Measurement station Roeleveense Plas (1953 – ~1995)
TNO used its measurement station Roeleveense Plas in Nootdorp both for underwater acoustic measurements (sonar measurements) and research in making submarines ‘invisible’ to radar waves (stealth) from 1953 until the mid of the ’90s:
Underwater acoustics research
During the development of sonar transducers, the measurement station in the Waalhaven, Rotterdam no longer met the requirements in the early ’50s:
- the water was not deep enough on the spot,
- the background noise was too high due to the proximity of ship traffic and ports, and
- the distance between the laboratory in The Hague and the port in Rotterdam was impractically large.
In 1953 TNO decided to create a new underwater acoustics measuring facility at the Roeleveense Plas (‘lake’) near Nootdorpnext to the A12 highway and the Hofplein line (nowadays the Randstadrail line E). The lake was a sand excavation from the ’30s. The excavated sand was deposited nearby in order to create a multi-level crossing with the A12, nowadays the Prins Clausplein. At that time there were already plans for a ‘Rotterdam connection’, the body of sand was never used for that purpose.
The Roeleveense Plas is a double triangular freshwater lake with sides of 300 and 400 metres (see the Google map below).
A new raft was developed. Two pontoons were transported from the Rotterdam Waalhaven to the Roeleveense Plas. The pontoons are former armour-clad doors that protected the German E-boat (Schnellboot) bunkers in the Rotterdam Waalhaven during the Second World War. The comparted pontoons are made of half-inch steel plates and sized approximately 4 x 3 x 2 metres (lwh). Therefore, there was headroom in the pontoon. The two pontoons were connected with a gap of 1 metre in between. A hoisting device was installed on the raft. On each pontoon, there was a hut: one hut for taking measurements and one hut to house a diesel engine and power generator.
At first, the raft was anchored in the middle between the highway and the narrowest point of the lake. On May 18 1954, the raft was moved to the centre of the right-hand triangular part of the lake.
For measurements with domes in the 1950s, the dome was lowered between the two pontoons with two pully blocks. Four metre long pipes mounted on the top of the dome gave a precise measure of how deep the dome was under the water level: 80 cm of pipe above the water surface meant that the top of the dome was 3.20 metre deep under the water level. The hydrophone was then lowered in the same way at the furthest corner of the raft. The transducer was lowered into the dome in the same manner until the pipe was 70 cm above the water surface; the top of the 60 cm high transducer was then 10 cm deep in the dome. The transducer could be rotated in the dome so that the directional deviation could be measured.
The raft was fixated near the narrowest part of the lake above its deepest point, then -18.5 metres, with four anchors. The detailed depth and underwater soil structure charts by the Nootdorps Pijnackerse Hengelsport Vereniging (NPHV) provide a good insight into the depth and soil structure of the lake. Click here for more graphs.
For experiments, the employees had to use a rowing boat and row from a jetty to the raft. This was a bit easier than in the Waalhaven with the high quay of the former German Schnellboote bunkers.
The swivel mechanism for the hydrophones and transducers under test originally came from the Hr.Ms. Paets Van Troostwijk. The swivel mechanism could be tilted in a way that the bottom side came above water to ease the mounting of a hydrophone or transducer. The swivel mechanism is still in use at the TNO Waalsdorp underwater acoustics basin.
The quiet environment and the large water depth benefited the quality of the sonar measurements, especially for frequencies below 500 Hz. This raft was also made suitable for measurements on sonar domes (a “dome” is a streamlined envelope of the transducer that serves to reduce the noise of the water flow). The diesel-driven power generator, however, made noise and interfered with the measurements. Moreover, in the winter it was quite a challenging task to start the cold engine. It had to be cranked up by hand. In 1955, an electric power connection was installed with the shore side.
The design of the raft should have enabled measurements on sonar domes. A “dome” is a streamlined enclosure of the transducer that serves to reduce the flow of noise. Unfortunately, it turned out that the measurement set-up was unsuitable for performing measurements on domes of the Friesland class destroyers.
A new raft for sonar trails (1961)
Initially, the sonar measuring facilities station met all the requirements. However, after a number of years, it turned out that a replacement of the raft was required. The development of the sonar technique led from the searchlight type of sonar with a single beam to panoramic sonars. Panoramic sonars are a combination of several fixed bundles of beams in a single transducer. At the same time, the development of sonars aimed at using lower frequencies. Both factors combined resulted in transducers and domes becoming considerably larger and heavier. As a result, it became impossible to transport the equipment to the raft per rowing boat.
In 1960, TNO decided to construct a larger raft in the same lake. At first, existing pontoons would be
refurbished for fl. 55,000. After the heavily rusted pontoons were cut open, their condition was so bad that it was decided to manufacture new, larger steel pontoons with a plate thickness of 6 mm. The new raft consisted of four pontoons of 4 x 2 x 1 meter (lbh) and two pontoons of 6 x 2 x 1 meter (lbh). Inside the pontoons contain three tectylated compartments. The pontoons were delivered, floated, and assembled on 1 November 1960.
