SIMULTANEOUS X AND KA BAND MARITIME SATCOM TERMINAL SUPPORTING BOTH MILITARY AND COMMERCIAL KA BAND COMMUNICATIONS John Logan Dr Chris Leat Director of Programmes Senior Antenna Designer EM Solutions Pty Ltd EM Solutions Pty Ltd Brisbane, Australia Brisbane, Australia Abstract—This paper describes the development and feed testing band diamond-series BUC also allows the user to select which results of a Tri-band on-the-move (OTM) satellite bands inside the 3GHz spectrum from 28GHz to 31GHz are to communications terminal. Using as a baseline an existing gymbal be electronically switched. mount and monopulse tracking algorithms, the COTM terminal is being developed for use simultaneously on X-band and Ka- Simultaneous X and military Ka operation can occur on band military satellites and also Ka-band commercial satellites such as Inmarsat GX. The feed has been designed to have military satellites such as WGS or Optus C1. However, the monopulse in both X-band and Ka-band (military and terminal also supports simultaneous operation on X-band and commercial) for precise tracking, eliminating the need for commercial Ka-band, if a satellite that supports these two mechanical scan tracking methods such as step track or conical bands is available, as shown in Figure 1. Polarisation switching scan. between LHCP and RHCP is handled by electronically switching the Tx and Rx for both bands. Keywords-Ka-band; X-band; simultaneous; COTM; WGS; EM Solutions I. INTRODUCTION High data-rate communications at sea depend almost exclusively upon on-the-move (OTM) satellite terminals. Yet suitable space, particularly with minimally obstructed sky- view, is precious on vessels. This paper describes a Satcom OTM maritime terminal with simultaneous X and Ka band communications capability being developed as a Concept Technology Demonstrator (CTD) for the Australian Defence Force. The terminal will support X, commercial Ka and military Ka-band high data rate communications. Figure 1 - The terminal allows simultaneous X and Ka-band communications The terminal will draw on existing monopulse and gyro- on WGS or Optus C1 stabilized gimbal technology developed for single band Ka and Ku antennas but adds the capability to employ monopulse If the terminal experiences congestion, rain fade, or tracking at either X or Ka-band using a new 1 metre antenna. unfavourable atmospherics on the Ka-band military payload, This provides a wide choice of satellites and robustness to the terminal will continue to use the X-band payload, as weather conditions. The monopulse tracking system will shown in Figure 2. provide rapid satellite acquisition directly from the beacon, very low pointing error, and low power consumption due to the The Tri-band terminal will closed-loop track the satellite inherent passive stability of the gimbal mounting.. beacon with either the X-band or Ka-band RF receiver automatically, depending on the beacon strength. II. SIMULTANEOUS X/KA BANDS The EM Solutions Tri-band terminal offers three bands: military X-band, military Ka-band and commercial Ka-band. The same feed and antenna supports all three bands without the need to swap any RF equipment. The EM Solutions Ka- The Ka-band tracking is expected to give the best tracking performance in clear sky conditions. However, the terminal will continuously monitor the signal quality of both X and Ka beacons and automatically use the band that provides the best pointing performance. The terminal can track common satellite beacons such as CW signals and lightly modulated telemetry signals that have a prominent CW component (e.g. WGS beacons). It is also possible to track data carriers (e.g. DVB-S2, QPSK) with the antenna control units (ACU’s) signal processing capability. Figure 2 - X-band operation can continue during Ka outages due to weather. Below deck equipment provides detection of Ka outage III. MONOPULSE TRACKING ON BOTH X AND KA-BAND The Tri-band terminal feed design not only supports X- band and both military and commercial Ka-bands, it also uses monopulse tracking in both X-band and Ka-band. This is a complicated achievement as the Ka-band horn must be placed inside the X-band horn without affecting the performance of one interacting with the other. This means that the Tri-band terminal can operate in Ka- Figure 3 - Operation on a single military band X or Ka. band (commercial or military) mode only and track a Ka-band only satellite, and similarly, operate in X-band mode only and IV. THREE AXIS BALANCED TERMINAL track an X-band only satellite. The X-band and Ka-band tracking are independent of each other. One of the most important features of the EM Solutions Tri-band terminal is the use of a