Block IIR satellite. The first launch in this series took place on July 22, 1997. Of the 21 planned satellites, 6 healthy units are in orbit, 1 suffered a launch failure, and 14 have not yet been launched. The first "modernized" Block IIR is scheduled to launch in 2003.

Modernization and the Move to GPS III

Steven Lazar

The numerous critical applications and infrastructures that have come to rely on GPS will require changes that cannot be accommodated by the system as originally conceived. Aerospace has been instrumental in defining a new system architecture that will assure that military, civilian, and commercial needs are met far into the future.

Enhancements to the Global Positioning System (GPS) have historically been driven both by technological advances and by user demand. The atomic frequency standard in the navigation payload has improved over the years, yielding a nearly threefold increase in ranging accuracy over original specifications. Hardware and software upgrades to the operational control segment have steadily reduced the positioning and timing errors attributable to satellite orbit determination. Rapid growth in the civilian market has spurred remarkable improvements in the performance, size, and cost of user equipment. Whole industries have sprung up to provide augmentation services to niche markets that will eventually include commercial aviation and maritime administration.

Nevertheless, the numerous critical applications that have come to rely on GPS will require changes that cannot be accommodated by the system as originally defined. For instance, efforts to modernize the second-generation (Block II) system focused on enhancing the space and control segments through retrofits to the original design, but these initiatives do not go far enough; rather, a substantially different approach will be needed to keep pace with the exponential growth in civil and commercial applications that rely on GPS balanced with the increasingly rigorous demands on the military side.

History of Modernization

The shift in GPS from an essentially military application to a dual-use system can be traced back to 1983, when Soviet fighter jets shot down a civilian passenger plane that had strayed into Soviet airspace. In response, President Reagan declared that GPS should be available for worldwide civilian use to prevent such catastrophes in the future. The specifics of civil use were established in a Memorandum of Agreement between the Department of Defense (DOD) and the Federal Aviation Administration (FAA) in 1992. As time went on, the disparity in quality between the new civil service and the legacy military service became apparent. In a report published in 1995, the National Research Council called for a second civil signal that would provide service equivalent to that previously enjoyed only by the military. This second signal would permit greater accuracy (by facilitating correction of ionospheric effects), provide a backup link in case of local interference, and allow more precise ranging measurements through its wider-bandwidth signal. On the military side, the National Research Council and other groups expressed concern over the ease with which an adversary could jam the relatively weak GPS signals. In addition, given the effectiveness of GPS in Operation Desert Storm, some analysts predicted that hostile entities would begin using it before too long.

America's GPS policy was firmly set forth in a 1996 Presidential Decision Directive that gave consent to the DOD/FAA plan to provide GPS for peaceful worldwide use, free of charge. Moreover, recognizing that GPS provided an increasing advantage in virtually every military operation and was becoming an integral part of the global information infrastructure, the Clinton administration also created an interagency board for the management of GPS. Still, there was no clear plan detailing what changes could be made most effectively and what part of the system should deal with the increasing service demands.

The Air Force initiated studies to evaluate the trade-offs between performance and cost among various alternatives for dealing with postulated threats to the system. The studies concluded that the signal-in-space used by the military had to be boosted by a significant amount to provide an operational advantage to the greatest array of users. Additionally, the need to prevent unauthorized use while preserving U.S. and allied use spurred studies to identify separate signals and spectrum allocations for military, civilian, and aviation use. At the request of DOD, The Aerospace Corporation led a study that looked for ways to separate the military signal without having to secure additional spectrum in the already crowded radio-frequency bands. The study concluded that frequency reuse within the existing bands would allow for a new military signal (with higher power) in the lesser-used outer portions of the currently registered bands. Thus, the existing civil signal at the primary frequency and the proposed new civil signal at the secondary frequency would remain unaffected by the new military signals, which came to be called the M code.

The first phase of the modernization of the Block II system will provide two additional signals, designated as the M code, on L1 and L2 for military use. The M code signals are designed to use the edges of the band with only minor signal overlap with the preexisting C/A and P(Y) signals. A second dedicated civil signal on L2 will be added to give civil users full dual-frequency service. In the next phase of modernization, another frequency (L5) with a new modulation type will be dedicated to high-accuracy use, fundamentally for aviation. The two components of the signal have been designated as I5 and Q5.

A vice presidential announcement in 1998 heralded the changes to the original L1 and L2 signals under the GPS modernization program. The new signal for aviation use (called L5) was announced in a subsequent White House release. The discontinuation of selective availability in 2000 was the easiest but most concrete portent of the changes that would come. As a result, the Block IIR program (already in production at the time) and the Block IIF program (in preliminary design) were modified to include gradual additions to the signals and power levels. Of course, these programs could not be significantly altered because of the maturity of the designs and the need to sustain the GPS constellation with regularly scheduled launches. Modernization of the control segment also got underway with the addition of new monitoring stations and processing techniques to reduce errors in positioning and timing.

