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Old 27th Dec 2012, 18:19
  #3335 (permalink)  
Danny42C
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Danny Finds his Way Home (with help from his friends).

Last time, I threatened to bore you. I am a man of my word.

From the very earliest days of radio, the possibilities of Direction Finding by this means had been exploited. But the old manual D/F rigs needed forever and a day to give you a bearing, and even then you were never quite sure that you hadn't got a reciprocal.

Enter the Cathode Ray Direction Finder. Fanfare of trumpets for the finest piece of navigational (ground) equipment that the RAF has had in half a century ? Not a bit of it ! I cannot find (Google/Wiki) a definitive date for its introduction in service, but in early '50s some places had got it, and some not, and Driffield was one of the lucky ones. And they were still going strong (as Commutated Antenna Direction Finders - UHF had come in) when I retired in '72. They may be going yet, for all I know....... Anybody ?

Some seven years after Driffield, a USAF Colonel came round Strubby to have a look at our gear. Uninterested in the old MPN1 Bendix radar truck (outdated US stock anyway), his gaze lit on the CR/DF tube. With growing fascination, he watched a highly skilled operator (modesty forbids) safely gather in a mixed bag of Hunters, Canberras and Meteors out of the winter murk (and feeding them into GCA if required) with practised ease. "That", said he finally, with obvious sincerity, "is the best Goddam aid I ever saw !" Hard to believe, but the USAF had (then) nothing to match it.

One of his compatriots had even more reason to admire the system at that time. The obvious extension of the service from simple airfield recovery was to be the bedrock of the VHF Emergency system (on 121.5). Three or more CR/DFs, widely dispersed, transmitted their bearings by landline to a Rescue Co-ordination Centre, There they flashed up onto a screen, intersected, and showed an instant "fix".

Our chap was very high in the Blue Yonder over the North Sea. The thing he was in spontaneously combusted and he had to get out very fast. He put out a Mayday, but got no further than "May" when the flames licked his bottom and he had to break off the conversation. But it was enough.

And it so happened (it really was his lucky day) that a S&R Helicopter was going out on an exercise, and was pretty well on the spot. It hung around until they saw him floating down and positioned themselves. They fished him out of the North Sea before he finished spouting out his first mouthful. (Pity they didn't have a sort of big butterfly net available, he wouldn't even have had to get wet). He sang the praises of the Limeys to his dying day (or so the story went).

As I have said, you called for steers repeatedly and, as everybody else in the upper airspace was doing the same, the poor Approach Controller was working like a one-armed paper-hanger. A way to automate the job was urgently needed; the boffins turned up trumps. They devised the Voice Rotating Beacon (VRB).

A ground transmitter near the airfield put out a rotating narrow beam signal. This was synchronised with a closed loop which carried a R/T recorded message in such a way that a listening pilot would hear only a short message, telling him his "Steer" (to the nearest ten degrees). With your fuel contents needles almost visibly moving down as you watched, this was a great comfort. It worked like this:

On its own frequency, (of course) you would hear faintly, (say) "Zero Three, Alpha", then much louder: "Zero Four, Alpha", then faintly again: "Zero Five, Alpha", then silence for half a minute while the radio "lighthouse" was going round, and then a repeat. "Alpha" was the ident, (there were several VRBs in the UK and it was advisable to be on the right one).

In the case given, you'd turn onto 040°. As you approached your field, the accuracy would increase and when you passed o/head, the QDMs would suddenly reverse and you knew where you were. Below cloud, you would have seen your field and joined visually.

The beauty of the idea was that it could serve an infinite number of customers at once (like a GPI). But this was to prove the Achilles' heel. A number of aircraft might be at the same height in poor visibility, pilots head down in the cockpit, fixed on their D.I.s, all homing onto the same point.

There were a number of spectacular near misses (as the system was not controlled in any way), but I don't think there was ever a collision. The risk was reduced by making people fly quadrantals, but could not be eliminated. Eventually the JP came into the schools, they got their own CR/DF and took over from Driffield and its VRB.

The VRBs were phased out, but not forgotten. The quicker thinking of our readership will have realised that the basic principle was ripe for development into the VOR, with only the voice element being replaced by a transmission which actuated the aircraft panel instrument.

The last VRB was in operation in the mid-'60s. I hunted it down on a quiet weekend ATC watch in Shawbury and found it (in RAFAC) somewhere in W. Africa (Ghana, I think). Long gone now, of course, but it was a Good Idea at the Time.

Now you have all been such good boys, and not fidgeted too much, so we shall go back to our Meteor next time.

Au revoir,

Danny42C.

(he has been at the Fine Ruby Port again)