The Missile Defense Agency's Space Tracking and Surveillance System
John Watson and Keith P. Zondervan
The Aerospace Corporation has provided systems engineering since the 1960s to the development of satellite systems designed to detect missile launches, track missiles in flight, and cue the deployment of ground-based interceptors. STSS is one of these systems.
The United States has had some form of missile defense since the 1950s. Early systems relied on ground-based and airborne sensors and interceptors, but later systems sought to incorporate space-based components. The addition of space layers to missile defense offered the potential for continuous global coverage and the ability to track objects from launch to reentry—all without issues of basing in foreign nations.
Space-layer defense concepts have included interceptors, lasers, and sensors, but only the sensing element has ever been operationally deployed—most notably, in the form of the Defense Support Program (DSP). These satellites scan the entire Earth from geosynchronous orbit by means of infrared sensors, looking for evidence of missile launches, detonations, and related phenomena. First launched in 1970, the DSP satellites are nearing the end of their service life; the final unit (23) was launched in November 2007. The decades of DSP collections have provided operations expertise, a database of target signatures, and processing innovations; these are the cornerstone of missile defense from space and provide a foundation for modern designs.
DSP satellites provide early warning of missile launches, but they were not originally intended to track missiles beyond booster burnout. Since the 1980s, various options have been considered for developing a missile-tracking satellite system. In the mid-1980s, the Strategic Defense Initiative Organization (SDIO) proposed a constellation of tracking satellites in low Earth orbit known as the Space Surveillance and Tracking System. This concept evolved within a few years to a modified concept and a new name—Brilliant Eyes. Development of this system was transferred later to the Air Force, which was tasked with developing a comprehensive system for early warning and tracking known as the Space-Based Infrared System (SBIRS). The system would have geosynchronous (GEO) and highly elliptical (HEO) components, known as SBIRS-High, to replace DSP, as well as a new low-orbit component, known as SBIRS-Low. Brilliant Eyes was thus restructured as SBIRS-Low.
STSS is designed to be the low Earth orbiter within the layered Ballistic Missile Defense System. It complements the geosynchronous DSP and SBIRS and other ONIR systems and provides tracking cues to systems on the surface. The STSS program is developed in phases, the first of which is the launch of two demonstrator satellites. The demonstrators will perform experiments and prove out systems and processes to establish a knowledge base for future operational designs. |
During its first decade, SDIO also went through transitions. It was renamed as the Ballistic Missile Defense Organization (BMDO) and its mission changed. New national initiatives provided the basis in 2002 for today's Missile Defense Agency (MDA).
The Brilliant Eyes program initiated the development of two nearly identical demonstration satellites known as the Flight Demonstration System (FDS). After development of the major subsystems, a change in the acquisition approach resulted in termination of the FDS effort and storage of the hardware. Instead, development of the operational system commenced. After spending only a little more than a year on this operational approach, the schedule was deemed unrealistic; the effort was terminated, and the decision was made to take the FDS satellites from storage and fly them as originally planned. The rebirth of FDS occurred in 2002 and coincided with the program being transferred back to the Missile Defense Agency with a new name—the Space Tracking and Surveillance System (STSS).
Changes in name reflected changes in mission for the overall missile defense system. Over the years, the primary focus shifted from national missile defense, to global protection against a limited strike, to theater missile defense, to protection of limited geographic regions and U.S. targets against a limited strike. The current vision for the Ballistic Missile Defense System (BMDS) is a layered defense, with the uppermost layers composed of the Air Force warning satellites, DSP, SBIRS-High, and other Overhead Non-imaging Infrared (ONIR) systems. These would be supported by the STSS operational constellation in low Earth orbit. The next layers would be the sea- and ground-based radars, each with its own altitude regime. Overall, the BMDS must have the flexibility to react to an evolving set of proliferating threats.
In 1984, Aerospace established a missile defense group at what was then the Air Force Space Division to support the space layers of SDIO. In the 1980s Aerospace supported a half-dozen major space systems focused on one or another missile defense mission. (There are actually four mission areas, and each imposes design requirements: missile warning, battlespace assessment, technical intelligence, and missile defense.) Aerospace engineers developed the first reference concept for use as a design starting point for Brilliant Eyes in the early 1990s. Aerospace remains a major contributor to the systems engineering of the space layers of missile defense for the United States.
The Role of STSS
STSS will be a constellation of satellites with both missile warning and tracking capability. It will improve the effectiveness of the BMDS by enabling earlier and more precise intercepts. When the constellation is fully deployed, STSS will provide stereo (two-satellite) coverage for determining target position information. The ratio of the radiation emitted by the target to that from the background radiation affects the capability of the sensor to effectively detect and track the target. The sensor will operate in multiple spectral wavebands, each tailored to detect observable phenomena particular to each segment of a missile's trajectory. The position-measurement precision is determined by the characteristics of the focal planes and the jitter environment in the telescope assembly.