The rectangular raft of 6 x 12 metres had an opening in the middle through which the sonar equipment under test and transducers could be lowered. I-beams across a part of the opening were used to stiffen the raft structure. The wooden deck of the raft was equipped with a roller shutter above the opening. The narrow-gauge rail system ran over the hatch as well.
The raft was connected to the bank of the lake by a floating bridge of five 6 x 2 x 1 metre (lwh) sized pontoons. The location of this new raft is clearly visible on the Google maps image above.
This new underwater acoustics facility made it possible to measure sonar equipment with maximum dimensions of 3 x 1.80 x 1.70/1.80 metres (lwh) and a weight of 5.000 kilograms. To this end, a narrow gauge lorry system with a turntable was installed on the new concrete bridge connection and the bank. The lorry system ended at a hoist construction at the Roeleveenseweg. Four plough-type anchors onshore and chains kept the raft and the floating bridge in place, 50 metres from shore and 60 metres from the lakeside. Under the raft, the water depth is 17.5 metres.
Because the new raft had steel constructions for the hoisting masts and a carpeted measuring and working hut, it lay deeper in the water than the last pontoon of the floating bridge. To achieve a smooth use of the lorry system across the connection, ballast was put in each of the bridge pontoons to ease the transport from shore to the raft and vice versa.
At the shore side of the raft, above the 2 x 10 metres gap between the pontoons, is a 13-metre high lattice construction containing a hoisting mast system. Objects up to 1.70 or 1.80 metres high (depending on the lorry used) can be lifted up from the trolley with the mast. The flange of the hoisting device can be lowered to 6 metres below the water surface (5.5 metres below the raft). Two hoisting cables are connected to an equator at the bottom of the mast and to an electric winch on the other side. This ensured an equal tension on each of the steel wires.
A maximum load of 5.000 kilograms could be lifted. The mast has two rotating columns. Those allowed 360 degrees rotation (independently of each other), either manually or with an electric motor. That motor is operated via a control panel in the measuring house.
A second hoisting mast is located at a distance of exactly 5.62 metres heart to heart, which relates to a 15 dB attenuation of sound under water making it easy to process measured results. That mast is made of square profile iron. A transducer of max. 150 kilograms could be lowered. A hand winch is used to lower or lift the mast. The mast lowers the measuring hydrophone to 5.5 metres below the water surface or 6 metres below the wooden deck of the raft.
The measuring raft is equipped with a three-phase power connection with sufficient power for the hoisting motor, a welding machine and the 12 kW transmitter for the 216TP5R transducer and other measuring equipment. A plastic hose of some 150 metres runs from the raft to the drinking water mains ashore. Unfortunately, the drinking water quality worsened over time.
The new raft was put into use in 1961. The old, Second World War-based, pontoon raft was transferred to the Marine Elektronisch en Optisch Bedrijf (MEOB) (MEOB), which used the raft for several years. They connected that raft with a bridge to shore. Later, the two Second World War-based pontoons were re-used as a base for a house on the other side of the lake until the end of 2019. Early 2020, the raft was moved to its original location and is in use by the Nootdorps Pijnackerse Angling club (NPHV).
TNO used the new raft and shore facilities for sonar experiments until the mid-1990s. The Netherlands Royal Navy became the new owner of the raft and related facilities.
In the ’70s, the facility was used as well to carry out measurements of transducers that could not produce short pulses. In that case, the switch-on and switch-off phenomena may dominate the long-duration response signal. The inset time of the transducer may also be longer than two milliseconds. In those cases, the in-house basin on the Waalsdorp Vlakte is not suitable for the measurements. The Roeleveense Plas measurement facility is.
The measuring facility Roeleveense Plas has only one problem, especially at the end of the summer. At that time, a strong temperature gradient has built up at a depth between three and six metres. On the surface, the water temperature can rise to 20 0C. Close to the bottom, the water temperature is about 8 0C. This temperature variation can adversely affect the accuracy of the measurements. In early spring the water temperature in the entire lake is very homogeneous, around 4 0C; a temperature that is eminently suitable for calibrating measuring hydrophones.
Radar Cross Section measurements and ‘stealth’ submarine masts
In the mid-’50s, the Royal Netherlands Navy develops the Dolphin-class and Potvis-class submarines, the so-called three-cylinder submarines. Several vertically movable ‘masts’ are located on top of a submarine: an attack periscope, a navigation periscope, an electronic emission scanner antenna, and a (passive) radar antenna. If required, a mast can be extended to just above the water surface. Moreover, a submarine has a ‘snuiver’ (sniffer or snort) mast to charge the submarine’s batteries for sailing stealthily underwater.
A submarine wants to remain undetected on the one hand, but wants to collect information as much and as long as possible using its visual and electronic sensors. Conversely, the Royal Netherlands Navy wants to detect enemy submarines as early and at best as possible.
Around 1956 the Royal Netherlands Navy asks TNO two questions:
- How do you reduce the possibility that an opponent can detect our submarine by radar?
