Remote Sensing: The start (1955 – 1960)


Remote Sensing in the Netherlands: The start (1955 – 1960)

Immediately after the Second World War, Remote Sensing (Earth observation, remote sensing) was only aerial photography, a mature technique of which the International Institute for Aerial Mapping and Geography (ITC) in Delft, the Netherlands could be counted as an exponent in the 1950s. On the other hand, there were also new observation techniques that emerged from the military reconnaissance in WWII. For instance, radar and the thermal imaging camera opened up other parts of the electromagnetic spectrum. Initially, the term Remote Sensing was even reserved for this group of techniques. These techniques were also referred to as ‘non-conventional air-sensing systems‘. Until 1968, these techniques were classified as they were being developed for military reconnaissance purposes.

Radar Observability

In 1955, the TNO-RVO Physics Laboratory started researching the topic ‘Radar observability‘ at the request of Professor Sizoo, the then chairman of the National Defence Organisation of TNO. That research was an extension of the infrared research that had already started. The question was: “What danger can a low-flying aircraft equipped with radar pose for the Netherlands?” 
A literature study was immediately started, in which in addition to the open literature – the well-known MIT series with all technical reports of radar research carried out during the war – the classified reports of the use of radar and acquired operational results were studied. Initially, only conventional circular scanning radars were included in the study. In parallel, TNO colleague Poley [1] conducted research in Norway, where the harbour of Bergen was viewed from a high point with an 8 mm radar and a 3 cm radar with the cooperation of the Norwegian sister organisation Forsfarets Forsknings Institutt (FFI).

The start of Remote Sensing in the Netherlands (1957 – 1960)

(as ‘flatlanders’) We looked for high points in the Netherlands to put radars on to mimic what a low-flying aircraft might observe. The high radio antenna towers that the PTT were building at the time were an obvious choice. At the end of 1957, for example, TNO started experimenting on the radio antenna tower in Goes, Zeeland with two radars: a 3-cm radar borrowed from the FFI and an 8 mm radar leased from Decca. The first radar had a resolution of 1 degree (antenna) with a pulse length of 0.1 µsec. The second radar had a resolution of 24 minutes (antenna) at a pulse length of 0.05 µsec. The images from the plan position indicator (PPI) scope were captured on film using a camera.


X-band radar image of Zeeland from the PTT antenna tower Goes at an altitude of approximately 100 m (1957)
X-band radar image of Zeeland from the PTT antenna tower Goes at an altitude of approximately 100 m (1957)

To get an impression of the echo strength (radar cross section) of the various observed objects, we used a weather balloon in a metal net as reference (spherical shape) and the radar IF amplifier which was reversed in known steps.

In 1958, observations were made of the PTT radio towers in Mierlo, Smilde, and Roermond and three other high places: the watchtower in Vaals, the Duno hill near Arnhem and the church tower of Hoog Elten (then the Netherlands, later Germany). The results of these experiments were very promising and were recorded in internal reports. In 1959, the reports were sent to the National Defence Organisation TNO (reports PhL 1959-2, 3, 4 and 5).
In the same period, Rijkswaterstaat was busy making wave measurements with the same Decca 8 mm radar as part of the Delta Works from the top of self-built towers [2].

Zendtoren Smilde
Transmission tower Smilde


St Vituskirche, Hoch_Elten - Door RomkeHoekstra - Eigen werk, CC BY-SA 4.0, RomkeHoekstra - Eigen werk, CC BY-SA 4.0,
St. Vituskirche, Hoch Elten. By Romke Hoekstra – Own work, CC BY-SA 4.0,

Internationally, TNO was allowed to discuss its ideas about radar in a fighter plane with the American Air Force attaché in the Netherlands. He listened to us with interest and only said yes or no. He was not allowed to say more. Yet he confirmed our approach. The same thinking had been used in the USA for the radar systems in the new fighter planes, including the Lockheed F-104 Starfighter ordered by the Netherlands. That was a creepy idea to him. After all, if you could invent this in the Netherlands, the Russians could do too.
He arranged a visit for TNO researchers to the USA. He also arranged flights by the US Air Force above the Netherlands to connect our antenna tower images with images made by their radars. The images thus obtained were handed over by the Laboratory to the Royal Netherlands Air Force, which used them for pilot training.

The next period is the period of internationally classified examination.

Acknowledgement G.P. de Loor gave a lecture on 7 June 1988 on the occasion of the 10th anniversary of the “Kring voor Remote Sensing” on 17 August 2009. He wrote down the basis for this text. His lecture also appeared in an interim version in the Remote Sensing Newsletter no.95 of December 2000 at the closing of the NRSP.

  1. J.Ph. Poley: “Note on the resolution of radar systems”; Tijdschrift Nederlands Radiogenootschap, vol.22, 1957, pp. 187 – 194.