Radar Bounced Off the Sun

 We are all familiar with the moon bounce, but the "sun-bounce" is much more obscure accomplishment.  The first successful attempt was transmitted on April 7th, 1959 at Stanford University. the radiomen on the job were Dr. Von Russel Eshleman, Lt. Col. Robert C. Barthle and Dr. Philip B. Gallagher, staff at Stanford University Stanford Electronics Laboratories. Their names appear together on a number of different astrological papers at Stanford:

1955 -  Regularly observable aspect-sensitive radio reflections...
1956 -  Analysis of a new type of radio scattering...
1957 -  Antenna array for studies in meteor and radio astronomy
1957 -  Meteor rate and radiant studies: experimental radio studies...
1958 -  Radio reflections from artificially produced electron clouds
1959 -  Theory of radar studies of the cislunar medium
1960 -  Radar echoes from the sun

The Stanford Electronics Laboratories group continued churning out white papers until 1961. It continued to make reports for NASA until at least 1973. It's notable that follows the departure of Gallagher. Barthe and Eschleman got all the press. Eschleman made the statement to the Stanford newspaper on Sputnik. Their quotes in Popular Mechanics weren't specifically credited.  [SOURCE] Sensibly Escheman and Gallagher went on to work for NASA and continued to write arcane papers that I barely understand for decades.

Their experiment required a 40,000 watt transmitter. The antenna consisted of 5 miles of wire spread over 11 acres of ground. To quote the Popular Mechanics article "...the sun was difficult to reach by radar because it was 93,000,000 miles away and because of the 'thunderous radio noise' arising from it's turbulent surface."  The signal's round trip took over half an hour.  

The signal was a 30-second bursts of dots and dashes that was perceptible despite the random noise of the sun. The radar echo didn't come from the sun's visible surface but it's outer corona. The return signals were recorded to magnetic tape for further study with an IBM 707 computer.  Anyone know where they have that recording at Sanford?

The experiment was repeated on April 10 and April 12, and the data was published in the journal Science on February 5, 1960. Popular Mechanics wrote it up in May 1960. More here.

Over The Horizon

The video above is Iran's over the horizon radar system. It's a wideband signal heard on 10 meters with a varying bandwidth of 60 KHz to about 90 KHz. It was hear later the same year at 28000 kHz with a 300kHz bandwidth. These are messy and were received all the way to Chicago, IL. The Russian OTH radar in Gorodezh (near Nizhny Novgorod) also causes strong QRM splattering from 10125 up to 14 MHz. The signals are received so far away it's often mistaken for jamming. But it's not. Last year Matt Waid at ARRL wrote a good column on the IARUMS reported increase in OTH radar signals (among other Eastern military radio traffic) [HERE] he wrote:

"At least some of these intruders were likely to be audible in other parts of the world. Monitors in Europe reported a Russian over-the-horizon (OTH) radar in Gorodezh on 14.108 MHz, causing strong interference daily and often exhibiting splatter. In addition the Russian Navy was reported active frequently on 14.192.0 MHz using FM CW.  ...some of the worst offenders are OTH facilities in Russia and Iran. The signals can result in broad swaths of noise in the 20 meter band"

So let's discuss what OTH is all about. OTH systems are a type of radar system with the ability to detect targets at very long ranges. This can be over 500 miles away. while this is not over the horizon as you might think of it at sunset, this is beyond the radar horizon, (the distance limit for normal radar.) There are several OTH radar systems most of which were developed during the peak of the cold war over the 1950s and 1960. 

The technology exploits a long-understood artifact of AM radio propagation, skip. With OTH, the broadcaster generates a powerful shortwave signal from a large transmitting antenna which reaches a receiving antenna beyond the horizon by bouncing off the ionosphere. A return signal (echo) is sent back from the receiving station by the same route. All other systems use low-frequency radio waves that due to diffraction follow the curvature of the Earth to reach beyond the horizon. The ionospheric refraction is the more common application.

But this time of refraction has two big limitations. Firstly the angle is limited to about 2 to 4 degrees off the horizon. Second they require enormous antenna arrays to transmit at this angle so the stations are expensive and immobile. The first development on this was done in the Soviet Union Engineers starting in 1946 under the Veyer research Project. The word is a surname, a city and the word for 'fan' so it's significance is unknown since the project was ultimately cancelled in 1949. A working model was developed in the U.S. under Dr. William J. Thaler at the Naval Research Lab. Their first experimental system, MUSIC (Multiple Storage, Integration, and Correlation) went online in 1955 at Cape Canaveral.

