by Steve Koutsoutis
The Defense Department has been experimenting with wideband commercial satellite communications for several years and has been leveraging consumer direct-to-home direct-broadcast-satellite technology to provide greater information capabilities to the warfighter. That effort, Global Broadcast Service, has been highly successful in recent years and has tremendous potential.
The GBS program uses commercial-off-the-shelf technologies in providing wideband data and real-time video products to a diverse user community, including tactical users. Tactical users, however, have expressed a great deal of interest in expanding this capability from fixed to mobile platforms. Several efforts exist in DoD to expand wideband SATCOM into a mobile scenario.
This article describes a research-and-development effort that my organization � the Space Technology Branch of Communication-Electronics Command�s Space and Terrestrial Communications Directorate � is undertaking. Our efforts include a prototype antenna subsystem and standard GBS components to experiment with and leverage into new areas for tactical ground-mobile users� benefit.
We designed a slotted waveguide, low-profile, mechanically scanned tracking-antenna system for receiving a Ku-band GBS signal from the SBS-6 satellite. We then installed the antenna on a commercial-utility-cargo vehicle equipped with GBS receivers, computers, monitors, test equipment and a trailer-mounted generator so we could test and evaluate the antenna�s performance while on-the-move.
The antenna has a gain of 36 decibels and only extends 14 inches (without the radome) above the CUCV shelter�s roof. Our prototype antenna wasn�t designed to provide operational-dish antenna margins, but to be used as a testbed basis to help us determine the characteristics and potential of selected collector and tracking technology as well as interaction with vehicle dynamics.
Our system has demonstrated the ability to remain locked onto a satellite (with a 23-megabits-per-second transponder) and the CUCV in motion up to 55 mph.
To date, the Army�s GBS efforts have mostly been limited to static platforms. Many exercises and demonstrations have been conducted that illustrate the concept of a wideband forward SATCOM link with a smaller return link for processing information requests and disseminating theater data products. The Army has experimented substantially in this area through its Force XXI advanced warfighting experiment series, Task Force XXI, Division XXI and other demonstration forums.
While much effort has gone into Phase I (Ku-band) and Phase II (Ka-band) GBS receive systems, operational antennas to date have been conventionally designed for fixed-site communications in accordance with the GBS operational-requirements document. However, it�s planned that a ground-mobile receive terminal be made capable of receiving and processing the GBS downlink while on the move for Phase III. For commercial DBS in the Ku-band, tracking antennas have already been developed � or are in development � for use on ships, airborne and land-mobile platforms using a variety of technologies. These include low-profile antennas required for airplanes and highly desirable for land-mobile vehicles. These commercial antennas don�t need as much gain as required for Phase I GBS.
In an effort to investigate near-term issues associated with GBS communications-on-the-move, CECOM initiated an effort to develop and integrate a Ku-band antenna into a CUCV. Desired objectives for this antenna�s development included:
|Compatible with low-cost production;|
|Capable of rapid acquisition and tracking while in motion on improved roads;|
|Quick turnaround-feasibility for the development cycle;|
|Capable of transitioning to Ka-band; and|
|Functionality in determining capabilities and limitations of antenna-tracking technology on military vehicles.|
We decided to base development on EMS Technologies� slotted waveguide technology, coupled with CAL Corporation�s positioner/tracker. This approach had the potential of meeting all our objectives. EMS Technologies had already successfully demonstrated variations of the required slotted waveguide antenna for Ku-band DBS and for some Ka-band experiments. Also, a similar technology had already been demonstrated for mobile DBS reception in Japan as described in "A Single-Layer Slotted Leaky Waveguide Array Antenna for Mobile Reception of Direct Broadcast from Satellite," IEEE Transactions on Vehicular Technology, November 1995.
In preparation for experiments and demonstrations of GBS COTM, we employed an Internet-protocol-router-based receive suite as the baseband component to process the Ku-band signal that the prototype antenna subsystem received. All equipment was installed on a CUCV.
|CUCV platform and trailer with prototype antenna installed.|
|Closeup of the prototype antenna subsystem.|
To close the link with a medium-power satellite such as SBS-6, it was necessary to incorporate an antenna with a relatively high gain and therefore a narrow beamwidth. The narrow antenna beamwidth made tracking requirements particularly difficult. The prototype tracking antenna system was delivered in February 1998 (less than one year from award). Brief descriptions of the key antenna-system components follow:
Antenna aperture. We chose as our antenna implementation a waveguide slot array incorporating a traveling wave feed structure. We designed the antenna to provide a receive gain/system noise temperature ratio of roughly 13 dB/Kelvin ratio. The final aperture configuration measures 20 inches by 30 inches and provides a "squinted beam" in elevation. The squinted beam allows the aperture to remain parallel to the horizon and still have its narrow beam directed at the satellite. The antenna was optimized for used in New Jersey and therefore was designed with a beam squint of 45 degrees (the elevation-look angle to SBS-6 from New Jersey).
