 As the threat of invasion was clearly felt in late 1941 , an idea for a series of secret observation posts ( first in Gibraltar and later in other places like Malta and Aden ) was put together under Operation Tracer . 
 As the threat of invasion was clearly felt in late 1941 , an idea for a series of secret observation posts ( first in Gibraltar and later in other places like Malta and Aden ) was put together under Operation Tracer . 
As the threat of invasion was clearly felt in late 1941 , an idea fa a series of secret observation posts ( first in Gibraltar and later in other places like Malta and Aden ) was put together under Operation Tracer .