In figure 2-13, notice that targets located at
position "A" are more than one beamwidth apart. The
radar therefore displays them correctly, as two
separate echoes. Also notice that some degree of
stretching is evident, in both echoes, due to partial
beam filling. Targets located at position "B" are
exactly one beamwidth apart and are displayed as one
large echo. As the beam rotates, there is no break in
returned energy between targets. As their energy is
merged, they appear to occupy the entire beam.
Position "C" illustrates poor azimuthal resolution and
target stretching caused by partial beam filling.
Range Resolution
Range resolution is the radar’s ability to display in-
line targets separately. Range resolution affects targets
along the beam, oriented behind one another. Targets
must be more than one-half pulse length apart or they
occupy the pulse together; their returned energy is
merged making it impossible for the radar to see their
separation. Targets too close together appear as one
and are displayed accordingly (stretched along the
beam axis). Range resolution is solely a function of
pulse length.
Pulse length is unaffected by distance, therefore
separation criteria remains constant.
In figure 2-14, a radar pulse is approaching two
objects (targets) that are one-half pulse length apart
(view A). In view (B), the pulse has hit the first target
and some of the energy is reflected back to the radar. In
view (C), the pulse has just reached the second target
and more energy is reflected back to the radar from the
first target. In view (D), the pulse strikes the second
target and energy is now reflected back from that
target: In view (E), reflected energy from the first
target continues to reflect towards the radar along with
the second target, which is now one-half pulse length
long. Its "front end’ is nearly coincident with the first
target. From this, we learn why it’s impossible for the
radar to tell where one pulse ends and another begins.
The radar sees one continuous signal. The slightest
increase in target separation will overcome this
limitation and enable the radar to display both targets
correctly.
Figure 2-13.—Azimuthal resolution and target stretching.
2-10
