Radio aids to navigation and bomb aiming

Radio aids to navigation and bomb aiming

If as they say necessity is the mother of invention then surely war is its father. And nothing proves the point better than the advances in electronics in both communications and aviation during the Second World War.

In our modern world there’s no such job as a navigator on aeroplanes; it’s all left to DME (Distance Measuring Equipment), GPS (Global Positioning System) and on-board computing devices to ensure that the aircraft stays on course.

It wasn’t always like that, at the start of the war much time and effort was invested by the RAF into training their navigators who themselves were some pretty clever “maths-savvy” individuals. Navigation skills were seen as essential since it was envisaged that much of the bomber’s war effort would be on the European mainland and that meant crossing the North Sea.

Pilots of single seat fighter aircraft were given some navigation training but since at the start of hostilities they would not be flying far from their home bases and for much of the time they would be flying in daylight, map reading skills and local area familiarisation rather than formal air navigation were seen as more useful, although designs were being investigated for radio aids for use by the single pilot which later resulted in a near automatic system for both bearing and ranging called VHF Omni-directional Ranging or VOR.

Once it was decided that the RAF bombing offensive should be by night, to avoid the enemy fighter aircraft which were causing such heavy losses on daylight raids, navigators were also trained in astro-navigation; steering by the stars.

The Bubble Sextant
Early tests showed that the maritime sextant; the device which is used to measure the angle between the horizon and an observable star was totally unsuitable for airborne use, largely because the horizon isn’t always visible due to cloud below the aircraft or hazy conditions, so one of the first advances was to redesign the sextant so that it would make the measurement against an artificial horizon; in this case a small bubble floating in a chamber containing alcohol within the body of the sextant. The next problem found to affect the accuracy of the airborne sextant was often due to air turbulence, so a small clockwork driven mechanical computing device was added which averaged out the effects so all the navigator had to do was keep the star in the bubble for thirty seconds or a minute while the sextant motor did its work.

Ironically German aircrews paid less attention to air navigation largely because of technology, they were using radio aids to find their way to their targets, although Britain was unaware of the fact at first.

Early results of RAF bombing raids showed a less than effective accuracy despite the claims of the crews to have bombed on their targets. The skill of the navigator was able to get the bomber into the vicinity of the target, but spot-on bombing was still more down to luck than good navigation. Many bombs fell up to five miles from the target.

Obtaining a fix by radio
The maritime method of determining ones position often used the system of taking a fix; measuring with a compass the angle to two or more visible fixed points such as headlands, lighthouses and so on then plotting those angles on a map; where the lines cross should be your position, however this isn’t possible from an aircraft way out over the North Sea.

Radio had been quite widely used for direction finding by shipping for some time but until successful airborne tests off the coast of Norfolk, Virginia in 1920 the size of the equipment was thought to be unsuitable for airborne use so it was redesigned with a small “loop” aerial fitted usually to the top of the fuselage, often in a streamlined shaped pod which could be turned either by hand or motor driven by the navigator or the wireless operator and the signal was then fed to the main aircraft radio receiver rather than a dedicated unit. Coupled with a fixed aerial on the aircraft a bearing was able to be taken on known broadcast transmitters and a fix could then be plotted on a map in the usual way.

Later updates and improvements led to the use of the ADF (Automatic Direction Finding) systems where the bearing to a Non-Directional Beacon or NDB could be presented to the navigator and pilot simultaneously on a radio compass dial, either alongside a magnetic compass or on a single dial of a dual read-out magnetic/radio compass design.

A similar reverse method could also be used where the aircraft could transmit a signal and two or more ground receiving stations could then take a bearing which could then be combined to provide a position.

However, locating the aiming point and successfully bombing relied on the bomb aimer not only becoming familiar during the pre-sortie briefing with the ground as it was laid out beneath the aircraft, identifying the target, feeding the necessary data such as wind-speed and direction, height and so on into the bomb sight then watching as the ground passed below until the target came into view under the cross-hairs, but being able to actually see the aiming point which could easily become obscured by smoke, dust or cloud layers.

There is no doubt that the aircrews were highly dedicated and proficient at their various duties, but post raid photo reconnaissance often showed that the target escaped damage and the bombing was inaccurate sometimes by miles, through no fault of the crews, but usually because of changes and difficulties encountered over the target area.

Radio beams for bomb aiming
At first it was thought that the problems encountered with MF and HF radio signals, particularly at night when conditions could be affected by “atmospherics” would make radio generally unsuitable for any aids to navigation but examination of downed German aircraft, secret recording of German pilot POWs and “Ultra” decrypts provided by Bletchley Park mentioning “bombing beams”. Churchill ordered an in depth investigation codenamed “Headache”.

