As with many other things, it was the growing threat of war with the Soviet Union that led to the rushing of Radar into service, with the first Finnish Radar Surveillance Systems set up over the spring and summer of 1939. The “Kotkansilmä” system was still fairly primitive and had been rushed into service on the basis that ”third best” would do if ”second best” wasn’t available. The “Kotkansilmä” NR01/38 looked nothing like later equipment – they were static systems with an array mattress antenna that could measure direction and distance to targets with an azimuth resolution of around 30° and an angular resolution of around 5 Degrees. The systems could pick up large groups of aircraft at up to 130kms and individual aircraft out to 70km.
By the autumn of 1939, equipment for 17 stations had been manufactured and assembled, with a chain of 8 “Kotkansilmä” air and surface surveillance stations had been rapidly constructed stretching 400kms from Lake Ladoga to Viipuri and along the coastline of the Gulf of Finland as far eastwards as Turku – stations were constructed in carefully positioned locations outside Tolvajarvi (the north-easternmost position), Sortavalla, Käkisalmi, Viipuri, Kotka, Helsinki, Hango and Turku. In addition, 6 further stations were hastily built on islands in the Gulf of Finland in conjunction with the existing coastal defence positions (at Bjorke, Lavansaari, Someri, Mäkiluoto, Örö and north of Turku at Lypertö). In addition, equipment for a three further stations was constructed and stockpiled for the refortification of the old positions on Utö, Kökar and the Aland Islands in the event of war. The radar sites were situated so that they overlapped, giving complete coverage over Lake Ladoga, the Karelian Isthmus and the Gulf of Finland. They were easily able to pick up signals from aircraft in the skies over Leningrad and Estonia. It was an immense construction job and a large expense for the Ilmavoimat, all of it authorised by Mannerheim before the equipment had been proved to be reliable. Interestingly, the radar stations themselves aroused little interest or curiosity as they were built, most seemed to assume they were simply new radio stations - and radio was a familiar device. With Finland moving to a war footing over the summer and autumn of 1939, numerous exercises were carried out to familiarize personnel with the system and its use. Initial attempts to use the radar system to direct Ilmavoimat fighters to discreetly intercept civilian aircraft did not go very well, but everyone learned, and “Kotkansilmä” proved its usefulness during further exercises in September 1939. Ground controllers successfully directed fighter interceptors to their targets three-quarters of the time. And the Ilmavoimat worked out largely by trial and error how to actually use the radar equipment - and it was the Ilmavalonta personnel who were assigned to work with the radar equipment who learnt to read and interpret the amorphous blips and squggles as they appeared on the radar screen in front of them.
The early “Kotkansilmä” Radar Stations were rapidly constructed over 1939, generally in Bunker Systems with towers to extend the range similar to the model illustrated above. Where stations were constructed on islands in the Gulf of Finland, they were co-located with pre-existing coastal defense batteries and defending infantry units. All radar equipment was fitted with demolition charges to be used in the event that the station was attacked and in danger of being captured.
A typical “Kotkansilmä” radar receiver room of 1939
As has been mentioned earlier, Somersalo was instrumental in driving through the operational integration of the early radars into an effective air defence system. All radar users learned sooner or later that such a powerful tool was of limited use without the proper procedures in place to make good use of it. Radar was a new thing and the Ilmavoimat had to learn by doing as well as by theorising. Fortunately for Finland and the Ilmavoimat, Eversti Richard Lorentz and his introduction of the “Havaitse-Punnitse-Ratkaise-Taistele!“ (Observe-Weigh/Consider-Decide-Fight!") loop into combat proceduresmeant that operational tempo was a prime area of focus for the Ilmavoimat. The availability of real-time radar data meant that the faster flow of operational data to fighter squadrons on the ground, as well as to fighters in the air, was of crucial importance and this was an area that the Ilmavoimat immediately focused in on.
As with many other industrial nations, there was a significant interest in Finland in the 1920s and 1930s in what is now known as the “Efficiency Movement”. Majuri Rautvuori and Eversti Lorentz worked closely with two Finnish industrial efficiency “experts” to identify ways and means to improve the flow of information and efficiently control and direct air defence. The end result was a major improvement in the flow of communications, the filtering and direction of information and the integration of the air defence chain of command. And in fact integration of the air defence chain of command had been one of the primary problems identified by Somersalo even prior to radar being introduced, when a mid-1938 demonstration of an early prototype radar system had gone comically wrong even though the radar system itself had worked perfectly. Somersalo was well aware of the importance of a unified command, and with Mannerheim’s support, the Ilmavoimat was given direct and unifed control over all assets related to air defence.
