Invisibility of submarines to radar (stealth)
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.