Principles of Modern Radar - Volume 2 1891121537

11.4 Code Selection and Grating Lobes Effects 519Aim: 5ç Aim: 0ç Aim: 5ç000−10−20−30−40−50−60−70−10−20−30−40−50−60−70−80−80−80−80 −60 −40 −20 0 20 40 60 80 −80 −60 −40 −20 0 20 40 60 80 −80 −60 −40 −20 0 20 40 60 80−10−20−30−40−50−60−70Residual Grating LobeFIGURE 11-26 Sub-arrays and grating lobes (left: ideal full array diagram; center: subarraydiagram; right: array factor with grating lobes, sub-array diagram, and final resultingdiagram superposed) Sub-arrays oriented towards broadside direction, aiming in direction 5 ◦ .0−10−20−30−40−50−60Full Array Sub-Array GlobalAim: 5çAim: 5çAim: 5ç00−10−20−30−40−50−60−70−70−70−80−80 −60 −40 −20 0 20 40 60 80−80−80 −60 −40 −20 0 20 40 60−80−80 −60 −40 −20 0 20 40 60 80FIGURE 11-27 Sub-arrays and grating lobes (left: ideal full array diagram; center:sub-array diagram; right: array factor with grating lobes, sub-array diagram, and finalresulting diagram superposed) Sub-arrays oriented towards the exact aiming direction 5 ◦ .−10−20−30−40−50−60The consequence for space-time coding is that, if the grating lobes cannot be tolerated, 8essentially three solutions remain:1. Using interleaved scanning, where each transmission is made through a directive antenna– with corresponding aiming of the sub-arrays (we assume that there is a phaseshifter behind each element in the array, as is the case in active antennas): this is asolution if the waveform is low repetition frequency, such that the signals are received,after each pulse, from the corresponding direction only – and then scanned for the nextpulse;2. Using intrapulse scanning, which also uses the transmit antenna only in a directive modeof operation (this is a solution only if the receiving antenna is correctly sampled);8 Depending on the application, such grating lobes may not be a real problem, if the receiving diagramis sufficiently clean. However, when the sub-arrays are identical on transmit and receive – which is thegeneral case, but not the only one – the grating lobes are an essential limitation to wide angular coverage.Such wide beam modes are indeed more appropriate for one dimensional coverage with linear sub-arrays(adjacent columns or rows)