Milstar Payload

30 Years on Orbit

The Milstar Communications Network — a series of advanced satellites linked to mobile ground terminals — provides assured command and control to U.S. forces worldwide.

The satellites carry Northrop Grumman-built low data rate payloads that provide the world's first onboard digital processing. Milstar satellites function as autonomous "switchboards in space," that let users communicate with each other anytime, anywhere in the world, at any level of conflict. These features, and others on a Milstar medium data rate payload with broadband capabilities, are crucial to successful operations on the modern battlefield and are not available through other military or civil communications networks.

Lockheed-Martin built and integrated the Milstar satellites. The first first Milstar satellite launched in 1994, the second in 1995. These first two flights, known collectively as Milstar I, housed a Northrop Grumman-developed and -built Low Data Rate (LDR) payload, serving core command and control activities. LDR offers secure, antijam, interoperable voice and data links to 2400 bps.

A Medium Data Rate (MDR) payload complements the LDR on Milstar II satellites (Flights 4, 5 and 6, collectively). Northrop Grumman developed and built the MDR antennas and digital processing subsystem for Boeing Satellite Systems, the MDR payload integrator. MDR extends the LDR architecture to higher data rates for tactical users into fixed sites and highly mobile terminals.

Low Rate Data (LDR) Payload

The Northrop Grumman-built Milstar LDR payload receives uplink user signals on nine EHF receive antenna beams. The uplink signals are demodulated and routed to destinations assigned by the onboard computers, which mark the world's first "bandwidth on demand" satellite communications. Data are also received via crosslinks from other Milstar satellites in the constellation. Received user data may be routed either to onboard destinations, to one of five downlink antennas, or to the crosslinks for connections through other Milstar satellites that form a "closed ring" enabling secure, Milstar communications from point to point anywhere on Earth.

The SHF downlink subsystem modulates and transmits the signal through the downlink antennas. A UHF subsystem provides interoperable UHF communications and a fleet broadcast channel. Milstar supports communications to man-transportable terminals.

Medium Rate Data (MDR) Payload

For the Milstar medium data rate (MDR) payloads that fly aboard Flights 4, 5 and 6, Northrop Grumman Space Systems supplies two types of antenna and the digital processing subsystem. MDR's data rates — as high as 1.55 megabits per second (MBPS) — accommodate voice and data, imagery and targeting intelligence at the 1.55 mbps rate on each of 32 uplink channels. This capability greatly increases the capacity of the Milstar system and support new missions, such as high-capacity links between Navy shore stations and ships at sea.

The MDR payload's Northrop Grumman-developed nulling antennas are capable, without any instruction from a ground station, of detecting and then countering enemy signal-jamming. This revolutionary capability was developed for Milstar II, with each MDR payload carrying two nulling antennas. The nulling antennas are well suited to Army coverage requirements, where many troops may occupy a relatively small theater of operations near the enemy. The Army, therefore needs coverage with protection against close-in jamming.

Six smaller Northrop Grumman-built MDR antennas produce spot beams and 1.55 mbps data rates without the nulling. These are called distributed user coverage antennas (DUCAs) and are best matched to the dispersed, global-type requirements of the Navy, which operates ships and shore stations around the world. Navy vessels and bases tend to be removed from the front and the threat of close-in jamming is much less likely.

Milstar I and II

The first two Milstar satellites, now operating on orbit, are referred to as Milstar I. They employ a low data rate (LDR) payload that transmits secure data at a maximum of 2,400 bits per second.

Flights 3 through 6 collectively are called Milstar II. Milstar II satellites will carry both LDR and medium data rate (MDR) payloads. The full constellation of six Milstar satellites is scheduled to be in orbit by 2001.

Northrop Grumman is the LDR payload integrator. Northrop Grumman also provides the MDR antennas and digital processing subsystem to Boeing Satellite Systems, the MDR payload integrating contractor; Lockheed Martin is the Milstar prime contractor.

Connectivity among Milstar satellites is established via space-to-space crosslinks in the 60 GHz band. Crosslinking will allow user communication networks to extend around the globe without retransmission through intermediate ground stations.

Benefits of Onboard Digital Processing

Quickly changing battlefield conditions and spontaneous requirements for interservice connectivity demand Milstar's highly sophisticated onboard network configuration capabilities.

Each MDR payload has eight antennas, each independently steerable and operating at extremely high frequencies (EHF). Terminal users may communicate with other users within the same antenna beam or with users located in other MDR antenna beams from any of the Milstar II satellites. The on-orbit digital router establishes and maintains links within and among beams and responds to users' changing and differing bandwidth requirements.

From each uplink beam the MDR payload processes the communication data, sorting and routing them to the proper downlink beams. If a data destination is found to lie outside the areas covered by a satellite's antenna beams, the payload routes the message via crosslink antenna to another satellite for downlinking.

