Just how does one navigate by radar?

Cartography in the Twentieth Century

Just how does one navigate by radar?

Postby John Horrigan » Sat 31 Mar 2012, 15:23

Can anyone shed some light on this World War Two RCAF map? I have no idea what I am looking at.

Top line: Scale 1 : 1,000,000 Reims Fixing Chart Radar Plotting Chart Edition of December 1944 London - München
Bottom line: C.S.G.S. 4587 ABC Published by War Office 1944 [I can't read the rest.]

The map was owned by a member of the Royal Canadian Air Force who flew in bombers during WW2. It includes penciled annotations showing what I assume is at least one bombing mission SW of Frankfurt, but I’ve no way of knowing when the annotations were made.

I have large (3M) images of this and several others if anyone would like a closer look.
I'm primarily interested in the base map. What are the curved lines? How are they used to navigate?

Many thanks - John
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Frankfurt.jpg
Radar map.jpg
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Re: Just how does one navigate by radar?

Postby Ian Williams » Wed 09 May 2012, 16:37

Looks like a parabolic radio base map. Read up LORAN http://en.wikipedia.org/wiki/LORAN or DECCA http://en.wikipedia.org/wiki/Decca_Navigator_Systemn navigation systems for more advanced equivalent systems. In WWII there was an early UK system called OBOE which required a pathfinders to follow a signal on a parabolic curve, and for a land based station to signal when the a/c was over target.
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Re: Just how does one navigate by radar?

Postby Terry J Deveau » Tue 15 May 2012, 20:08

I'm not exactly sure about these specific maps, but in general, if you are receiving synchronized radio time signals from two separate transmitters, and you compute the difference in the time of reception between them (say, in milliseconds) a line on a map that represents a constant time difference is a hyperbola. So if you measure the time difference between two such signal receptions from two such transmitters, and you have a map with hyperbolas of constant time difference for those two transmitters drawn on the map, then you know that you are somewhere on that specific hyperbolic line.

Now if there is a third transmitter, and second set of hyperbolic lines for the time difference between that signal and one of the first two, and you do the same thing for the second time difference, then you can fix your position precisely where the two hyperbolas intersect.

Terry
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Re: Just how does one navigate by radar?

Postby John Horrigan » Thu 28 Jun 2012, 17:08

Thank you both for your help. The Wiki links were a great foot in the door and things make much more sense now.
There appear to be six transmitters generating three sets of parabolic lines, but only four towers. The central tower, marked in white, seems to have three transmitters. It's located just outside Reims, hence the Reims Fixing Chart.
Thanks again - John
Attachments
ABC - LL corner.jpg
Reims Tower Array.jpg
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