An Area Air Defence Centre (Ilmapuolustusaluekeskus): Tellers on the balcony overlook the plotting table and vertical long-range handover board. At the end of the balcony a Leading Observer acts as Post Controller.The introduction of the “Kotkansilmä” Radar Stations led to extensive improvements to the overall system that was already in existence, extending the range and allowing Controllers to provide real-time direction to Ilmavoimat fighters as they moved to intercept incoming or outgoing Soviet aircraft.
Perhaps the most significant operational innovation made by the Ilmavoimat was the introduction of the “Filter Room”. Since all personnel and records were under tight security, it is only recently that the story of the Ilmavoimat’s Filter Room has come to light – this was an aspect of the war that was even more obscure and tightly secured than the Nokia Radars themselves. Operationally, the large network of Observers generated a mass of information that was handled through Ilmapuolustusaluekeskus (Area Air Defence Centres) and the introduction of the Radar Stations introduced a flood of additional real-time data which needed to be assessed and used in real-time to be most effective. To be used most effectively, the mass of raw information generated by the radars had to be processed before it could be presented to the Operations Room. This processing was carried out by the “Filter Room”, which was the nerve centre of the Radar and visual surveillance system. The complexity of the Filter Room task cannot be overstated. Much depended on the Filterer’s detailed knowledge of the performance and limitations of each individual radar and their confidence in the ability of the crews on watch. The Filterer’s ability to correlate the information quickly and assess the probability of the true radar picture and of the visual observations being called in underpinned the successful operation of the whole radar system.
But even so, the Filter Room itself was only a part of the Air Defence Control System that was put in place. And the Ilmavoimat’s Air Defence Control System was, by late 1939 and the start of the Winter War, state of of the art and probably the best in the world, operationally at least on a par with the RAF’s (and given the superiority of “Kotkansilmä” over the RAF’s Chain Home network, likely better overall). Illustrated below is the Ilmavoimat’s National Air Defence Centre at Mikkeli. This was constructed rapidly over summer 1939, incorporating the latest developments in Ilmavoimat monitoring and control procedures and equipment. The control centre consisted of a large concrete bunker with integral machine and plant rooms, a telephone exchange, telephone equipment room and an operations room with intercept cabins in raised galleries around the reporting room floor. Personnel working in the Centre included 'Plotters', 'Filterers' and 'Controllers'. Designed with the assistance of the noted Finnish architect, Alvar Aalto, the Mikkeli National Air Defence Centre was both futuristic looking and highly functional, designed around the filtering and flow of information with which to control the air war. Each Sector Air Defence Centre controlled its own area and they were manned 24 hours a day through out the entire war period as was the Mikkeli National Air Defence Centre where the entire air battle situation was monitored and from which overall direction was given to the individual sectors.
The Alvar Aalto-designed reporting room at the Mikkeli National Air Defence Centre . The general situations map (left) is replicated in Sector operations maps. It displays all information received from the Observer Corps and the Filter Room. The nationall situation map indicates positions of all aircraft, both Ilmavoimat and Soviet. The room is overlooked by three cabins with angled windows where the officers or the decision-makers sat (the Chief Controller, Fighter Controller and Anti Aircraft Liaison Officer & Searchlight Controller). The map table with its real time display provided a snapshot of events, giving the decision-makers the information they needed to bring their fighters into the battle.
The reporting room had two horizontal plotting tables with a vertical 'Tote' board detailing the status of flights and raids which could be viewed from the elevated controller's cabin. Seven plotting tables at Mikkeli showed the regional situations, the one in the illustrations above and below gave a view of the national picture.
Mikkeli National Air Defence Centre Operations Room –photo taken shortly after the start of the Winter War.
Reports from radar stations and observers would be called in by phone. By combining overlapping reports from adjacent radar stations and observers and then collating and correcting this information, accurate details of incoming or outgoing aircraft could be obtained. This process was called “filtering” and took place in Sector Filter Rooms each of which was attached to an Ilmavoimat Fighter Group. 'Filterers' received radar, visual observation and pilot sighting information by telephone and radio, aggregated this information in real time and piped the information through to the Ilmavoimat Lotta Plotters in the reporting room wearing headsets. Personnel in the Filter Rooms predominantly consisted of young women who worked in eight hour watches in the underground bunkers (perhaps predictably, these young women would become known as the Beauty Chorus). To avoid errors and delays, clear Finnish was insisted upon. The Controllers were almost all recruited from the Helsinki Stock Exchange, typically because they were accustomed to making swift decisions under extreme pressure.
The Filter Room
The Filterer’s duty “was to decide the moment when a few successive plots, with all their possible inaccuracies, might be considered as a reliable track, fit for the operations room to act on for fighter interception”. Constantly on the horns of a dilemma, the Filterer had to decide whether to pass information to the Plotter on the strength of her first guess, or to wait for another plot in the hope that it would confirm or confound her suspicions. Acting quickly gained valuable time but risked fighters taking off on false information. Waiting for more information reduced the chance of interception. It was a stressful position and one that demanded a high level of intelligence and decisiveness and an ability to make decisions quickly and accurately. Speed was of the essence since there was a shortage of fighter aircraft, trained pilots and fuel. Without radar and the speed and skill of the Filter Room personnel, the outcome of the Air Battle could have had a very different outcome for Finland.
