An Overview of Ground System Operations

Marilee J. Wheaton

Without a functional ground system, the most sophisticated satellite constellation would be little more than orbiting debris.

A generic space-system architecture encompasses a space segment, a launch segment, a user segment, and a ground segment. The space segment consists of the primary mission satellites, payloads flown on other satellites, and any relay satellites. The launch segment includes the launch vehicle, launch range, payload adapter, and launch site services (such as range safety and payload integration and processing). The user segment comprises the end consumers of satellite data—the users, user terminals, and related services such as data processing. The ground segment encompasses the hardware and infrastructure that enable the space assets to conduct their mission successfully.

A generic system architecture identifying space and ground elements. The mission control center is the nucleus, managing and operating the overall mission.

Although the space segment often receives the most attention, without the ground systems to maintain them, space assets would be essentially useless. The ground segment supports the launch facilities when placing satellites into orbit, and, after initial checkout, helps keep them there, responding to tasking and supplying useful data, which it gathers, decodes, analyzes, and distributes. Simply put, the ground system is the conduit through which users receive the benefit of the space assets.

The ground segment comprises various facilities and installations, generally categorized in terms of ground stations, ground centers, and sometimes, user elements. The ground stations house the antennas and equipment that enable communication with the space assets—for example, RF receivers, transmitters, and power generators. Ground centers include operational nodes such as the mission operations center, payload operations control center, satellite operations control center, and of course, the mission control center, which plans, monitors, and controls the execution of the overall mission. User elements may be located within the control centers, but typically are in the hands of users dispersed across the globe.

Ground Systems Functions

The primary functions of the ground centers include mission management, space-system asset command and control, and mission data processing. Most of these are implemented by a mix of computer and communications hardware, firmware, and software. Embedded in these functions are support functions such as training, data archiving, and enterprise management.

Mission management focuses on planning and scheduling, from long-range planning of architectures and configurations, through preplanning and scheduling of mission activities, to real-time reactions to events and changing circumstances. Specific tasks would include constellation management, payload tasking, resource allocation, maintenance scheduling, data processing and distribution planning, schedule dissemination, and evaluation of system performance.

Space asset command and control includes all the activities and functions that enable a spacecraft or payload to perform its mission, from launch and operations through deactivation. Space-to-ground communications are required for satellite operations, so the command and control facilities must include hardware for antenna control, signal processing, encryption and decoding, and related functions. A major aspect is telemetry, tracking, and commanding, or TT&C, which ensures that satellites can relay health and status reports to the ground systems, receive commands from the ground, and control space subsystems and payloads based on commands and status. Telemetry refers to automated health and status measurements generated by the spacecraft, which are collected, packaged, and transmitted to the ground. Tracking (or orbit determination) is the determination of orbital parameters based on angular position, range, and range-rate measurements. Telemetry and tracking employs subsystem experts engaged in short- and long-term trending, calibration, and resource management. Commanding refers to signals that are transmitted to the spacecraft to affect operations and reconfigure subsystems and payloads. Anomaly detection and response applies engineering resources to isolate the causes of anomalies, define mitigation efforts, and gauge the success of the response. In the satellite operations center, telemetry is processed and analyzed, commands are generated, and their execution on the satellite is verified. Satellites are kept in the appropriate orbit and orientation with stationkeeping and attitude corrections, which require maneuver planning.

Telemetry, tracking, and commanding (TT&C) consists of self measurements generated by space asset, calculations of the spacecraft's orbit, and directions for actions executable by the subsystems and payloads.

Mission data processing is provided by the mission data chain, which includes all the functions and activities related to the information gathered in space, from the time it arrives on the ground from the sensor or processor until the products or communications generated from it reach the end users. Essentially, this is the space mission's goal. The functions in the mission data chain start with data capture and archiving, then proceed through data conditioning and preprocessing, mission product generation, dissemination to users, and exploitation. Included in these steps would be the implementation of mission algorithms, data analysis, and product validation.

These primary mission tasks require many support functions, such as data archiving, enterprise management, network management, maintenance, testing, training, and simulation. Testing and training activities include certification testing, operator training, exercises, launch rehearsal, and simulation of the processors, spacecraft bus, payloads, and interfaces. Also critical is the information infrastructure and tightly controlled networks that connect the different functions and facilities within the ground system and with the external world of end users, launch facilities, tasking authorities, and other programs.

Design Trades

Some mission requirements can be fulfilled in either the ground or space segment. Generally, it's not an either/or decision, but a question of how much capability goes to which segment. Trade-offs include cost, schedule, technology readiness, security, and even political considerations. Aerospace conducts comprehensive trade-off studies to help system architects decide, for example, whether data should be processed on orbit or on the ground, whether to use satellite crosslinks or build more tracking stations, and whether to increase satellite transmitter power or use larger terminals (see sidebar, The Concept Design Center Ground Systems Team).

