Digital Technologies: Digital Sonar Calibration

Sonar Calibration


Around 1969 the four three-cylinder submarines of the Royal Netherlands Navy were equipped with new digital fire control, the HSA M8/17, manufactured by Hollandse Signaal Apparaten (HSA). The boats were also equipped with new torpedoes, the electrically powered and wire-guided Mk37 mod 2. The old unguided Mk8 torpedo also remained part of the armament.
The new weapon system consisting of fire control, sonars, periscopes and torpedoes would be evaluated. This was done by the Laboratory of Electronics Development for the Armed Forces (LEOK). A fire control system was temporarily placed at the LEOK in Oegstgeest. Equipment was developed for recording exercises onboard the submarine HNLMS Zeehond. In 1969 and 1970 exercise programs were run in Scottish waters with two LEOK staffers on board.
In the 2nd half of 1970, the official evaluation of the developed equipment took place at AUTEC, the US Navy Atlantic Undersea Technical Evaluation Center, on Andros Island in the Bahamas. Two LEOK staffers were involved in this evaluation. During the sonar calibrations, it was found that errors were not normally distributed around the zero line. There was a relatively large angle-dependent fixed error. This was also known to the Royal Netherlands Navy but nothing was done with it at the time.
We then proposed to develop equipment for recording sonar calibrations and then correcting for angle-dependent bias in the fire control.


  • NP – Navigation periscope
  • AP – Attack periscope
  • LWS 10 – Passive sonar (two consoles connected to the same sonar array)
  • LWS 30 – Option to connect and measure the new sonar which was under development
  • PWS – Active sonar
  • LAS – Sonar for passive range detection


Sonar registration automate with FACIT papertape punch
Sonar registration automate with FACIT papertape punch


The recording device

The “viewing angles” of periscopes and sonars were measured with synchros and passed on to the fire control. All devices were equipped with two synchros, a coarse system for 360 degrees measurement, and a fine system with 36 or 16 rotations over 360 degrees. All these synchros were also connected to the backside of the recording device during calibration (see photo on the back with MilSpec plugs). The angular readings were digitised with 14-bit synchro-digital converters in the device. During a measurement, the digital angle values ​​were recorded in a 5-hole paper punch tape on the FACIT papertape punch.

Sonar registration automate (backside)
Sonar registration automate (backside)


The procedure

During a calibration run, the submarine slowly turned in a small circle while a target ship was followed both by one of the periscopes and the sonar to be calibrated. Measurements were taken every approximately 2 degrees. At the command of the officer at the periscope, all information was recorded in papertape after a push on a push button. The papertape was delivered to the LEOK (and later the FEL-TNO). Subsequently, the information was read and processed by a computer. All measurements of the difference between periscope and sonar as a function of the angle to the vessel’s axis were graphed. Any ‘outliers’, ie obvious procedural errors in the measurement, were removed: “The readings came from the periscope servos, that’s coarse/fine stuff. Sometimes I saw values that I thought I had the wrong coarse value …” Then the mean deviation per sector was calculated by the program using the least-squares method of smoothing and averaging the measurements. There were 32 sectors all around. It was quite a job to convert a correction value to a digital ‘diode or not’ on a PCB. The program was in CORAL, the NATO standard language in those days. In the end, the layout came out for a PCB with a set of diodes which incorporated the correction. The end result was sent back to the Submarine Service of the Royal Navy.

The original idea of ​​the LEOK was to record the results in an EPROM, an electronically programmable memory chip. With a print card and a few lines of program code in the fire control system, the systematic correction for sonar measurements in the fire control could then be processed. EPROMs were fairly new at that time, the early 1970s. However, HSA did not find it reliable. Their solution was printed circuit boards into which small metal brackets were inserted to mark a digital 0 or a 1. At least eight printed circuit cards were needed per fire control unit. Plus a laborious procedure to manually record the results of the paper calibration in the print cards. HSA eventually got its way. Modification costs per fire control amounted to approximately 21,000 guilders, a considerable amount at that time.

Magnetic tape for registrations was used during the second sonar evaluation in 1975. The procedure was maintained until the four 3-cylinder boats and the two Zwaardvis-class boats were taken out of service, the last one in 1995. For TNO therefore also the burden of keeping a paper tape reader, even if these were now obsolete for all other purposes.