Radar developments period 1946 – 1965
The period immediately after the war (1946 – 1947)
With the cooperation of the Navy Radio Service investigated technicians from the Physisch Laboratorium PTT provided the ‘accessible’ English and German radar systems in the first half of 1946. Particular attention was paid to the mechanical construction. A visit was also made to the radio and radar masts at the island of Voorne. Through the intervention of Von Weiler, the Navy’s laboratory received a defective 10 cm (S-band) radar set. It was examined, repaired and put into operation for tests.
In the second half of 1946, the Technical Staff of the Royal Netherlands Army requested the PTT Physics Laboratory to make two damaged AA No. 4 Mk. 3 (“GLAXO”) mobile 200 MHz radar sets of the Canadian army operational again. This radar had four Yagi antennas that had to be manually rotated with a wheel. The Plan Position Indicator (PPI) was mechanically linked to the radar. Microwave components are scarce at the time and often only available in the army dump. Despite these difficulties, the laboratory succeeded in making the two radar sets operational again.
Development of the long-range air warning radar LW-01 (1949 – ~1952)
On 7 June 1949, the Netherlands Royal Navy ordered the Navy Radio Service and the TNO Physics Laboratory in The Hague to develop in cooperation a long-range air warning radar (LW-radar) in the 25-cm/L-band. The radar had to be installed on board the Navy carrier and the planned Holland class destroyers. The prototype of the radar had to be ready in time for series production, to install the LW radar simultaneously with the other radar systems. Just at the time of the order, the L-band magnetron type 5J26 became available on the market. The magnetron was manufactured under license by multiple manufacturers.
The work on the transmitter and receiver was carried out at Oegstgeest while the antenna development took place at the Physics Laboratory. The LW-radar system had to be built from scratch because a lot of components were not available on the market yet and had to be designed and engineered at the laboratory. The LW-radar had the interest of the Air Force as well. In an early stage, the industry became involved which led to the production of the LW-radar by Philips Telecommunication Industry and Hollandse Signaalapparaten. This LW-01 radar has been the start of a very successful series of LW-radars in use with the Netherlands Navy.
The production version of the LW-01 antenna was 11 metres wide and 4.88 metres high, with a weight of 1.45 metric tons. The maximum reach of the LW-01 radar was 259,3 km. The dead zone of the radar was 700 metres. The LW-01 was intended for use with two cruisers and the Dutch aircraft carrier HNLMS Karel Doorman. The LW-02 with an antenna span of 6.5 metres was built for the new destroyers. Its reach was 139 km for targets at a height of 11 km.
Magnetron development (1953 – 1955)
The lack of microwave components also led to the successful development of a magnetron at the Physics Laboratory (1953 – 1955). After this success, a start is made to develop a tunable high-power magnetron. This would make it possible to change the transmitted frequency to avoid jamming. Technologically, the project is a major assignment. After some years of research, the project was closed when magnetrons became commercially available.
After 1950, Radar R&D was assigned to the LEO(K). The TNO Physics Laboratory will continue its research into anti-radar reflection techniques, which already started in 1947. This research includes the development of materials that absorb electromagnetic waves in the radar frequency band. That can be considered the early start of stealth technology in the Netherlands.
The Physics Laboratory starts research on anti-radar techniques such as the development of materials with properties of absorbing electromagnetic waves in the radar frequency band. This can be considered to be the early start of stealth technology in the Netherlands. In 1951, the development of radar-absorbing material (RAM) started. In close cooperation with Philips material is discovered that absorbs radar in a broad frequency band.
The scarcity of microwave measurement equipment leads to the design and development of special systems that are of great value to the measurement of objects covered with RAM. This measurement radar is also used for determining the radar reflection of objects in free space and above water. In 1958, a radar cross-section (RCS) measuring facility was created at a lake near Nootdorp. See Nootdorp radar facility.
Around 1957, the Armed Forces tasked the Physics Laboratory TNO to work on “Radar observability”. This became the first activity on Radar Remote Sensing in the Netherlands. The Armed Forces were interested to know what an aircraft equipped with radar for navigation and mapping “sees” when flies at low altitudes. Ship navigation radars with wavelengths of 3 cm (X-band) and 8 mm (Ka-band) were used to carry out the research. Because no aircraft were available, the radars were mounted on TV towers under construction. In this way, radar PPI pictures were obtained at a height of about 100 m. The results of the project on flat land were discussed with the US Air Force. These discussions contributed to the requirements for the Navigation And Situation Awareness Radar (NASAR) of the Starfighter for flying at low altitudes.
Improvements of the LW-radar: automatic receiver selector (~1960)
The operational use onboard leads to several suggestions for improvement of the radar system to be investigated by the LEOK. A serious problem encountered is the vulnerability of the LW-01 to jamming.
At the end of the fifties, the Navy wished to improve its radar capability against Electronic Counter-Measures (ECM). Moreover, if possible, the Navy wants an enhanced detection of targets in clutter. LEOK has to develop solutions.