Researchers from Aerospace confirmed that the most efficient means to generate the high-power M-code signal would entail a departure from full-Earth coverage, characteristic of all the user downlink signals up until that point. Instead, a high-gain antenna would be used to produce a directional spot beam (several hundred kilometers in diameter). As a result, the necessary power could be directed specifically toward areas of interest, reducing the amount of amplifier power needed on a satellite and limiting potential interference on the ground. Originally, this proposal was considered as a retrofit to the planned Block IIF satellites. Upon closer inspection, program managers realized that the addition of a large deployable antenna, combined with the changes that would be needed in the operational control segment, presented too great a challenge for the existing system design. As a result, the focus shifted from a modification of this existing contract to a new start program. That new start was granted by Congress in 2000 and came to be called GPS III.

A conceptual drawing of a GPS III satellite, created by Aerospace's Concept Design Center.

Assured Delivery

The suggested target levels for GPS III accuracy and signal availability were found to be incremental improvements over what could possibly be achieved by a fully modernized Block II constellation and updated control segment. What is needed most from GPS III is the assurance of such performance in the face of both deliberate attack and normal degradation of the system components. Such assurance can be attained in part through a more robust operations center as well as more secure ground-to-space and space-to-space links. As for the high-power spot-beam, assured service is not just a matter of delivering the enhanced signal on the ground; rather, it implies the ability of the system to respond to changing signal requirements in an operationally timely manner. The spot-beam signal must be designed to allow users to acquire the signal with the requisite accuracy, along with ancillary products such as updated navigation messages. Meeting these needs may entail additional infrastructure, control elements, and communication networks.

Assurance also means the ability to respond quickly to service anomalies and unanticipated disruptions. This type of assurance is known as integrity. For the civil user, integrity means that the system can be trusted in safety-of-life applications such as vehicle surveillance and guidance. In the near term, this level of integrity will be added as an external overlay to GPS by the FAA's Wide Area Augmentation System, a network of ground reference stations that correct for GPS signal errors and provide information regarding the health of each satellite. Potential performance and cost benefits can be achieved by integrating these functions in one navigation system.

For the military user, integrity might mean that a GPS-guided weapon can be trusted to complete its mission with an acceptably low likelihood of collateral damage. Toward this end, a goal for GPS III may be a high degree of self-monitoring, both within the space vehicle and within the constellation. This layered integrity approach, which may also include ground monitoring and a custom integrity signal provided by FAA for civil and commercial aviation, has the potential to meet dual-use needs. The same safety features may efficiently meet civil needs for safety-of-life services and military needs for guiding weapons to their targets reliably and effectively.

Spectrum Challenges

Of the various design challenges driving the specific enhancements planned for GPS III, spectrum considerations are among the most important. The high-power M-code military signal is particularly sensitive and will have to be added judiciously. At the upper primary frequency (L1), the signals from a GPS satellite compete with other GPS signals that share this band as well as with signals from the Wide Area Augmentation System and various European augmentation systems. At the secondary frequency (L2), signals must coexist with surveillance radar systems, and Aerospace has supported studies to determine the degree to which current and future signals can do so (see sidebar, Frequency Reuse and Signal Modernization). Although the higher-power signal will have sufficient bandwidth within the registered frequency, it may still prove difficult to prevent the total signal from spilling over the occupied bandwidth.

The current GPS constellation has signals on two frequencies (L1 and L2) with P(Y) code modulation dedicated to military use. L1 also has the C/A coded signal that is the primary signal for civil GPS users. Among the factors considered in selecting the optimal frequency and bandwidth for a space-to-Earth signal suitable for high-accuracy ranging are attenuation through the ionosphere, rain attenuation, code rate for accuracy, and limits of digital circuitry and radio-frequency components. As part of the GPS modernization efforts, Aerospace reexamined these tradeoffs and found that the current C/A and P(Y) codes at the L1 and L2 frequencies are in fact optimal choices.

Spectrum issues affect the uplink, downlink, and crosslink frequencies as well. The uplink and downlink bands used for telemetry, tracking, and control (TT&C) are subject to reallocation to permit increased commercialization of government bands for emerging mobile-satellite systems. The directional crosslinks, used for satellite-to-satellite communication and potentially for intersatellite ranging, represent a significant departure from the current ultrahigh-frequency implementation, which is essentially nondirectional. The current crosslink signal is not situated in a properly allocated band and can suffer from occasional interference as a result of the broad satellite-antenna coverage at that frequency. While modernization of the Block II serves as a first step in addressing some of these issues, the fundamental design changes that can truly deal with the changeover in frequency bands are beyond the scope of the existing programs and configurations.

Integrated Dual Use

GPS III offers the opportunity to depart from previous designs, rather than simply add to them. The FAA Wide Area Augmentation System, Coast Guard National Differential GPS, and numerous global monitoring networks operate independently of the main GPS infrastructure, and the operational control segment does not fully benefit from the superior availability and orbit determination capability of these civil networks. A potential innovation in GPS III would be to incorporate the products of such civil networks (with the appropriate safeguards) into its operational database. International concerns could also be met by incorporating monitoring and integrity information from other countries and regions. The use of host country messages in a local area can satisfy the need for sovereign countries to maintain control over their regional radio navigation aids within the context of a service carried on GPS.