A standard commercial waveguide low-noise-block downconverter with a noise figure of about 0.75 dB was used for the prototype antenna. The antenna�s gain was measured on the near-field range at Electromagnetic Sciences. The observed gain closely matched the predicted gain of 36 dB. The combination of the very highly efficient antenna aperture and a low noise figure LNB produced the necessary G/T to close the link with some margin for tracking experiments.
Tracking/positioning system. The GBS COTM tracking/positioning system�s basic design was derived from a commercially available L-band SATCOM system. It uses inertial-rate sensors in pitch, roll and yaw as well as received-signal-strength indicator inputs to maintain tracking. Sensor alignment is critical for accurate tracking.
The mobile-receive terminal is configured similarly to a fixed-receive terminal except that it�s on a moving platform, the CUCV (a Chevrolet pickup truck with an installed S-250 shelter). The CUCV has a separate trailer attached as a platform for an alternating-current power generator. We eliminated need for an attached trailer by using a high-power inverter in the CUCV.
For the GBS COTM application, a bandwidth allocation of 1.544 mbps was established for the unclassified encrypted data broadcast from the GBS testbed. This bandwidth allocation is due to the baseband equipment�s constraints. The actual data is a repetition of several text files several times per hour. Also, a stream data session was established in which data was continuous and could be output to the workstation display, saved to hard drive or both.
This data channel, along with the standard broadcast products (Cable News Network video, for instance) comprised the 23 mbps composite data stream GBS COTM downlink suite received. GBS Joint Program Office personnel established this data channel and specifically scheduled broadcasting for this Army experiment.
We initiated and completed a feasibility development project within eight months to demonstrate principles and capabilities of a low-profile antenna for Ku-band GBS. Our test results indicated substantial improvements in satellite-acquisition time, compared to manual pointing. The system typically acquired in less than two minutes. We demonstrated system tracking with limited vehicle motion; this marked the first time the SBS-6 signal has been acquired and tracked on a vehicle. To improve on-the-move performance, we experimented for six more months to fine-tune the positioning algorithm, better characterize the sensors and improve circuitry. These tweaks enabled high-speed lock (while driving up to 55 mph) on the satellite.
We didn�t design this antenna to provide an operational-dish antenna�s margins, but to be a testbed basis and help us determine characteristics and potential of selected collector and tracking technology, as well as interaction with vehicle dynamics. We built a slotted waveguide antenna using rate sensors and return-signal-strength indicator for positioning, with a motion range of 360 degrees in azimuth and plus-or-minus 15 degrees elevation at a squint angle of 45 degrees. This squint beam in elevation allows the antenna to lie at low-tilt angles on the positioner assembly and still be aimed at the satellite, reducing the system�s profile.
The antenna has a gain at 12 gigahertz of 36 dB and a G/T of 13.5 dB/K over the GBS band of 11.7-12.2 ghz. It�s made up of four subarrays with overall dimensions of 30 inches by 20 inches. Respective beamwidths are 1.4 degrees and 3.5 degrees. Sidelobes in both azimuth and elevation are down by 13 dB. At maximum tilt, the antenna�s profile � including a radome � is 16 inches.
By comparison, the antenna commonly used for nonmobile reception of the Phase I GBS Ku-band signal from the SBS-6 satellite is a 0.92-meter dish with a gain of 38.5 dB and a two-degree beamwidth.
What�s needed now is to get the Army requirements community energized into defining technical specifications we actually require for mobile operations vs. on-the-pause communications using wideband (Ku-band and higher) satellites. These requirements should be fed into the next-generation payload designs so reasonable antenna dimensions can be used, with enough margins, for mobile operations. The immutable laws of physics need to be considered before the satellite is launched, not after.
Mr. Koutsoutis is employed by CECOM�s Space Technology Branch. His current work, in the role of technical-program manager, includes leading mobile SATCOM projects: GBS COTM (Ku-band) and MUBLCOM (hand-held voice/data terminals, low-earth-orbit satellite, airborne relay, ground control and experiments with all DoD services). For CECOM, he has worked communications efforts in the areas of communications security, fiber optics and SATCOM. He holds a bachelor�s degree in electrical engineering from Rutgers College of Engineering and a master�s in electrical engineering from Fairleigh Dickinson University. Koutsoutis has also completed the coursework towards a doctorate at Stevens Institute of Technology. He has received many awards, including technical employee of the year.
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