Knickebein
As part of the “Headache” investigations, some odd radio signals were detected on a frequency around 40MHz which turned out to be a beam system emanating from the occupied continent. After some measurements and further investigations it was found to be two narrow radio beams. During these investigations, on one day plotting the directions of the beams were crossing directly above the Rolls-Royce engine works at Derby.

The two beams could be “steered” or pointed towards any target in Britain and the German bombers just flew along one of the beams until they met the crossing beam, and at that point they dropped their bombs. Each beam transmitted a series of dots; dot – dot – dot and when they reached the crossing point the signal changed to a double dot sequence; dot dot – dot dot – dot dot.

Efforts were then put in place to counter the German bomber’s beams, this time codenamed “Aspirin”. Since the actual radio beam could not be bent, extra information was transmitted by low power British ground stations to confuse the enemy bomb aimers.

Eventually the ground station operators became so adept at interfering with the beams that it is said that they could control where the bombers would drop their payload. As a side effect of these counter-measures, because the German crews weren’t where they expected to be when they dropped their bombs and turned for home and since they weren’t properly trained in aerial navigation unlike the RAF, many didn’t see Germany again, running out of fuel over the sea.

Not only were there efforts to counter the German beams but also to design a beam system as a navigation and bomb aiming aid for the RAF. The first system to become available fairly early on in the war was GEE followed some time later by OBOE. Along with the increased amount of radio equipment being proposed for RAF Bomber Command, there were the weight considerations that all the new equipment would pose, however Bomber Command was at that time early on in the war flying limited operations whilst re-equipping with the newer four engined “heavies” able to carry not only a larger bomb load for greater distances but also easily allow for the installation of the new radio aids coming on stream.

GEE
Gee was a system based on a number of radio beams which made an invisible radio grid over the ground by which a suitably equipped aircraft could determine it position with a greater degree of accuracy than previously available. Radio signal propagation was not fully understood at the time and at first it was thought that its range wasn’t going to be far enough to be of any great use, however tests conducted in June 1940 showed that far from being short range, Gee was usable by aircraft flying at 10,000 feet at ranges in excess of 300 miles. The system was technically challenging and not easy to disrupt; based on “hyperbolic navigation” (yeah, me neither) and heavily dependant on signal timing. Post war the Gee system was upgraded and used quite widely by commercial airlines until the last Gee transmitter was finally switched off sometime in 1970.

OBOE
Oboe was a twin beam system similar to the German Knickebein system of steerable aerials but unlike the German system Oboe was a pulse based system. By using a transponder in the aircraft the timing of the pulses could be used to interrogate the transmitter station to recover distance information. The beam width was apparently only 35 yards wide.

Oboe proved to be extremely accurate; the MkI had a quoted accuracy of within 120 yards at 250 miles. In his book, "Most Secret War", British physicist R. V. Jones wrote, "As it turned out, Oboe was the most precise bombing system of the whole war."

Continuous development of Oboe kept it ahead of German efforts to jam it. By the time they had worked out how to do it the next mark of Oboe came on stream.

During “Operation Manna”; the supply of food to the Dutch, Oboe enabled the canisters containing supplies to be dropped within 30 metres of the aiming point.


H2S Airborne Radar
Developments in ultra high frequency equipment, in particular the Klystron valve and the invention of the Cavity Magnetron led to the introduction of a ground mapping radar set small enough and light enough to be carried by an aircraft with a large enough power output and resolution fine enough for presenting the bomb aimer with an electronic map of the ground irrespective of cloud density which allowed for much greater accuracy at night and above the clouds.

H2S was in its early days fitted to the Pathfinder aircraft whose job it was to drop the coloured flares over the target in order to mark the aiming points for the main bomber stream following closely behind. As it became more widely available to squadrons it became standard equipment for the Master bomber at first then it was brought into more general use as more crews were trained.

H2S equipped aircraft can easily be identified with a large streamlined “blister” housing on the underside of the fuselage which contained the dish antenna unit.


Other ground based beacon systems

CONSOL
An American designed radio navigation aid called Elektra was further developed by Germany and called “Elektra-Sonnen” using a system of electronically driven aerials sweeping a signal across the sky and timing tone pulses to indicate the angle to the transmitting station. The system was easy to use, required no special equipment and was surprisingly accurate; fixes of within a quarter of a degree were possible at 1000 miles.

Sonne was a very long range navigation aid and at first it was widely used by U-Boats in the Atlantic. It was compromised when the U-Boat U-505 was captured allowing a full understanding of the system.

