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Thread: Question on MRU radar stations

  1. #11
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    Hi Peter,


    The fine line difference between mobile and transportable - not forgetting that if we need to move the transportable unit by air we had better not disposed of the Short Belfast that was the only aircraft that it could be transported in!


    During some work that I did on Chain Home it struck me that I knew very little about how the early CH stations operated, assuming that it was not much different to the dustbin lid full of glowing maggots that could be visually understood in a sweep of the beam.


    The first eye opener for me was that the Chain Home beam was not a 4 degree cone that swept but a 100 degree stationary fan that returned echoes from everything in front and behind at the same time.


    The operator had to manually look at the fan in steps like looking at a complete room through the letter box with a telescope. No wonder that being inattentive caused information to be missed.


    This is a description of what the screen operator did in a 1941 Chain Home station from Signals Volume V:


    "Observer
    This Radio Operator sat opposite the cathode ray tube wearing a Head and Breast Telephone set and observed the echoes on the tube.


    He (sic) dealt with them in turn starting with the echo nearest the ground ray on the left-hand side of the tube.


    He first sensed the echo to determine whether the aircraft was in front or behind the C.H. station and then side-sensed it to decide which side it was of the line-of-shoot.


    He then placed the manually controlled range pointer on the left-hand edge of the echo and took a bearing by swinging the goniometer until the minimum signal strength was obtained.


    When the operator pressed two switches which actuated the electrical calculator this caused a grid reference (controlled by the settings of the positions of the range marker and the goniometer dial) to appear on an illuminated display panel situated in front of the teller.


    The Observer then switched to the height aerials and repeated the performance causing a height to appear below the grid reference.


    A further set of switches indicated the estimated number of aircraft in the raid.


    As soon as the complete set of information was displayed the Observer announced the name of the R.D.F. station by telephone to Stanmore Filter Room followed by the track number or identification and I.F.F., if showing."


    In the 1940 version the electrical calculator was a person with calibration tables derived from autogyro test flights and a pencil, who adjusted the information from the Observer before it was sent to Stanmore.


    Quite a series of steps to be done to cover the entire 100 degree line-of-shoot when all the time the hostile/friendly was advancing at 300 miles an hour.


    All this goes a way to put the errors by the No.254 MRU operators into context. No real time for calibration, overlong watches, equipment blind spots and permanent echoes in distracting places on the oscilloscope style display.


    You can see what an advance in situation visualisation that the PPI display brought.


    Regards
    Ross
    Last edited by Ross_McNeill; 5th March 2014 at 18:46.
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  2. #12
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    Hello Ross,

    Thanks very much for your responses. I think that the Department of National Defence history directorate here in Ottawa has a copy of Signals Vol. 5 and I’m trying to borrow it now.

    The Signals Vol. 5 footnote you’ve quoted says that “The technical equipment A.M.E.S. No.254 (M.R.U.) reached Colombo from Great Britain on 23 March 1942 and was working by the evening of 25 March. The plotting line was serviceable two days later.” This agrees fairly well with the AMES 254 ORB, except that the latter says that their technical equipment came with them from Egypt, on a separate ship from the unit’s personnel.

    The statement that “The failure to detect the raid on 5 April was attributable in part to the very troublesome permanent echos and to gaps subsequently discovered in the vertical polar diagram of the station.” is new for me and very interesting. Understanding it stretches my technical knowledge a little bit:

    -By ‘permanent echoes”, do they mean that at certain bearings/altitudes/ranges there was always a blip and an enemy formation in it would be undetectable?

    -By “gaps … in the vertical polar diagram” do they mean that no signal was being emitted, or no echo could be received, on certain bearings? (I suppose that these gaps were “subsequently discovered” because the station was operational for only about 7-10 days before the Japanese attacked and too few calibration flights had yet been flown to find them.)