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The acquisition sensor on board the demonstrator satellites, operating in the shortwave infrared waveband with a fixed revisit rate, with coverage provided by spacecraft motion, provides detection, acquisition, and booster tracking. Onboard processing provides a handover of acquisition information to the track sensor. The track sensor can operate in a number of wavebands and provides tracking in the midcourse. | ||
Each satellite will have a track sensor that will create new or extend existing "track files" of targets. The tracking data will be sufficiently accurate for launching ground- or ship-based missile interceptors.
Development Strategy
STSS is being designed for an operational capability after 2012 to defend against an evolving threat. STSS will use a spiral development approach, in keeping with the evolutionary acquisition guidelines established for the BMDS by the MDA. The development philosophy is based upon a buildup of knowledge derived from a series of tests, experiments, and block design changes. The space vehicles will take advantage of prior missile-defense and early-warning system designs and experiments. Space-based experiments and demonstrations—such as NFIRE (Near-Field Infrared Experiment), launched in April 2007—will provide infrared phenomenology data and laser communications crosslink performance data as input to the concept design process (see sidebar, NFIRE).
STSS development is divided into phases. The first has been focused on deploying a two-satellite demonstration constellation, scheduled for launch in 2008. The demonstration satellites will conduct a series of functional and performance tests and experiments to prove the overall system concept, including the ground segment. Satisfactory orbital performance will pave the way for the flight and ground software upgrades and ground hardware upgrades of phase two. These efforts will provide a basis for the design of an operational STSS constellation, with initial launches predicted in the 2013–2015 timeframe. The concept development of the demonstrator satellites started in the early 1990s and ended in the late 1990s with components and subsystems delivered to SBIRS-Low, then under Air Force management. MDA assumed custody of these deliverables and cognizance over STSS in 2002.
The demonstration satellites will be used to observe single test missiles launched within their fields of regard. A full constellation would provide continuous coverage of threat regions and be able to track multiple missiles in simultaneous flight. Deploying larger constellations rapidly and cost-effectively would require an emphasis on producibility and would need to capitalize on economies of scale.
Demonstration Satellites
The pair of STSS demonstration satellites will be placed in a circular low Earth orbit separated by a central angle of 35 degrees (to enable stereoscopy). The power system is sized to provide for mission operations.
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Simulations of a long-range engagement with a representative target rocket show that the position and velocity accuracy required by ground-based interceptors is satisfied by STSS. These requirements change with the progress on the trajectory of the missile and are met even without stereo viewing (although computation requirements are increased with viewing by a single satellite). |
The STSS demonstration has eight mission objectives; four are considered fundamental to success, and four involve exploitation of the data or directed actions while on orbit. The critical objectives are: to demonstrate the ability to acquire, track, discriminate, and report missile threats; to demonstrate the ability to autonomously hand over the acquisition signal to the tracking sensor; to demonstrate the ability to hand over the tracking data to another space vehicle; and to verify the operation of the uplink, downlink, and crosslinks. The four ancillary objectives are: to perform MDA-directed data and contingency operations; to provide missile defense and missile warning for evolving STSS concepts; to provide data for use in assessing the feasibility of a tracking-only constellation with acquisition provided by a separate constellation; and to provide opportunities to explore approaches for closing the control loop for firing MDA weapons.
The two STSS demonstrators will assess both missile warning and tracking capability, viewing targets both below and above the horizon and providing stereo coverage for target position information. Each satellite has a missile acquisition sensor and a track sensor. Both the acquisition and tracking sensors capitalize on the magnitude and spectral content of the heat energy radiated by a missile. The acquisition sensor detects missiles in boost phase using its shortwave infrared waveband. The sensor signal is processed onboard for relay to the ground and handover to the track sensor. The tracking sensor has a narrow field of view and operates in multiple spectral wavebands, all of which are tailored to detect observable phenomena particular to each segment of a missile's trajectory. For example, its "see-to-the-ground" and short-wave infrared bands are designed for tracking rockets in their boost phase. The midwave infrared is designed to track the upper stages of rockets and small engines of postboost vehicles. Mid/long-wave and long-wave infrared, with their sensitivity to cooler targets, are used for midcourse tracking and are designed to provide space surveillance as well. The track sensor data are used to create a "track file" that is sent to the ground for use in engaging the target. A separate visible sensor supports midcourse tracking and discrimination of sunlit objects and can provide space surveillance capability as well.