The technical challenge was that a magnetron has a noncoherent nature and limited frequency tunability. Therefore, the use of the Doppler effect is not possible and the possibilities for jamming suppression are limited. Nevertheless, the Navy had several special types of radar receivers able to suppress jamming or at least reduce its harmful effects. Depending on the situation, one could switch between receivers to obtain the best result concerning detectability and false alarm rate.
These types of receivers were:
- A linear receiver for the highest sensitivity in benign conditions.
- A video frequency filter (VFF) to prevent saturation of the screen in case of continuous wave (CW) or spot noise jamming.
- A logarithmic receiver with a pulse length discriminator (PLD) to detect targets in clutter.
- A Dicke Fix receiver followed by a limiter to prevent a high false alarm rate [“Dicke Fix” is a technique that is specifically designed to protect the receiver from ringing caused by noise, fast-sweep, or narrow pulse jamming]
- A CCM-2 receiver, a combination of logarithmic PLD and Dicke Fix (3 + 4), against the combination of clutter and jamming.
Because of the rotation of the antenna, it is not easy for the radar operator to switch in time to the receiver with the best result. Therefore, the development of an automatic system is ordered.
The criteria for the automatic receiver selector are sensitivity and false alarm rate, which are measured during each sweep. The false alarm rate is measured by setting thresholds. The receivers with the highest false alarm rate are switched off for that sweep. From the remaining receivers, the output of the most sensitive receiver is selected. This is achieved through the introduction of a series of artificial echoes growing from weak to strong and subsequently counting the number of detected echoes for each receiver. The receiver with the highest number is selected as the receiver with the best result. Series production of this automatic selector at PTI Huizen started around 1960. The centre frequency of the radar also increased from 15 MHz to 30 MHz at that period.
Procurement and improvements of the long-range air warning radar ER438 with an amplifier chain (~1960 – 1965)
Around 1960, the Royal Netherlands Air Force procured the ER438 long-range air warning radar from the French company CSF. This L-band radar does not make use of a magnetron as a transmitter but of a chain of microwave components. The radar’s peak output power was 3 MW with a pulse length of 4 msec and a pulse repetition frequency of 200 Hz. The transmitted frequency could be fixed or jump from pulse to pulse.
Starting with a carcinotron as a frequency source, several microwave amplifier tubes follow (so-called travelling wave tubes (TWTs) or ‘Tube Propagation de l’Onde’ (TPO) in French), and a cross-field amplifier (CFA) as the output stage. A carcinotron is a frequency generator where the applied voltage determines the output frequency. Therefore, the carcinotron frequency can quickly change. This enables frequency agility and “système écoute”. The latter is an ECCM feature. The system listens first to ensure that the selected transmission frequency is not jammed. In the event of a jammed frequency, a different transmission frequency is quickly selected.
The LEOK was asked to assist with the procurement of the radar and help with the acceptance test of the radar. The radar is part of the NATO Air Defense Ground Environment (NADGE) early warning system.
After the radar becomes operational, the Dwingelo Radio Observatory issues complaints. As the observatory carries out measurements on a special frequency in the L-band, the LEOK has to develop an absorbing filter. In 1962, the filter was built in the waveguide between the transmitter and the antenna to reduce the radiation at this specific frequency to an acceptable level. The dispersive filter had to be developed and has been constructed with all-pass filters consisting of capacitors and inductors. A sonic dispersive delay line from General Electric later replaced this filter.
Although the radar was equipped with a “Système écoûte”, the radar was still vulnerable to broadband jamming. For that reason, LEOK developed additional anti-jamming equipment in 1965. The modification of the radar comprised the following elements:
- Sidelobe canceller
- Anti-jamming receivers
- Automatic selector
For sidelobe cancellation use was made of an auxiliary antenna. The signals received via the main antenna and from the auxiliary antenna are subtracted. The echoes received by the main lobe of the main antenna, however, turn out to be too strong to cancel. Therefore, all the jamming and clutter received via the side lobes of the main antenna have to be cancelled while the echoes received via the main lobe are detected.
Radar pulse compression and the need for increased transmission power
Upon request of the Air Force, an investigation started into pulse compression with possible implementation in the ER438 in 1963. An experimental system is set up. As a transmitter, use is made of two TWTs from the ER438. The first TWT had an output power of 1 W while the second TWT had an output peak power of 1 kW (see photos above). Further use is made of the LW antenna at the LEOK. The pulse compression signal is a chirp with a pulse duration of 10 µs and a linear frequency modulation of 2 MHz resulting in a compressed pulse duration of 0.5 µs. The dispersive filter has to be developed and constructed with all-pass filters consisting of capacitors and inductors. A sonic dispersive delay line from General Electric later replaces this filter.