    “Another contributory cause of the failure was an unequally divided watch-keeping roster resulting in operators continuing on watch with diminished alertness.” I guess they’re implying here that the operators on watch between 0700 and 0730, when the Japanese should have been detected, were tired at the end of a longer than average shift. That’s interesting. A Catalina sighted the approaching Japanese carrier force the previous afternoon and the airfields went to “standby” at 0400 , so you’d think that the CO would have been on hand to make sure everyone was alert and on the ball, and that, if anything, extra hands would have been on deck.


    Cheers,

    Rob

  3. #13
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    Hi Rob,

    The Chain Home RDF/Radar of the 1938-42 era can be described as the best that could be delivered in the best time rather than the best that could be developed over an extended delivery time.

    It was generally a grouping of existing solutions in the 1934-1936 period that were adapted/extended to be included in the RDF system and came with inherent problems that persisted in the final system.

    Work at the turn of the century showed that an object would reflect a portion of the continuous wave that struck it.

    What was not clear to the Air Ministry was that an aircraft structure would reflect enough energy to be picked up by a receiver and this was part of the reason for Dowding asking for the demonstration using the existing continuous wave broadcast transmitter at Daventry.

    The eventual transmitter design used in Chain Home from 1938 varied in power output but used the same polar transmission diagram.

    The transmitter would project a lobe of pulsed radio energy in front of the station as well as a lesser amount behind and to the side of the line of shoot of the transmitter installation axis.

    Everything that is illuminated by the pulse in front of, behind and to the side, returns the initial energy. Returned energy that has passed the station can also be re-reflected off objects behind the station giving ripple effect echoes that can be received for quite some time after the initial pulse.

    Figure 1 of this excellent site shows a plan view of the lobes associated with a transmitter.
    http://www.radarpages.co.uk/mob/ch/chainhome.htm

    Moving onto the receiver, for Daventry this was the device used by Watson-Watt for picking up the energy from lightning discharges associated from Storm Clouds. They had already used a CRT display to give a display of the energy spike received. Using a pair of aerials and looking at the slight deviation in time the spike was received was used to give left or right direction and a bearing.

    What the gear used at Daventry did not give was a range or height.

    The pulsed transmitter allowed the range to be calculated from the time from emission of pulse to return of signal but that also means that echoes are also displayed as well as the first return.

    The operator needed be both skilled at operation of the set and have experienced the return from locality aircraft to be able to recognise the characteristics of false echo and discount them. This is why additional staff was not the easy answer. The establishment limited the number of trained operators and the "set time" was needed to give them the skills in radar target identification.

    For Chain Home four receiver masts were used allowing a quick means of sensing the target echo eg signals arriving first at the pair of masts behind the line of shoot could be discounted immediately and then sense left or right could be made quite quickly.

    However the bearing part needs quite a bit of operator skill and memory. When multiple targets are being processed the operator needs to associate the snapshot view they have with a previous snapshot to report a movement of a previous target or the appearance of a new one on a similar bearing.

    The last part of the picture is height and this was derived in a similar manner to bearing but not from different masts but two aerial arrays on the same mast.

    Figure 4 shows the vertical polar diagram this gives
    http://www.radarpages.co.uk/mob/ch/chainhome4.htm

    As you can see there is a gap between the lobes where reflections of any aircraft are not picked up by either aerial pair.

    In normal operation the operators will become aware of ground reflections/echoes and can discount them when a target is seen but until they have time on the set in the location all must be considered as targets or all as echoes.

    In a similar manner calibration flights and balloons will identify the gaps in the polar lobes and the operators will be aware that targets transiting these areas will reappear at a position that can be estimated. Also targets that approach in the gap of a lobe will suddenly appear at a much shorter range than most other targets detected in the lobes.

    The sea level blank area for vertical lobes was the biggest limitation of Chain Home and it was why the Chain Home Low was developed to plug the gap of aircraft in the 5,000 ft and 25 to 30 miles area.

    CH sets are only one part of the solution to coastal radar, they need CHL and CHEL to fill the gaps. For looking inward or across land a whole new set of gear is needed to be effective.

