We take for granted the fact that TomTom tells us the way and that the weatherman says whether it will be sunny or cloudy later on in the day. However, we hardly stop to think about the error margin in both applications. Lindenbergh: ‘Satellite navigation in the car is accurate down to a few metres. The box on your dashboard does not know if you are driving on the main road or the parallel road next to it. The same is true for the very hard to predict thunderstorms. Even the best weatherman cannot tell you if the main act at the Pinkpop music festival will be rained out thanks to a thunderstorm.’
Water vapour is the spoilsport. GPS satellites transmit radio signals to earth. These are dispersed or delayed by water vapour in the atmosphere. The same is true for signals from the ASAR instrument onboard the European earth observation satellite Envisat. This radar instrument transmits signals to the earth and then picks them up again a little while later. On the outward and return journeys the signal is influenced by water vapour in the atmosphere. This leads to deviations in the measurements of ocean currents, ice caps and the topography of land masses. Lindenbergh used another instrument on Envisat, the Medium Resolution Imaging Spectrometer (MERIS), to investigate how water vapour in the atmosphere is distributed. With the help of data from two wavelength bands, an estimate is made of the quantity of water vapour between the satellite and the earth. ‘We cannot see how much water vapour there is at any specific altitude above the earth. Yet in this case the following maxim applies: better to know something than nothing.’
GPS ground stations provide a second way of measuring the water vapour. By taking into account the position of the different GPS satellites, the altitude of the ground station and the delay of the signal, a map of the water vapour in the atmosphere can be compiled. Besides MERIS 25, Lindenbergh used GPS stations throughout the Netherlands. Such maps are less detailed than the MERIS maps but are more up-to-date. They provide continuous measurement data, whereas MERIS needs 3 days to map the entire earth once.
‘Thanks to this research we can improve the accuracy of the GPS and ASAR measurements using the MERIS data. And vice versa we can improve MERIS using the GPS data’, says Lindenbergh. ‘This is the first time we have mapped the distribution of water vapour from two viewpoints: both from the ground and from space.’
The more accurate water vapour data shall not only benefit meteorology and satellite navigation. Climate researchers cannot wait to get their hands on the data, as water vapour is the most important greenhouse gas in the atmosphere. In addition to this, many modern surveying applications make use of satellite measurements. Buildings, gas fields and dikes are monitored from space. Lindenbergh: ‘Our method of estimating water vapour using satellite data can be used in a wide range of applications. This will result in considerably more accurate data.’
Roderik Lindenbergh carried out his postdoctoral research at Delft University of Technology with financial support from the User Support Space Research programme, which the Netherlands Space Office manages on behalf of the Netherlands Organisation for Scientific Research (NWO). This programme enables researchers working in the Netherlands to use the scientific infrastructure in space. The programme runs from 2007 to 2011.