"Must be under twenty-one years of age, with quick reactions, good at figures - and female". These were the prerequisites for members of the Lotta Svard seeking a commission as Officers working in the Filter Room. This secret section of the Ilmavoimat's Defence programme in the Winter War and WW2 has never had the recognition it deserves. Nevertheless, it was one of the greatest aids to the protection of Finland and to Ilmavoimat air operations over the whole war period.
The reason that very few people even today have heard about this is the extreme secrecy which the personnel maintained about its work and its influences. Only in recent years have the restrictions been lifted and wartime members of this close group been freed from the silence imposed upon them.
The work done by the Lotta Svard Filterer Officers of the Filter Room, the Movement Liaison Officers and the Plotters who were responsible for calculating and rectifying the position, and identifying the hundreds of tracks of hostile and friendly aircraft leaving and approaching the coasts of Finland, was vital.
It is time that their valuable contribution to the Battle in the air be recognised. The Filter Room was the nerve centre of the “Kotkansilmissä” Radar system. It received information from the many Radar stations which formed a chain of protection around our coasts. This information, due to the early primitive forms of detection equipment and the possible human error of its operators needed to be instantly corrected, co-ordinated and displayed on a huge map table of the area concerned, in a form suitable to be passed on to the Operations Rooms. Without this essential link, the Radar information at that time could not have been used.
This cleaned-up (or filtered) information we have all seen in many films where Ilmavoimat Ops room plotters move coloured arrows around a map table, using a long pole-like contraption. It always appears so peaceful and bears no relation to the hectic activity that really occurred in the Filter Room.
From this information, orders were given by the Senior Officers observing from the gallery above the Ops table, for air raid warnings to be sounded in threatened areas, fighter squadrons to be scrambled, incoming hostile aircraft be intercepted and returning bomber aircraft in difficulties monitored so air-sea rescue boats could be directed to their assistance, should they ditch. The AA gun sites and Searchlight stations also relied on this information.
Finland was divided into Air Defence Sectors and there was a Filter Room for each. These were manned twenty-four hours daily from the commencement of hostilities on the 30th November 1939 until after peace was declared in Europe. Personnel were formed into four watches. Sometimes it was impossible through illness or shortage of trained personnel to maintain this and a three-watch basis was instituted. This meant leave was impossible and other than eating, sleeping and working, there was little time left.
Personnel had two fifteen minute periods when possible during the watch for a refreshment break. The food available varied considerably. Sandwiches of raw cabbage were offered throughout the night watch for weeks on end at one particular station, almost leading to a mutiny! Working conditions were often difficult and unpleasant. Many of the centres were underground where ventilation and heating left much to be desired.
The pressure of work depended upon the amount of aircraft activity and also the region involved. Naturally the northern areas were not as busy as the Filter Rooms covering the Gulf of Finland and the Karelian Isthmus. The weather too was a key factor in the activity to be expected so meteorological advices were posted constantly.
The requirement for the Filterer Officers to have quick reactions was patently obvious. They had to sort out the correct position of the aircraft from the various overlapping Radar station plots which covered the same aircraft responses. They needed to estimate both height and number of aircraft, as well as direction from information given, having intimate knowledge of the siting of the Radar stations involved and judging their accuracy. All of this had to be done with great speed.as the aircraft themselves were constantly moving on to new positions. It was found that male Filterers, mostly well over thirty years of age and unfit for front-line service for various reasons, were far too slow during periods of intense activity and they had to be removed from the table!
For the displayed information to be of value to the Operations Room, it had to be as up-to-date as possible. This meant that in times of the greatest activity, a Filterer Officer had to estimate and display salient information on up to fifty different tracks within a minute. The mental stress and physical strain were intense under these conditions and when the personnel came off watch, tension was invariably high. Throughout the meal supplied when coming off duty, the atmosphere was almost hysterical as they gradually unwound. Quite often however tired, sleep was impossible.
The mixture of backgrounds amongst the members of the Filter Room officers was amazing. Most of the senior male officers hailed from the Helsinki Stock Market where they worked as jobbers and brokers. It was an inspiration on the part of Somersalo to choose these men for the positions as Controllers and Movement Liaison Officers. All personnel involved had to have quick reactions, good mathematical ability and be physically very energetic. The women chosen ranged from psychology or science students, young actresses, society debutantes, grammar school high flyers to daughters of famous people - novelists, painters, musicians and vicars. But they were all dedicated to their work, intelligent and articulate.
