The radar equipment was fundamental to operating the aircraft, it was a prime aid for navigation as well as for allowing the crew to deliver their weapon on target.

‘Shifting’ the display Interpreting the low level picture Photos of the radar display

The main equipment operated by the Navigator Radar (the crew position I occupied) was, as his role suggests, the H2S radar and the NBS bombing equipment. In many early documentary films of WW2 one can see radar displays used by those who were using early radars to detect incoming German fighter aircraft. These would usually show a single trace with a ‘blip’ at some range indicating an ‘echo’.  The radar in the Vulcan was picture produced by a rotating aerial producing a Plan Position Indicator (PPI) display whereby ‘returns’ would produce a brightness on the screen, the shape and intensity of which depended on radar settings and the ground feature producing the return.  An example can be seen in the photos below.

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The H2S set in the Vulcan had a 9” CRT display, and the radar could be set to have a scale like a map, i.e. 1/8mill, 1/4mill, 1/2mill and 1Mill. At these settings the radar had a range of 23 nautical miles (nm), 46nm, 92nm and 184nm, respectively. However, because the display was only 9” this meant that the range from the centre of the PPI to the edge only showed a range on each scale of approximately 7.5nm, 15nm, 30nm and 61nm. To see further than these ranges on each scale the ‘time base origin’ of the display (the aircraft position which in the photos below is the dark circle in the centre of the screen) has to be ‘shifted’ from the centre.  This shift is applied to the coils of the CRT by varying the voltage applied to them by movement of the 626 control stick, this effectively varies the voltage to the coils so moving the time base origin from the centre and often ‘dragging’ it off the screen so that the maximum range of the radar on any setting may actually now appear in the centre of the screen.  So the operator could leave the time base origin in the centre of the PPI in which case on any particular range setting he would only see the reduced ranges of 7.5, 15, 30 & 61 nautical miles mentioned above and the target would appear to fly towards the centre of the screen at a speed representative of groundspeed or he could use ‘shift’. In this case the time base origin would be shifted, or dragged, by the 626 (making the increased ranges of 23, 46, 92 and 184 nm available) so that the target could be dragged to the centre of the screen and the aircraft (represented by the time base origin) would appear to move from the dragged position to the centre of the screen, the target.  The difference is easier to see in operation than it is to put into words.

The photo above right shows the H2S PPI display with the 626 control stick in front of it. By moving the 626 in any direction it will move the ‘centre of scan’ or the ‘time base origin’ of the PPI display.  This shift allows more of the range along track to show on the display.  This is shown diagrammatically above left.  The centre of scan has been moved to bottom left of the screen, and on the 1Mill and 1/2mill ranges the PPI shows a heading marker and 10 nm range markers. The target or fix-point at ‘X’ is some 70 miles from the aircraft position, almost on track. If the 626 is moved bottom left a little more then the centre of scan will ‘drop off’ the PPI but the target (‘X’) will have been dragged onto the screen and will be visible as a radar return.

This is a good shot taken from one of Ken Townsend’s photos. It is from XL360 at Midlands Air Museum, Coventry.  The antenna controls are on the left with the PPI in the front. PPI scale selector is on the right of the screen showing 1/8 1/4 1/2 and 1 million scales. For those who don’t know, the orange pad is for the nav radar to rest his head on when he is working the radar.  The pad is on the R88 camera which takes a film of the radar picture all through the bomb run.

Interpreting the low level radar picture.

One of the basic skills that the Nav Radar has to acquire is the interpretation of the radar picture, this is fundamental to the role. The picture produced by the radar and presented to the Nav Radar on the CRT can vary in a number of ways, some of them under the control of the Radar.  As has been mentioned elsewhere the Radar can tune his picture and the returns he gets by selecting the aerial into a sweep mode, by varying the declination of the aerial (aiming it downwards rather than straight ahead, and by tuning brightness and contrast as you would a TV picture.  These things can vary the image painted by the radar. There are other things that are outside the control of the operator. At low level any high ground around the aircraft, or tall structures, will cause a 'radar shadow' so for example a ridge of high ground three miles to starboard, parallel to track will cause a bright return on the 'scope' showing the ridge three miles away. However, as the radar cannot see through the high ground, the radar will be black from three miles to the edge of the screen unless there is another ridge some miles away and taller than the nearest one. In this case there will be a bright smudge for the first ridge, then a black area, then another smudge from the further ridge. Were the aircraft to climb so that it was higher than the nearest ridge, then the black area between the two ridges would fill in as the radar became able to paint the ground between the two ridges.  As the example shows, the radar picture changes with the relationship between the height of the aircraft and the ground features around it, it will also change as the range from the features changes, so the 'aspect' between the aircraft and the features changes, so changing the shape of the feature on the radar.  The skill the Radar needs to acquire is how to predict in his flight planning how the features on a two dimensional map will translate into a radar picture in the air, on his heading and at his height, he then needs to know how that will change as the aircraft flies past or over those features.