When Britain discovered the system, it was found to be so useful they did nothing to disrupt it but in the interests of security renamed it “CONSOL” and started to use it too. RAF Coastal Command was in need of a system with an even greater range than Gee and the use of CONSOL became so widespread that they became dependant on it.

Post war the system became known universally by its UK name and was available to anyone with a radio receiver, a Consol chart and the ability to count; no other special equipment was needed. In 1946 an additional transmitter site was commissioned at Bushmills in Northern Ireland, working with the other established stations at Stavanger, Norway, Ploneis in France, Lugo and Seville in Spain it was the radio navigation aid of choice for many pleasure and fishing boats due to its low cost of implementation.

The European CONSOL system was augmented by two additional Atlantic coast sites in the USA where it was known as Consolan. The USSR also built some Consol stations, such was its popularity.

Consol remained in operation until the late 1980s with the last station at Stavanger closing down in 1991.

LORAN
Loran (LOng RAnge Navigation) was an American development of the British Gee navigation system. Operationally Gee had a range of about 400 miles whereas the Loran development was useable up to 1,200 miles. The RAF adopted the early Loran system for the very long distance operations outside the range of Gee.

Both systems used the “hyperbolic navigation” technique and were widely used post war. The last Loran station being closed down in 2000 in favour of the satellite based GPS system, although there are some moves afoot to bring an updated version of Loran online to act mainly as a ground based back-up navigation system should the GPS fail or become unusable.

Other radio aids and developments

IFF (Identification Friend or Foe)
Both British and German forces used their own types of IFF, in the case of the British version each aircraft carried a special radio transmitter and receiver combination called a “transponder”.

Its purpose was to listen for the signal put out by the Chain-Home RDF stations and when that was received to automatically transmit a signal set up with the “key of the day” which would then be received at the Chain-Home station thereby identifying them as friendly. The premise being that aircraft not responding with the correct key, or not at all weren’t friendly and would be attacked.

Well, that was the theory, but it took no account of wrong keys or damaged/faulty IFF transponders.

British military scientists turned the German IFF system back on them. Mosquito night fighters were fitted with an IFF transmitter codenamed “Perfectos” designed to trigger the transponder on the German night fighters giving the Mosquito an indication of where the German was. As they say “End of sports”.

“Darky”
Darky was a low powered HF radio back-up homing system working on a frequency of around 6MHz which allowed returning aircraft to be “talked” back to their home airfields by voice.

“Squeakers”
Barrage balloons were a hazard to both the enemy (as intended) and to the RAF; more than a few returning bombers were brought down or damaged by running into barrage balloon cables at night. Marking them with lights would have defeated the object but a system of notifying RAF crews of the hazard was developed using small low powered radio transmitters attached to the cables which would emit a squeaking noise in the pilot’s earphones to warn approaching RAF aircraft so that they could take avoiding action.

Airfield approach radio beacons and ILS (Instrument Landing System)
The use of very narrow radio beams wasn’t just used for directing aircraft onto a target. Two beams set to overlap slightly could be used to guide an aircraft along a track directly in line with the runway. Deviation off the “centre-line” either way would produce a different signal in the pilot’s earphones; the morse code letter “A” one way and “N” on the other side. The centre-line signal was a continuous tone.

A similar system turned on its side could be used to produce “glide-slope” information; the angle of the slope was usually set to something around 3 degrees.

The ILS was able to provide the pilot with a sort of tube of radio beams which if followed correctly would guide him straight down to the threshold of the runway.

Three more beam transmitters were also used with the beams set directly vertical under the centre-line. One transmitter was located at a distance of nine miles out which gave the pilot a distance indication from the runway. This was known as the outer marker. Another beam transmitter was set up in the same way at a distance of three miles and called the inner marker. The middle marker was at six miles out from the runway.

Later developments of the ILS provided a dial on the pilot’s instrument panel which presented the centre-line and glide-slope as a pair of crossed needles giving a visual indication of where the “tube” down which he had to fly was in relation to his own position and attitude.

ILS is still used today, although it is now combined with other radio aids into the automatic landing system.

Security considerations
During hostilities many of the secret developments installed in aircraft flying over enemy territory were fitted with a barometric triggered incendiary device to completely destroy the internal workings in the event of a crash landing behind the lines. It wasn’t always that successful, allowing German scientists to reverse engineer some of the British developments, as indeed we did to them. There was also the embarrassing moment when the wireless operator or navigator forgot to disable the incendiary trigger only to see a puff of smoke emitting from the equipment as they touched down on their home runway - Oops.