Satellite Communications-On-The-Move (COTM), or, sometimes referred to as Satellite-On-The-Move (SOTM), has come a long way in the past few years. The satellite industry has taken many technological leaps forward in order to provide high bandwidth, global satellite coverage on a moving platform.

Spread spectrum waveforms allowed the use of even smaller aperture antennas without violating adjacent satellite interference regulations. Global, distributed Network Management Systems (NMS) allow Internet Protocol (IP) satellite routers to maintain a consistent set of IP subnets despite roaming. However, no technology has been more instrumental in enabling COTM than modem-to-antenna integration. With fixed very small aperture terminal (VSAT) installations, antenna integration is simple. An installer points the antenna, adjusts polarity, runs a 1dB compression test, and locks the entire system down with just a wrench as the required tool.

This is not the case on a COTM platform. In a COTM system, the modem and antenna must work closely together. This is especially true for long duration vehicles, such as ships and aircraft, which are likely to traverse satellite coverage areas. In a COTM system, the antenna and satellite router must communicate current geographic location, satellite handoff, beam quality values and other information if a beam switch is to be successful.

The first challenge encountered in trying to integrate COTM antennas and satellite routers is agreeing on a communication language. At the dawn of the COTM age, antenna manufacturers and satellite router vendors each had their own proprietary protocols. These protocols were never designed to work together.

Some vendors offered a bundled solution. A bundled solution did allow for more convenient integration but severely limited end-user choice as to the best available technology and mission flexibility by locking the user into a proprietary solution.

To help enable the market and to achieve best-in-class airborne solutions, an open protocol was required. This market need drove the development of the Open Antenna to Modem Interface Protocol (Open-AMIP). Open-AMIP has now been widely adopted by a number of maritime and airborne antenna manufacturers as the protocol of choice for their antenna control units (ACUs).

In some cases, however, vehicle or mission requirements may dictate the use of an antenna which has not yet adopted the Open-AMIP standard. In this case, a protocol translation between the satellite router and antenna will have to be performed. This protocol translation can be accomplished through specialized software running on a PC platform. In order to facilitate this protocol translation, some satellite routers built for COTM applications have a PC104 CPU built-in so the translation software can be run locally to the router. (PC104 Single Board Computer formats are for specialized computing environs, most often within an extreme environment. This form factor enables the use of a rugged system that’s customized without having to wait for months of design work.)

True system level antenna integration is much more than just a communications protocol, of course. System level integration requires the satellite router and the antenna make intelligent decisions regarding when to change beams, and in the case of overlapping beams, making the correct choice of beams.

Knowing when to switch beams and selecting the correct beam cannot be done by simply monitoring out-route signal quality, as signal quality can degrade for reasons other than reaching the edge of the beam. To make an intelligent decision on beam selection, the system must be aware of the relative signal strengths and the EIRP contours of all the beams from which a vehicle could possibly derive service. This data must then be cross-referenced with vehicle path and speed in order to initiate a beam switch.

Once a system has been completely designed with global, beam signal quality maps, the other vexing problems of satellite COTM solution can be tackled. One such issue is known as the skew angle problem. Skew angle refers to off axis radiation from an oblong or rectangular, flat panel antenna.

The transmission pattern of a rectangular, flat panel antenna is itself non-symmetrical. When properly oriented with the satellite orbital arc, there is minimal adjacent satellite interference (ASI). However, when an aircraft banks or changes direction, the orientation of the radiation pattern changes—such can cause an unacceptably high degree of ASI.

This skew angle problem is most apparent when an aircraft or ship is close to the equator. By leveraging a map server with the relative strengths of the beams with an overlay of the skew angle of an antenna as a function of latitude and coupling a robust adaptive in-bound channel mechanism, skew angle ASI can be effectively mitigated

The final element of modem to antenna integration is the human element. Satellite routers and antenna control units are each, in their own right, complex equipment. Once integrated, troubleshooting can become an exercise in finger pointing at different vendors. In addition, an operator in an aircraft or maritime vessel must have situational awareness of the communications links. Information such as time to beam switch becomes critical to providing a satisfactory experience for the end-users. Fortunately, graphical displays have been developed which can provide not only a moving map with EIRP contours overlaid but a health status of the satellite router and antenna control unit.

Future developments for satellite router and antenna integration are focusing on the seamless hand-over needed for the spot beam architecture of the new High Throughput Satellites (HTS). Much of the development done previously can be leveraged onto HTS platforms—however, the overlapping nature of the spot beams allows for the flexibility of make before break.

About the author
Karl Fuchs is vice president of technology for iDirect Government Technologies (iGT). He joined iGT in 2004 as the director of sales engineering, just as the satellite-based IP communications company was expanding its very small aperture satellite (VSAT) market presence into the federal government and international Internet Protocol (IP) networking world. He now works as the vice president of technology. With more than 20 years of experience in technology and with the federal government, Fuchs leads iGT’s team of federal systems engineers and serves as chief architect for new product integration.  Prior to joining iGT, Fuchs was director of systems engineering at Nortel Networks, where he oversaw the Verizon account team of systems engineers, leading the design of IP, frame relay, asynchronous transfer mode (ATM) and dense wavelength division multiplexing (DWDM) networks. Before joining Nortel, he designed IP and ATM networks for Sprint and the federal government. Active in the satellite industry for more than 10 years, Fuchs has contributed editorially to numerous publications and has been a featured speaker at leading industry events.