Dear Colleagues, could you please help to solve a WW II mystery related to the H2S operation and a special duty flight? I am referring here to the Pickaxe project, which resulted in the British-Soviet collaboration to insert secret agents into continental Europe on behalf of the NKVD. My interest is related to the failed Whiskey mission of April 20/21, 1942, involving a H2S equipped Halifax Mk II, # V9976, of the 138th RAF Sq., with the mixed Polish/British crew. The flight was to deliver two Soviet NKVD agents into Austria, which was likely beyond the capabilities of the Soviet special duty aviation at the time. Hence, Moscow applied for help to the British under the Pickaxe agreement. Unfortunately, the plane crashed into the ground in dense fog in Bavaria on April 21, 1942, at about 01:10, killing the crew and the passengers. The crew members names are as follows.
F/O Ryszard Zygmuntowicz, PAF - Pilot - killed
W/C Walter Ronald (Wally) Farley, DFC, RAF - 2nd Pilot - killed
F/O James A. Pulton, RAFVR (161 Sqdn) - Gunner - killed
Sgt Czeslaw Madracki, PAF - Navigator (Flt Engineer?) - killed
F/Sgt Bronislaw Karbowski, PAF - Rear Gunner - killed
F/Lt Antoni H. Voellnagel, PAF - Flight Engineer (Navigator?) - killed
Sgt Leon Wilmanski, PAF - Air Bomber (W/Op?) - killed
Sgt Mieczyslaw Wojciechowski, PAF - Wireless Op (Gunner?) - killed
The sources are confusing the names of the Soviet agents, who were passengers of the flight. Here are the names that appeared in the publications, but note that there were only two agents on-board:
Peter Stari(t)sky aka Peter Schuulmburg or Schulenburg aka Rudolf Hofstädter, a German, who became an NKVD agent in 1940
Sevolod (Vsevolod?) Troussevitch aka John Traun aka Johann Traunn, an NKVD agent since 1940
Or rather:
Franz Löschl aka Franz Mayer aka S. Troussevitch, an Austrian born in Vienna in 1913 - killed
Lorenz Mraz aka Ing. Rudolf Hofstädter aka Peter Staritsky, an Austrian born in 1908 - killed
Apparently, both agents were typesetters by their profession, and were to operate in Austria in the defense industry.
Unfortunately, the aircraft hit a hilltop of the Blauberge mountain south of Wildbad Kreuth, Bavaria, which is south of Munich (near Tegernsee lake), on way to the drop zone in Austria in dense fog. There are rumors that one or both agents could have survived the crash, but lost their lives during an encounter with a German woodsman.
My question is related to the accuracy of the H2S radar in the ground mapping setup. This equipment was apparently used for navigation as the aircraft was flying as low as possible, trying to avoid German radars. Some sources sugessted that the reflection from the Tegernsee lake could have been misinterpreted as far as the altitude is concerned, and the aicraft technically made a controlled flight into terrain. Is such an explanation plausible? Or maybe someone has another explanation on the reasons of the tragedy related to the accuracy of the H2S?

More information on the flight:
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The basic operating concept of H2S is that vertical objects cause very large returns due to them forming a partial corner cube. Radio signal hitting the object is reflected back, but so is any signal hitting the ground in front of it, which reflects off the ground, into the object, and back to the receiver. This makes areas of water appear completely black as the radio signal is reflected forward and away from the aircraft, unless there is a ship, in which case it produces an extremely powerful return.

It was while testing a new anti-shipping radar using a PPI display that the mapping concept was immediately evident. For mapping, the most reflective surfaces are mainly seen from the vertical walls of houses and other buildings, dams, towers and similar objects. Water remains invisible, while forests produce an intermediate return. By comparing different areas the operator can guess what sort of terrain they are flying over.

So a mountain in front of you will produce a very large return.

