Earlier, we had briefly mentioned Tigerstedt’s friendship with the British scientist and inventor, John Baird, as well as their mutual interest in early infrared technology (“Noctovision”) and Tigerstedt’s participation in one or two of Baird’s experiments. At this stage it is also perhaps worth mentioning a book written in 1931 called “Television: Today and Tomorrow” (by Sydney A Moseley and H J Barton Chapple with a foreword by John L Baird, published by Sir Isaac Pitman and Sons, London Second Edition 1931). The authors of the book describe Baird's demonstration on Box Hill and were clearly excited by the naval and military possibilities. They foresaw this infrared technology as providing a capability for night and fog-bound vision that was, in reality, only achieved by Radar during World War II.
We also know that on having joined Nokia Radio and with R&R funding having been made available, after his initial work on the Suuritehotaisteluvalonheitin device and on the Portable Combat Radio, Tigerstedt again visited the UK, where he spent considerable time with Baird over a two month period. Almost nothing beyond a few handwritten notes remains as evidence of what the two men worked on or discussed, but the little we we do know suggest that Tigerstedt and Baird focused primarily on radio wave detewction systems and on applications for infrared technologies. Unfortunately the meetings themselves were never documented and it is now, this far removed in time, unclear as to who contributed what ideas and to whom – but what we do know is that after his return, Tigerstedt began what was almost literally a frenzy of work on two different projects – one of which would become the Nokia Radio-wave Detection System, while the other would eventually become known as the “Verenimijä Project.” The Maavoimat R&D Oversight Committee proved receptive to both ideas and by late-1937 had agreed to provide Tigerstedt with funding – his earlier successes which were now beginning to come to fruition almost guaranteed him support for any new proposal with military applications.
The Nokia Radio-Wave Detection System
In the period between 1934–1939, nine nations developed, independently and mostly in secret, radio detection systems that used the reflections of pulsed radio signals from metal objects to determine the range and distance of said objects. These nine countries were the United States, Great Britain, Germany, the USSR, Japan, the Netherlands, France, Italy and Finland. In addition, Great Britain had shared their basic information with four Commonwealth countries: Australia, Canada, New Zealand, and South Africa, and these countries as a result also developed indigenous radar systems. During the war, Hungary was added to this list. Over the course of WW2, France and the Netherlands were removed from the equation early on, Italy somewhat later and towards the end of the war Hungary was in ruins – but by the end of hostilities, the United States, Great Britain, Germany, the USSR, Japan and Finland had a wide diversity of land- and sea-based radars as well as small airborne systems.
Finland was the only small country out of those with such capability at the start of the war which both survived and continued to develop its own indigenous capabilities. And in this, Eric Tigerstedt was one of the keys to Finland’s success – and part of thre reason for Tigerstedt’s success was his ability to gain a rapid start through leveraging his personal contacts within the scientific community in the UK, Germany and the Netherlands. As we had previously mentioned, Tigerstedt had long had a good working relationship with the Philips subsidiary in Finland, Fenno Radio. As a result of this and his visits to Philips in the Netherlands (made as he began work for Fenno on radio designs), he was aware of the early research that Philips had been conducting in the Netherlands on radio-based detection systems (as well as earlier research and work on similar systems) and his personal contacts with Baird in the UK gave him some “inside information” on the UK’s Chain Home project.
As has been mentioned a number of times, Finland had strong engineering links with Germany, many Finnish engineers (including Tigerstedt) had studied in Germany and Finnish Radio Manufacturing companies had both formal and informal links to German companies such as Telefunken and Lorenz. Also, radar research in Germany in late 1937 was not a particularly high priority and it was a fairly straightforward process foir Tigerstedt to use his contacts in Germany with Telefunken and Lorenz to gain privileged information on the German research that was going on. It was thus not surprising that in early 1938, Tigerstedt, with his contacts in these countries, was able to quickly bring himself up to date on the progress that had been made in work on radio detection systems and incorporate this into his own program, leapfrogging Nokia (and Finland) into a position more or less on a par with the then leaders in the field. Tigerstedt wrote up his findings on the current state on radio detection systems and the possibilities for its used in an internal document for Nokia Radio R&D entitled “How far recent advances in scientific and technical knowledge can be used to strengthen the present methods of defence against hostile aircraft” which, despite its title, also looked at how these methods could be used for naval gunnery, both from ships and for the coastal defence batteries.
Funding was allocated to Nokia almost immediately for a Project to build a trial radar system. At this stage, one must keep in mind that given the way that electronic warfare characterizes today’s battlefields, it is sometimes difficult to remember that scientists only discovered the existence of radio waves a little over one hundred years ago. The history of radar actually starts with experiments by Heinrich Hertz in the late 19th century that showed that radio waves were reflected by metallic objects. And by 1904 a young German engineer named Christian Hulsmeyer claimed his patented “telemobiloscope” could transmit radio waves and receive their reflections of passing objects.
