Bill Silvey wrote:

So, why is the radar so limited in it's coverage? The blind spot directly
in front of the radar is particularly puzzling. I mean, if I'm in A-A, the
whole antenna does TWS or of course Boresight modes or whatever.


When a Doppler ground mapping technique is being employed the
direction of your line of sight matters a lot. The difference in
Doppler between targets A and B is very small. On the other hand, C
and D are in the position of max Doppler difference and are much
easier for the radar to resolve. The size of this difference is not
the only factor involved, but it's very important.

-- A
B



C D

For instance, the B-1B Offensive Systems manual says, "The HI-RES
ground map processing uses Synthetic Aperture Radar (SAR) techniques
and as such must look off the flight vector by [deleted] degrees or
more to get a good resolution map. A HI-RES map can be taken near the
flight vector, but the quality will be very poor."

Problems also occur at steep lookdown angles. The signal processing
becomes sensitive to altitude errors for such targets. The B-1B manual
has an elaborate discussion of all the factors involved in getting a
top quality ground map.

Often, sophisticated radars will have a "real beam" mode which
functions much like an old fashioned pulsed radar. The B-1B does: "The
primary use for Real Beam ground maps is to provide mapping capability
near the velocity vector (aircraft track) where HI-RES maps cannot be
made, and for large area mapping."

--

Paul Hirose
retired USAF radar maintainer