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Defense Engineers Design New Orbital Switchboard

February 2002
By Robert K. Ackerman
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In with the new does not necessarily mean out with the old.

The U.S. Defense Department’s new generation of military communications satellites will be both forward-looking and backward compatible. They will introduce state-of-the-art capabilities with flexibility for upgrades, and they will be able to interoperate seamlessly with existing Milstar satellites.

The new satellites will provide all of the Defense Department services with both tactical and strategic communications capabilities. They will be fully compatible with U.S. Army forward force deployments that require over-the-horizon command and control, and they are designed to operate in various hostile environments that would deter most other systems.

An extremely high frequency (EHF) capability will offer greater capacities and higher data rates than Milstar. The user will see faster response times and higher content capabilities that can allow exchanges of imagery, including videoconferencing. And, the new satellite’s smaller size and lighter weight will allow it to be launched on a medium launch vehicle such as a Delta or an Atlas.

Known as Advanced EHF satellites, these follow-ons to Milstars I and II are being built under a $2.7 billion firm fixed price contract. The schedule-driven program is slated to begin launching its first satellites in 2006.

Lockheed Martin Space Systems, Denver, is the prime contractor for the program, and TRW Space & Electronics Group, Redondo Beach, California, is the subcontractor in charge of the payload. Clayton Kau, TRW Advanced EHF payload program manager, explains that TRW’s role encompasses all payload electronics, including both hardware and software as well as the antennas, radio frequency electronics, integration hardware, cross-links and digital electronics.

Emanuel Dimiceli, program director for Advanced EHF at Lockheed Martin, describes the new orbiter as a full-process satellite rather than merely a transponder. Onboard processing enhances the satellite’s capabilities without degrading its communications performance.

Dimiceli states that the Advanced EHF satellite design represents an increment of capability advance over the Milstar II design. It will satisfy worldwide continuous coverage at 10 times the capacity of heritage Milstar systems. Where Milstar I largely was conceived as a strategic communications satellite, Milstar II incorporated a medium data rate designed to serve tactical needs of the Army.

With the Army moving strongly ahead in its transformation efforts, its requirements were a major driver in the development of the new satellite’s requirements. Dimiceli adds that, to the extent that Advanced EHF was designed to serve Army needs, it should be able to serve the other services as they undergo their own transformations.

Four satellites are planned for the full constellation to provide complete worldwide coverage. They will be launched incrementally to interoperate with on-orbit Milstars. These satellites also will work with the next-generation terminals that the Army and the U.S. Air Force are introducing to their forces.

Dimiceli relates that the new satellite was designed around several key performance metrics. These included antijam capabilities, low probability of intercept, performance in nuclear scintillation and cross-link capabilities with other satellites in the constellation.

Antijam capabilities are incorporated in the satellite’s waveform, which Dimiceli describes as an extension of the earlier Milstars’ waveform. While the waveforms defend against noise jammers, the satellites also incorporate special antennas designed to handle high-power jammers, especially sanctuary jammers in theater that might be difficult to destroy by conventional means.

Communications security will be ensured through the use of the common criteria for information technology security evaluation. Dimiceli explains that designers are applying these standards for security requirements. Applying the common criteria approach does present some challenges, as it generally is geared for small systems and products. Advanced EHF satellite designers are trying to apply it to a large system of systems.

The system architecture has been designed for multilevel security with many users wanting to operate at all levels from unclassified to top secret. Dimiceli relates that four separate targets have been established that involve different combinations of security. These security targets would encompass various elements of the system.

Networks can be established independently through a set of protocols established in the original Milstar system. This allows the increased flexibility necessary for rapid worldwide deployments.

Dimiceli also notes that the new satellite design permits more localized planning of resources. Instead of having a totally centralized planning system, the new system assigns resources in blocks that are managed by individual commanders in chief.

Another new facet of this satellite constellation is its mission planning element. This element provides tools to military planners to allow them to configure the satellite to support various missions and scenarios, and it enables significant flexibility on assigning resources. The first terminals to use this system will be new terminals currently being deployed by the Army.

A key innovation is the use of phased array antennas for some communications links. The use of these antennas has been examined in commercial applications, but their introduction to the military satellite communications arena represents a leading-edge application, Dimiceli states. The technology allowed designers to incorporate the time-sharing beams necessary for military requirements.