Christian Hulsmeyer’s “telemobiloscope”
….and Christian Hulsmeyer,
He suggested that such a device could prevent collisions at sea or aid navigation. He gave many public demonstrations in Germany and the Netherlands of the use of radio echoes to detect ships so that collisions could be avoided. His device consisted of a simple spark gap used to generate a signal that was aimed using a dipole antenna with a cylindrical parabolic reflector. When a signal reflected from a ship was picked up by a similar antenna attached to the separate coherer receiver, a bell sounded. During bad weather or fog, the device would be periodically "spun" to check for nearby ships. The apparatus detected presence of ships up to 3 km, and Huelsmeyer planned to extend its capability to 10 km. It did not provide range (distance) information, only warning of a nearby object. He patented the device, called the telemobiloscope, but due to lack of interest by the naval authorities and by shipping companies at the time the invention was not put into production. Huelsmeyer also received a patent amendment for estimating the range to the ship. Using a vertical scan of the horizon with the telemobiloscope mounted on a tower, the operator would find the angle at which the return was the most intense and deduce, by simple triangulation, the approximate distance. This is in contrast to the later development of pulsed radar, which determines distance directly.
Representatives of shipping companies flocked to various demonstrations in Germany and the Netherlands and were impressed that the device could detect objects up to a range of approximately five kilometers. But there were no buyers. At the time, investment capital was scarce in the maritime industry, wireless telegraphy already offered a means of communication between ships (as well as a form of direction finding by taking cross bearings on shore stations) and the legal and technical relationships between this use of radio waves and the patented wireless telegraphy of the Marconi monopoly was unclear to shipowners. That is, budgets were tight, other devices that seemed to fulfill present needs already existed and it took real imagination to sort out the nature and possibilities of something so new. The key to the timing that turns a discovery or invention into a successful innovation usually lies in whether laymen can envision its possibilities. And in this case, it would take more than the promise of safer navigation at sea.
Although radio became a significant feature of WW1, the idea that later became radar languished in obscurity. Radio waves form a portion of the same electromagnetic spectrum as visible light and travel at the same speed. The difference lies in the lower frequencies and thus longer wavelengths of radio waves. Like light, they reflect well of most metallic surfaces, but longer wavelengths allow radio to penetrate fog, clouds and many solid objects. In contrast to light, they are also partially trapped within the multi-layered canopy around the earth known as the ionosphere. This initially attracted researchers and inventors to the low frequency end of the spectrum, where they could bounce waves of thousands to hundreds of meters long off the canopy for long-range communications. Into the period of WW1, sparking devices were used to generate such waves (this incidentally accounts for the way the German word for spark, Funk, is still embedded in many terms dealing with radio communications such as Rundfunk for broadcast radio and the corporate name, Telefunken).
Such devices offered an on-off form of communication and were well-suited to morse code at sea and on land as a backup to wireless telegraphy, or even in airships with multiple crew members where someone could operate the wireless telegraph. The increasing importance of single-seater aircraft encouraged the development of electron tubes that amplified signal strength and made wireless voice telephony possible. These “thermionic valves” as the British called them, offered stable continuous frequencies, as well as ways of modulating a sound wave onto a carrier wave. The advantages of voice communication that would keep both hands of a pilot free were obvious. The naval requirement for radio voice communications was less pressing, even where wireless was useful, such as in the case of convoys or submarines.
In general, the Germans made more extensive use of radio on both sea and land than the British in WW1. They believed that it offered a way to transmit commands simultaneously to as many units as had receivers without the bottlenecks and delays caused by telegraph relays or telegraph switching stations. They relied on encryption rather than radio silence to mask their communications. What they did not realize was that the British were particularly effective in direction finding and other signals intelligence techniques, as demonstrated by their success in deducing the movement of the German High Seas Fleet out of its ports – in May 1916, a British advantage that resulted in the Battle of Jutland and almost in the destruction of the German Fleet. On the other hand, the Germans effectively confused the tactical situation by transferring the call sign of their flagship to shore before sailing. The basic electronic warfare pattern of measure, countermeasure, and counter-counter-measure was already becoming apparent. The Germans actually had the opportunity to build a radar capability at this time, but when the German engineer Hans Dominik shared the reports of his tests of a Stahlenzieler (“ray-aimer”) with the Imperial Navy in February 1916, the Navy responded that the device still needed six months development and therefore would not be useful in the war.
The engineer and scientist Hans Dominik joined the "Department of Lighting and Power" at Siemens & Halske in 1900. Following a comprehensive project on electrification in mining for the Paris World Exhibition, he was made manager of the Literary Department (public relations), a post he held for around a year. From 1917 Dominik worked as an engineer in the area of telegraphy. He was also a writer, and from 1922 published a series of technological utopian novels. The most important works of this author, who was also hailed as the "German Jules Verne", include the novels "Die Macht der Drei" (1922), "Die Spur des Dschingis-Khan" (1923), "Der Brand der Cheopspyramide" (1926), "Befehl aus dem Dunkel" (1933), "Der Wettflug der Nationen" (1934), "Atomgewicht 500" (1935), "Himmelskraft" (1937), "Treibstoff SR" (1940) and his memoires "Vom Schraubstock zum Schreibtisch" (1942).
When the Great War ended, radio development continued apace. Growing commercial possibilities tugged developments along, despite the economic ups and downs of the postwar era. A wide range of manufacturers, products and techniques for continuous wave propagation emerged. By 1930 the American technical lead in the immediate post-war years had begun to dissipate, but the Americans and British were ahead of the French, Italians and Germans in market development. When the Nazi’s came to power in 1933, they mass-produced simple and inexpensive sets known as the Volksempfanger (“peoples receiver”) with which to hear the voices of Hitler and Goebbels. Worldwide, radio broadcasting and reception moved from the phase of innovation to that of diffusion.
The Volksempfanger (“peoples receiver”) – an example of the diffusion of new technology. Radar would follow the same process, moving from an innovation at the start of WW2, to widespread use by the end of the war.
