Range Resolution

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