6. Tide-prediction machine
North West Europe was a new environment for most of the Allied troops. Invading it required a broad evaluation of all the factors that would affect the effectiveness of the operation. They would use the sea for the landings, so the tide was a major factor. Some troops would land from the air, which would be impacted by the sky conditions. Landing at night would use the moon for light. The actual battle on D-Day would need a conducive weather.
Planners of the June 1944 invasion took all these factors into consideration. One of the solutions hatched out of this rigorous process was the tide-prediction machine. The naval operations needed calm waters. Their ground troops also needed to land at low tide to escape the German beach obstacles. To achieve the best combination of these factors on the D-Day, the planners had to consult widely to get the required best combination of all these factors. They consulted some meteorologists and other experts in the process. Eventually with the help of a British mathematician Arthur Thomas Doodson, the Allies managed to reveal tidal patterns using a specially modified machine. That is how they arrived at the 5th to 7th June window as the exact timeline for the operation.