How Did OBOE Work?

[The Old Fogducker]

Hello Everyone:

I saw a TV program recently on the Canadian History Channel which included a short explanation of the impact of OBOE on bombing accuracy. This show was produced in the UK and was quite educational.

It left me with several questions regarding through cloud accuate bombing techniques, some of which were answered by searching on the Internet.

OBOE and GEE were quite amazing systems for their day with many similarities to flying a DME arc approach today. To obtain accuracy within 35 yards with the electronic timing involved was nothing short of "weird science" in it's day, which must have been a great step forward in the development of the electronic circuits, particularly so in the days of vacuum tubes, manufacturing component tolerances, and the mechanical stability required of both the airborne equipment and ground stations.

The only question remaining is how did the aircraft receive the dots and dashes indicating it was left or right of the arc on the desired inbound track?

Was that a function of the Cat station by manual means after the operator noticed the track error from the transponder, or an automatic system of some sort when the aircraft varied from the "slot" on the CRT display? Or was it perhaps some form of null system to cancel the dots and dashes or combine them into a single "on course tone" when the aircraft was "in the slot" waiting for the signal from the Mouse Station.

This part is intriguing to me....the rest all makes perfect sense and is fairly easy for me to visualize. After all, flying an accurate DME arc can be quite a challenge from time to time .... remaining within the centre of a 100 ft wide track would likely require some concentration!

Thanks for taking the time to help me figure this out.

Regards,
The Old Fogducker

[BEagle]

Have a look at http://www.radarpages.co.uk/mob/navaids/oboe/oboe1.htm

Pretty amazing for something dreamed up over 60 years ago!

[The Old Fogducker]

Hi Beagle:

That was somewhat helpful, but still struggling with the mechanics of what would prompt the change in the fly left/right audio signals based on aircraft position.

Fog

[BEagle]

"The train of pulses from the tracking station, which commonly has a recurrence frequency of 133 pulses per second is modulated by Morse signals to convey information to the aircraft. Thus if the aircraft is at too short range a series of dots is sent; if at too great a range, a series of dashes is sent. Additionally various letters in Morse code are sent to notify the pilot when he is within certain distances of the prescribed track, and other letters to denote certain distances from the release point."

I guess this means that, since it's a secondary radar system controlling only one aeroplane at a time, the ground station interrogates the airborne transponder and deduces its range. If less than the pre-computed radius, the signal is then modulated with dots, if greater then it's modulated with dashes. The pilot manouevres accordingly until equi-signal is received.

[Self Loading Freight]

One way of doing it would be to have one circuit generating dots all the time, and one doing the same with dashes. The transmitter sends a signal and chooses which circuit to use depending on the reply from the transponder. That's just timing - if the signal from the aircraft is earlier than it should be, the transmitter chooses the dot modulation; if it's later, then the transmitter uses dashes. If it's right on time, then neither.

The electronics behind that isn't complicated - triggering actions based on when pulses arrive is bread and butter stuff - but this was the first time it had been done reliably to that level of accuracy (as far as I know). One of the many nice things about the system is that the stuff on the aircraft isn't anything special: the latency of the transponder has to be well known (and may be swamped by propagation variables anyway - I wonder what the major errors were), but otherwise it's just basic radio gear.

Another thing I've wondered about - was Oboe chosen as the code name because the 133pps pulse train sounded like one?

R

[PAXboy]

Currently I am reading Bomber by Len Deighton and, by coincidence, have just passed the chapter that involves the Mossies on Oboe duty on the night of the fictional raid.

It makes reference to the system being able to handle a maximum of two a/c in a ten minute window.

My father was on night fighters but in attack roles, rather than pathfinder. So he was involved in Gee and Serrate, not Oboe.

[The Old Fogducker]

Hello Folks:

Thanks for adding to the info stream on this topic.

Regrettably, I'm still no further ahead than when I began.

I recall from the TV program that a CRT was used to display the time differences between the aircraft's actual position and desired, but I guess the question could be further defined to be "Was it a manual or automatic system?"

The operator could easily have operated a toggle switch to signal the dots or dashes based on the AC being left or right of track. This manner of controlling would be somewhat similar to a GCA procedure, except using dots & dashes to take the place of the controller's narrative I used to hear in my headset .... "a little low, decrease you rate of descent, correcting nicely ....etc"

Also, it could have been done by sensing circuits based on the arrival time of the transponder signal vs the desired time interval, with the automatics making the selection between dots and dashes.

Anyone with any more definitive info to add?

It may be this level of detail about the OBOE System is now lost to us "youngsters" in aviation.

Regards,
Fog

[Onan the Clumsy]

I thought intersecting DME arcs was H2S.

[Gainesy]

H2S was an air-to-ground mapping radar.

[Daysleeper]

Called H2S cos of the stink over how late into service it was :D

[Onan the Clumsy]

Oh yeah. Wasn't that where Cunningham was testing an air to air system and the a/c got into a spin. As it was being recovered, he noticed that he could see streets and buildings etc.

[henry crun]

Havn't heard that story before Onan, do you have a source ?

I think history shows that H2S was a development of the early airborne ASV radars and not a chance observation such as you describe.

The invention of the magnetron made airborne centrimetric radar a reality.
Two experimental models, for Coastal Command called ASV 111 and for Bomber Command called H2S, were developed more or less simultaneously in 1941.

Daysleeper is nearly right. Lord Cherwell subsequently asked the development team why they had not pushed the work and when they made various excuses he said, "it stinks, it stinks".
This prompted the thought of Hydrogen Sulphide but they did not expect Cherwell's obvious question which was, "Now why did you call it that ?"
An unnamed boffin replied "please sir, Home Sweet Home". :D

[Onan the Clumsy]

Henry I think I read this when I was but a lad. It was in a paperback, but I can't remember if it was an autobiography, a biography, or just a story of the development of nightfighting and the development of radar interception. I'd go and look, but all my books are tucked away in an attic in England somewhere. :(

[Biggles Flies Undone]

Somewhere in my loft I’ve got a copy of ‘Very Secret War’ by R V Jones. Think it covers this stuff but may well be out of print now. If anybody would like to borrow it, drop me a message.

[Simtech]

BFU,

Think you mean "Most Secret War" by R.V. Jones. It's been reprinted in paperback by the Wordsworth Military Library, ISBN 185326699X, U.K. price £4.99.

This is one of the best books I have read about the intelligence war 1939-45, and also includes details of how the various radio navigation systems worked.