As to resolution, H2S was on the order of 3 degrees, meaning a single object like a radio mast would be displayed as a short arc on the display, about 5 degrees wide. It was useful for determining the difference between river and land, open land and cites, but that's about it. That said, a hill or mountain in front of you would definitely be visible, and obvious enough to avoid.

However, the range of the system is a function of altitude. As the ground directly below the aircraft is at right angles to the radar, it always produces an enormous return that overwhelms anything in the area. As such, H2S had a control, the "center zero", that eliminated signals at the current altitude, so that this area of the display was always blank, leaving the rest of the display much easier to read.

So my suspicion is that if the aircraft was at low altitude, H2S would have been effectively useless, completely overwhelmed by the ground return. There would be a thin annular area around the aircraft, further than the zero area but limited to the maximum forward angle (which I don't know, but likely something like 10 degrees or so) which would leave a very small area visible. Its entirely possible there was no visible area at all.

I find it difficult to imagine how it might even be used in this role - the TFR and TA radars built in the 1960s were specifically designed to shoot directly forward and even up with an extremely thin beam in order to avoid the ground reflection and be useful at low altitudes.

The issue of the lake is an interesting one. A lake basically looks like the center zero, so I can understand possible confusion there if the aircraft flew into the lake. But it appears the aircraft flew into the hills, which to my mind suggests they didn't see them and that suggests the radar was not being used or the range where objects were visible was simply too small to react.
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Dear Maury Markowitz, thank you very much indeed for the detailed reply.

Blauberge mountain has the top elevation of 1,862 m (6,109 ft), while Tegernsee lake has the surface elevation of 725.50 m (2,380.2 ft) according to Wikipedia. It seems that the Halifax #V9976 crashed just 25 meters short of the top of the Blauberge mountain. Perhaps the pilots were desperately pulling the aircraft up to avoid the obstacle but to no avail. If the weather was reportedly foggy, I assume that they could have used radar for navigation.

It's likely that the H2S setup on the #V9976 was purely experimental. Reportedly, the H2S was officially tested on another Halifax B Mk II, #V9977, only on April, 23, 1942, that is only two days after the Blauberge crash. The #V9977 eventually crashed on June, 7, 1942, killing all occupants. (See: https://aviation-safety.net/wikibase/wiki.php?id=74458).

Some Polish researchers claimed that the Soviet agents on-board the #V9976 to be inserted into Austria were to assassinate Adolf Hitler, but no references are available, especially from the Russian archives, on the true character of their mission. The drop zone was anyway near Linz. Without the release of British and Russian files on the mission and reading the reports of the German police who inspected the crash site, we can only speculate.

As a Post Scriptum, the spouse of one of the deceased agents, Hildegard Mraz, was also working for the Soviets. In 1943, she was parachuted into occupied Poland, from where she got to Austria, where she acted as a radio-woman of a Soviet spy ring. Arrested by the Gestapo in early 1944, she was forced to play a role in a Funkspiel against the Soviets for about 14 months. Following the war, she was tried for treason by the Soviets, and sent to GULag. She was officially pardoned only in 1965, and she died in Vienna in 1997. Apparently, the Soviets informed the CP Austria only in 1957 that Hildegard's husband Lorenz Mraz had died during a mission behind the enemy lines during WW II.

I am adding two more references on the V9976 flight:
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It's likely that the H2S setup on the #V9976 was purely experimental. Reportedly, the H2S was officially tested on another Halifax B Mk II, #V9977, only on April, 23, 1942
Well all such units were purely experimental at the time, it was not until early 1943 that they were operational and in April I suspect there were no more than two flyable units available, along with some bench models.

The question, for me, is what this aircraft was doing with an H2S aboard. At this time, there was a huge debate in the RAF about whether or not the magnetron should be allowed anywhere near the continent because the Germans would be able to easily reverse engineer it.

And yet, here it is being flown over the continent long before it even entered production? That seems like madness!

Is there any possibility the system was not operational and they had simply removed the magnetron and rhumbatron for this mission, if not the entire system?
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