The military need for ever greater distance ranges led to ever-higher transmission power. Peak power is limited by transhipment problems in the microwave tube. Initially, this problem is solved by filling the microwave tubes with SF6 gas. This resulted in only a limited improvement. Another possibility to increase the transmission power is to use longer transmission pulses. However, this adversely affects the distance resolution of the radar, making it difficult to distinguish two closely spaced objects in the same direction.
This challenge is solved by the LEOK by applying a modulation of the transmit pulse. For example, by linearly varying the frequency of the transmit pulse over time. When a reflection from an object is received, the signal is applied to a dispersive filter. This dispersive filter has the property that the processing time varies linearly with the frequency. In this way, the long pulse is compressed into a short pulse, allowing distinguishing closely spaced targets.
Radar equipment calibration and monitoring device (KORA)
In the sixties, radar techniques became more complex, partly due to modern electronics. The possibilities have expanded enormously. Military use of radar also showed developments that are important from a tactical-strategic point of view. For that reason, radar test and control equipment has also been expanded considerably. For testing, monitoring and calibration it is necessary to imitate an environment that is realistic for the radar. Relying on real targets in a real environment to test and fine-tune all functions of a modern radar would make these procedures too costly and lengthy.
In 1963, the LEOK developed the “KOntroleapparatus RAdardevices” (KORA) target trajectory simulator to automatically test and calibrate radar devices with a fixed microwave frequency. The radar under test is operated about 35 meters away from a movable horn antenna that acts as a target simulator. The pulses emitted by the radar are received by the horn antenna, processed and sent back in the direction of the radar. The processing of the pulses is the essence of the simulator. Different target distances can be simulated by delaying and attenuating the received pulses before they are transmitted to the radar. The adjustable time delay and attenuation are directly related to the target distance. A small target will give a weak echo. The sensitivity of a radar determines which target can still be detected at a certain distance. This sensitivity can be tested by giving the received radar pulses a fixed delay time before they are returned and gradually increasing the attenuation.
Finally, to check the tracking capacity of the radar, it was possible to move the horn antenna. The radar will follow this movable target and will provide control voltages to, for example, a gun to be connected. In practice, these control voltages can be passed through a fire control computer. For the tracking capability test, the control voltages can be compared to the voltages derived from the moving target simulator itself. The difference between these can be registered and gives a good impression of the tracking errors of the radar. This can then be done for various target velocities, simulated distances and echo strengths with or without a fire control computer.
When the fire control system is switched on, the hold-up and pitch angle for a gun can be registered.
The KORA simulator has been in daily use since 1964 in the Simon Stevin barracks in Ede, where all tracking radars of the Royal Netherlands Army were tested with this simulator. To test the target tracking radars for dynamic behaviour, a mill with two blades has also been added to the KORA. A transponder was mounted at the end of each blade. The blades started to turn and then it was checked whether the radar locked on the transponder and continued to follow the rotating target.
The Armed Forces continuously asked to extend the detection range of radar and the detection of even smaller targets. Using the Doppler effect, small targets could be detected despite the unwanted clutter by environmental reflections, such as precipitation (rain, hail), buildings, afforestation, and waves. By using the difference in Doppler frequency of flying objects and clutter, signals can be separated. In a radar clutter suppressor, the Doppler effect is used to weaken the clutter and to make the weak echoes visible in the weakened clutter. The use of the Doppler effect is not easy with magnetron-based radars. A magnetron does not have a very stable frequency of its transmission pulses as a result of which the Doppler frequency varies from pulse to pulse. The laboratory, however, developed a smart circuit to overcome this challenge.
Digital processing and visualisation
In parallel, LEOK worked on the digital processing of radar data. In 1963, the Radar Information Processing Equipment (RIVA) project used a digital computer of one’s design. This computer processed the acquired radar information using a video integrator and generated control data for steering the height radar antenna.
More information on the digital processing of radar target information and its presentation on radar screens can be found on the Digital Technologies: Radar image processing page.
The ‘follow-up’ to the RIVA project was the 3D simulator project for the Royal Navy (from 1965 to 1970). The 3D simulator injected simulated targets and clutter into the processing equipment of the 3D radar that was under development.
More on the computers utilised can be found in Computer history: LEOK period 1961 -1974.
The developed 3D radar was installed at the Tromp class frigates of the Netherlands Royal Navy. The radar system combined the aforementioned radar functions of search, tracking, direction-distance-altitude, anti-jamming and Doppler.
- KORA – “ROERING” Mededelingenblad van de Vereniging van Ingenieurs der Marine, Jaargang 10, Nr 2, Dec 1973
- Radar Development in the Netherlands: 100 Years Since Hülsmeyer, van Genderen, P. (2004)
- Radar en aanverwante onderwerpen 1954-1975: Het samenspel van marine, onderzoek en industrie, Ir. C.M.N. Belderbos CDR E bd, in “Herinneringsboek van het Korps Elektrotechnische Dienst van de Koninklijke Marine 1950-2014” (2014)
- AA No. 3 Mk.4 (Glaxo)