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  1. #1
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    Default Question on MRU radar stations

    I am trying to figure out why AMES 254, a MRU (Mobile Radio Unit) radar station, did not detect the Japanese aircraft approaching Colombo on 5 April 1942. MRUs had directional antennas, much like the Chain Home stations, so I have to wonder if maybe it was pointed toward the southwest instead of south, which is the direction the Japanese flew in from. Can anyone say how wide an arc a MRU could cover? For example, if oriented to the southwest could it pick up aircraft approaching from the south? Also, to what extent, if any, was range reduced toward the edges of the arc which could be covered?

    Thanks,

    Rob

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    Of the two radar stations only one was operational (Trincomalee). The one at Colombo was still being installed and so could give no warning of raid. It was only when visually observed that alert was given.

    However the early, albeit confused, report from the Catalina patrols had meant that Hurricane pilots were at readiness sitting in the cockpits.

    Weather also played a factor with conditions at Colombo of 8/10 to 10/10 cloud with a base of 2,000 feet. Heavy tropical rain was falling with the associated gusts.

    Regards
    Ross
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    Hello Ross,

    In fact, I have the ORB for AMES 254, a 2 May '42 cable from Layton to Churchill and the AHB narrative called "Radar in Raid Reporting" which all say that AMES 254 became operational on 28 March. Secondary sources which say otherwise may have confused AMES 254 with the other radar unit in the Colombo area, AMES 524, which was still being installed on 5 April. The 222 Group report on the Colombo raid says that the early warning system "failed" on 5 April and not that there wasn't one. The AMES 254 ORB notes that its Lewis guns were in action on 5 April but does not say whether or not the radar detected the approaching aircraft.

    Cheers,

    Rob

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    Hi Rob,

    My info comes from two different sections of the AHB Narrative Far East Campaign Vol III India Command.

    The most pertinent being in the section entitled "The Attack on Colombo". Actual references and words used are:

    IIJ50/14/1 D of Ops Folder cited as source of Colombo not having operational Radar

    IIK18/9 and IIJ50/14/6

    "34. The radar system was still in process of installation at Colombo and no information regarding the approach of enemy aircraft was therefore received until they were actually sighted."

    This suggests that no unit at Colombo was operating on the 5th.

    Regards
    Ross
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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

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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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  7. #7
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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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  8. #8
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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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    Ross,

    Thanks for alerting me to the AHB narrative on the Far East Campaign. I’ll have to get my hands on a copy of it. However, I’d point out that the AHB narrative on radar says the opposite, and as it’s specificly on radar I think it should be the more reliable – and it was completed five years after the war instead of only two. Furthermore, I’d say that the AMES 254 ORB has to be acknowleged as the primary source for what AMES 254 did, and it notes very clearly that:

    -its personnel arrived at Colombo on HT Talma on 18 March and its equipment arrived on another ship at 0900 on 22 March. My the end of the 22nd the “R” mast was laid out ready for assembly at the Ridgeway golf course, about seven miles north of Ratmalana

    -assembly of the R mast started at 0600 on 23 March

    -the “T” mast was erected on 24 March

    -the station became operational at 1800 on 25 March, but without telephone lines to the operations room

    -on 28 March the station was connected by telephone to the operations room, making it fully operational

    -the section of the ORB covering April gives no indication that it detected the approaching aircraft on 5 April but there is a note saying “Normal Operations throughout month”, which indicates that it was in operation on the day of the raid.

    There is also, as I mentioned, a 2 May cable from Layton to Churchill in which he reported that “M.R.U. Ridgeway [AMES 254] installed 25th March experienced interference from hills. Range approximately 60 miles.” As C.-in-C. Ceylon, Layton would have got this information from 222 Group, so this is a reliable primary source. His cable to Churchill may not have been available to the AHB in 1947.

    I also have the ORB for No. 20 Operations Room, which was the ops room to which AMES 254 was connected. It notes that “the early warning system failed to provide information on the Japanese aircraft until they were very close to Colombo.” So the AHB Far East narrative is right that there was no early warning but almost certainly wrong about why that was the case. I think the explanation remains to be determined, and I’m just wondering if part of it might be that the station's non-rotating directional antennas might not have been pointing to the south. The Japanese approached from a little east of south. They made landfall about 60 miles south of Colombo and approached it over the land and not over the sea.

    Rob

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    Hi Rob,

    One that needs a great deal of spade work to get the originals and try to fathom out why the words were written. I suppose failure/non operational can mean different things to different people.

    What is clear is that Colombo was on alert and that various reports had drawn attention to the location of the Japanese task force so I would be surprised if radar the could be turned was pointed in any direction other than ordered.

    Catalina report on the evening of the 4th reported the large enemy force approacing from the South East some 360 miles out. Despite the loss of this aircraft and others elements of the force were shadowed during the night until lost in the early morning due to adverse visibility.

    In the early hours of the 5th the CinC East Indies station gave orders for dispersal of shipping from Colombo to the north and the west including Dorsetshire and Cornwall to Addu Atoll.

    Catalina regained and reported contact with one Japanese Battleship and two Cruisers at 07:00 on the 5th, subsequent messages sent in error were quickly corrected but there is no indication that any carriers were reported as part of the fleet composition.

    AOC Ceylon had assumed that the carrier position was associated with the erroneous report 85 miles east of the 07:00 Battleship/Cruiser report.

    A Blenheim strike was assigned but these could not take off until after the first Japanese air attack at 07:40 hrs on Colombo Harbour and a second wave of approx 40 aircraft going for Ratmalana where the RAF/FAA aircraft were either airborne or dispersed when the aerodrome was hit.

    So shipping dispersed, part of the Japanese force being shadowed in contact and aircraft in the process of taking off.

    Regards
    Ross
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