Another story from the past...

Back in the late 60's when I was a Flight Engineer on the 707, navigation was, shall we say not so precise as it is now with inertial navigation and GPS.  Flying the North Atlantic in those days, the spacing was very different than the way it is today.  We flew tracks that were 120 miles apart laterally and 2,000 feet in altitude and 10 minutes in long track.  The main navigation was Doppler navigation which used a special antenna in the nose of the 707 that put out four beams.  Two forward at an angle left and right and two more rearward also with an angle.  This was designed to be able to measure ground speed and drift.  Most of the time it worked pretty well, but when the sea was fairly flat as it is sometimes in the summer the Doppler was not able to receive a good return of the signal and would loose speed and drift information and revert to DR (dead reckoning).  >:(

In that case, we would use LORAN as a back-up to get a fix and plot our progress.  If you're not familiar with this system, it was really old school.  The radio stations for LORAN consisted of a master station and a slave.  The master would put out a signal that would move in azimuth over time and when the signal pointed at us, that would give us a LOP (line of position). Then you would plot the slave and it too would give you an LOP.  Using both would give you a "FIX."  The pilot doing the navigation used an oscilloscope mounted on the aft pedestal to line-up the two signals by dialing in a delay between the two beams. On a good day it was okay, but you had to plot fast to get a good fix since you were moving at 600 kts.  Remember, this system was designed for ships doing maybe 20 kts. But, here's the kicker, in times of solar activity - wait for it - yeah, you guessed it it was almost impossible to get a good signal - so back to DR! 

This moved us to the next system of back-up navigation...  look for other contrails!  :o