    Regards
    Ross
    Last edited by Ross_McNeill; 23rd March 2014 at 13:24.
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  4. #14
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    Rob, Hi,

    Radar (the Dark Arts of The Wiggly Amps) can behave in some very queer ways (many connected to meteorology in one form or another). One of these is ducting! Ducting occurs when the temperature and/or humidity in the atmosphere changes rapidly with height. This can cause (sometimes severe) changes in either the propagated signal, or the return signal (quite apart from the bog-standard polar diagram - as per Ross' post). This, in turn, can be used for either offensive, or defensive, purposes provided ‘we’ are aware of whatever ducting is occurring. I suppose one of the modern instances is that navy ships have their radars mounted as high as possible – to get max range. But just above the sea surface the temperature/humidity changes rapidly – the dreaded Surface Duct. Ships’ radars – trying to see ‘down’ as well as ‘up’ – are refracted by this duct, and can often ‘see’ little at very low level. If you can fire a missile under the duct – say, an Exocet as in the Falklands - then you stand a good chance of it being undetected until it’s too late!!

    Some smart radars could alert you when a target was moving or not. MTI – Moving Target Indicator. On some early versions the wet leaves on the trees in the nearby copse in a fair wind could cause the system to alarm!!

    And don’t even begin to venture into the history/evolution of the dreaded Radar Fuse for the Dropshorts!!!

    I would think that it’s all improved since I was involved in the late 1980’s, but for those fighting a war in the infancy of radar it would – I suspect – have caused a considerable flow of Anglo-Saxon 4-letter words from Operators, Supervisors, and Boffins, etc, etc.

    And when your operator has done 8 hrs on duty on the scope (ok, possibly rotated with others), then 8 hrs on Guard Duty, then 8 hrs eating/sleeping, you can get a very significant fall-off in attentiveness after only a few days! I know I did - and I didn't have to do Guard Duty.

    HTH
    Peter Davies
    Last edited by Resmoroh; 23rd March 2014 at 14:11.
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  5. #15
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    Quite right Peter on the Met effects on the propagation, all that you said and more - to the benefit and to the determent on a day by day basis.

    Used to work on Tropospheric scatter links in the 80s that was used and tweaked with the met effects to wring that little bit more unlisted performance out the boxes.

    The polar charts are good for estimates but amazing the changes that can happen (and be predicted).

    On an island at the eastern end of the Med an aircraft on the ground needed two engineers, one local and one at Cottesmore, to talk one on one - no secure comms. MoD passed the request on to UK met magician who came back with a VHF freq and a 15 min time slot in 3 hrs time.

    Sick aircraft was dragged out to a remote pan and ground power applied. At the appointed time Med bod keyed the mic not expecting any result on the short range VHF and was answered in clear by bod at Cottesmore doing the same on aircraft on the pan there.

    Regards
    Ross
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  6. #16
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    Ross, Hi,

    Ah!, Troposcatter and OTH “Woodpecker”, etc! Sounds like you and I may have come perilous close to colliding in our previous incarnations? TCW? But it was un morceau d’urine (so to speak) to get, in the field and away from STC predictions, some reasonable frequency info by using Chirpsounders, etc. Most of us didn’t understand the Peruvian Nose-Flute Music that underlay it all – but we could look at the results and say (HF comms) “OK, we’ll go for 4’s until 0300, and then switch to 14’s”.

    Good days

    Peter
    Meteorology is a science; good meteorology is an art!
    Septimus, quia ego vado transgressus esai levabit pons!

  7. #17
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    Hello Ross,

    This is a belated thanks for your numerous helpful posts on this.