- What is the optimum angle at which naval patrol aircraft, at that time the Grumman S-2 Tracker and later the Lockheed P-2 Neptune, can detect an enemy submarine with their radar despite sea clutter?
To answer these questions, three topics were worked on in parallel:
- The development of theoretical models.
As all kinds of multipath effects occur when trying to detect submarines, only the strongest signal reflection path is considered when modelling. At the same time, that is the signal level that one’s own submarine must suppress as much as possible.
Model development was complex at that time, all the more so since there were no computers yet.
- The development of Radar Absorbing Materials (RAM) together with Philips.
RAM reflects considerably less of the incoming radar radiation. However, it is not easy to achieve a low reflection in a broad spectrum with RAM.
- (Re)designing and optimising the masts that protrude above the water surface from time to time in such a way that they, in combination with the RAM, provide the smallest possible radar reflection surface (radar cross-section (RCS).
In 1958, a radar cross-section (RCS) measuring facility was installed at the Roeleveense plas in Nootdorp. That facility helped to answer the questions. Measurements are needed for validation of the theoretical models and to analyse the suitability of the designed mast form factor – RAM combination in practice. Based on the validated theoretical models, work was performed on optimising the design of the submarine masts in terms of invisibility for detection by radar.
Ultimately, we measured three generations of the snort with RAM (Dolphin, Swordfish, Walrus classes of submarines) and two generations of periscopes and electronic emission scanner antennas (Dolphin and Swordfish classes). For the Walrus class, only the optimal design of the snort was researched, not the RAM developed by TNO and Philips.
The facility consists of a 20-metre high lattice constructed tower onshore and, at 132 metres distance, a pole underwater with a vertically movable platform attached to it. That platform can be lowered to two metres below the water surface. A stepper motor is mounted on the platform that drives a horizontal turntable of 50 cm diameter on which a mast section to be examined (attack periscope, navigation periscope, electronic emissions scanner antenna, snort), or a reflector for calibration is placed. By rotating the object on the turntable over 360 degrees, asymmetrical mast parts can be measured from any angle. The object to be examined can also be positioned higher or lower above the surface of the water by remote control from the measuring cabin onshore.
The lattice construction tower was equipped with a lift with a measurement cabinet. The measuring radar in the cabinet could be moved vertically up and down with the lift. The measuring radar could also be tilted with a motor. Therefore, measurements could be made on a submarine mast protruding above the water surface at different verticle angles and rotations. The motors of both lifts and the rotation and tilt devices could be operated remotely from a shed (“pre-processing cabin” in the figure below). In addition, there was a raft and a floating hoist. The raft was used to reach the underwater pole.
The narrow rail track that leads to the sonar raft was extended in the opposite direction from the hoisting construction at the Roeleveenseweg to the landing stage of the raft next to the lattice constructed tower for radar cross-section measurements.
After several dives, the Navy divers want to go home quickly. Their Zodiac hits the underwater pole at high speed. The pole didn’t give in. The Zodic did … with a leak.
Radar Cross Section and sea clutter
The Nootdorp radar cross-section measurement facility was unique within the NATO membership. It is striking that the very secret measurements took place in full visibility of the public on the bicycle path along with the facility and the traffic passing by on the A12.
The combination of prominent modelling, the optimised mast designs, and the secret RAM developments all supported by this measurement facility led to a very low detection probability of Dutch submarines in the radar domain. It should be noted, that optimising a mast design is not only a matter of RCS and RAM. The effects of the water flow around the mast have to be considered as well. This aspect required tests with scale models in the towing tank of the MARIN.
The following research and development path has been used:
- After the theoretical model development, validation tests took place at the RCS-measuring facility. Work was performed on optimising the mast designs using the results of the scale model tests at MARIN.
- Subsequently, sailing tests were conducted. In addition to Dutch Navy aircraft, English Avro Shackleton aircraft from RAF Coastal Command took part. They tried to spot the submarine’s modified mast designs in the high seas and to measure whether the modified masts were really less detectable.
- Endurance tests.
- Production of definitive designs, not only for Dutch submarines, but also for 15 Norwegian Kobben class submarines (1964-2005). Four of these 15 submarines were taken over by the Danish Navy in the ’90s.
In consultation with the Dutch Ministry of Defence, knowledge development on these research domains was shared with England and Norway as part of the Anglo Netherlands Norwegian Cooperation Program (ANNCP) task 1.6.
Similarly, ANNCP 1.19 collaborated on research on sea clutter, the disturbance of radar reflection due to the undulating sea surface. By understanding this phenomenon, and after optimisation the angle of incidence and filtering of the reflected radar signal, a submarine mast reflection could be better detected. On the other hand, by understanding the phenomena, a submarine can make itself “more invisible”.
This research and development would not have been possible without international cooperation. Thanks to the ANNCP cooperation, TNO was able to use a Norwegian measuring post, a 450-metre high measuring platform on the Stadlandet peninsula. The sea there is the roughest sea in the world on which radar measurements can be made from shore.