 The demonstrator satellites provide above-the-horizon stereo (two-viewer) coverage in the green area, mono (single-viewer) coverage in the yellow areas, and no coverage in the red area at any given instant. Stereo viewing provides better track accuracy than mono viewing. Stereo coverage and coverage radius are dependent upon target altitude, which in turn determines whether the target is viewed against an Earth background (below the horizon) or a space background (above the horizon). While the demonstrator satellites are not technically operational, increased numbers above two would provide a rapid increase in coverage capability, especially for stereo viewing. (Data by C. Kobel, The Aerospace Corporation.) |
The Missile Defense Space Experimentation Center (MDSEC), a part of the Missile Defense Integration and Operations Center at Schriever Air Force Base in Colorado, will receive and process data from the satellites. The tracking data generated onboard each satellite and downlinked to the MDSEC will consist of 2-D object-sighting messages. These will be fused by the ground-data processor to produce 3-D tracks, which will also generate a menu of target attributes. The 3-D tracks will be sent to the Command and Control, Battle Management, and Communications (C2BMC) system for evaluation. The MDSEC will uplink additional tracking cues to the satellites. The tracking data generated will be sufficiently accurate for launching ground- and ship-based interceptor missiles.
The demonstration program calls for a 12-month mission. During this year, the two satellites will undergo a battery of system functional tests as well as performance tests using a set of test targets characteristic of real threats.
Connectivity is provided through the Air Force Satellite Control Network (AFSCN) in near real time, using a high-capacity recorder on orbit. An onboard crosslink relays data and commands between the demonstrators. A limited amount of real-time experiment activity is envisioned during the system tests.
The acquisition sensor has a horizon-to-horizon field of regard with a fixed scan and revisit rate and can detect boosting objects using its shortwave infrared waveband. The tracking sensor may be cued by the acquisition sensor or by commands sent through AFSCN. Each waveband/altitude-regime combination is programmed with its own revisit rate.
 The design of the STSS demonstrators is based on the observables generated by a ballistic missile from launch to reentry. Demonstrator capability is based on a robust menu of available sensor wavebands, which are selected during all phases of a missile's trajectory. The demonstrator design was intended to collect data for further system refinements, both for the sensor and its onboard data-processing and communications systems. The satellites report to and are controlled by a missile defense center on the ground, which integrates the information coming from other systems. |
The MDSEC provides a centralized, collaborative environment enabling on-orbit space operations and experiments to exploit and integrate space sensor capabilities and data into the BMDS. It investigates BMDS capabilities of government ONIR systems and provides common infrastructure/capabilities to support BMDS space-layer maturation.
The Aerospace Corporation has been intimately involved in the science and engineering of space vehicles used in missile warning and defense for nearly 50 years. Since the 1960s, Aerospace has been a stakeholder in the success of every space program with a missile-defense-related mission. Aerospace support has included advice on acquisition strategy as well as advice and data to other laboratories and agencies for their programs. Some of the earlier space programs were discontinued and their functions folded into programs that answer an evolving set of national security requirements. The principal programs in which Aerospace provides support today include DSP, SBIRS (both HEO and GEO), and STSS—and this involvement spans the entire life of these developments, from concept definition to early on-orbit operations and mission operations.
 The track sensor in the STSS demonstrator satellites was specifically intended to capture any observables that might be useful in later operational designs. The multiple sensor wavebands provide many experimental options for the collection of missile observables. The track sensor can receive visible data through a separate optical train. Contextual information may continue from the acquisition sensor as well (courtesy of Raytheon). |
Aerospace Contributions
Aerospace has provided a decade of analytical effort to STSS, using industry standard and Aerospace-developed models and simulations. Aerospace contributions to STSS include the independent assessment of system performance, mission assurance functions and the use of integration and test principles. System performance is benchmarked using the Aerospace-developed System Performance Evaluation Tool. The test approaches include "test like you fly" and "day in the life" testing, which is designed to exercise sequences of mission threads, with a primary focus on flight software functioning with flight hardware, using full optical-system test equipment. The scope of the test includes acquisition of target and handover to the track sensor. The goal is to assess system performance over representative mission scenarios, including the tracking of multiple targets. Further, as part of Aerospace's role as an independent evaluator, anomaly reviews are provided to program office leadership.
Conclusion
The Ballistic Missile Defense System will need to know the position and velocity of a target during all phases of its flight: boost, midcourse, and reentry. Each of the sensing systems and their interceptors are designed for one of these segments. STSS will play a critical role in the development and deployment of the operational integrated system. The STSS demonstrator satellites are the first step leading to the necessary knowledge to put into orbit a tracking constellation providing for precise deployment of interceptors and the resultant increased probability of target kill.
Aerospace has supported the STSS program through several changes to mission requirements and mission concepts. From the start, Aerospace defined a set of intrinsic capabilities for the satellites that have survived all the changes to the space mission and national policy. STSS and its demonstrators are thus able to contribute to missile defense today and to each of its participating elements in the future.
Approved for Public Release 08-MDA-3476 (19 MAY 08)
