# Multifunction RF Systems for Naval Platforms
## 2 Requirements for Naval RF Functions
### Communication Function Requirements
Given the simplicity and compactness of these SATCOM antennas, their contributions to ship system life-cycle costs and ship RCS are already very small compared to those of other RF functions. Consequently, there is no obvious motivation to add UHF and L-band SATCOM services to an AESA-based MFRF system. Current shipboard terminals for SATCOM services at the higher X, K\({}_{\text{u}}\) and K\({}_{\text{a}}\) bands typically use parabolic reflector antennas, commonly referred to as \"dish\" antennas. An example of this is the Thales SURFSAT-L naval SATCOM terminal [13], which is a dual-band system that can accommodate any two of the three specified bands. Consequently, a minimum of two such terminals is needed to cover all three bands. The antenna is isolated from ship roll, pitch and heading changes through a gimbal mounting with 3-axis or 4-axis inertial stabilization. It is noted that the diameter of the parabolic reflector needs to be in the order of 1-2 m to achieve the required gain at the frequencies of interest. Consequently, the RCS presented by such an antenna to a threat radar may be significant. For example, the maximum RCS of a parabolic reflector with a diameter of 1.5 m would be approximately 15 dBm2, as seen by an L-band threat radar [14]. These terminal antennas also occupy large physical footprints-the Thales SURFSAT-L antenna assembly above deck, including the radome, gimbal mounts and a parabolic reflector with a diameter of 2.1 m, has an overall diameter of about 2.7 m, a height of 3 m, and a weight of 380 kg. These considerations motivate incorporation of the X-band, K\({}_{\text{u}}\)-band and K\({}_{\text{a}}\)-band SATCOM services in a MFRF system. A communication service referred to as Tactical Common Data Link (TCDL) is another common requirement for naval vessels. TCDL is a secure K\({}_{\text{u}}\)-band data link developed by the US military to send data and images from airborne platforms to surface platforms. It utilizes the Common Data Link (CDL) waveforms and protocol that have been mandated by the US Department of Defence as the wideband communications standard for transferring intelligence, surveillance and reconnaissance (ISR) sensor data between both manned and unmanned platforms. A typical example of a currently available shipboard TCDL antenna is the CPI SST-100 [15]. The entire above-deck antenna assembly, including a radome, stabilization gimbal mounts and a parabolic reflector antenna with a diameter of 0.8 m, has an overall diameter of 1 m, a height of 1.1 m and a weight of 200 kg. An antenna such as this would be needed for each independent TCDL link. Studies within the US Navy have indicated that at least four independent TCDL links are currently required, with future projections of up to 24 independent links [16]. Given the physical size of these parabolic reflector assemblies, the amount of available deck/superstructure space on a ship significantly limits the number of TCDL links that can be currently supported. Migration of the TCDL function to an MFRF system would alleviate this constraint, by virtue of the flexibility afforded by AESA use. To summarize then, the communication functions that are considered for candidates in a MFRF system comprise the SATCOM services in the X, K\({}_{\text{u}}\) and K\({}_{\text{a}}\) bands, and the TCDL service. Referring to the frequencies of operation for these functions in Table 1, it is noted that the Rx and Tx operations are conducted over separate non-overlapping frequency sub-bands. This allows simultaneous reception and transmission, referred to as full duplex communication, on one antenna without causing self-interference. There is some further discussion provided on this issue in Section 3. The instantaneous bandwidth requirements listed in Table 1 correspond to the channel bandwidths that are specified for the different communication services.