    I am currently writing an article I'm calling A First Look at Why the British Fighters were Caught on the Ground at Colombo. I'll probably offer it to the good folks at http://www.combinedfleet.com/kaigun.htm for posting in their Articles of Interest section. A portion of it attempts to explain why AMES 254 did not provide timely warning of the approaching raid and the main portion of this part currently reads as follows:

    Sufficient information is available for some informed speculation as to why AMES 254 provided no timely warning on 5 April. In the first place, we know from a cable from Layton to Churchill that AMES 254 “experienced interference from hills” and had a range of approximately 60 miles.46 This was not the best that could be expected from this type of radar. With similar hardware, AMES 272 picked up the Japanese aircraft approaching Trincomalee on 9 April at 91 miles, for example.

    As noted, we also know that AMES 254 was located at Ridgeway. Ridgeway is about six miles north of Ratmalana, where three of Colombo’s four fighter squadrons were based, and one mile east of the improvised airstrip at Colombo’s racecourse, where the fourth, 258 Squadron, and the Blenheim-equipped 11 Squadron were based. The leading element of the Japanese formation, which of course was approaching from the south, was a chotai of nine Zeros from the carrier Hiryu. The Model 21 Zero cruised at just over 200 mph, so Hiryu’s chotai probably covered the roughly 54 miles between the nominal edge of AMES 254’s coverage and Ratmalana in about 17 minutes.

    Seventeen minutes should have been enough time, one would think, to get word to the airfields before the enemy arrived overhead, and of course the Vals and Kates were a few minutes behind the first Zeros. There was probably no way that AMES 254 could have provided enough warning for the Hurricanes and (especially) the Fulmars to get to altitude, but it should have been possible to at least get them off the ground.

    After the war the Air Ministry’s Air Historical Branch produced a number of “staff narratives” on the RAF’s wartime operations. Signals Volume V: Fighter Control and Interception makes two relevant comments. The first is that “The failure to detect the raid on 5 April was attributable in part to the very troublesome permanent echoes and to gaps subsequently discovered in the vertical polar diagram of the station”.47 To deal with permanent echoes the operator needed be both skilled at the operation of the set and have experienced the return from local aircraft to be able to recognise the characteristics of false echoes and discount them. AMES 254 had gained operational experience in Egypt and presumably had adequately trained operators, but, having been operational at Colombo for less than two weeks, it is unlikely that all its operators were yet familiar with local conditions. It’s pure speculation to say so, but it is perhaps possible that the operators on duty on 5 April may initially have taken the real echoes from the enemy aircraft as being false echoes.

    The reference to “gaps … in the vertical polar diagram of the station” may require some explanation. AMES 254’s transmitting antenna array consisted of parallel dipole elements. In the photo at http://www.iwm.org.uk/collections/item/object/205210707 of a Mobile Radio Unit (MRU) such as AMES 254, the transmitting mast is on the left and it can just be made out that there are three parallel dipoles pointing toward the camera and another three diploes perpendicular to them. Each of the six diploes radiated its own signal and thus had its own lobe. The lobes overlapped but there could also be gaps between them. In the height/range diagram in the figure below,48 a small gap can be seen between lobes ‘A’ and ‘B’, meaning that aircraft flying at the relevant altitude would be detected at a shorter range than at a lower or a higher altitude. In the case of AMES 254 on 5 April the altitude at which these gaps occurred is not known, and consequently it is not known whether or not the leading Japanese aircraft fortuitously flew in at an altitude at which they would be detected at something less than 60 miles out. However, the quote from Signals Volume V seems to imply that this was the case.


    [Fig. 4 from http://www.radarpages.co.uk/mob/ch/chainhome4.htm will be inserted here.]

    The second relevant comment in Signals Volume V is that “Another contributory cause of the failure was an unequally divided watch-keeping roster resulting in operators continuing on watch with diminished alertness.”49 This clearly implies that the operators on duty when the Japanese came within range had been at their consoles too long. In this context it is important to note that the operators did not have a PPI (Plan Position Indicator) display, the radar ‘scope familiar to everyone today, which displays both range and bearing. At Colombo the operators had “A scopes”, which gave only range.50 Working out the bearing, height and strength of an approaching formation entailed a number of steps which needed to be completed in sequence, and fatigue would no doubt slow things down and tend to result in errors.51

    [A short summary will be inserted here.]

