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A radar ghost still haunts
 Q: During the Korean War, about the time of the landings on Inchon, we picked up a contact on
our surface radar at about 15 miles. The target closed on us but no visual contact was made. The
target plotted a collision course (constant bearing closing) and plotted right through our ship
without visual contact. The ship went to "General Quarters" at about 8 miles out. The contact was
truly a ghost. How does this happen? --William Wantuck, Houston, Texas... (Incident aboard the USS
Montrose APA 212 circa 1950.)
A: Reconstructing the 'crime' a half a century later injects some uncertainties. Nevertheless, here's probably what happened: Your
World-War II vintage radar did not cope with far away signals due to weird atmospheric effects and misinterpreted returning signals as a
ship propelling toward you on collision course.
[Russ McDonald, USS Montrose APA-212 Association] The amphibious Attack Transport APA-WQW/LPA Montrose
As you know, radar transmitters send out a pulse, wait a set time interval, then send out another pulse, and so on, continuously sending a
train of pulses. When a pulse hits a target and bounces back toward the ship like an incoming echo, the listening radar receiver records the
signal, determines the distance to the target, and displays the result as a blip on the radar screen. This system works fine as long as the
radar doesn't send a second pulse out before all the returning signals have arrived from the first pulse. See the "oops" in the figure.
 However, sometimes Nature throws a curve ball. Normally temperature decreases with altitude.
Occasionally, though, the air is hotter higher up than near the Earth's surface; this is called a temperature
inversion and acts as a lid on air rising from layers below it. Such a condition forms a duct that guides
radio waves over great distances with little loss of signal strength. These ducts can be stable for many
hours and can extend hundreds of miles.
[Peter Wittenberg, Boeing] The gray "ghost" returns before the next pulse sends
Consequently, when your radar pinged out a pulse, the signal entered the temperature inversion duct, zoomed along the sea's surface,
traveled much father than expected--perhaps a hundred miles or so to elevated topography near Inchon, and returned. The return arrived
long after the radar sent its next pulse. That's trouble.
The radar receiver blithely assumed the return was from the pulse the transmitter just sent. Suppose the radar has an unambiguous range
of 135 nautical miles (nmi). The Inchon heights are, say, 150 nmi distant. Then the radar misinterpreted the actual distance as a nearer
one, thinking that the return was due to the second pulse. It thought the target was 150 - 135 = 15 nmi instead.
At about 2 a.m., you, the operator, looked at your screen and saw a blip that indicated a ship 15 nmi away, about the same size as the
Montrose. Worse, the Montrose was heading toward Inchon and so the distance steadily narrowed as your radar pinged signals off the
distant stationary heights near Inchon, received the ever- quicker returns, and continued to misinterpret the distance.
At eight nmi the radar blip indicated such danger the ship went to "General Quarters." The electronic gonging sounded throughout the
ship, followed by a human voice: "General Quarters-General Quarters-all hands man your battle stations." But no oncoming ship
appeared even though the blip on the screen got ever closer. Finally, the target blip was at zero distance (actually 135 nmi away, in our
example).
"Bang!" Someone said in the CIC (Command Information Center). The blip proceeded in closer. The radar display cleared the blip as
"outside maximum radar range."
That's probably what happened. Other theories are possible but this is the most likely.
Note: I'm indebted to a team of sleuths tracking down the answer to this one: Bill Case (radar engineer since 1956) at Northrup
Grumman, Kate Marks at the Historical Electronics Museum, Bill Wantuck the radar operator on the Montrose, and Peter Wittenberg
(radar designer) at Boeing.
(Answered by April Holladay, science correspondent, Apr. 17, 2002)
Further Surfing:
Peter Wittenberg, Boeing: Intro to radar
The Historical Electronics Museum
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