Many friendships were forged under these conditions and remain close until today. Now the strictures of secrecy have been lifted, it is surely right that their dedication should be recorded. The importance of their work in the defence of Finland against the Soviet forces together with their contribution to the successful invasion of Europe and the ultimate overthrow of Hitler's forces should be made known.
http://www.youtube.com/watch?v=Gq8y-O6eANU
For the interested, here is a documentary on the Filter Room and how the whole process worked. For the purposes of this alternative history, visualize this as an old Finnish documentary about the Ilmavoimats Filter Room
The Operations Room
In the center of the Operations Room was a real-time event gridded map table providing a visual representation of the skies above Finland. Plotters used various counters on the map table to show the location of friendly and enemy aircraft. The Lotta Plotters would plot all planes based on received information from the filter section. This gave a pinpoint position, the expected course, the estimated height and number of aircraft and whether Identification Friend or Foe (IFF) was indicated. Enemy planes were tracked and plotted as they took off in Soviet-controlled areas, in real-time. Each radar station was allocated a colour and position plots, numbered one to five and colour-coded, signified time received. Filterer Officers - or 'Filterers' - filtered the information and passed it on to the Plotters who updated the position of the counters and placed an arrow giving direction and position. Plotters used metal poles with magnetic tips to manipulate the arrows which were colour coded in coordination with an Ilmavoimat sector clock with 5 min colour sectors to show the most recent plot information.The information on each track was displayed on a magnetised metal plaque. Every change was reflected by the counters on the table. For example, if a raid split into several smaller raids, direction changed or number of aircraft was updated.
From the elevated cabin overlooking the Operations Room, a Controller would oversee the entire situation whilst an Allocator would allocate fighters and intercepts to individual fighter controllers, who would then control the radar interceptions. The controllers and their assistants were situated in cabins behind the controller. There would be direct phone lines to the wider air defence organisation, manned by assistants who passed on the resulting information by phone to all who needed to know, including Air Raid Warning Officers, Observer Corps, AA Artillery, Air Sea Rescue Centres and Group and Sector Operations Rooms who would then order fighter interceptions of the hostiles. Radio operators were also located on a balcony overlooking the map, and relayed instructions directly to a particular squadron, or more typically, a remotely located station relaying information to a group of aircraft. Above them were status boards, consisting of a series of lights showing the current status of a particular squadron, on the ground, in battle, returning, etc. Overall direction of the battle was directed by commanders who thus had "instant access" to a picture of the battle as a whole, with the National Air Defence Centre at Mikkeli having a view of the air situation over the entire country in real time.
The senior Fighter Command Officer could tell at a glance from his seat in the Cabin above how many air attacks were forming up, how big they were, what the course was as well as how many Ilmavoimat Fighter aircraft were available and at what state of readiness they were. As the system was honed in the last months of 1938, Eversti Lorentz had experienced fighter pilots placed in the Filter Room for a short tour of duty alongside the filterers so that the Pilots could gain confidence in the system and then spread this confidence to the other members of their squadrons. A futher benefit was that the Pilots could also explain to the Filterers what the Pilots could and could not do, and what they needed to know, the object being that the Pilots must have absolute confidence in the instructions they received from the ground, even if they came in the form of the voice of a young woman who had never been up in an aircraft in her life.
Mikkeli Chief Controllers Cabin. The tote can be seen through the cabin window in the reporting room.
The decision-makers would use a second user-interface model the tote board, after the horseracing tracks ''Totalisator" board. The board indicated which squadrons in what sectors were in contact with the enemy, and those disengaging to refuel and rearm. It also indicated the operational state of readiness of squadrons held in reserve that were "available" in 30 minutes, at "readiness" in five minutes, or at "cockpit readiness" in two minutes to engage in immediate battle, as well as what was in the air. The board had dozens of electric lights that ran the full length of a wall. Beneath was the map room clock with its colour coded time intervals.
An Ilmavoimat Lotta supervisor maintained the wall mounted tote board and added additional data to the arrows such as a contact number and a classification of the contact as Hostile or Friendly. The tote board would have listed the local Ilmavoimat fighter squadrons, the aircraft available and their status. With this model decision-makers had a means to respond by determining what resources were available and how they could be deployed. They could tell from the coloured counters the time segment they were dealing with. They would then pass directives to the individual squadrons and pilots. They positioned the fighters at the required operational heights to be the most effective.
Air Defence Sector Control
Sector Control Centres were similar in layout and design to the National Air Defence Centre at Mikkeli, albeit somewhat smaller.
Radar-Lotta’s monitoring airspace over the Karelian Isthmus from the large Finnish radar station outside Viipuri
Room layout of a Sector Air Defence Control Centre
Detailed layout of equipment and personnel in the operations area of a Sector Air Defence Control Centre.