And now, before we move on to look at the Finnish Radio-Detection System Project, it’s more than likely a good idea to describe the basics of Radar itself. And as Radar is basically an evolution of a radio system, it’s probably useful to define the basic elements of a radio system first. A radio system consists of a "transmitter" that produces radio waves and one or more "receivers" that pick them up, with both transmitter and receiver(s) fitted with antennas. The very earliest "wireless telegraphy" radio systems used a transmitter that simply generated a burst of radio energy by opening an electric circuit containing an inductive coil with a telegraph key, causing a spark. The radio waves propagated through space and set up an electric current in a receiving antenna, which in turn closed a relay switch. Messages were sent using Morse code. The problem with this simple scheme was that the transmitter generated waves over a wide and indiscriminate range of frequencies, with a single receiver picking up and mixing up transmissions from every transmitter in line of sight. This problem was solved by fitting each transmitter with a "variable oscillator" -- an electronic circuit that generated electrical signals at different frequencies, as set by a knob turned by the transmitter operator. A receiver picked up this signal with its antenna, with the signal run through a "variable filter" -- an electric circuit consisting of an inductor coil and a variable capacitor that could be set by a knob to block out all frequencies except one.
This scheme allowed multiple transmitters to operate in a given area without mutual interference. The transmitter operator set the transmitter oscillator to a given frequency or "channel", and then used a telegraph key to gate the oscillator output on and off into an amplifier circuit, which drove a high-power signal out the antenna. The receiver operator set the receiver filter to the same channel. The receiver picked up radio waves on all frequencies and amplified them. The amplified received signal was run through the variable filter, and then into a "detector" circuit to convert high-frequency signals into a direct-current signal to activate the relay switch. The detector included a "rectifier", a one-way valve for electricity that eliminated half the waveform, with this rectified signal then passed through a "low pass filter", consisting in the simplest case of a resistor and a capacitor that smoothed the received signals into pulses.
In a simple "amplitude modulated" voice radio, the voice of a user is converted into an electrical waveform that controls or "modulates" the amplitudes or "envelope" of a variable oscillator signal. The modulated signal is then amplified and transmitted over an antenna. The oscillator frequency is known as the "carrier" frequency, since it "carries" the audio signal. A receiver picks up the signal with its antenna, using a variable filter to isolate the desired channel. The signal is then amplified and passed through a detector circuit to extract the original audio signal. The audio signal is amplified and driven to a loudspeaker. A radio channel is actually not a single frequency but range of frequencies. Although the details are beyond the scope of this post, the frequency range or "bandwidth" of a channel is roughly proportional to the amount of information carried by the channel.
Transmitter output power is measured in watts, or (as far as radar is concerned) more usually kilowatts (kW, thousands of watts) and megawatts (MW, millions of watts). Receiver "sensitivity", or the ability of the receiver to amplify received signals, is determined in terms of "decibels" while the amplification factor is commonly referred to as "gain". A radio receiver, particularly one that is built into a vehicle and is moving around, may be picking up a transmitter signal that varies in strength. That means that the volume of the radio output will tend to fade or grow continuously, requiring the listener to keep adjusting the volume control. A circuit known as an "automatic gain control (AGC)" helps correct this problem by measuring the average received power of the signal and adjusting the receiver gain to ensure that it stays as constant as possible.
A transmitter needs an antenna to send its radio signal, and a receiver needs an antenna to pick up that radio signal. The simplest form of antenna is the "dipole". Suppose the electrical output of an oscillator is directed down two conductors, not connected at the ends. This will radiate EM energy from the open-circuit ends. It radiates energy much more effectively if the conductors are bent at the ends to form a right angle, with each bend being a quarter-wavelength long relative to the oscillator output. This is a "half-wave" dipole. It is not only effective in generating radio waves at a particular frequency, it is also effective in picking them up. This is true in general of all antennas: they are "reciprocal", working much the same in transmission or reception, just in different directions. A single conductor can be used as well; this is the "monopole" antenna used in portable radio receivers and the like. By itself, a dipole or monopole antenna "broadcasts" in all radial directions evenly, or in other words it is an "omnidirectional" antenna.
It could be turned into a "directional" antenna by placing it in the center of metal parabolic dish, with a small reflector above the dipole to bounce the signal back into the dish for transmission in one direction. This configuration is familiar from the modern satellite-TV receiver, though instead of a dipole radio energy is usually just dumped into the dish through an open "horn", either fed through the dish or under the bottom of the dish. It's really very much the same as using a parabolic mirror to focus light, only the wavelength of radio signals is longer. While parabolic dishes are usually circular, creating a focused "pencil" beam, elliptical or cylindrical dishes with parabolic curvature can also be used if the radio beam needs to be focused along one axis but not along the other, or in other words has a "fan" configuration. The beam width is normally defined by a "3 dB" law, with the boundary of the beam defined as the surface where the power of the beam at the center falls by 3 dB. Another simple way to create a "directional" antenna with a dipole is to mount it within a row of parallel conductive rods, with the rods of decreasing length to the "front" of the dipole (relative to the direction of focus) and of increasing length to the "back" of the dipole. This type of antenna is known as a "Yagi-Uda" or just "Yagi" antenna. Such antennas are referred to as "end-fire antennas", since they are directional along their long axis. Dipole antennas, in contrast, are directional at a right angle to their plane.
