Update readme.md

data processing/data/readme.md

 ## this folder contains various raw data
 
-[POS519_UW_dship.dat](https://git.geomar.de/tobias-steinhoff/Poseidon-POS519/blob/master/data%20processing/data/POS519_UW_dship.dat):
-underway data from ships system
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+#[POS519_UW_dship.dat](https://git.geomar.de/tobias-steinhoff/Poseidon-POS519/blob/master/data%20processing/data/POS519_UW_dship.dat):
+
+underway data from ships system
+variable | unit | description 
+:--- | :--- | :--- 
+Excel time | | 
+matlab time | | 
+SYS.STR.Course | deg | course over ground
+SYS.STR.HDG | deg | ship's heading
+SYS.STR.PosLat | deg N | Latitude 
+SYS.STR.PosLon | deg E | Longitude
+TSG.SBE21.Salinity | PSU | salinity calculated from conductivity with ship's thermosalinograph [SBE21](https://www.seabird.com/)
+TSG.SBE21.t68extern | degC | seawater temperature at water intake, SBE38
+TSG.SBE21.t68 | degC | seawater temperature inside SBE21
+Weather.PDWDA.WindDir_true | deg | true wind direction
+Weather.PDWDA.WindSpeed_true | m/s | true wind speed
+Weather.PDWDA.Air_pressure | mbar | air pressure
+Weather.PDWDA.Humidity | % | humidity
+Weather.PDWDA.WindDir_rel | deg | wind direction, relative to the ship
+Weather.PDWDA.WindSpeed_rel | m/s | wind speed, relative to the ship