A standby generator was generally housed in an adjacent building (standby set house). The final installation had a single rotating aerial array with the transmitter and receiver housed in an underground well beneathIn order to provide communication between the controllers and the intercepting aircraft, two VHF/UHF multi-channel radio transmitter and receiver blocks were built at remote sites to stop interference and swamping of the radio signals by the radar arrays. The Centre also provided information for anti-aircraft gun sites within the area controlled by the Centre.
The experiences of a Finnish radar operator
The recruitment of radar operators was not an easy task because of the need for secrecy. Recruitment officers could not be told what the Lotta girls would be doing and were informed simply that a new secret signals unit attached to the Ilmavoimat needed women recruits with a university degree or other higher education qualification, and they were asked to refer likely candidates to the Ilmavoimat recruiting officer. Most recruitment was from within the Lotta Svard organisation, but some recruitment also took place at the universities and through personal contacts. While little detail could be given to potential recruits, the requirement that they take an oath of secrecy and be subject to very stringent security checks generally intrigued them greatly.
Operators read the screens of a Nokia NR04/39 radar (as Radar became more widely used, the previous informal ET designation was replaced with the NR designation, taken from Nokia Radiomittaus, with the 04 representing the model number and /39 the year the type first entered service
Siiri Vasström was one of those excited young recruits shortly before the start of the Winter War. A university student at the University of Helsinki with a middle class background (her father was a Doctor and a staunch member of the Suojeluskuntas, her mother and grandmothers on both sides of the family were long-time Lotta Svard members). Siiri herself had joined the Pikkulotta’s as soon as she was old enough and at 17, she had transferred to the Lotta’s. On first attending the University of Helsinki in 1938, she had transferred to the University Lotta’s and started Signals training on radio equipment. She proved adept with radio equipment and to her surprise (and excitement), in mid-1939 she was approached by a senior Signals-Lotta and asked if she would like to be part of a secret Ilmavoimat Signals Unit that was being set up. After some thought, Siiri agreed and was accepted and advised that her training would start at the beginning of the Summer Holidays. In the event, she did not return to her University studies until 1941, by which time she had been promoted to Kapteeni and commanded a Radar Station herself.
Her initial training course lasted eight weeks; the first half in Helsinki and the second in the large Radar Station that had just been constructed outside Kotka. In Helsinki, the recruits were accommodated in a Girls Boarding School which had been taken over by the military for Lotta Signals training. Of course, they met many other women undergoing Signals training and were simply accepted as a different unit within the Signals Corps and the other girls were, on the whole, were not curious about what they actually did. Like almost all Finnish girls who had attended school through the last half of the 1930’s, Siiri had undergone Military Cadet training and was more than familiar with the military. Unlike other militaries of the era, the Finnish forces wasted very little time on training in the things every recruit was expected to know. Instead, after a day of kit issue, Siiri started her Signals training with a full day lecture from Major-General Aarne Somersalo, the Commander of the Ilmavoimat and Eversti Richard Lorentz, the head of Fighter Command. “The fact that Kenraaliluutnantti Somersalo and Eversti Lorentz gave us so much of their time, and explained what we were to do and our role in the Air Defence organisation in such detail impressed and motivated us all tremendously,” she wrote in her post-war autobiographical book, “One Woman’s War” by Siiri Vasström (Gummerus, 1954).
After the first two days, training began in earnest in the actual operation and maintenance of the new Radar equipment. Training also included a one week familiarization tour of the new Air Defence Operations Centre at Mikkeli, the heart of Finnish air defence as well as a week at an actual operating Radar Station. Much discussion took place among the girls as to whether it was better to be posted to a station or to Mikkeli - not that they were given a choice! Those working in Mikkeli had an overall picture of what was happening throughout the country, whilst those at a station knew only what was showing up on their own screen. “Mikkeli girls”, by working fairly closely with Military Headquarters, were often privy to secret information about special operations, major battles, the detection of Soviet naval ships and submarines, aircraft losses, etc. Such information, being totally secret, only reached the stations long afterwards, if ever. Conversely, many station operators (but not those on the islands in the Gulf of Finland) worked close to Finland’s major cities and also had all the city amenities to draw upon during their hours of leave whilst the “Mikkeli girls” lived in an isolated outpost in an unspoiled environment, with only the forest and military headquarters on their doorstep.
Siiri Vasström was always a “station girl” and her favourite, and the station where she spent much of her time over the course of the Winter War, was on the island fortress of Suursaari. To reach it, she had to travel by train to Kotka and and then travel on a fast Patrol Boat to Suursaari. “The boys on the Patrol Boats used to like giving us girls a thrill and they would ask us up onto the bridge and then make the run at full speed, which was tremendously exciting.” At Suursaari, which was securely guarded, military passes were checked and they were driven to the Central Radar Station, which was carefully concealed and mostly underground. “It had been very rapidly built in 1939 and it was brand new and to start with, all you could smell was new paint and oil. But there was always a good clean wind of the sea and after a while you didn’t notice the paint smell at all.” On arrival at the Station the girls were met by the station commander and with him, the OC Lottas, who was not much older than the girls in her charge. They were taken to their quarters - which were shielded by a standard split pole and wire fence – which the girls promptly called the “chastity fence”! A short distance from the camp and looking rather like a large garden shed was a wood and stone hut. This was the entrance to the underground Central Radar Station which monitored the airspace around Suursaari, and at the time Siiri arrived it was equipped with one of the brand new Nokia NR02/39’s. Each of the Artillery Batteries was also equipped with its own Fire Control Radar which was shorter ranged and tracked ships rather than aircraft, and they were also assigned Lotta teams to run their Radar.