A more sophisticated approach is to obtain a directed focus by using an antenna with multiple dipoles in a grid arrangement, with the focus obtained by interference effects. Such "dipole arrays" were common with early longwave radars. Arrays can be made with end-fire antennas as well, and very significantly as slotted plates, with radio energy fed through the slots. The slots act as dipoles, though while the polarization of a radio wave generated by a dipole is in line with the long axis of the dipole, it's at a right angle to the long axis of the slot. Slotted planar arrays are very popular these days, since shrewd slot arrangements allow them to be much more efficient than simple parabolic antennas, which will waste about two-thirds of the energy pumped into them. A well-designed slotted planar array will waste less than a third of the energy. Directional antennas are characterized by a factor known as "antenna gain". This is simply the ratio of the focused beam power to the same broadcast power sent through an omnidirectional antenna. For example, if the focused beam has 50 times the power of an omnidirectional antenna with the same transmitter power, the directional antenna has a gain of 50, or 17 dB.
The larger the receiver dish, the greater the receiver sensitivity, since it creates a bigger "bucket" or "eye" to collect radio waves. However, the longer the wavelength, the bigger the dish has to be to focus the radio waves, and conversely the more focused the beam, the bigger the antenna. Another minor related fact is that the dish doesn't have to be solid. It can be a mesh, just as long as the mesh grid spacing is less than that of the radar operating wavelengths. This makes for a lighter antenna, and also one not so easily disturbed by the wind. Directional antennas don't always generate all their radio output in a nice neat directional beam. Interference between transmit signals may generate "sidelobes" that cause unwanted transmissions to the sides of the beam, or a "backlobe" in the reverse direction. The sidelobes and backlobe can rob the main lobe of energy and of course corrupt the directionality of the beam, generating and receiving signals in unwanted directions. Proper antenna design minimizes the power lost by sidelobes and backlobes.
Antenna Propagation Pattern: Sidelobes are a particular nuisance in radars, since they can produce false returns and can pick up radio interference, including deliberate interference produced by countermeasures systems. In radar systems, the ratio of sidelobe to main beam power is generally kept to less than -40 dB, or 1:10,000. This is done in arrays by carefully arranging the power levels of the array elements, with more power in the center elements than at the elements along the edge. There are a number of "aperture tapering" schemes to define the proper arrangements of power levels
A Simple Pulse Radar System
The best way to explain radar is to imagine standing on one side of a canyon, and shouting in the direction of the distant wall of the canyon. After a few moments, an echo will come back. The length of time it takes an echo to come back is directly related to how far away the distant canyon wall is. Double the distance, and the length of time doubles as well. Given that the speed of sound is about 1,200 Kph (745 Mph) at sea level, then timing the echo with a stopwatch will give the distance to the remote canyon wall. If it takes four seconds for the echo to come back, then since sound travels about 330 meters (1,080 feet) in a second, the distance is about 660 meters (2,160 feet).
Radar uses exactly the same principle, but it times echoes of radio or microwave pulses and not sound. Like a wireless telegraphy set, a simple radar has a transmitter and a receiver, with the transmitter sending out pulses, short bursts, of EM radiation and the receiver picking them up. In the case of the radar, the receiver is picking up echoes from a distant target, with the echoes timed to determine the distance to the target. Early radars simply used an oscilloscope to perform the timing, with the detected return signal fed into the oscilloscope as a "video" signal, and showing up as a peak or "blip" on the display. An oscilloscope measures an electrical signal on an electronic beam that moves or "sweeps" from one side of a display to the other at a certain rate. The rate is determined by a "timebase" circuit in the oscilloscope. For example, the sweep rate might push the sweep from one side of the display to the other in a millisecond (thousandth of a second). If the display were marked into ten intervals, that would mean the sweep would pass through each interval in 0.1 milliseconds. While this would be shorter than the human eye could follow, the sweep is normally generated repeatedly, allowing the eye to see it.
Since EM radiation propagates at 300,000,000 meters per second, or 300,000 meters per millisecond, then each 0.1 millisecond interval would correspond to 30,000 meters, or 30 kilometers (18.6 miles). If the sweep on the scope is "triggered" to start when the radar transmitter sends out the radio pulse, and the sweep displays a blip on the sixth interval on the display, then the pulse has traveled a total of 180 kilometers (112 miles). Since this is the round-trip distance for the pulse, that means that the target is 90 kilometers (56 miles) away. The trigger signal provides synchronization, so it can be regarded as a type of "synch" or "sync" signal. The sweep is called a "range sweep" and the output of the display is called a "range trace".
The amplitude of the return also gives some indication of the size of the target, though the relation between return amplitude and target size is not straightforward, as discussed later. It would also be nice to know what the direction to the target is, in terms of its "altitude (vertical direction)" and "azimuth (left to right direction)". This is a bit trickier to describe, but no more complicated in the end. Some early radars, like the famous British "Chain Home" sets that helped win the Battle of Britain, simply transmitted radio waves from high towers in a flood over their field of view, and used a directional receiver antenna to determine the direction of the echo. Chain Home actually used a scheme where the power of the echo was compared at separated receiver antennas to give the direction, which astoundingly actually worked reasonably well. Other such "floodlight" radars used directional receiver antennas that could be steered to identify the direction of the echo. Incidentally, a radar that uses receive and transmit antennas sited in different locations is known as a "bistatic", or in the more general case "multistatic", radar.