Iran didn't get into the OTH game until 2013 with their Sepehr project. It is purported to have a range of over 1500 miles. Sepehr  is the Iranian word for "sky" which at least makes more sense than 'fan'.

Google has a Radar Chip

The ATAP team at Google I/O recently unveiled a radar chip. The chip is about the size of your thumbnail and functions as a radar detector. It can detect small, and even subtle hand motions which may allow more nuanced interactions with hand-held and wearable devices.  It's called Project Soli.

GEE

In 1935, the British began developing GEE. It was developed by Robert Watson-Watt at the Air Ministry and introduced by the Royal Air Force (RAF). The idea of hyperbolic navigation known in the 1930s, but the first trials didn't happen until 1941.  Starting in 1942 these radio signals were used to guide the WWII carpet bombing of Germany. Germany began it's attempts at jamming the GEE system in 1943 but these were largely ineffectual. So what's hyperbolic navigation?

Hyperbolic navigation is a class of radio navigation systems based which determines position based on the  microsecond difference in timing between the reception of two signals.  While you need three points to triangulate a location in 3 dimensional space, the surface of the earth is effectively a 2 dimensional plane. But hyperbolic navigation requires the plotting all of the potential locations of the receiver for the measured delay. This produces a series of hyperbolic lines (a parabola) on a chart. Taking multiple measurements narrowed this down. GEE was the first such system. 

Imagine two terrestrial radio stations located 186 miles apart. At light speed (the speed radio signals travel) their signals can reach each other in exactly 1 millisecond. So Station A emits a pulse, this pulse is received at Station B and it triggers the transmission of another pulse 1 ms later. But a ship in motion at a third location will receive these at odd intervals because of the distance from each station. But knowing their interval, they can plot a circle around each transmitter, and at their points of intersection (parabolas) they can determine their possible location down to a set of two. Then they had to use traditional navigation to pick which of the two. The source of that problem... is time.

In order to measure the exact time it took for a signals to reach the receiver, the receiver must know the exact time the signal was transmitted. But reliable millisecond time-keeping wasn't possible in the 1930s. The most common clock to use with GEE was a crystal oscillator. But that drifts about 1 to 2 seconds in a month. That may sound trivial, but 2 seconds is a distance of 372 miles... that's the difference between Bermuda and the Carolina coast. That's a big deal. (This problem wasn't overcome until the advent of atomic clocks in the 1960s.) So instead of absolute time, differential time was used. Your distance from the two transmitters was unknown, but the difference between their signals was measurable on an oscillograph and the delay curves were available on a chart.

GEE signals were all sent on the same frequency which made it difficult to distinguish the original signal from the response signal. Chains of GEE stations were built in the UK, France and northern Germany. Ultimately it was replaced by VOR systems and LORAN. Some of the British GEE equipment was used in the later GEE-H system which operated at the 20-80 MHz range. The last GEE chain was shut down in 1970

The irony in all this is that hyperbolic navigation was originally developed by Germany. Meint Harms lectured on the topic as a masters student at Seefahrtschule Lübeck, a navigational school. After becoming a professor of Mathematics, Physics and Navigation at the Kaisertor in Lübeck he began to demonstrate models of the system which he patented in 1932.

Radio Proximity Fuses

In the late 1930s the proximity fuse was developed in the UK.  The Germans were also working in the same arena and had developed some models but the Brits beat them to it. They had been working on a variety of developments to increase what the military called "air defense efficiency."

Into this technological melee they dropped William Alan Stewart Butement. In 1931 he and P. E. Pollard, invented a shipboard radio device for the detection of ships. Today we call it radar. It operated at 600 MHz and using pulse modulation was able to detect ships100 yards away. The Navy wasn't big on it, but others saw potential. By 1938 Butement had ramped up his invention into large scale devices that could be used from land to protect the coast.

