Blip-to-scan Ratio - Avoiding Detection

Avoiding Detection

Soviet early warning radars of the 1950s, such as the P-20 "Periskop", were little changed from the earliest examples operated by Germany and Britain during World War II. Their initial design parameters did not account for either the speed or altitude capabilities of jet aircraft. This created the opportunity for aircraft to avoid detection by exploiting blip/scan ratios.

With lower-flying, slower aircraft, recognizing and tracking an aircraft by its radar blips was a straightforward process. Consider an antenna that spins once every ten seconds detecting a target aircraft moving at 1000 km/h. With every blip the aircraft will have moved almost three kilometers (1000 km/h = 278 m/s). On a display showing the example radar's entire 300 km radius this represents movement of only 0.5% across the display's face (600 km diameter), producing a tiny line segment between the two dots that is easy for an operator to interpret as an aircraft rather confuse for clutter.

But if the target speed is increased, its movement becomes more pronounced on the scope, making it less recognizable and more difficult to track. At Mach 3 (3500 km/h at 25,000 m) the same ten seconds of movement represent over 1.5% of the display's face. At this point the slowly moving dot turns into a series of individual spots, which can more easily be mistaken for clutter. Additionally, since the spots are separated on the display, the returns no longer "add up," potentially reducing the returns to the same level as background noise.

An operator seeing a line of small dots across their screen might eventually recognize the return as an aircraft. To frustrate even this, aircraft were designed to fly as high as possible. Recall that the radar's scanning beam is fan-shaped when seen from the side. This beam only scans high altitudes at long ranges. This means that a high-altitude aircraft is only visible to the operator in a narrow ring at the extreme outside of the scope. Crossing this area quickly would result in only a few dots, which might not be enough to recognize, let alone interpret.

And thus the concept of using the blip/scan to avoid detection. A high-speed, high-altitude aircraft might fly over early warning radars without being recognized. Moreover, even if an operator recognized the blip as an aircraft, the small number of returns and fast movement across the display would make it difficult or impossible to manually calculate a trajectory for relay to intercepting aircraft.