The Suursaari Central Radar Station was manned by about forty personnel, all but four of them Lotta’s (the Fire Control Radar Stations had a complement of about twenty, the difference being the rather larger complement required for the Central Radar Station’s Filter Room). The few men shared the simple recreation and eating quarters with the women and, after war broke out, also shared the Spartan underground accommodation quarters as the time taken to get from the Lotta Camp to the various Radar Stations, especially at night and in winter weather, could prove critical when all personnel were required.
Plotting tracks in the Suursaari Operations Room
At the Suursaari Central Radar Station, the radar girls went on shift in teams of four, each shift lasting four hours. While one girl watched the screen, two had various tasks to perform, including marking all the air and surface plots on a map of their area while the fourth telephoned the plots through to “Mikkeli” (there was a secure undersea cable with a radio backup link). After or before night shifts, the operators slept in a room adjoining the Plotter room. In case of need, there was always one male technical staff on duty and the one entrance to the radar station was guarded day and night by a heavily armed Infantry Section from the garrison (as was each of the Fire Control Radar Stations). No one without the proper pass and password could get into the bunker and as all personnel were known by sight to the guard sections, security was always good.
Siiri was assigned to Suursaari from mid-September 1939 and found there was little in the way of recreation. “We had time off but there wasn’t much we could do. By then Suursaari was heavily garrisoned and more artillery positions and defensive positions were being prepared all the time. I think when I got there they were just finishing off installing some of the old Battleship guns they bought from the French which gave the Russians a horrible surprise when they tried to attack us. To us girls, it seemed like the island was bristling with all sorts of artillery and AA guns and there were large areas we couldn’t walk into because of all the barbed wire and landmines. There were even tunnels dug into the rocks along the shoreline for torpedo boats to hide in until the last minute and the Ilmavoimat had cut an airstrip with underground hangers dug into the rock to hide aircraft in. The whole airstrip was camouflaged by these large nets and it could be opened up and hidden in minutes. The Maavoimat boys used to call Suursaari their “unsinkable battleship.”
When we had time off the infantry boys always liked to see us and they used to joke around and flirt with us. And of course we always got invitations to dine in at the Officers Messes, which we always enjoyed. There was a lot of rivalry between the Maavoimat, Ilmavoimat and Merivoimat Officers to invite us to their messes. Us girls all enjoyed ourselves tremendously. Of course, it all changed a few weeks later when the fighting started and so many of our friends died or were injured in the fighting. But that just made all of us even more determined to do our jobs the best we could and support our boys. And we did, we were never offline for a moment, we knew we were the Kotkansilmissä and there was no way we were going to let our boys down.”
The visible part of one of the Suursaari Fire Control radar stations. Suursaari was a critical position, strategically located to command the entry and exit of ships from Leningrad and a chokepoint in the Gulf of Finland and so was an initial focus of the Soviet Navy in the early days of the Winter War. The strength of the Suursaari defenses resulted in disastrous losses for the attacking Soviet naval forces. A British newspaper correspondent called Suursaari “The Gibralter of the Baltic” and it was as accurate a description as any.
Siiri Vasström served on Suursaari through the entire Winter War, by the end of which she had been promoted to Yliluutnantti and was the Radar Station Commander.
Kotkansilmä - a last observation
While the early Finnish Kotkansilmä system was able to warn of enemy aircraft approaching southern Finland and the Gulf of Finland coast, once having crossed the coastline the Lotta Svärd Ilmavalonta organisation continued to provide the only means of tracking their position until mid-1943, by which time the entire country was covered by radar surveillance. Throughout the Winter War and most of the remainder of the Second World War period, the Lotta Svärd Ilmavalonta organisation continued to complement and at times replace the defensive radar system by undertaking all inland aircraft tracking and reporting functions, while the radar and radio-monitoring systems provided a predominantly coastal and southern border-oriented, long-range tracking and reporting system. In addition, the early equipping of Finnish AA Gun Batteries with radar controlled guns (achieved by mid-1940) and the use of proximity fuses had a significant impact on the effectiveness of Finnish AA Gun fire – which had already been proved highly accurate in the first nine months of the Winter War.