As for the increasing crowding of the electromagnetic spectrum, some of the proposals identified in GPS III make it possible to meet the growing operational need within the confines of protected national and international frequency allocations. In the case of the high-power military signal, the use of a highly directional spot beam and better spectral and geographical tailoring of the signals could allow the system to satisfy this goal. High-frequency ground-to-space and space-to-space communication links have been recommended by the research and contractor teams to create a survivable network for the command and maintenance of the system under variously challenging conditions.

Acquisition Innovation

Not all of the innovations in the GPS program have been purely technical. As a departure from the traditional way in which the Air Force acquires space systems, GPS III was designated as a "pathfinder" for a new process. In the past, similar acquisition programs had to endure numerous meetings of separate process teams to gain acceptance of the acquisition strategy. In contrast, GPS III will undergo a review by a single body, the Independent Program Assessment team, under the aegis of the Air Force's Space and Missile Systems Center. The Independent Program Assessment team will gauge the readiness of the program to pass the respective milestones and report to the milestone authority. Composed of representatives from numerous other programs, the Independent Program Assessment team brings diverse viewpoints and expertise from numerous quarters and prepares other program offices to transition to this new way of doing business.

The early GPS receivers were quite bulky—and in fact had to be worn like a backpack. In contrast, today's receivers are extremely compact and easy to integrate with other portable electronic devices. Shown here are the Street Pilot III (top right) and eTrex Summit (bottom right) from Garmin. (Photos: U.S. Air Force and Garmin)

The Aerospace Corporation has been instrumental in conducting trade studies among a host of proposed architectural elements for the Block II modernization and GPS III design efforts. Aerospace researchers working with the corporation's Concept Design Center have been analyzing constellation size, number of planes, spacecraft design, and ground-segment configuration in regard to performance and total life-cycle cost. This capability has been used to independently validate the results of contractor studies and to create a government technical baseline. The baseline helps refine requirements that will serve to specify what GPS III should be and allow the Air Force to estimate the program's cost and funding cycle going into the next phase of the acquisition.

With the help of DOD's Center For Systems Acquisition Development, Aerospace has also conducted critical risk assessments at various stages of the acquisition program. More recently, the potential risk of some of the new features for GPS (although they are not new in other space programs) such as the large spot-beam antenna and higher-frequency crosslinks have been examined more closely. The interdisciplinary Aerospace research team has used its database to alleviate some concerns and to help formulate prudent development and production schedules for planning and cost estimation.

The transition of the operational control segment has also been identified as a focus area, and Aerospace has been assisting in concept development and advanced planning. The safe transition from the control of the current constellation to the more advanced and networked operations for the future satellites is a key focus area for architecture explorations and risk reduction.

Flexibility and Growth

One of the chief lessons learned from the modernization program is that the process of getting design changes developed, tested, and phased into the operational system is both deliberate and slow. The remarkable longevity of the GPS satellites has meant that for large and economical satellite acquisitions, new requirements and technology updates take a long time to implement. When designed in from the beginning, flexible elements can be readily accommodated in the architecture. Furthermore, planning for future growth with a margin of flexibility in processor capacity, memory, power, mass, and thermal capacity can allow for improved or even new payloads and missions to be accommodated quickly and economically.

Artist's rendition of a Block IIF satellite. The first unit is scheduled to launch in 2005. (U.S. Air Force)

The American GPS is not without competitors. In fact, in the 1980s, the Soviet Union launched its own version called GLONASS, although this system has fallen into disrepair since the breakup of the Soviet Union. More recently, the European Union has decided to develop another global navigation satellite system, called Galileo. U.S. policy is still evolving with regard to Galileo, but the question of whether to be competitive, complementary, or fully interoperable with foreign systems like Galileo underscores the need for flexibility as well as improved services for future GPS manifestations.

Indeed, the international competition to create the preeminent global navigation satellite system may be decided by the ease with which a system can either establish or adopt service standards. Given that the frequencies, signal structures, and protocols of the various potential services in Galileo have not been fully defined, timely decisions on the part of the United States with respect to GPS III may set the pace in establishing these standards. Accelerating the program, maximizing the civil benefits, and including provisions for international cooperation in the operation of GPS III can build confidence on the part of equipment manufacturers and local certifying agencies. Most important, providing the technical and operational backing for the political assurances of superlative and reliable service will allow the United States to make sure that GPS remains the leader in the global navigation satellite arena.

Conclusion

It would be presumptuous to try to predict all the ways that GPS will evolve in the next 30 years; however, it's safe to say that GPS will continue to play both a visible and supporting role in virtually all commercial, civil, and military enterprises. The modernization of Block II will achieve noticeable performance improvements, and augmentation systems will provide an added dimension of safety for applications such as air travel and marine navigation. GPS III promises to consolidate these and further advances. With more capable and efficient design and management, the future GPS will help the military achieve its objectives reliably and discriminately while safeguarding the trust of all of its users.

To Summer 2002 Table of Contents