In 1939 Butement attempted to improve anti-aircraft guns. It's hard to hit a moving target. He had a better idea.  His plan was a very compact  Radio Direction Finder (RDF) unit placed on the projectile. It would then trigger the detonation when near the target. This was not simple. The RDF had to be small and also durabel enough to be fired out of a cannon. This circuit included glass vacuum tubes. Somehow just over a year the United States was manufacturing projectiles with a proximity fuse. This was also called a VT (variable-time) fuse.

This was not an induction trigger. It wasn't sensing a ferrous body. It was detecting the reflection of radio signals. A later improvement was The transmitter which used the shell body as an antenna and sent out a continuous wave at around 200 MHz. As the shell approaches a reflecting object, (a ship, a plane, the ground) the reflected signal created interference. That pattern changes with proximity. As the objects get closers the signal moves in and out of resonance as the reflected signal length changes; half a wave is resonant, so is a quarter, an eighth etc.

This causes a small oscillation of the radiated power and consequently the oscillator supply current of about 200–800 Hz, the Doppler frequency. This signal is sent through a band pass filter, amplified, and triggers the detonation when it exceeds a given amplitude. Later in life, Butement said that he considered the proximity fuse as his most significant accomplishment.

The Radio Control of "Little Boy"

Most people assume that bombs work in real life like they do in Looney Tunes cartoons—they fall out of a plane and they explode upon impact. Those are ballistic bombs.  That is not how all bombs work.  Many operate by radio control. I am not referring to missiles that are steered by radio control. Many times of bombs used by the U.S. military detonate depending on their distance to the ground and that is determined by radio. More here.

On Monday, August 6th, 1945, at 8:15 AM, the Atomic Bomb "Little Boy" was dropped on Hiroshima by an American B-29 bomber, the Enola Gay.  The code names of the first two nuclear bombs were "Fat Man" and "Little Boy."  I only read recently that they were radio controlled. the book E=MC² by David Bodanis detailed the whole arming mechanism. I quote it here at length:

"...Weak radio signals were being pumped down from the bomb to the Shina Hospital directly below. Some of the radio signals were absorbed in the hospitals walls, but most were bounced back skyward. Sticking out of the bomb's back, near the spinning fins, were a number of whiplike thin radio antennae. Those collected the returning radio signals, and used the time lag each took to return as a way of measuring the height remaining to the ground. At 19,000 feet the last rebounded radio signal arrived."

He doesn't mention it above, but another plane flying in formation with the Enola Gay dropped measuring instruments by parachute. I found aversion of the story in the New Yorker that incorrectly describes this device like a timer. "...wires on top of the device were attached to a solenoid unit on the roof of the bomb bay. When the bomb dropped out, the wires came loose from switches inside the clock-box—the brain that told the bomb to drop for forty-five seconds before detonating."  It's correct that a solenoid switch started the sequence, but the count down clock was a back up system. More here.

The radio described by Bodanis was the AN/APS-13 radar unit.  While I can't find a source on the frequencies used by the equipment in this case I do know that a standard AN/APS-13 operates at 410 - 420 MHz with a receiver IF of 30 MHz. The British called these "Archies." One source I read claimed that it detonation was triggered only when two AN/APS-13 radar units identified the critical altitude. They were trying to prevent premature detonation. With the score of the number of other back up systems that seems totally plausible. Some sources claim there were as many as four. More here. They were used as tail radar in allied planes in WWII. This is the device that actually triggered the altimeter fuse. The Duxford Radio Society has pictures of a restored unit here. The device is automated and self contained.

Some of this is still classified today, and I suspect this was not an off-the-shelf AN/APS-13. That device wasn't designed to measure distance. It was a tail warning device, i.e. it warned a pilot that an aircraft was approaching him from the rear. It's effective range was given as 2,000 to 2,500 feet transmitting and receiving a fan-shaped beam behind the airplane. More here. You know the rest of the story but let me quote this from the Yale Avalon Project, and recommend that you visit their website.

"At 8:16 A.M., the Tokyo control operator of the Japanese Broadcasting Corporation noticed that the Hiroshima station had gone off the air. He tried to use another telephone line to reestablish his program, but it too had failed. About twenty minutes later the Tokyo railroad telegraph center realized that the main line telegraph had stopped working just north of Hiroshima. From some small railway stops within ten miles of the city there came unofficial and confused reports of a terrible explosion in Hiroshima. All these reports were transmitted to the Headquarters of the Japanese General Staff."