decca navigation system

DECCA NAVIGATION SYSTEM


Is a hyperbolic navigation system which allowed ships and aircrafts to determine their position by receiving radio signals from fixed navigational beacons.

First developed by the Royal Navy during WW II.

It was established in the United Kingdom after World War 2 and is later used in many areas around the world.

DECCA NAVIGATION SYSTEM (DNS)

It is operated by measuring phase differences between continuous signals from a master and slave stations.

These differences were then related to hyperbolic lines printed on a chart.

By plotting the readings from two pairs of hyperbolas at any particular instant, user could plot their position instantly.


The system used groups of at least three shore based transmitter stations called chains operating under low frequencies from 70 to 130 kHz radio band.

Each Chain comprised of one Master and two or three Slave stations, usually 80 to 110 km from the Master Station.


The accuracy of DNS ranged from 50 meters during daytime to 200 meters at night time.

It could decrease up to 800 meters as the distance from the baseline increased.

Accuracy was also affected by seasonal effects which generally reduced the accuracy by a factor of 6 to 8.


The maximum daytime range for DNS was 300 to 400 miles with a reproducibility of 200 meters.

At nighttime, accuracy was guaranteed out to be 240 nm by the British Admiralty.

DECCA NAVIGATOR

PRINCIPLE OF OPERATION

Chains - consists of a number of land-based radio beacons organized into chains.

Master station and three Slave stations, termed Red, Green and Purple. Ideally, the Slaves would be positioned at the vertices of an equilateral triangle with the Master at the center.


Master station – provides the 'master' signal which was used by its associated Slave stations to derive signal frequency and timing sequences.

Loss of a Master would disable a station whilst loss of a Slave would reduce accuracy.


Master-Slave Distance = 60~120 Nmiles

It operates by comparing the phase difference of radio signals emitted by several radio stations.

Pattern – refers to the set of hyperbolic lines of position


Receivers identified which hyperbola they were on and a position could be plotted at the intersection of the hyperbola from different patterns, usually by using the pair with the angle of cut closest to orthogonal as possible.

DECCA NAVIGATION PRINCIPLE

FREQUENCY

Station Harmonic Frequency (kHz)

Master 6f 85.000

Purple Slave 5f 70.833

Red Slave 8f 113.333

Green Slave 9f 127.5000

COMMON FREQUENCY (LCM) FOR EACH MASTER/SLAVE PAIR

Pattern Slave Harmonic

Slave Multiplier

Master Harmonic

Master Multiplier

Common Frequency

Purple 5f *6 6f *5 30f

Red 8f *3 6f *4 24f

Green 9f *2 6f *3 18f

LANES AND ZONES

Decometer - An adding-type phasemeter which adds up the total number of degrees of phase shift between two signals.

Lane – interval between two adjacent hyperbolas on w/c the signals are in phase.

Zones – group of lanes usually with 18 green, 24 red, or 30 purple lanes per zone

TYPICAL LANE AND ZONE WIDTHS ON THE BASELINE ARE SHOWN IN THE TABLE:

Land or Zone Width on Baseline

Purple Lane 352.1 m

Red Lane 440.1 m

Green Lane 586.8 m

Zone (all Patterns) 10563 m

DECOMETERS

MULTIPULSE

20-second cycle simultaneous wave transmission for each of the four Decca frequencies with a brief period of 0.45 seconds.

Provides automatic method of lane and zone identification by using the same phase comparison techniques described above on lower frequency signals.

decca navigation system

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