Radar Recollections - A Bournemouth University / CHiDE / HLF project

 
 
 

The Plan Position Indicator

Men, machines and raw materials are always precious and this was certainly the case in 1940. A way had to be found to resist the onslaught of numerically superior Luftwaffe.

Maneuvering an aircraft into a tactical fighting position

One of the ongoing problems with airborne (AI) radar is that the detection range was never much more than 3 miles and if the aircraft were below 10,000 feet, this figure reduces to 2 miles due to ground signal reflections. If ground controlled interception (GCI) was to work effectively, it would be essential that the ground team could accurately direct the fighter onto the target and into a strategically advantageous position, ideally 1 or 2 miles behind the enemy plane.

The night bombing raids presented a particularly difficult problem because visibility on a moonless night might be as little as 300 yards. It was essential that the most advantageous firing position was calculated rather than left to a "who sees who first" method. Therefore the ground control team needed to have a system whereby they could 'look' at the whole battle and move their forces accordingly.

A Lone SpitFire...
A Lone SpitFire in pursuit of 2 Heinkels during the Battle of Britain.
In pursuit of 2 Heinkels during the Battle of Britain.


With the introduction of rotating antennae on the transmitters and receivers used for CHL and Naval radars it was now possible to build an oscilloscope that displayed a trace which swept a radius from the centre of the screen and rotated about the centre in time with the rotating aerial. The phosphors used to coat the screen were especially chosen to produce a long 'afterglow', so that each trace remained visible to be refreshed with each cycle of revolution.

All that was needed now was to draw on (or overlay) this special screen with the map of the area scaled to coincide with the known radar range. The radial time base display developed by G.W.A.Dummer was tested and eventually the PPI was born and within a year was made compact enough to be installed in aircraft as well as used for the original ground-based applications.
By mid-1940 CHL was rapidly evolving from a defensive instrument into a potent offensive tool that gradually gave the RAF pilots a tactical advantage over the attacking aircraft.


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