In addition to sorting and routing messages, the digital processing subsystem performs other key functions such as demodulation of the EHF signal, authenticating and granting user access, and dynamically configuring payload resources (antennas, receivers, processors, etc.) to establish networks and provide bandwidth on demand.

To perform these complex functions, the MDR digital processing subsystem relies on 14 custom application-specific integrated circuits and 397 large-scale integrated (LSI) circuits, all fabricated in CMOS technology. This figure represents a decrease of 37 percent from the 630 custom LSI circuits required for each LDR payload.

Fast, Flexible, MDR Flight Software

The software that controls MDR payload resources, like the processors it runs on, delivers higher performance at reduced cost than its flight-proven counterpart in the LDR payload.

The software is responsible for managing all MDR payload resources (uplink, downlink, and crosslink) employed for user communication. The software acts as a "switchboard in space," dynamically changing the routing path of communications data, based upon user requests.

In addition the software performs all the multisatellite coordination with other Milstar satellites to support worldwide communications without resorting to intermediate ground links.

The ability to improve performance and functionality while lowering development risk and reducing cost stems largely from two key factors. One is the selection of the Milstar Advanced Processor (MAP) as the MDR resource controller. MAP is a high-performance, general-purpose computer, developed for the Milstar program and used in other capacities on Milstar spacecraft. The other risk- and cost-reduction factor is the extensive reuse of LDR processing design and software code, which shortened development time dramatically.

In addition, Milstar requirements and external interfaces were already well defined by the time MDR development began.

Although software in the MDR and LDR payloads provides similar functionality, the MDR software incorporates a number of important system improvements. MDR software:

• Employs a centralized database architecture. This means that database information about users is not duplicated on multiple satellites. Instead, service operations are coordinated by a single satellite with full service information. Centralizing this function simplifies multi-satellite coordination and lowers the amount of crosslink traffic devoted to system administration "overhead."
• Adds a new "fencing" capability. Fencing ensures that designated user groups in the three military services can procure access to allotted satellite resources. In times of crisis, fencing guarantees these user groups Milstar availability.
• Increases system flexibility. The software is both configurable and uploadable from the ground. This feature provides the ability to add software patches and improvements "on the fly," or to replace the software with an updated version.
• Supports LDR-aided acquisition of the MDR payload. This feature permits LDR users to log onto the MDR payload.

Savings in Time, Weight and Power

Payload engineers took advantages of advances in microelectronics technology to pack more processing power into smaller digital units, or "configured items," as the boxes are called. For example, the MDR digital processing subsystem requires only two configured items to perform demodulation and routing functions. The LDR payload management subsystem (PMS) required four configured items to perform the same two functions. Overall, the MDR's digital subsystem weighs about 40 percent less than its equivalent configured units on the LDR PMS. The MDR units consume only half the power of their LDR counterparts.

The MDR design also paid off in savings of time and manpower. The integration and acceptance of the MDR digital processing subsystem took less than three months, about half the time required to complete the same tasks for the LDR PMS. In addition, the size of the crew needed for integration and test was reduced by one-fourth. Add the savings in support personnel and the manpower required for integration and acceptance testing was cut in half.

MDR Digital Processing Demonstrates Continuous Milstar Improvements

The MDR payload not only boosts Milstar performance by giving forces in the field the wideband services they need for high-speed communications and dynamic network configuration. It also demonstrates the Milstar program's ongoing progress in providing greater performance from electronic payload units that weigh less, consume less power, and require fewer people to build and test. All of which is necessary to meeting the U.S. military evolving communications needs in the post-Cold War world.

Milstar MDR Nulling Antennas: Ensuring Secure Tactical Military Communications

The medium data rate (MDR) payload of Milstar II satellites — delivering data to field terminals at T1 rates of 1.544 megabits per second, or more than 25 times the speed of a 56 kilobit-per-second computer modem — will make possible secure, real-time transmission of voice, image and data among tactical users. Northrop Grumman-developed adaptive nulling antennas on each MDR payload protect Milstar MDR communications against electronic jamming by enemy forces. Each "nuller" is a fully autonomous antenna system that continuously maximizes user signals while minimizing jammer signals.

Extending the Range and Ensuring the Protection of Military Tactical Communications

Desert Storm demonstrated the fast pace of today's battlefield, where highly mobile military units rapidly move beyond the range of ground-based line-of-sight communications. The trend is accelerating: the U.S. Army says its first digitized division in 2000 will control an area 600 times larger than the conventional division of 1984.

Extending the Range and Ensuring the Protection of Military Tactical Communications

Desert Storm demonstrated the fast pace of today's battlefield, where highly mobile military units rapidly move beyond the range of ground-based line-of-sight communications. The trend is accelerating: the U.S. Army says its first digitized division in 2000 will control an area 600 times larger than the conventional division of 1984.