third axis which is referred to The monopulse tracking in both bands reduces the power as cross-elevation (this is additional to the traditional two consumption of the terminal, as there is no need to other axes of elevation and azimuth). The concept of “letting mechanically scan the antenna to perform conical scan or step the antenna remain still” has been used throughout the design track pointing, and also increases tracking performance. of the system and this can be seen by the use of contactless low friction direct drive motors rather than having gearboxes Table 1 - Frequencies for TE11 and TE21 Modes or drive belts. Band Transmit (uplink) Receive (downlink) units Having this balanced axis is important as it allows the min max min max antenna’s inertia to keep the antenna still, despite motion of X 7.900 8.400 7.250 7.750 GHz the vehicle underneath it. Thus, the antenna naturally tends to maintain pointing even while on a moving vehicle. This is not Commercial Ka 29.000 30.000 19.200 20.200 GHz the case for a two-axis (azimuth over elevation) system. Two- Military Ka 30.000 31.000 20.200 21.200 GHz axis systems have difficulty tracking satellites directly above the antenna (i.e. perpendicular to the base of the antenna). This problem is often called “key-hole effect”. TE11 The key-hole effect requires two-axis terminals to rotate rapidly in azimuth when tracking satellites that are directly overhead. This results in degraded tracking performance TE21 (tracking) when the satellite is at high elevation angles. Note that the When tracking a satellite with both X and Ka band transponders, the terminal can switch tracking between the X attitude (ie. Pitch, roll and yaw) of the vehicle is important, as and Ka beacons without introducing significant pointing the key-hole effect depends on the elevation angle to the errors. This allows the terminal to switch from X to Ka satellite relative to the base of the antenna. So, even if a two- axis terminal can operate properly if driven on level roads, it tracking at the end of a Ka outage (e.g. due to weather) may suffer from the key-hole effect if driven on sloping without disturbing X band communications. terrain, or across uneven terrain that causes the vehicle to to 10dB more stringent than other specifications. The mask pitch, roll and yaw. shown in Figure 5 and Figure 6 is the WGS mask. V. OPERATION ON COMMERCIAL KA-BAND The X-band pattern is not required to meet this mask as the antenna at X-band is less than 50 wavelengths in diameter. The third band of the Ti-band terminal is commercial Ka- band for use on services such as Inmarsat GX. The terminal can switch electronically to the 29-31GHz band. Again all polarisation switching is performed electronically. In commercial mode, the embedded GX (or commercial) modem is given control of satellite selection and all RF settings and the modem will communicate with the terminal’s antenna-control-unit via OpenAMIP. Figure 6 - TE11 Pattern at X-band VII. FEED PERFORMANCE The feed itself is a coaxial design with the X-band components consisting of circular waveguide and a corrugated horn. The Ka-band components are added to the assembly by using a dielectric rod in the centre of the X-band horn to shift Figure 4 - Operation on commercial Ka band satellites including Inmarsat GX. the Ka-band through the X-band feed section without Commercial satellites with military Ka-band transponders are also supported disturbing it. VI. ANTENNA PERFORMANCE The antenna and feed modeling was performed by EM Solutions using a hybrid approach with FEKO software. The feed and sub reflector both used a method of moments technique and the main reflector was modeled using physical optics. Figure 7 - Feed Horn with Dielectric Rod The input to the feed is a section of circular waveguide about 22mm in diameter which tapers down to the diameter of the dielectric rod. The diameter of the dielectric rod in the fully loaded part of the circular waveguide is less than 1cm, which is sufficient to allow the TE21 Rx mode at 20GHz to propagate. The TE21 signal, which has a sharp null at the centre of boresight, is used for tracking. Its network at Ka band is located on the transmit side of the dielectric rod antenna, so the rod and circular waveguide tapers have to be invisible to the 20GHz TE21 mode while not exciting any Figure 5 - TE11 Pattern at Ka-band other higher order modes. One of the most challenging parts of the of the feed and antenna networks was meeting the antenna pattern mask to Antenna measurement patterns were performed on the satisfy MIL-STD-188-164. The back lobe requirements are up X/Ka-band feed using a 1.2m semi circle test jig as shown in Figure 8.