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Thread: Question on MRU radar stations

  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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    Ross,

    Thanks for alerting me to the AHB narrative on the Far East Campaign. Ill have to get my hands on a copy of it. However, Id point out that the AHB narrative on radar says the opposite, and as its 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, Id 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 Im 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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    Ross,

    I have done some more research and have more or less answered at least part of my original question. I was hoping to find a specific figure in degrees indicating how wide an arc the signal from the directional antenna array could cover (e.g., 90 degrees). I didnt find any specific figure but did learn that the signal floodlit a wide frontal area and that the diploes had reflectors, so that most of the signal was directed to the front. The radiation pattern may therefore have looked liked Figue 5 on the page at http://www.hp.com/rnd/pdf_html/antenna.htm. If this is the case, then maximum range was attainable across an arc of 40 degrees and close to maximum range for at least a further 40 degrees, so pretty good range across an arc of at least 80 degrees.

    I agree with you that AMES 254 was not going to go mucking about with its antenna orientation on 4 April after Birchall sighted KdB approaching, especially as it had taken them three days to put their masts up in the first place. Considering that Colombo was some way up the west coast of Ceylon and taking into account Ceylons position in the Indian Ocean, it would have been clear to 222 Group that Japanese carrierborne aircraft could attack Colombo from west, southwest, south, or any bearing east of south. Since ground-based observers could give some warning of an attack approaching from the south or anywhere east of south, Id be sorely tempted to point my radar to the southwest. Also, if I were expecting an attack from the south Id want to put my early warning radar to the south of my main fighter base and not seven miles to the north of it.

    To summarize this scenario:

    -Layton says that AMES 254 had a range of 60 miles
    -if the antenna was pointed more than 40 degrees off due south there would have been some reduction in range. A 10 percent reduction would make the range 54 miles
    -subtracting the seven miles by which AMES 254 lay to the north of Ratmalana leaves 47 miles. The Japanese aircraft were approaching at about 150 MPH, so there would have been less than 20 minutes left to get a warning through to No. 20 Ops Room and for it to get a scramble order through to Ratmalana and Racecourse.

    This is of course only one possible explanation. As you say, more digging is needed.


    Rob

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    I'd like to note that I was apparently wrong to state in this thread from a year ago that MRUs had directional antennas. It seems that MRUs and TRUs had the same electronics as Chain Home stations but different "all round" antennas.

    The last photo on page 9 of the document at http://www.dunwichmuseum.org.uk/resl...0Dunwich11.pdf shows the crossed horizontal dipole receiving antenna. I also have a photo, not in the public domain, of the transmitting antenna. It also had crossed diploes, but it looks like there were six of them - three parallel to each other facing one direction and another set of three at right angles to them. The photo at http://commons.wikimedia.org/wiki/Fi...45_CH15200.jpg shows a MRU in operation, with the transmitting mast on the left and the receiving mast on the right, and it is clearly visible that the antennas are small arrays attached to the mast and not slung between them.

    I think that the radiation pattern from the transmitting antenna was probably like the pattern shown as view "A" in figure 4-39 in the document at http://www.rfcafe.com/references/ele...-4-41-4-50.htm. If this is correct then the coverage from a MRU or TRU station was as near to omnidirectional as makes no difference and antenna orientation was not a factor in the Japanese attaining surprise at Colombo.

    If anyone is in a position to confirm or refute any of this I'd be grateful to hear from them.

    Thanks,

    Rob

  9. #9
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    Hi Rob,


    You may like to have a look at Signals Vol 5.


    The footnote on page 62 says:


    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. 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. Another contributory cause of the failure was an unequally divided watch-keeping roster resulting in operators continuing on watch with diminished alertness.


    Regards
    Ross
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  10. #10
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    Ross (et al)
    That footnote was clearly drafted by someone who had suffered all the vicissitudes mentioned. I felt for him – even after all this time - having had much the same, more modern, experiences. The vertical polar diagram anomalies referred to were exploited in our Cold War (and subsequent) Tactical Decision Aids (TDAs) whereby (to put it simply) WE could SEE (or forecast!) where the gaps were in THEIR radar coverage/returns and send OUR a/c into those gaps! But getting a big AMES up and running in 48 hrs was good going! Compare that with transporting 1 ACC (in its TINSMITH incarnation) which took near half the Herc fleet!!! Made 1 ACC about as tactically mobile as Windsor Castle!!
    HTH
    Peter Davies
    Meteorology is a science; good meteorology is an art!
    Septimus, quia ego vado transgressus esai levabit pons!

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