Floodlight radars were quickly abandoned. They spread their radio energy over a wide area, meaning that any echo was faint and so range was limited. The next step was to make a radar with a steerable transmitter antenna. For example, two directional antennas, one for the transmitter and the other for the receiver, could be ganged together on a steerable mount and pointed like a searchlight, an arrangement that is sometimes called "quasi-monostatic". The transmitter antenna generated a narrow beam, and if the beam hit a target, an echo would be picked up by the receiving antenna on the same mount. The direction of the antennas naturally gave the direction to the target, at least to an accuracy limited by the width in degrees of the beam, while the distance to the target was given by the trace on the A-scope.
Of course, it was realized early on that it would be more economical and less physically cumbersome to use one antenna for both transmit and receive instead of separate antennas; it was possible to do so in theory because a radar transmits a pulse and then waits for an echo, meaning it doesn't transmit and receive at the same time. The problem in practice was that the receiver was designed to listen for a faint echo, while the transmitter was designed to send out a powerful pulse. If the receiver was directly linked to the transmitter when a pulse was sent out, the transmit pulse would fry the receiver.
The solution to this problem was the "duplexer", a circuit element that protected the receiver, effectively becoming an open connection while the transmit pulse was being sent, and then closing again immediately afterward so that the receiver could pick up the echo. This was done with certain types of gas-filled tubes, with the output pulse ionizing the gas and making the tube nonconductive, and the tube recovering quickly after the end of the pulse. More sophisticated duplexer schemes would be developed later. The receiver was also generally fitted with a "limiter" circuit that blocked out any signals above a certain power level. This prevented, say, transmissions from another nearby radar from destroying the receiver. After this evolution of steps, the result is a simple, workable radar. It has a single, steerable antenna that can be pointed like a searchlight. The antenna repeatedly sends out a radio pulse and picks up any echoes reflected from a target. An A-scope display gives the interval from the time the pulse is sent out and the time the echo is received, allowing the operator to determine the distance to the target.
A simple Pulse Radar System
The transmitter emits pulses on a regular interval, typically a few dozen or a few hundred times a second, with the A scope trace triggered each time the transmitter sends out a pulse to display the receiver output. The number of pulses sent out each second is known as the "pulse repetition rate" or more generally as the "pulse repetition frequency (PRF)", measured in hertz. The width of a radar pulse is an important but tricky consideration. The longer the pulse, the more energy sent out, improving sensitivity and increasing range. Unfortunately, the longer the pulse, the harder it is to precisely estimate range. For example, a pulse that last 2 microseconds is 600 meters (2,000 feet) long, and in that case there is no real way to determine the range to an accuracy of better than 600 meters, and there is also no way to track a target that is closer than 600 meters. In addition, a long pulse makes it hard to pick out two targets that are close together, since they show up as a single echo.
PRF is another tricky consideration. The higher the PRF, the more energy is pumped out, again improving sensitivity and range. The problem is that with a simple radar it makes no sense to send out pulses at a rate faster than echoes come back, since if the radar sends a pulse and then gets back an echo from an earlier pulse, the operator is likely to be confused by the "ghost echo". This is usually not too much of a problem, since a little quick calculation shows that even a PRF of 1,000 gives enough time to get an echo back from 150 kilometers (95 miles) away before the next pulse goes out. However, as mentioned propagation of radar waves can be freakishly affected by atmospheric conditions that create ducting or other unusual phenomena, and sometimes radars can get back echoes from well beyond their design range.
This can be confusing, because a pulse will be sent out and a return will be received very quickly, indicating that the target is close. In reality, the target is distant and the return is from the previous pulse. This is called a "second time around" return. Given a PRF of 1,000, then a target 210 kilometers (130 miles) away will appear to be only 60 kilometers (37 miles) away. Similar confusions could be caused by returns that arrive from long ranges after more than one additional pulse, resulting in "multiple time around" returns. Of course, a simple pulse radar also has "blind ranges" or "blind zones": if our example radar is trying to spot a target exactly 150, 300, or 450 kilometers away, the return will arrive when the next pulse is being sent out and the radar will never spot it.
To deal with such "range ambiguities", radars were designed so they could be switched between different PRFs. Switching from one PRF to another would not affect a "first time around" echo, since the delay from pulse output to pulse reception would remain the same, but the switch would make a ghost return from a current pulse jump on the display. Suppose our radar could be switched from a PRF of 1,000 Hz to 1,250 Hz, and is trying to track a target 210 kilometers away. At 1,000 Hz, the maximum range is 150 kilometers and the target appears to be 60 kilometers away, but at 1,250 Hz the maximum range is 120 kilometers (75 miles) and the target return jumps to a perceived range of 90 kilometers (56 miles). The fact that the target range jumps when PRF is changed reveals the range ambiguity; adding perceived range to the maximum range for each PRF setting gives the actual range.
The Radar Range Equation
The information provided so far give enough information for understanding the "radar range equation", the most fundamental formula for radar operation. As its name implies, it gives the possible maximum range of a radar, as determined by the following factors:
• Electrical noise. This is a function of environmental noise, which tends to be unpredictable, and the noise inherent in the electronic systems of the receiver. A radar pulse echo return must be above the noise threshold for a target to be detected.
• Transmitter power. As mentioned, this is a function of pulse power and PRF, as well as antenna gain.
• Receiver gain. This is a function of the receiver antenna gain and the sensitivity of the receiver electronics.
