novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

### Understanding Decca

Hi folks the following paragraph is from wikipedia;

"The Decca Navigator System consisted of a number of land-based radio beacons organised into chains. Each chain consisted of a Master station and three (occasionally two) 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 centre. The baseline length, that is, the Master-Slave distance, was typically 60~120 nautical miles. Each station transmitted a continuous wave signal that, by comparing the phase difference of the signals from the Master and one of the Slaves, resulted in a set of hyperbolic lines of position called a pattern. As there were three Slaves there were three patterns, termed Red, Green and Purple. The patterns were drawn on nautical charts as a set of hyperbolic lines in the appropriate colour. 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."

Even delving further into this and checking back on notes i can still not grasp it, can anyone simplify how this navigation technique works?

Woodreau
Posts: 1441
Joined: Mon Sep 24, 2001 6:44 am

### RE: Understanding Decca

Same site that you went to for Omega but different section:

Decca Overview
http://www.jproc.ca/hyperbolic/decca_oview.html

Picture of how to compute position using the phase difference/hyperbolas.
http://www.jproc.ca/hyperbolic/decca_red_green_lattice_map_b1.jpg
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.

Pihero
Posts: 4318
Joined: Mon Jan 31, 2005 5:11 am

### RE: Understanding Decca

Decca is one of the navigation systems based on hyperbolic interference , like the CONSOL, the LORAN (s) and the Omega.
The interference between fixed phase from a master transmitter and three slaves generate phase interferencer in the form of a succession of hyperbolae. The position is obtained at the intersection of two hyprebolae from two stations (or more ). it is computed electronically,; or manually - even with counting the "bips " by ear on Consol.

The precision of these systems was incredibly high (Decca was the basis of the moving map used on Tridents et al in the late sixties / Early seventies.
Contrail designer

longhauler
Posts: 5433
Joined: Sat Mar 06, 2004 12:00 am

### RE: Understanding Decca

 Quoting Pihero (Reply 2):The precision of these systems was incredibly high (Decca was the basis of the moving map used on Tridents et al in the late sixties / Early seventies.

I recall seing a BEA Trident cockpit with the moving map. Someone told me the Viscount 800s were also so equipped, is this true? Were any other aircraft equipped with Decca Moving Maps?

Were the exported Tridents also equipped with moving maps?
Just because I stopped arguing, doesn't mean I think you are right. It just means I gave up!

novice
Topic Author
Posts: 90
Joined: Tue Aug 07, 2012 2:55 pm

### RE: Understanding Decca

thanks for the link woodreau some very good information in it, that explaination does make it simpler pihero though i have read over alot of it an stilln get stuck on bits espically hyperbolic lines   though guess i'm reading into it to much,
Thanks for the help guys

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

### RE: Understanding Decca

 Quoting novice (Reply 4):though i have read over alot of it an stilln get stuck on bits espically hyperbolic lines   though guess i'm reading into it to much,

Don't get too worried about the hyperbolic part.

Think of it this way...you've got a master and a slave broadcasting the same signal at the same time, only they're 60-120 miles apart. As a result, unless you're exactly the same distance from both stations, the signals will be out of phase (the peak of one signal will hit you at a different time than the peak of the other). The delay between the peaks (the phase difference) and the time delay from the stations (the transit time) is related to how far away, and in what direction, you are from the stations.

If you solve out all the equations, it turns out that if you connect all the points with the same time delays and phase different you get a curve called a hyperbola. But the fact that it's hyperbolic isn't that important to actually using it...you just need to know that your charts have a bunch of curves on them and, by picking up the signals, you can tell which curve you're on. Since you get a different curve from each pair of stations, you just find any pair of curves that you're on and the intersection of those curves on the map must be your location.

GPS basically uses the same technique, except instead of hyperbolas it gives spheres. You find a sphere from each satellite and you must be at the location where the spheres intersect.

Tom.

smittyone
Posts: 1336
Joined: Thu Feb 09, 2012 12:55 am

### RE: Understanding Decca

Man, this brings back some great memories. Until very recently I had one of my old LORAN interpolator cards lying around, must have lost it in the last move. If I ever find it again I will keep it in my wallet.

I can clearly remember feeling so 'high tech' plotting LORAN fixes at sea...we always plotted the hyperbolic lines vice reading the computed lat/lon off the display. Good stuff.

Pihero
Posts: 4318
Joined: Mon Jan 31, 2005 5:11 am

### RE: Understanding Decca

 Quoting tdscanuck (Reply 5):GPS basically uses the same technique, except instead of hyperbolas it gives spheres. You find a sphere from each satellite and you must be at the location where the spheres intersect.

I like your explanation to the non-techs of our group.
Actuallyu, youi've just revealed the basics of navigation : bar the case in which one uses a radial / dme distance, an aircraft position is always at the intersection of two - or more - curves , whether it's radial/radial, a set of three astro lines, most of the radio nav systems...
The only exception is the INS - or rather IRS system... the result is a position derived from three fixes.
Contrail designer

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

### RE: Understanding Decca

 Quoting Pihero (Reply 7):Actuallyu, youi've just revealed the basics of navigation : bar the case in which one uses a radial / dme distance, an aircraft position is always at the intersection of two - or more - curves

I'd argue that even radial/dme is intersection of two (albeit very different) curves. The radial gives you a particular straight line from the station and the dme gives you a circle (around the same station). Find intersection of the circle and the line and voila!

Tom.

Woodreau
Posts: 1441
Joined: Mon Sep 24, 2001 6:44 am

### RE: Understanding Decca

 Quoting Smittyone (Reply 6):I can clearly remember feeling so 'high tech' plotting LORAN fixes at sea...we always plotted the hyperbolic lines vice reading the computed lat/lon off the display. Good stuff.

I remember plotting LORAN on the chart as a big pain in the butt, but that's all you had that gave you immediate feedback once you got out of sight of land.

Other than that all you had was a dead reckoning plot where you plotted the LORAN fix and reset your DR plot off that.

 Quoting Pihero (Reply 7):Actuallyu, youi've just revealed the basics of navigation : bar the case in which one uses a radial / dme distance, an aircraft position is always at the intersection of two - or more - curves , whether it's radial/radial, a set of three astro lines, most of the radio nav systems...

Otherwise known as Line of Position (or LOP). Your location (always where you were) is at the intersection of your lines of position. You can get a position from a single Line of Position by advancing a prior Line of Position, but in order to do that you have to keep a dead reckoning plot.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.

Pihero
Posts: 4318
Joined: Mon Jan 31, 2005 5:11 am

### RE: Understanding Decca

 Quoting tdscanuck (Reply 8):I'd argue that even radial/dme is intersection of two (albeit very different) curves.

Hats off ! Of course You are right.
Contrail designer

Pihero
Posts: 4318
Joined: Mon Jan 31, 2005 5:11 am

### RE: Understanding Decca

 Quoting woodreau (Reply 9):Your location (always where you were) is at the intersection of your lines of position.

You are right to point at the fact that unti the avent of the Decca / Loran associated to a moving map, and obviously the inertial navigation and the GPS which continuously update your position, air navigation used to be done by dead reckoning only between a known position we passed some time ago and a point ahead.
Contrail designer

### Who is online

Users browsing this forum: Mr AirNZ, phllax, sevenair, william and 26 guests

### Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos