# Multifunction RF Systems for Naval Platforms
## 2 Requirements for Naval RF Functions
### Radar Function Requirements  
The naval radar function can be broadly divided into three main subfunctions: _volume_ search (VS), _horizon_ search (HS), and _f_erminal illumination (TI). VS involves scanning the upper hemisphere above the ship deck to detect and track air targets primarily, although the coverage volume extends down to the horizon to also allow detection of larger surface vessels. Because air targets can be relatively fast, the VS subfunction is designed to provide a long-range detection capability so that incoming air threats are registered as soon as possible. HS more specifically focuses on detection and tracking of targets at low elevation angles out to the horizon, including both smaller surface targets and low-flying air targets. The design of this subfunction is driven primarily by the requirement to detect the supersonic sea-skimming _a_nti-ship _m_issile (ASM) threat with sufficient warning time to deploy countermeasures. Compared to VS, the search ranges and volumes in HS mode are relatively small, but significant challenges exist in dealing with very small ASM RCS values, low-angle sea clutter, multipath interference nulls, and anomalous propagation conditions such as surface ducting. TI is a radar subfunction that supports onboard fire control systems when semi-active missiles are deployed. In this event, the radar must illuminate the target during the missile terminal guidance phase, so that the missile seeker can home in on the reflected signal. As indicated in Table 1, VS typically operates at a frequency within L-band or S-band. These are the bands of choice for long-range detection since signals at these frequencies suffer less attenuation from precipitation than would be the case at higher frequencies. Conversely, HS most often employs higher S-band or X-band frequencies for primarily the following two reasons:  
* At lower frequencies, there is a wide null in the radar propagation factor that forms in the elevation plane for small target elevation angles, due to multipath interference between the direct target signal return and the specular reflection of the signal return from the ocean surface. This would make detection in HS mode of certain targets like small surface vessels or sea-skimming ASMs particularly difficult [6]. Multipath nulls are present for this scenario at higher frequencies as well, but are narrower in elevation angle, and therefore have less of an adverse effect. * The use of higher frequencies allows beamwidth and antenna gain requirements to be met with a smaller and lighter antenna, since beamwidth varies inversely as frequency, and antenna gain varies as the square of frequency. An important benefit of a smaller antenna is that it can be installed at a greater height above the ship's deck to extend the distance to the horizon as much as possible, which is desirable in HS mode. Finally, the TI function operates at an X-band frequency for compatibility with the seekers typically employed in semi-active missiles. Traditionally, multiple radars operating in different frequency bands have been installed on a vessel to collectively provide VS, HS and TI functionality. For example, an S-band radar would be employed for VS and HS, while a separate X-band radar would be used for TI. Alternatively, an X-band radar would provide both HS and TI, and another radar operating at L-band or S-band would be utilized for VS. Since radar returns are simply replicas of the transmitted waveforms, the instantaneous bandwidth requirements listed for the VS and HS subfunctions reflect the instantaneous bandwidths of their transmitted waveforms. The waveform bandwidths are selected to achieve a desired range resolution, where the highest obtainable range resolution \(\Delta R\) is given by \(\Delta R=c/2B\), with \(c\) being the speed of light and \(B\) being the waveform bandwidth. The required instantaneous bandwidth for the HS function is generally somewhat larger than that for VS, because HS employs transmitted waveforms with higher range resolution.