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Discussion 
the requirements for electronic aids in different classes of air survey and 
especially in aerial mapping. An account is given of trials and of the operational 
use of the system as an aid to various types of air survey. The paper concludes 
with a consideration of the limitations of the present system and of certain 
future developments. The material is grouped under the following headings: 
Origin and Development of the Decca Navigator 
Original Invention — Wartime Development by the Decca Record Company 
and the British Admiralty — Post-war Role of the Permanent Decca Chains 
for Marine and Air Navigation: Use for Surveying — Concurrent Development 
and Deployment of the System Specifically as an Aid to Survey Operations. 
Outline of the System 
The Continuous-Wave Phase-Comparison Principle — Characteristics of the 
Radio Frequencies Used — Range, Accuracy and Coverage — Basic Form of 
the Equipment — the Flight Log. 
Use of Mobile Decca Chains 
Mobile Decca Chains — Practical Equipment Details — Hydrography, Marine 
Oil Exploration and Coastal Trilateration — Two-Range Decca — The Track 
Plotter — Terrestrial Exploration and Surveying — Low Level Air Operations 
— Helicopters — Short Base Decca. 
The Decca Navigator as an Aid to Air Survey 
Scope for Navigational Aid and Electronic Fixation — General Navigation 
— Tracking Methods: Repeatability — the Homing Method — the Plotting 
Method, Manual and Automatic — the Decca Flight Log: Characteristics, 
Capabilities and Limitations — Fixing: the Requirement — Effects of Terrain 
and Altitude — Performance Obtainable — Equipment: Ground Stations — 
Aircraft Installations. 
Future Developments 
Problems to be Overcome — Propagational and Instrumental Variations — 
Correction Procedures — the Decca Mark 10 Principle — a New Approach to 
High Precision Fixing for Photogrammetry. 
C. Powell: Une aide radioélectrique à l’observation aérienne. 
L’aide radioélectrique décrite dans cet exposé est le système de localisation »Decca Navigator». 
Un bref rappel des origines et des premiers développements du système et de l’exploitation qui en 
a découlé dans le domaine de la navigation aérienne et maritime est suivi d’une vue générale des 
principes de bases et des caractéristiques essentielles. Les équipements utilisés pour les observations 
sont alors décrits. 
L’exposé traite ensuite des aides électroniques à l’observation en général et, plus particulièrement, 
du système Decca en montrant de quelles différentes manières ce système peut être utilisé pour 
répondre à des besoins déterminés. 
Le problème de l’observation aérienne est examiné et 1’ auteur précise les caractéristiques deman 
dées aux aides électroniques dans les différentes utilisation de l’observation aérienne et, en particulier, 
dans le cas de la cartographie aérienne. L’exposé fait ensuite un bref compte-rendu des essais 
effectués et des emplois opérationnels faits dans différents cas d’observation. 
La conclusion concerne les limitations de système actuel et certains développements futurs. 
Prof. A. Bjerhammar: We have not only been interested in optical methods 
but also in quite electronic methods. At an early stage of the development here 
in Sweden was tested a design which I found rather similar to this one we now 
heard about. It was a design from the Swedish National Research Council and 
it gave an accuracy in the order of about 10 cm which seems to correspond to 
this as we have heard about today. But these were measurements over rather 
favourable countryside or perhaps it was in most cases over sea. And as far as