Water in the form of lakes or rivers can show well on the radar, while the water itself is not producing a useable return, the river or lake banks do.  However, this can look very different flying over Canada in winter, something we did a lot from Goose Bay in Newfoundland. In winter the lakes are frozen over, and the ice is then covered in snow, which blends well with all the other snow! In this situation (similar to Russia in winter when they were the Cold War enemy) planning fix points or offsets using land/water features such as headlands in a lake that look great on a map may well not show at all on the radar.  Again, it's all down to learning to interpret the picture, and a bit of experience. The first trip I made to Goose Bay in winter I made a mistake that I am sure other Radars have made before me.  As we approached the Newfoundland coast I wanted a long range fix on the coast to get the Plotter an eta update for coasting in. I saw what I believed to be the radar returns from the Canadian coast and took what I believed to be a fix.  The Plotter got a bit of a shock as it indicated we were some 30 miles closer to Canada than he expected us to be.  After a few minutes we realized that I was in fact getting returns from the sea ice which was following the shape of the coast, not the coast itself.  I was ready for it on the next trip.

The photo below shows a little of what I mean. On the right of the photo, ‘X’ is some ground rising to the right and beyond that what appears to be a frozen lake, ‘Y’. The lake is likely to be in the radar shadow of ‘X’, and unlikely to show.  Also ‘Z’ is a ridge rising in the middle of the photo, almost on the nose,  and going to the right of the photo. This ridge will show as a radar return, but all beyond it will be in shadow. On the PPI display this means that while the low ground left of the nose is all likely to show, the radar picture to the right of the nose will have a bright return from ridge ‘X’, shadow at ‘Y’, bright return from ridge ‘Z’ only a couple of miles away, then nothing but shadow.  A very different picture from the birds eye, two-dimensional view, given by a map of the ground they are flying over. The photos further down this page give a good idea of the radar picture over Canada.

Photo above courtesy of Andy Leitch.

The image below of the map and the two PPI display photos below that were all taken from a file in the PRO,  (AIR 2/14578)   J Dillon

The map and the photos are from a B.C.D.U. trial that was done in Canada in 1961 looking into the problems of navigation and radar interpretation at low level.  The trials were done in a Valiant using an older version of H2S than the one we had later in the Vulcan. The PPI display photos however are very representative of the display we would have had.

The map above and the two Plan Position Indicator (PPI) photos show the aircraft at 5418N and 6329W, over Canada.  This is a very sparsely populated area, with almost no roads, but thousands of small lakes. This makes navigation difficult, especially in winter when the lakes freeze over and then get covered in snow.  At that point most land features are difficult to read.  You can see what the terrain is like from the pilot’s seat on another page.  Although it is difficult to see on the map, points ‘A’, ‘B’ & ‘C’ show small islands or headlands on small lakes, you can then see these same points on the PPI photos, but from different heights.  The position of the aircraft is the large black cross in the middle of the map, on the PPI displays it is the bright spot at the centre of the screen. The bright line going from the centre to the bottom right is the leading edge of the ‘sweep’ of the antenna.

The photo above shows the PPI display of the area from 20,000ft, while the photo below is of the same area from 500ft. It gives a good idea of the change in the radar shadow as you go lower.

These two photos show the difficulty of reading a PPI display, especially at low level, and the problems there can be in relating the display to the map. It is a bit of a black art, but it got easier the more we did it!

In the photo above you can see that there is a thin circular ‘marker about half the diameter of the screen, also, coming out from the centre is a thin ‘marker line at an angle of about 45^o these two thin markers are intersecting over a small bright radar return.  In the top PPI photo taken from a higher level you can see that in the position indicated by the marker intersection in the bottom photo, there is a lake (dark area) with some small islands and headlands. The marker intersection in the bottom photo is over one of these and I would suspect the operator was using this as an offset to guide him to positions ‘A’ & ‘B’.