By Trygve Halsne, Thomas Lavergne and Mari Anne Killie. Norwegian Meteorological Institute.
The image above is captured by the Sentinel-2A satellite, one of the workhorses in the European Copernicus Earth Observation program. The image is centered close to Lindeberg, in the middle of Lillestrøm and Gardermoen, and the question reads: Can you spot the train? No? Then read more!
About the Sentinel-2 images
The Sentinel-2 satellites carries a multi spectral imaging sensor capturing images of the earths surface up to 10x10 meter pixel resolution. This resolution is in fact too coarse for spotting the train which is about 3 meters wide. Although, it is quite evident that we can spot the train in the middle of the image with a blue, green and and red dot. So how did we do this?
The trick is related to the trains velocity. All images captured by the satellite consists of 13 spectral bands. We use three of this (red, green and blue) to generate a color composite i.e. the image above. Each frequency band acquisition is separated with a time difference of about 0.5 seconds.
The train moves significantly between the aquisitions of the three frequency bands. Hence, when composing a color composite, the signal from the train has moved between each band inherent in the composite. Thus, we can see a blue, green and red dot in the image.
The same principle also applies for airplanes, but then the altitude also plays a role. Read more about that here.
Available for Norwegian users
The Sentinel-2 constellation is part of the Copernicus family. It consists of two satellites, A and B, and will be providing us with satellite imagery for a long time into the future. The same applies for the Sentinel-1 satellites which captures "images" with a RADAR instrument and the Sentinel-3 satellites monitoring color and temperatures for ocean and land as well as altimetry.
The Norwegian Meteorological Institue coorporates with the Norwegian Space Centre in development of the National Ground Segment (NBS - Nasjonalt BakkeSegment), where we make all of these sateliite products available for norwegian users. Currently, we have tons (or peta bytes) of data in our storage facilities which are free and open for everyone. You made the first correct step by entering www.satellittdata.no where you can locate your data of interest.
Can you decide the trains direction?
Last question: Can you decide the direction of the train?
The answer is that it's travelling northwards against Gardermoen. We know this since the frequency bands for blue, green and red are acquisited in that order respectively with a time delay of 0.527 seconds between blue and green and 1.005 seconds between blue and red. If you measure the pixel displacement you can estimate the trains velocity, which in this case is somewhere between 150 and 200 km/h. If you look more closely on the images, you can recognize the signals from cars and/or trucs on the E6 highway up lef corner of the image, and draw the conclusion from knowing that cars drive on the right hand side in Norway.