There are situations in which using a magnetic compass is unreliable. This is the case inside a military vehicle (plane or tank). Indeed, the presence of continuous electrical circuits and masses of metal disrupts the compass needle. On a ship, this problem also exists, but it can be corrected in port: the masses of metal being always the same on board during the voyage, we can correct the deviation by positioning other masses of metal near the needle which compensate for the effect of the ship’s mass. It is more difficult to do this in a column of tanks, because they move relative to each other. To escape the influence of metal, we calculate one meter for one ton of steel. Thus, you have to move more than 30 meters away from a 32-ton Sherman tank, which cannot be done while rolling and can put the crew in danger. Furthermore, travelers in the desert at the beginning of the 20th century had noticed that compasses could also be disrupted by the frequent presence of iron ore deposits as yet unmapped. For all these reasons, the compass was of limited use.
However, knowing the North-South direction was all the more important as the troops engaged in the Sahara during WWII could not navigate by sight, using the reliefs on the ground to find their way: either there were none, or they were not on the map. The lack of precise maps complicated the problem.
Finally, these troops were often engaged too far from radio sources which could have given them direction (radio direction finding), as do ships approaching the coast. And they could not have used their own radio resources, at the risk of being spotted by the enemy. Hence the use of solar compasses during the Second World War, especially on the North African front.
Several models of solar compasses were used by the American, British or even German armies, on land or in the air: