# Multifunction RF Systems for Naval Platforms
## 2 Requirements for Naval RF Functions
### Radar Function Requirements
Better range resolution leads to a smaller effective radar resolution cell, since the resolution cell area \(A_{c}\) for low radar grazing angles is given by \(A_{c}\approx\Delta R\theta r\), where \(\theta\) is the azimuth beamwidth and \(r\) is the radar range. A smaller radar resolution cell provides higher tracking accuracy, but more importantly for HS mode, improves the detection of slow-moving surface targets due to the fact that the amount of low-angle sea clutter returns that compete with target signal returns in the resolution cell containing the target is reduced. The larger instantaneous bandwidth required for HS mode implies a larger receiver bandwidth, which, as indicated previously, leads to a resulting increase in competing system noise power during the detection process. However, in the case of radar, the signal-to-noise ratio (SNR) is not affected under these circumstances, as long as the waveform bandwidth is matched to the receiver bandwidth, and matched filtering, often referred to as pulse compression, is applied upon reception. Under these circumstances, the signal enjoys a proportionately higher pulse compression gain that allows the SNR to be maintained despite a larger value of system noise power. With regard to the TI function, the instantaneous bandwidth requirement is negligible, since the transmitted waveform is essentially monochromatic. The receiver dynamic range requirements for the VS and HS subfunctions are determined by the ratio of the strongest expected clutter-plus-target return power to the weakest expected target return power. Dynamic range is not relevant for the TI subfunction since this radar mode has no receive component. The naval radar function generally has the highest EIRP requirements relative to the other RF functions under consideration. For VS and HS subfunctions, the required EIRP is driven primarily by operational requirements that dictate maximum detection ranges for different types of targets. The operational requirements for VS typically involve longer detection ranges compared to HS, but on the other hand, some targets of interest in HS may have relatively smaller RCS values, especially for the ASM case. The required EIRP for operation of the VS subfunction at L-band is somewhat less than that for operation at S-band or X-band because of the better propagation characteristics of the lower frequencies, as noted earlier. It should be pointed out that the array receive gain \(G\), also factors into the radar range equation to determine target detection capability of the VS and HS subfunctions, in that the SNR of the received radar signal is proportional to \(EIRP\times G_{r}\). Consequently, increasing \(G_{r}\) allows EIRP to be decreased proportionately without affecting detection ranges. For simplicity, the EIRP requirements listed in Table 1 for VS and HS are based on the assumption that the array receive gain is the same as the transmit gain, which would typically be the case for radar. The EIRP specification for the TI function is determined by the signal strength of the reflected target illumination required by the missile seeker for terminal guidance. The requirements for one-way beamwidths listed in Table 1 are approximately the same for VS and HS radar subfunctions. The minimum achievable beamwidth is inversely proportional to array dimensions, so the beamwidth requirements directly impact the size of the AESA. For the VS function, required beamwidth values mainly derive from operational tracking accuracy requirements and required scan revisit times. These factors are considerations for the beamwidth specification of the HS function as well. However, as discussed earlier, the impact of radar resolution cell size on signal-to-clutter ratio for detection of surface targets additionally plays a role in arriving at the HS beamwidth requirement. In the case of the TI subfunction, there is no independent beamwidth requirement; the beamwidth used is just a consequence of the array size needed to provide the transmit gain that allows the EIRP requirement to be met.