.

Since 2002 the Met Office has been developing a GNSS processing capability with the main objective being to process as many GNSS sites as possible and deliver the results with the minimum time delay possible. Met Office has worked in partnership with Ordnance Survey and the Nottingham Geospatial Institute at the University of Nottingham to develop an automated system managed by the Met Office for GNSS data retrieval and processing. This system was successfully rolled-out for operational use in May 2007.

The processing systems at Met Office HQ in Exeter, use raw GNSS data and, combined with other information, such as satellite clock and orbit information, compute the column total amount of water vapour vertically above the GNSS station. The resultant water vapour estimates are then assimilated into global, European and UK-scale Numerical Weather Prediction (NWP) models. Assimilation impact trials have demonstrated positive impact of GNSS data in not only precipitation, but also in cloud cover and surface temperature forecasts. Data is also displayed as 2D plots for forecaster use and put onto the Met Office database for archiving and future case study work."

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