Ground system architectures are complex, and a decision affecting one component will generally affect many others. For example, an increase in the amount of onboard data processing could enable a decrease in the downlink bandwidth. This could conceivably improve data latency and increase user satisfaction. On the other hand, the amount of memory needed on board would increase, affecting the complexity, mass, and power of the space vehicle itself. This in turn might limit the choice of launch vehicles. Ground processing might be slower, but could take advantage of technology upgrades, permitting maintainers to upgrade hardware periodically; hardware on satellites can generally not be replaced.

Another important design consideration involves operations and maintenance. In this case, the investment of a higher development cost could result in lower operational cost or improved system performance. For example, the use of fault-tolerant hardware and software should increase reliability and require fewer maintainers. Automated deployment of software upgrades should result in fewer site visits, and automated training could lead to a reduction in training costs. On the other hand, automation can add a layer of complexity and risk, since not all anomalies or events can be predicted in advance, and redundancy can add another layer of cost.

The planned location of a specific ground node can also be a design trade. The drivers for the number and location of ground stations include mission considerations such as constellation visibility, the number of satellites to be contacted, and the number and duration of contacts per satellite. Other potential drivers include politics, environment, infrastructure, feasibility of satellite crosslinks, and requirements for availability and survivability. For example, some locations that provide good sight lines and accessibility might be prone to earthquakes or hurricanes. Tracking stations in foreign territory would require diplomatic agreements, and those in remote locations might require additional facility security or a high level of automation.

Challenges and Risks

The acquisition of ground systems involves the development of large, complex software-intensive systems that are increasingly dependent on commercial off-the-shelf (COTS) hardware and software for major pieces of functionality. Recent experience has shown that heavy use of COTS software may seem efficient and economical in theory, but can in practice create problems that lead to schedule delays, performance shortfalls, and cost overruns. Ground systems also contain sophisticated external interfaces that link them to users, and these interfaces can be a source of engineering problems, particularly as the operating systems and hardware evolve. For many space missions, the ground system will at some point undergo a transition from legacy to new components—and this transition must be seamless. Sometimes, the use and reuse of legacy components, and their integration during system transition, are not adequately planned, and this can adversely affect the mission.

An important aspect of the ground systems expertise that Aerospace provides is in the up-front planning for acquisitions—particularly in regard to the requirements and architecture definition. It is paramount to a successful acquisition that the operations concept and requirements include the ground segment in the initial planning phases. Given the high total cost of ownership of these systems, sustainment must be emphasized from the very beginning. Information assurance across the enterprise (or several enterprises) is another challenging area, and here, Aerospace is taking a lead role by developing policy and practices and providing guidance on technology and implementation.

As with the space segment, active risk management for the ground segment is a major contributor to mission success. Potential risk factors include poor estimation of the effort required, inadequate understanding of technology maturity, overemphasis of COTS products and reuse of existing assets, failure to consider sustainability, difficulty in obtaining regulatory compliance, and even the incidence of bad weather.

Potential risk can be mitigated by ensuring accessibility for fixes throughout the lifetime of the ground system, designing a robust and flexible architecture that can accommodate changes, making judicious use of advancing commercial technology, and adhering to an evolutionary design approach. Benefits of the evolutionary strategy include an early operational capability, reduction of risk by focusing on smaller system attributes, accommodation of relatively flat development budgets, flexibility to incorporate better technology when ready, and quick response to evolving user needs.

Future Trends

What is driving the future trends in ground system design and implementation? Certainly, the changing technology, especially as the rate of change continues to increase. Interoperability is another major driver, as evident by the increasing emphasis on systems-of-systems engineering and conformance to recognized standards. The amount of software in ground systems continues to grow, and with this growth comes greater demands on program management for successful execution of development projects involving several million lines of code. Increasingly, these demands are being met through COTS and open-source components, a trend that is likely to continue. Data rates and volumes are also rising—already reaching gigabit data rates and petabyte data volumes. Effectively managing and exploiting such large quantities of data will require sophisticated tools and agile processors. End users are highly distributed and are increasingly demanding information and knowledge—not just raw data. The need to supply useful information packages while meeting low latency requirements will profoundly affect ground system design and modernization for the foreseeable future.

Tight budgets are also exerting an influence, and ongoing operation and maintenance costs will be the subject of greater scrutiny. Other trends include the commingling of commercial and defense systems, such as in the communications, meteorology, and launch vehicle areas. A decrease in the number of experienced ground station and mission center operators is spurring a need for improved training, increased system autonomy, enhanced human system interfaces, and additional tools and simulators.

Summary

As perhaps the most flexible element in a space system architecture, the ground segment is often called upon to accommodate unplanned changes or errors in the space segment design or implementation. But even though ground systems are easier to modify, their functionality can never be taken for granted; rather, system architects must engage in a fairly comprehensive investigation of ground system expectations from the earliest concept development stage. Modern national security spacecraft can perform remarkable feats, but without equally sophisticated ground systems, they would be unable to achieve their critical mission goals.

To Spring 2006 Table of Contents