• Attenuation due to range. The power of a radar beam will fall off with the square of distance. Since the radar must pick up the return echo of the transmit pulse, which also falls off by the square of distance, that means that the strength of a return pulse falls off by the fourth power of the distance to the target.
• Target "radar cross section (RCS)". The RCS of a target is effectively its reflectivity to radar. RCS varies with the material being illuminated, for example metal surfaces tend to be more reflective than plastic surfaces, and with the physical configuration of the surfaces. A smooth surface tends to be less reflective than a jagged rough surface. The RCS of a target tends to be highly variable, depending on the viewing angle of the target. An aircraft that is very bright to radar from one angle may be almost invisible from another, and its radar return may change drastically as it flies around.
• Atmospheric attenuation. This is the trickiest of all the factors to estimate, since it can vary wildly given different atmospheric conditions. It is usually just given as a flat constant, since it is hard to do much better in practice.
This gives a simplified version of the radar range equation:
power * gain * RCS
----------------------- > noise
attenuation * range^4
There are many variations of this equation, usually providing greater detail or modified to demonstrate the capabilities of different radar configurations. The basic idea is simple: the capability of a radar to detect a target is directly proportional to its transmit power, its receiver gain, and the RCS of a target; and inversely proportional to the atmospheric attenuation and the fourth power of the range.
Early Radar Technology
Search Radars, PPI & Height Finders
The sort of simple pulse radar system described above was more or less what was available at the beginning of World War II, and was used on ground sites and on ships. WW2 led to improved radar technologies and an explosion of radar applications and types. One of the early improvements was to build a radar that could automatically sweep around the sky to search for intruders. The early floodlight systems could cover a wide sector of the sky, but as mentioned they were inefficient. A simple steerable radar with an A-scope display was more efficient, but it had to be manually steered to find a target. Building an improved radar that could be swept around 360 degrees was a bit tricky, since it implied that the electrical connection between the antenna system and its associated electronics had to freely rotate, and designing reliable "rotary couplers" was troublesome.
It also implied a different type of display, the "plan position indicator (PPI)", also known as the "polar plot indicator". The PPI is a circular display, with a sweep rotating around the center in sync with the transmitter antenna, and the return for a particular angle displayed along the display sweep. As the sweep rotates around the center of the display, it paints an image of what the radar "sees" all around it. The display uses has a "long-persistence phosphor" that allows the image to linger after the sweep has passed, fading away just before the sweep comes around to refresh the image. A PPI display can be thought of as something like an A-scope being spun around in a circle, with a single A-scope trace on each radius of the circle.
The PPI is the popular concept of a radar display, commonly seen in TV shows in which a mysterious or dangerous intruder is moving closer to the center of the display, where the heroes are, with every sweep. (In some shows, they use a PPI even when the radar doesn't have a rotating antenna.) In the early days, the radar did little processing on the return echoes, and so it was up to the operator staring at the PPI to figure out what the display actually said. It wasn't necessarily the case that there was a simple bright blip where the intruder was; there would often be sources of "clutter" in the radar sweep, such as flights of birds, swarms of insects, and other obstructions to the radar beam. Incidentally, the time it takes for the antenna to rotate 360 degrees and for the sweep to correspondingly move all the way around the display is referred to as the "update rate". One of the classic examples of such a radar was the US Navy "SG" shipboard radar, which was a 3 GHz / 10 centimeter system with a horizontal parabolic antenna. It could provide a "map" of threats and obstacles around a vessel on its PPI display. One of the issues, if not necessarily a problem, with the "search radar" scheme described above is that it gives the range and azimuth to the target, but not its altitude -- it is a "two-dimensional" or "2D" radar. That was okay if the search radar was being used by a ship or a coastal site to track other ships, since their altitude was of course at sea level, but not so good if the search radar was tracking aircraft.
The search radar didn't really need to determine altitude by itself. Its major function was just to provide a warning, and to do that it was best designed to generate a long-range beam in a "fan" configuration that was very tall and thin, with the radar essentially throwing out a cylindrical "wall" of radio waves with each sweep, through which intruders must pass. Once an intruder was located, a separate, steerable "height-finder" radar could be pointed in the direction given by the search radar to determine the intruder's altitude. The height-finder radar generated a beam that was very short but wide, exactly the opposite of the search radar. The two radars effectively formed "crosshairs" that pinned down the precise coordinates of the target. Height finders were often designed to "nod" up and down to search for a target. The US Navy "Mark 22" or "Lil' Abner" radar was a classic example of such a height-finding radar, with a vertical antenna like a peel of a slice of an orange and nodding operation. Of course, over the long run improvements in radar technology allowed development of a single radar that could determine both the azimuth and altitude of an intruder. Such a radar is of course known as a "three-dimensional" or "3D" radar.
Tracking Radars, Lobe Switching, Conical & Helical Scanning
The search radar / height finder radar combination was fine for vectoring fighters against intruders, but antiaircraft guns needed a single radar that could zero precisely in on a target and track it. As mentioned earlier, a radar's accuracy is a function of the angular width of its beam. A radar beam can be thought of as something like a radar "spotlight", with a very narrow spotlight beam able to more precisely pin down the direction of a distant target than a broad one.
