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This is what Wikipedia says near the end of its radio altimeter article:

The radio altimeter first showed up in the German Junkers Ju-87 "Stuka" dive bomber which was equipped with one for automatic pullouts in the dive bomb run which usually consisted of an 80-90 degree dive. The Stuka pilot would set the radio altimeter to 750m which was connected to the bomb release and automatic pull out. The radio altimeter would drop the bombs and pull the Stuka out of the dive at the set altitude to a level flight. This was invented because the pilots would black out on the pull out, usually for 2 to 5 seconds.

No citation, and I would be skeptical anyway. I know the Stuka had an autopilot that pulled out of a dive, but I thought it was activated manually by the pilot dropping his bomb. I figured he would just drop his bomb at whatever altitude he judged by his own eyes, rather than watching some altimeter instead of the target.

I'll also note that Wikipedia is not self-consistent here. If you scroll up just a few paragraphs, it says:

In 1924, American engineer Lloyd Espenschied invented the radio altimeter. In 1938, Bell Labs put Espenschied's device in a form that was adaptable for aircraft use.[13] In 1938 in co-operation with Bell Labs, United Air Lines fitted a radar type device to some of its airliners as a terrain avoidance device.[14]

Two citations there. So I don't know what the author means exactly when saying it "first showed up" in the Stuka.

I would have thought such a radio altimeter would have been very difficult without the cavity magnetron, invented in Britain and sent to America in the Tizzard Mission of 1940 September.

On the other hand, I have come across some night fighter radars that Germany had, such as the Lichtenstein Radar that did not need cavity magnetrons. That was not used until 1942, though.

So did the Stuka really have some radio altimeter? If so, when? Was it really the B version that saw action in 1939? What frequency and power did it use? How big was it?

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    $\begingroup$ A radar altimeter doesn't require nearly as powerful a transmitter as an air defense radar. The ground is a much larger target than an aircraft, and it's much closer. So you don't need a cavity magnetron. $\endgroup$ – pericynthion Oct 19 '17 at 20:08
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    $\begingroup$ WW2 also had radar-fused anti-aircraft shells, to further blow your mind en.wikipedia.org/wiki/Proximity_fuze#World_War_II $\endgroup$ – Erin Anne Oct 20 '17 at 21:35
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It looks like the actual date is 1942. The 'Nachtschlachtgruppen' aircraft were given Ju 87 D aircraft with numerous upgrades including the FuG 101 radio altimeter.

This would make sense because to use the regular contact altimeter the pilot must have knowledge of the height of terrain above sea level and then add to that when preparing his aircraft for dive bombing. A radio altimeter should always give absolute altitude above the terrain. At night trying to estimate your height above terrain is basically impossible. It's either completely black or a sea of lights beneath you.

I found this information on page 58 of "Junkers Ju-87 Stuka", ISBN 978-83-89450-49-4. More information this book and excerpt is found here: http://mmpbooks.biz/ksiazki/46

Some brief information about the Fug 101: https://airandspace.si.edu/collection-objects/radio-altimeter-german-fug-101-1500-meter-type

https://forum.keypublishing.com/showthread.php?132745-FuG-101-Radio-Altimeter-S-101-Transmitter-Unit

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In places where Stukas were doing their thing there was no ATIS to give you field elevation etc (QFE/QNH) to enable accurate determination of altitude.

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    $\begingroup$ but there were maps, and determining elevation of the target was one part of flight planning. You know what maps are? No need to charge batteries, no boot time, just a big piece of paper with lots of information on it. $\endgroup$ – Peter Kämpf Oct 19 '17 at 6:51

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