In combating the Soviet Air Force over the course of the Winter War, Kotkansilmissä gave the Ilmavoimat a huge advantage – while the Soviets had a small number of their own RUS-1 radars deployed experimentally, the Soviets were not even aware of the Finnish Kotkansilmä radar networks existence or of its capabilities – with consequent effects on Soviet aircraft losses. Indeed, with the late addition of the Island stations to the network, the Finns managed to cover a good part of the Gulf of Finland as well as the Karelian Isthmus in depth, and these were the two avenues over which most Soviet air attacks on Finland were launched. And for Ilmavoimat aircrew, thoughout the Winter War (and through WW2), there was nothing quite so reassuring as the sound of the cool calm voices of the young Lotta Svärd Controllers on the Radio offering instructions, relaying information on enemy aircraft and in the event that Ilmavoimat pilots were shot down or had to bail out, calling in Search and Rescue aircraft to recover them.
The Cavity Magnetron
While everyone except the Finns and the British were working on radar at a relatively leisurely pace, the Nokia team was pushing the technology as hard as they could but making only slow progress. Fortunately, while the work struggled forward, a pair of young Ph.D researchers at the University of Helsinki who were somewhat incidentally being coached and mentored for their thesis by Eric Tigerstedt in yet another of his roles, Vilho Räikkönen and Erkki Riipinen, had come up with a new invention that would help make radar more effective all up and down the line, and put Nokia well ahead of the both the USSR and Germany in radar for the rest of WW2.
Shorter wavelengths provided a number of advantages for radar technology, including finer resolution, a tighter beam, and greater immunity to noise. However, there was simply no technology available in 1939 to generate radio waves of sufficient energy at short wavelengths. Nokia, in collaboration with a small number of other interested parties in Finland, had set up a special team to investigate radar that would operate at ten centimeter wavelengths, in the microwave band. A Fenno Radio team was assigned to work on a microwave receiver, while a team from the physics department at the University of Helsinki was to work on a microwave transmitter. The University of Helsinki effort was led by a Swedish-Finn named Marcus Backström. Räikkönen and Riipinen were not at the heart of the transmitter development project. In the fall of 1939, they were simply trying to develop microwave detector circuits. To test their designs, they had to generate microwaves for their circuits to detect.
Every now and then people who are newcomers to a field make a great discovery, simply because they don't know what works and what doesn't. Räikkönen and Riipinen didn't know much about generating microwaves, so they set about learning how. There were two devices available at the time for the task. The first was the "magnetron", which was basically a classic vacuum diode with a magnetic field placed across it. The interaction between the external magnetic field and the electron flow through the tube produced microwaves. The other was the "klystron", much more recently invented by the brothers Sigurd and Russell Varian at Stanford University in California, and based on a "resonant cavity" through which streams of electrons flowed. Backström 's team believed the klystron was the solution for short-wavelength radar.
Räikkönen and Riipinen didn't want to spend a lot of time and effort generating microwaves for test purposes. They focused on the less sophisticated magnetron simply because it seemed simpler to work with. As they learned about the magnetron, however, they realized that they could combine features of the magnetron and the klystron and come up with something new. Working on a shoestring budget, the two men pieced together their new "cavity magnetron", as they called it. The core of the cavity magnetron was a thick copper cylinder, with a large central tunnel bored through it. Six smaller tunnels, or "resonant cavities", were bored around the central tunnel, and connected to the central tunnel through slots running down their length. The copper cylinder was positively charged, forming the "anode" of the tube. A metal conduit was inserted down the central tunnel. The conduit was negatively charged, forming the "cathode" of the tube. The cylinder assembly was sealed at the ends, and a magnetic field placed across it.
Under the combined influence of the electrical potential between anode and cathode and the magnetic field, electrons circulated in the central tunnel, producing electromagnetic radiation in the resonant cavities. The electromagnetic radiation from the cavities coupled together in the central tunnel, interacting with the electron flow to efficiently extract energy from it with high efficiency. Physicists working with the device would later describe it as a kind of "whistle", where the flow of electrons generated electromagnetic waves of a specific wavelength, just as the flow of air through a whistle generates sound waves of a specific wavelength. The frequency of a whistle is dependent on its size, with a big whistle generating a low sound and a small whistle generating a shrill one. Similarly, the frequency output of the cavity magnetron was dependent on the size of the cavities. The cavities had a diameter of 1.2 centimeters (a little under a half inch), confining the electromagnetic radiation to produce "standing waves" at 9.1 cm (3,300 MHz or 3.3 gigahertz / GHz). A tap was bored through the side of the cylinder to provide an outlet for the microwave energy generated inside. Räikkönen and Riipinen performed the first microwave transmission using their cavity magnetron system on 21 February 1940. Within a few days, they were lighting up fluorescent tubes from some distance away, which indicated power output on the order of 500 watts. They found this unbelievable, and rechecked their figures and experimental setup. Nothing was wrong. The cavity magnetron was an entirely unexpected leap forward in microwave technology. The cavity magnetron was so promising that Backström's group abandoned their work on the klystron to work with the new device and Tigerstedt himself joined the team to work on the transmitter. Through the spring of 1940, they continually improved their crude microwave transmitter into something resembling an operational system, with a maximum output power of 15 kilowatts, three orders of magnitude greater than the output power available with any other device. The cavity magnetron opened doors to new technological possibilities. Nokia Radar received its first cavity magnetrons on 19 July 1940. Schonland’s team quickly put together a microwave radar system operating at 9.1 centimeters, though the technology was referred to as "10 centimeter" for convenience, and tracked an aircraft with it on 12 August 1940. The next day, the radar tracked a technician riding a bicycle carrying a tin sheet. Ground "clutter" would have simply blinded any long-wavelength radar under such circumstances. Microwave radar had arrived, not quite in time for the Winter War but certainly in time for Nokia to make some giant steps ahead prior to early 1944 when Finland reentered WW2.
