Most Interesting
lwd wrote:I thought angle accuracy was the problem but this implies the range accuracy of US sets was better. I'm also wondering just how Kongo's officers came by this intelligence.
The general idea that radar has more problems with directional accuracy may be because of the grey area of accuracy and resolution. A radar in many cases can have excellent bearing accuracy but poor bearing resolution. Resolution is the ability to distinguish among one or more closely grouped targets.
The azimuthal resolution is a function of how wide the beam is, often described as 1/2 power beam width. This is in turn a function of wave length/ effective antenna size:
WL/Apeture(x corrective coefficient)
As can be seen, by using shorter wave lengths, or larger size antennas, improved bearing resolution can be attained.
In terms of bearing accuracy, accurate enough to be useful for firecontrol, requires lobe switching of some type so that the bearing accuracy is measured in small fractions of a degree. Without lobe switching the bearing accuracy can be at best ~1. The British 284 had a bearing accuracy of 0.75* using max signal, not accurate enough for firecontrol, for example. In 1935, Kriegsmarine Ordnance set the specifications for Seetakt for fire control usage to match that of their optics, such that the resolution for range to be no greater than 50 meters, and the bearing accuracy at least 0.2*, for example.
Looking at the USN Mk3/4 as an example, we find that its directional accuracy is within 0.2* when lobing, but the trade off is very poor bearing resolution. This is because lobe switching in this case causes the effective size of the antenna to be 1/2 of what it is when using max signal. The beam width and the bearing resolution becomes on the order of 12*-15*. This explains why it had problems distiguishing targets and for spotting the fall of shot for bearing.
The British 284M used a type of lobe switching that had small effect on the bearing resolution, so it's bearing resolution was 4.5*, as oppossed to 15* for the MK3, while lobing. However, the 284M's display setup made it very difficult to spot the fall of shot directionally as well. If the stabalised director, that the radar's antenna was mounted to, was aimed directly at the target; the 284M would cause the pip to hold steady on the A-scope. If the director was aimed slightly off to the left or the right, then the pip would flicker at different rates. The directional accuracy of 284M could be as good as 0.08* (better than Mk8) but the practical bearing accuracy was also limited by the precision of the director the antenna was mounted to. The accuracy of the lobe switching indication and antenna aim could be checked by occasionally rocking the director back and forth.
The USN Mk8 using a 10cm wave length could attain a beam width of 2* while using a relatively compact antenna array of 42 plastic cones. This is compared to 15* for the Mk3. By moving to 3cm wave length in Mk13 (Mk8Mod3 is a Mk8 converted to Mk13) the beam width could be further reduced and even better bearing resolution attained. However, practical problems could be caused by too small of a beam width. For example, the British 274 used 10cm wave length with a relatively large antenna. This produced a beam so focused that even at extreme ranges; the shot could fall mostly outside of the beam, and so it could not spot the fall of shot.
Range accuracy is a function of the how precise the elapsed time of the pulse to and from the target could be measured. The British Type 284M to use an example once again, attained a range accuracy of 120 yards read directly off the A-scope, but 240 yards normally. The USN radars provided range accuracies that were a % of the range. For example MK8 was 0.01% (10x more accurate than the USN's optics) the range +/- 15 yards. So the range accuracy at 30,00 yards was +/-45 yards.
Range resolution in most cases (but not all) is a function of the pulse duration. The British 284M used a pulse duration of 1us resulting in a range resolution of 150 meters ( Mk3 was 400 yards by way of comparision). For the Japanese radars with extra long pulse widths of up to 10 us, results in a range resolution of 1500 meters, unless they were using some more advanced method.