These antennas provide an added degree of flexibility on how capacity can be made available to users, he continues. They are designed for lower rate capacity, and they can form beams of different sizes for spot coverage.

Kau explains that the satellite employs two types of phased array antennas: a single unit for EHF uplink and two identical antennas for super high frequency (SHF) downlink.

Six different antenna types dot the satellite’s exterior, but not all of them are phased array. For example, the special antijam antennas are dish-shaped, while others were incorporated for relatively fixed short-term coverage.

A multibeam nulling antenna is an extension to the nuller that originally was developed for Milstar. This dual-frequency-capable antenna performs EHF uplink and SHF downlink. Two earth-coverage antennas—one for uplink and one for downlink—are similar to the standard earth-coverage antennas on Milstar.

The satellite’s cross-link subsystem comprises two antennas on opposite sides of the spacecraft. These provide direct cross-links among the Advanced EHF satellites and their Milstar progenitors. Six gimbal-drive antennas provide spot beam coverage.

Kau explains that these cross-link communications improve on Milstar data rates by a factor of six. An Advanced EHF satellite will use the slower Milstar rate of 10 megabits per second to communicate with the older orbiters, but it will transition to the higher data rate of 60 megabits per second when communicating with another Advanced EHF satellite.

The most significant improvement over Milstar capabilities may be the new satellite’s data rate. Each Advanced EHF satellite can move data at 8.2 megabits per second. This order of magnitude increase, Kau relates, requires next-generation monolithic microwave integrated circuits (MMICs) as well as new digital application-specific integrated circuit (ASIC) technology.

The next-generation ASICs, which are conventional metal oxide semiconductors (CMOS), are key elements of this system. Kau notes that these CMOS chips can contain from 200,000 gates to up to 4 million gates on a single device. These ASICs help reduce the weight and the power consumption of the systems, which permits increased functionality without any corresponding increase in size or power.

Kau continues that some of the ASICs feature a computer incorporated in a single chip. This eliminates the need for assembling multiple ASICs to perform a computer function.

The MMIC chips are fabricated with indium phosphide, which provides the low-noise front ends that are necessary for meeting requirements. Other key elements include high-speed and high-precision analog-to-digital converters.

Each satellite constellation will have a capacity of 1 gigabit per second. Dimiceli notes that this requirement is configured for anticipated uses in the 2010 time frame.

Even with this advance, the satellite is backward-compatible with Milstar. It combines the low and medium data rates of Milstar’s waveform with the higher data rates.

Kau relates that roughly half of the satellite’s software is backward-compatible, and this involves migrating proven Milstar software to the new orbiter’s computers. The rest of the software features similar software protocols that enable the higher communications rates. Kau explains that work remains to add newer protocols that have evolved over the past two years and provide more functionality.

While the phased array antennas and the digital processors represent significant hurdles to be overcome, the biggest challenge may lie in the sheer magnitude of the software that remains to be developed, Dimiceli offers. Ground-based software may comprise more than 1.5 million lines of code, half of which will involve the mission planning element, and the space vehicle may require 500,000 lines of code.

Payload integration, especially of the many different types of antennas, also will be a challenge, he adds. Designers are working to ensure that self-interference is not a problem among the multitude of antennas.

Among the system’s characteristics are multiple demodulators, data rates and signal paths. These raise issues of timing across all of the payload elements.

The major cost driver for the system is its coverage requirements, Dimiceli relates. The mix of beam types and threshold coverage requirements threatened to push the cost up during the design phase. Engineers worked to determine the best mix of phased array antennas to provide capacity and gimbal dish antennas to achieve necessary coverage. Without cost constraints, phased array antennas would have dominated the design, Dimiceli allows.

The satellite’s design permits hardware improvements to be incorporated easily into future generations, Dimiceli notes. One upgrade might involve the inclusion of more phased array antennas. Advances in antenna technologies may open the door to this upgrade in future years, which would enable greater capacity and higher data rates with only minor changes on the platform design.

Some on-orbit upgrades can be performed by uploading software. This can be accomplished in a matter of days, Dimiceli says. This time line is similar to that of a force deployment into a theater of operations, he notes, which is the basis for this requirement.