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Understanding Decca  
User currently offlinenovice From United Kingdom, joined Aug 2012, 90 posts, RR: 0
Posted (2 years 1 month 1 week 6 days 22 hours ago) and read 3124 times:

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?

11 replies: All unread, jump to last
 
User currently offlinewoodreau From United States of America, joined Sep 2001, 1043 posts, RR: 6
Reply 1, posted (2 years 1 month 1 week 6 days 3 hours ago) and read 3000 times:

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.
User currently onlinePihero From France, joined Jan 2005, 4453 posts, RR: 76
Reply 2, posted (2 years 1 month 1 week 6 days 1 hour ago) and read 2971 times:
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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
User currently offlinelonghauler From Canada, joined Mar 2004, 4992 posts, RR: 42
Reply 3, posted (2 years 1 month 1 week 6 days ago) and read 2950 times:

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?



Never gonna grow up, never gonna slow down .... Barefoot Blue Jean Night
User currently offlinenovice From United Kingdom, joined Aug 2012, 90 posts, RR: 0
Reply 4, posted (2 years 1 month 1 week 5 days 16 hours ago) and read 2874 times:

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


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 5, posted (2 years 1 month 1 week 5 days 14 hours ago) and read 2848 times:

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.


User currently offlineSmittyone From , joined Dec 1969, posts, RR:
Reply 6, posted (2 years 1 month 1 week 5 days 14 hours ago) and read 2845 times:

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.


User currently onlinePihero From France, joined Jan 2005, 4453 posts, RR: 76
Reply 7, posted (2 years 1 month 1 week 3 days 19 hours ago) and read 2703 times:
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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
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 8, posted (2 years 1 month 1 week 3 days 14 hours ago) and read 2662 times:

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.


User currently offlinewoodreau From United States of America, joined Sep 2001, 1043 posts, RR: 6
Reply 9, posted (2 years 1 month 1 week 3 days 12 hours ago) and read 2643 times:

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.
User currently onlinePihero From France, joined Jan 2005, 4453 posts, RR: 76
Reply 10, posted (2 years 1 month 1 week 3 days ago) and read 2598 times:
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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
User currently onlinePihero From France, joined Jan 2005, 4453 posts, RR: 76
Reply 11, posted (2 years 1 month 1 week 2 days 23 hours ago) and read 2596 times:
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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
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