There was a way to get accuracy much better than the actual width of the beam. Radar antennas typically emit electromagnetic radiation in the form of a teardrop-shaped "lobe", tapered at the sides and broad at the tip. Trying to pin down a target in a single broad lobe is troublesome -- but suppose the radar transmitter actually has two antennas, toed out slightly relative to their mutual centerline, and the transmitter alternates sending pulses, sending a pulse with one and then the other consecutively. The radar operator can then steer this antenna array until the alternating returns are the same size, meaning the target is on the centerline. Since the edges of the lobes are relatively sharp, this allows relatively precise location of the target. The error signals provided by the difference in the two lobes can be used to control servo motors that guide the radar along the track of the target automatically: if the signal is stronger in one lobe than the other, the antenna is steered in the direction of the stronger lobe until the two signals balance.
This scheme is known as "lobe switching" or just "lobing" and it is a form of what is called "angle tracking". Some early anti-aircraft radars used horizontal and vertical lobe switching to target intruders. A good example was the US "SCR-268" anti-aircraft radar, which was developed alongside the SCR-270 search radar mentioned and which shared some of the same technology. The SCR-268 operated at 100 MHz / 1.5 meters. It was somewhat clumsy-looking, featuring a transmit antenna, a vertical lobing receiver antenna, and a horizontal lobing receiver antenna, all mounted together on a single gun-type mount. The transmit antenna was in the form of a 4 x 4 array of dipoles; the vertical receiver antenna was a 2 x 6 rectangular array of dipoles, mounted with its long axis vertical on the right; and horizontal receiver antenna was a 6-by-4 array of dipoles, mounted with its short axis vertical on the left. The SCR-268 had a beam width of 2 degrees in both the horizontal and vertical directions, and a maximum range of 36 kilometers (23 miles). As awkward as it looked, the SCR-268 was actually a fairly good piece of gear by the standards of the time, and would remain in first-line service for gun laying and searchlight direction late into the war, its retirement mostly being driven by the fact that the Germans figured out how to jam it.
Some shipboard radars used to direct naval guns for firing on surface targets got by with only horizontal lobe switching. Anti-aircraft radars were then refined to a more sophisticated scheme for lobe switching, known as "conical scanning". This involved a parabolic dish antenna with a radio "feed" element that was slightly offset from the centerline. The feed element was rotated at a low rate to generate pulses slightly skewed from the centerline, with the dish steered until the returns were all equal. Some of these radars also had "helical scanning", which sounds the same but was actually something different, meaning that the entire radar dish spun around in a helical pattern while it was searching for a target, something like the way a height-finder radar nodded up and down. Once the targeting radar found a target, it stopped helical scanning and used conical scanning to pin down its precise location.
Originally, anti-aircraft targeting radars simply gave aim points for anti-aircraft guns. The scheme was quickly improved so that the error signals from the tracking radar not only steered the radar antenna, they steered the gun automatically as well. Since the gun had to "lead" the target to score a hit, it couldn't point in exactly the same direction as the radar antenna, with an analog computer in the loop calculating the proper lead for the gun. The result was an improvement in lethality by an order of magnitude or more. The classic example of such a gun-laying radar was the US "SCR-584", which was a microwave set with a circular parabolic dish using helical and conical scanning. It was linked to a heavy antiaircraft gun through an analog computer system. The technology has been considerably refined since WW2, one of the prominent examples being the well-known and highly effective Soviet-Russian ZSU-23-4 "Shilka" tracked antiaircraft vehicle, with quadruple 23 millimeter automatic cannon in a turret mount, guided with speed and accuracy by an automatic radar fire-control system.
AI, ASV & AEW Radars and Radar Displays
If fighters were sent up against intruders in daylight and clear weather, ground-based radars could generally get them close enough to perform an interception by eyeball. However, there was little chance of finding an intruder visually at night or in bad weather, and so night fighters carried their own radars, allowing them to target intruders after being vectored to their vicinity by ground search radars. Night fighter or "airborne intercept (AI)" radar had limited maximum range -- and early sets had long pulse widths, giving the radars a long minimum range, meaning they had to be near a target to find it with radar and then could easily lose it while trying to close in on it. Another problem with these early night fighter radars was that they operated at long wavelengths, making them difficult to focus into a narrow beam. The problem with such a wide beam was not really limited angular accuracy; night fighter radars used lobe switching and conical scanning to obtain useful targeting precision with relatively long wavelengths, obtaining enough accuracy to find a target even with a wide beam. The major difficulty with all early pulse radars was that they had no "discrimination". If the radar pulse hit something, anything, the echo came back and showed up on the display. That meant that if a hostile aircraft was low to the ground, reflections from the terrain or "ground clutter" kept it invisible to radar. A wide beam meant that ground clutter remained a problem at relatively high altitudes, lost in the noise; the need to produce an AI with a narrow beam with an antenna that could be carried in a night fighter was one of the drivers of microwave radar.
The classic AI radar was the US "SCR-720", a 3 GHz / 10 centimeter set used in the Northrop P-61 Black Widow night fighter. The US SCR-720 remained in first-line service into the early 1950s in improved versions. Work on radars that could be carried by patrol aircraft to hunt for ships and submarines in the dark and bad weather went on in parallel with the development of AI radars. Such "air to surface vessel (ASV)" radars were useful because a ship target was big enough to be picked out of the clutter returned from the surface of the water, though unsurprisingly the clutter got worse when the weather was worse and the waves were higher. Microwave radars were also useful for ASV, since their higher resolution allowed them to better pick out targets. Systems were developed that linked into the ASV radar to automatically release bombs during low-level attacks on shipping.
Following the development of ASV radars, other radars were developed for targeting air strikes against cities and other area targets on land. These were very crude bombing aids, since they really couldn't do much more than distinguish between dry ground and bodies of water, and only worked well when the target could be identified by lakes or the confluence of rivers. Once again, microwave radars were preferred since they gave a higher resolution image. Such bombing radars used a PPI display to give a map of the terrain below. The radar had to compensate for the fact that radar echo returns farther away from the center of the display were fainter, distorting the radar "image", and so the receiver sensitivity was adjusted to be greater at greater angles. This is known as "cosecant-squared" operation, since that's the mathematical function used to determine the gain function. It was actually implemented by modifying the antenna to provide the cosecant-squared pattern, with the antenna designed with different inner and outer curvatures. The cosecant-squared configuration was also used in naval search radars, to allow the radar to pick up targets at higher altitudes while avoiding pickup of sea-surface clutter.
Late in WW2, the first "airborne early warning (AEW)" radar systems were built. Since a search radar was blocked by the horizon and suffered from surface clutter reflections, the idea was to put a radar in an aircraft that could fly at a high altitude to give it a wide view and greater freedom from ground clutter. The radar itself was less of a challenge than the issues of relaying the radar information to the ground station or aircraft carrier that was operating under the AEW "umbrella". The grandfather of AEW radars was the US "AN/APS-20" radar, which was initially deployed at the end of WW2 on a modified Grumman Avenger torpedo bomber with a bulging radome under the belly. The AN/APS-20 would actually remain in service, with improvements, into the Vietnam War era and, to a lingering extent, well beyond. Its best-known platform was the RC-121 Warning Star, a military modification of the Lockheed Super Constellation four engine piston airliner, with the AN/APS-20 in a belly radome and a height-finder radar in a dorsal radome.
IFF, Radar Beacons and Radar Reflectors
The invention of radar to track aircraft immediately led to the issue of how to distinguish "friendly" aircraft from "hostile" aircraft. If an aircraft was just a blip on a PPI scope, there was no telling from that information if it was an enemy that had to be destroyed before it was too late, or if it was an "friendly" who got lost and was now in great danger from his own side. The answer was to create a scheme, known as "identification friend or foe (IFF)" that allowed electronic identification of a target. A radar site or a fighter could have a radio system known as an "IFF interrogator" that sent a specific signal to an aircraft. The aircraft would, in turn, have a radio system called an "IFF transponder" that picked up the interrogator signal and gave a proper coded response to identify itself as friendly. Incidentally, IFF was also used on ships.
IFF is a tricky issue, since an enemy can not only use IFF to impersonate a friend, but can also trick friendly aircraft or ships into giving away their presence by interrogating IFF. This is the IFF challenge: protecting one's own IFF while trying to compromise the enemy's. Early IFF systems were actually interrogated directly by radars, but as radars evolved into a wide range of different types, that meant that an IFF transponder had to be able to respond to all the different types of radars. That not only made the IFF transponder complicated, it made it easier for an adversary to compromise the IFF system. The solution was to develop specialized interrogator systems designed to be used as an accessory on a radar, with an IFF antenna "piggybacking" in some way on the radar antenna. This scheme was embodied in the British "Mark III" IFF transponder, which became an Allied standard during the war.
The idea of having a transponder that replied to specific radar signals was a dead end for IFF but had its uses elsewhere, leading to the parallel development of "radar beacons". These beacons were just transponders that could be used to mark an airfield, or could be carried by advance parties to mark paratrooper landing zones or amphibious landing beaches. The same approach could be used to give the distance to a fixed station. The idea is conceptually very simple: an interrogator sends a radio pulse to a transponder, which then replies, and the round-trip time is determined to give the range between interrogator and transponder. The approach is very similar to radar, and in fact such a radar beacon scheme is often called a "secondary radar". The British developed a precision bombing system based on secondary radars named "Gee-H" that permitted highly accurate "blind bombing", at least for aircraft with a line of sight to a fixed base station in friendly territory.
As noted above, radar beacons were often used to mark drop zones and landing beaches. The advantage of a radar beacon was that it did not advertise its presence to the enemy, only "speaking when spoken to." There were times when that wasn't really a concern, for example to mark rocks that were to be avoided by a landing force, and a cheaper marker could be used, called a "corner reflector" or more formally a "radar signature enhancement device". This was just some panels of metal joined together in a kite-like configuration to create nice sharp corners that could reflect radio waves.
Radar Reflector: The problem with this type of corner reflector was that it was somewhat bulky and inconvenient.
Continuous Wave Radars
Simple continuous wave detectors and radars were discussed above. During the war, "proximity fuzes" were developed for anti-aircraft gun shells, allowing the shells to be triggered when they passed within a lethal radius of a target. Coupled with radar-guided automatic tracking, the proximity fuze helped boost the lethality of antiaircraft guns by a large factor. Such fuzes could be thought of as CW radars, but that's stretching the definition of the term "radar": they simply generated a continuous radio signal and at close ranges, the "near field", the presence of a target would "load down" the oscillator generating the signal and changing its frequency of oscillation, which triggered the fuze. The term "proximity detector" seems more suitable.
Anyhow, for what it’s worth, that’s the basics covered for the WW2 period and slightly beyond. With that in mind, we will next now take a quick look at the state of Radar world-wide in 1938 and 1939 (the period over which Eric Tigerstedt and the Nokia R&D Team worked to develop Finland’s early Radar Detection System.