Piers
That's a reasonable point, and quite well presented (;-)), but the
height channel is relative as well as absolute. All you want is a
flat surface leading up to a recognisable point at approximately the
same elevation (let's say within 5m). This would ensure that your
fix was properly ranged if you were using the radalt for ranging.
Navigators would normally take a height fix as well as an azimuth fix
and see how their height channel was doing. If it were within say
20ft (sorry, the kit is in imperial not metric units) he might reject
the height part. If it were say 50ft out he might accept it to get
the height channel under control; there are many imponderables in an
inertial system. Even GPS is not wonderful: the kit in the mighty
Tutor, a differential GPS system, is normally over 100ft adrift in
elevation at known positions such as the takeoff point.
But for radar-ranged level bombing attacks with unguided weapons, 15m
would be OK. The height channel is only really critical when
carrying out visually aimed attacks. The depression from the
horizontal for a visual sight (the head-up display (HUD) is used for
ranging and it is a small depression at low level; and the tan of a
small angle is very small indeed and causes floating-point errors in
the range calculation. So pilots were rarely allowed to take fixes
unless the radar or laser were used for direct ranging through the
visual mark ... and visual bomb attacks from low-level (150-200ft at
500 knots), with its high sight-line rate and scope for errors, were
done for the practice for targets of opportunity rather than a
planned attack. Everything happened very quickly for a pilot in that
environment, whereas the navigator could start his scan from 15miles
away. Remember that the radar had its slant range converted to plan
range to reduce height errors.
Now, the best of all worlds was the F-16 with Doppler Beam Shift (for
accuracy of ground mapping on a measured offset with radar fixed-
target track for ranging. It's all very interesting stuff ... why
have you all fallen asleep? ;-)
Chris
~~ >-)-
C M I Barker
Cambridgeshire, Great Britain.
+44 (0)7092 251126
www.threeshoes.co.uk
homepage.mac.com/zuiko
On 21 Dec 2005, at 17:29, Piers Hemy wrote:
> Interesting post about lighthouses, Chris, but the last bit
> surprises me.
> Does the Admiralty supply RAF with tide tables? I wouldn't have
> thought your
> chaps would want to be so dependent on the Senior Service. Or
> perhaps 15
> meters/res up or down makes no difference to you. Unless you are
> landing!
>
> ;-)
>
> --
> Piers
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