The MDR payloads are a key "range extender" of the enhanced digital communications that 21st century U.S. military forces will employ.

Yet, the very feature that makes satellites so desirable for military communications — line-of-sight access over a large area — makes satellite uplinks vulnerable to enemy jamming. Jammers may be 100 or even 1000 times more powerful than the ground terminals of friendly forces.

The MDR adaptive nulling antenna instantly detects enemy jamming and counters it within a fraction of a second by placing a "null" in the jammer's direction. (In antenna terminology, a null is a direction from which an antenna collects very little energy.) This adaptive action minimizes the strength of jammer signals received at the satellite, while maximizing the strength of desired communications signals. The null effectively "screens out" jammers, and all this happens automatically with no interruption to communications.

Nulling is required at MDR data rates to ensure the integrity of the communications data. (Milstar's low-data-rate (LDR) payload, with much lower data rates, relies entirely on frequency-hopping, spread-spectrum techniques, and error correction to protect messages.)

The MDR Antenna Suite

The MDR payload's antenna suite is designed to meet the diverse needs of Milstar users. The Army, for example, deploys large numbers of troops in a relatively small theater of operations, where the bulk of communications is in theater. Therefore, the Army needs to cover a small geographic area with one or two payload antennas. The Navy, by contrast, operates ships on every ocean and has shore stations around the world. The Navy needs as many antennas as possible covering the portion of Earth visible from the satellite.

Close-in jamming (where jammer and friendly terminals are both located within a single antenna beam) is a serious threat to the Army, which tends to operate near the front, close to enemy jammers. The Navy often operates from bases and carriers some distance from the front, where the jammers are often out of beam.

Two nulling antennas onboard each MDR payload are capable of negating the effects of both in-beam and out-of-beam jammers. In addition, the MDR payload carries six smaller antennas that produce spot beams without nulling. These are called Distributed User Coverage Antennas (DUCAs). Generally speaking, the nulling antennas are best matched to theater-type requirements, while DUCAs better match those of dispersed global users such as the Navy.

Adaptive Nulling: How The Nulling Antenna Works

Milstar satellites and ground terminals employ a spread-spectrum approach in which the signal hops in pseudo-random fashion from frequency to frequency within an assigned bandwidth. User terminals communicate with the satellite using a secure frequency hopping pattern shared by the terminal and the satellite.

In the absence of jammers, user signals received by the nulling antenna fall within an expected distribution of frequencies. When a jammer terminal begins operating within the satellite's spot beam coverage area, its radiated power does not follow the satellite's frequency-hopping pattern. As a result, anomalies appear in the nuller's power distribution curve, revealing the presence of jamming signals.

The antenna takes immediate steps to eliminate these unwanted signals. It calculates appropriate signal-weighting factors for the power received at each antenna feed to determine the position of the jammer and produces a null in that direction.

The MDR nulling antenna consists of four major components:

• Multibeam reflector and feeds. The MDR nuller produces 13 essentially non-overlapping narrow spot beams. Its design is based on a 40-inch offset-Cassegrain reflector, which illuminates a 13-element feed array. Each antenna is gimbaled and can be independently steered to any position on the visible Earth.
• Beamformer. A millimeter-wave beamformer provides sum and sample signals to a radio frequency combiner, which establishes the received radiation pattern. The pattern, of course, includes nulls that block out jammer signals.
• Correlator. The correlator constantly monitors each of the 13 EHF feed inputs, determines whether a jammer is present, and then computes baseband error weightings.
• Digital processor. Error signals produced by the correlator are passed to the processor. The processor updates the beamformer weights to drive the errors toward zero. As error weightings coming from the correlator are progressively reduced, the beamformer shapes a null in the antenna pattern in the direction of the jammer.

Northrop Grumman also developed the algorithm for the nulling antenna processor. The Northrop Grumman-patented algorithm determines the weight updates from the correlator error signals. The algorithm's performance, along with the speed of the processor, is essential to the nulling antenna's ability to counter jammers.

Nulling: Key to MDR Performance

The complex signal-processing algorithms required for adaptive nulling have existed for some time. Only in recent years, however, have advances in microelectronics enabled engineers to design a fully autonomous nulling antenna system light enough and compact enough to fly aboard a spacecraft.

Northrop Grumman delivered the first flight nulling antenna to MDR payload integrating contractor, Boeing Satellite Systems, in October 1996.

The MDR nulling antennas do far more than simply receive RF signals. Each antenna is a complete feedback control system designed to continuously maximize desired signals while processing out jamming signals. With nulling antennas in operation, the Milstar MDR payload can push data rates to 1.544 megabits per second. Or, by switching to lower data rates, it can receive signals from small, low-power ground terminals. Or it can operate at some intermediate combination of data rate and terminal power — all without sacrificing anti-jam performance.

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