The Imperial War Museum Replicates a WWII Bomber’s H2S Radar Display

Visitors can play with the pioneering technology that helped turn the war

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Ask a layperson about technology developed during the second World War and you’ll likely get answers referring to the invention of atomic bombs, long-range ballistic missiles, or perhaps digital code breaking. Ironically, radar is often forgotten, despite it having a dramatic and direct impact on aerial combat during the war, and being a much bigger part of modern daily life than, say, atomic bombs or rockets.

A Duxford Radio Trust team at Britain’s Imperial War Museum Duxford is offering visitors some hands-on experience of World War II radar by giving them the warts-and-all view of the radar display as seen by a navigator on a Lancaster bomber. Thanks to an amazing hodge-podge of technology, visitors can operate the dim displays of the influential British H2S radar system on a simulated flight.

The H2S was the first ground-scanning radar to be mounted on an aircraft, and made it possible to bomb long-range targets at night and in bad weather from 1943 on. As a first-generation piece of technology, it had a number of interesting quirks, such as a blank circular zone at the center of its display, with a radius that reflected the plane’s altitude above the ground. The operator had to manually adjust an altitude-compensation dial to eliminate the zone.

The most direct way to ensure that visitors would have an authentic experience would be to use a vintage H2S indicator display. Unfortunately, while the Imperial War Museum does have an original display in its collection, it is illegal to modify it in any way. However, “we did have supplies of the old cathode-ray tubes,” explains John Moore, the technical lead on the H2S project, along with “a container full” of thermionic tubes. “So we built modern circuitry around the old [cathode-ray] tubes, but as far as we could using the original method of doing everything. Which was quite fun.”

Because the static, ground-based exhibit would have no actual radar hooked up, Moore worked out a way to feed the display a simulated signal that mimics what a navigator would have seen as he flew over actual terrain. Moore took a 4096 × 4096 grayscale satellite image centered on the English Channel, and wrote a program running on a laptop that uses the image to generate 670 radial traces for any given position and altitude. While using a two-dimensional satellite image does not duplicate such phenomena as the radar shadows produced by mountains, it does produce plausible results for the relatively flat terrain surrounding the English Channel. The digital radar traces from the laptop are then fed into a custom-made radar waveform generator via a USB link. This generator in turn adds some simulated noise and feeds an analog signal into the H2S display.

The laptop software runs under the elderly Windows XP operating system because of the FPGA chip at the heart of the waveform generator. An older 208-pin surface-mount FPGA was selected, since current chips use a ball grid array that’s impractical to solder by hand. Fortunately, Moore had used the chip earlier in his career as an EE and had some code and device drivers that could be reused for the simulator. But the design required an older USB communications chip that used a driver not available on more recent versions of Windows.

The resulting setup is so faithful to the original that many visitors find the display—intended for use in the dark bowels of a bomber—difficult to see under the bright lights of the museum, so a removable hood was added with a webcam. This allows the display to be projected on a brighter screen. Building the replica simulator has given Moore a keen appreciation for the original creators of the H2S: “It has taken us about two years to develop [the replica]—longer than the guys that actually developed [the H2S] from scratch at the time. God, those guys were clever.”

This article appears in the April 2019 print issue as “Replica Radar.”

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