Editing Decca Navigator System (section)

=== Lanes and zones ===
[[File:Admiralty Chart No 1607 Thames Estuary Southern Part, Published 1967.jpg|300px|thumb|right|A 1967 [[Admiralty chart|Admiralty]] Decca Chart of the Thames Estuary, marked with red and green lanes and zones.]]
Early Decca receivers were fitted with three rotating ''Decometers'' that indicated the phase difference for each pattern. Each Decometer, which could be read to a resolution of a centilane, drove a second indicator that counted the number of lanes traversed &ndash; each 360 degrees of phase difference was one lane traversed. In this way, assuming the point of departure was known, a more or less distinct location could be identified.

The lanes were grouped into ''zones'', with 18 green, 24 red, or 30 purple lanes in each zone. This meant that on the baseline (the straight line between the Master and its Slave) the zone width was the same for all three patterns of a given chain. Typical lane and zone widths on the baseline are shown in the table below (for chain 5B):

{| class="wikitable"
|-
! Lane or Zone!! Width on Baseline
|-
| Purple lane || 352.1 m
|-
| Red lane || 440.1 m
|-
| Green lane || 586.8 m
|-
| Zones (all patterns)|| 10563 m
|}

The lanes were numbered 0 to 23 for red, 30 to 47 for green and 50 to 79 for purple. The zones were labelled A to J, repeating after J. A Decca position coordinate could thus be written: Red I 16.30; Green D 35.80. Later receivers incorporated a microprocessor and displayed a position in latitude and longitude.