    Here are the relevant footnotes:

    46. Layton to Churchill, 2 May 1942, IZ 155, 1353Z/2, in AIR 20/4693.
    47. Signals Volume 5, footnote on page 62, quoted in Ross McNeill post of 5 March 2014 at http://www.rafcommands.com/forum/sho...radar-stations.
    48. From http://www.radarpages.co.uk/mob/ch/chainhome4.htm, downloaded April 2014.
    49. Signals Volume 5, op cite.
    50. For an explanation of radar display types, see A.P.1093C, Vol. I, Chap. 3, at http://www.vmarsmanuals.co.uk/archiv...093C_Vol_1.pdf.
    51. The process is described in a passage from Signals Volume 5 quoted by Ross McNeill in his second post of 5 March 2014 at http://www.rafcommands.com/forum/sho...radar-stations. The author is indebted to Ross for his insights.


    Your information was very significant for this article, so I thought I'd give you a sneak preview. Of course, please feel free to comment on it.

    Thanks vey much,

    Rob

  8. #18
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    Just a little footnote!
    I think something should be mentioned about the problems of erecting the masts on which all these Tx/Rx antennae could be hung.
    In one incarnation I was part of the RAF Tac Comms Wing. We went around the globe from war to exercise to war, etc (wherever it/they happened to be!). The basic ‘pole’ was the SPM 30 – a 30 ft mast made up of 6 x 5 ft aluminium tubes that slotted into each other, and was guyed by 4 x guys. These could be erected with little technical skill. Two of those and you could hang all manner of fancy dipole/longwire aerials on them. The next size up was (IIRC?) the SPM 40(?). Same principal. But, again, could be erected with a sufficiency of (non-technical?) labour.
    Next up was the SPM 80!! This was a big bu$$er. You had to lay it out on the ground and then turn it through 90-degs (twice) using Turfor winches to get the damn thing upright! And when we were going to hang log-periodic antennae (the dreaded Grainger log-periodic) from it then EVERYBODY on the erection team HAD to know what they were doing or folk could get killed. Sgts (and even Cpls!) in the Aerial Erector Trade would (should, and did!) order people about (some of whom were Junior Electrical Engineering Officers with Degrees!!).
    And then, when we’d got the bl00dy thing up, and connected the antennae to the Tx/Rx’s we would find that the RN Ship we were supposed to contacting was now in a different sea/country/continent (delete as appropriate)!! And we had to move the log-periodic to a new azimuth!! Much cussin' & swearin'!!
    What I’m trying to exemplify is that all these technicalities are relatively harmless PROVIDED everybody who knows what they’re doing is there and does it. These huge, early, mobile(?), radar masts must have been a right cow to get sorted out. Add to that the temp/humidity in Ceylon, and it’s wonder that they got anything up!
    HTH
    Peter Davies
    Last edited by Resmoroh; 21st April 2014 at 19:27.
    Meteorology is a science; good meteorology is an art!
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  9. #19
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    Rob,

    I should perhaps mention that the quote from Signals Volume V that Ross mentions is actually a footnote on page 63, not page 62. Sorry to be pedantic, but I thought it important to ensure this is cited correctly.

  10. #20
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    Ian, thanks for the clarification. Appreciate it.

    Peter, I can certainly relate to your experiences. For several years in the 70s and 80s I was an officer in what had been called the Royal Canadian Corps of Signals before the Canadian services were unified in 1968. Our linemen did the type of work you talk about, especially the erecting and maintenance of large permanent radio and radar masts. Of course our radio operators had to put up whatever kind of mast was needed for their tactical radios. For any mast which needed to be guyed, even a small one, there was a real art to putting it up, especially in the dark. We were fairly good at it, but on one we occasion messed it up, the mast came down and one section was snapped in two. I was in charge on that occasion!

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