It should be no great surprise, given the fact that radar itself arose simultaneously in several countries, that several other nations discovered the cavity magnetron at about the same time. Two Soviet engineers, N.F. Alekseev and D.D. Malairov, developed the technology in the late 1930s and actually published a description of it in a public technical journal in 1940. The fact that it wasn't kept a secret indicates the importance, or lack of it, assigned to it, all the more so because the Soviets took secrecy to an extreme. Soviet radar design was badly hampered by bureaucratic indifference and incompetence, plus the fact that a number of first-class engineers were arbitrarily purged and sent off to forced labor camps, from which many never returned. The USSR didn't exploit the cavity magnetron until after the Western Allies had put it into extensive use, and Soviet radar designs lagged badly. They received radar gear from Britain and the US, building copies when it seemed like a good idea, as they did with GL Mark II. Postwar Soviet radar work was almost all based on technology provided by the Western Allies.
Magnetron technology was also invented roughly in parallel in Switzerland, France, and Japan. In a further irony, the Japanese design was based on a device built in the mid-1930s by an American engineer, Arthur L. Samuel of Bell Labs that he never got to work very well. Samuel did make a major contribution to early Allied longwave radar work by designing a high-frequency triode vacuum tube known as a "doorknob" for its appearance. It appears that Räikkönen and Riipinen, whose naivete in electronics was obvious, knew little or nothing of any efforts similar to their own being carried out outside of Finland. Ironically, both the Americans and the Germans had worked hard before the war to come up with a device to generate high power, short wavelength radio signals and missed the cavity magnetron, while the Finns and the British basically stumbled onto it by accident at almost the same time and both ran with it.
The cavity magnetron would be at the heart of the revolutionary new Nokia 10cm Radars, the NK08/42 which were manufactured specifically for the Finnish Navy. Their small parabolic aerials projected a very narrow radar beam which was ideal for detecting S-Boats, Submarine conning towers and sometimes even their periscopes – and they were largely instrumental in ensuring that the Finnish Navy completely dominated the Baltic Sea. Even following the end of the Winter War, the Finnish Navy refused to lift the blockade of Leningrad and maintained complete control over the Baltic north of the line Osel Island – Stockholm. Both the USSR and Germany were advised that any German or Soviet vessel identified in Finnish-controlled waters would be sunk on sight and Swedish naval vessels were permitted only with advance notification. And indeed, a number of both Soviet and German warships and submarines were sunk, almost all after the start of Barbarossa. Soviet submarines attempting to leave Leningrad for the southern Baltic were sunk and, as the Germans advanced north, German warships supporting the invasion of Estonia that transgressed the line and did not turn back were also sunk, resulting in a distinct cooling in the German attitude to Finland.
And aside from the politics, one of the factors that was uppermost in Finnish naval thinking at this time was of course the effectiveness of their NK08/42 Radar fitted on their destroyers, corvettes and the Patrol Boats that maintained Finnish dominance of the northern Baltic through the war years.
As a note of interest, today almost every home has one of these secret magnetrons - inside the microwave oven!
http://www.youtube.com/watch?v=4h1ESUz2H3E
Next: Radar for AA Guns and for the Merivoimat
Early in 1939, with the first production air and surface surveillance sets having been completed, the Nokia R&D Team would resume work on a mobile radar system for use with AA Guns, with a second version designed and developed for Naval Fire Control.
Mobile AA Gun radar was trialed in mid 1939 and volume production started in the first quarter of 1940 after teething problems had been worked through. The system was installed on a four-wheel trailer with a theoretical distance limit of about 25 km. The angular resolution was 0.5 degrees and error in distance 25 meters.
By the spring thaw of 1940, all Finnish Navy surface warships had also been equipped with a naval version of “Kotkansilmissä”.
We will look at both of these in the next Post.