Critical Issues in Spectrum Management for Defense Space Systems

Albert "Buzz" Merrill and Marsha Weiskopf

Competing claims on the available radio-frequency spectrum have placed the Department of Defense in a difficult position. Military satellite communications must provide unique warfighter mission support, operational security, and high capacity on demand. But with commercial interests demanding more of the usable spectrum, these goals will require strong leadership and vision to attain.

The last two decades have seen major changes in spectrum management. The state of affairs 20 years ago was one of widely available bandwidth that could be used simply by filling out the proper paperwork. This clear channels paradigm was gradually replaced by a more contentious environment, wherein focused analytic studies were needed to allow users to coexist without conflict. Further increase in spectrum demand now requires users to share spectrum in an actively cooperative manner. The next decade may see assignment of priority based on a system's value to national and global interests. All this places a greater burden on the spectrum-management community as well as the spectrum-management process, which must anticipate and balance future military, civil, and commercial interests on both the national and international level.

The Allocation Process

The worldwide communications community follows an established Table of Allocations that identifies how different portions of the spectrum may be used. In the United States, the National Telecommunications and Information Administration (NTIA) and the Federal Communications Commission (FCC) create additional rules, regulations, and notes that further complicate the matter. The NTIA answers to the President and manages spectrum use by the U.S. government, determining spectrum-use viability prior to program approval. The FCC answers to Congress and performs a similar role for U.S. civil and commercial organizations. No other major nation separates these functions to the extent that the United States does.

The radio-frequency spectrum extends from about 3 kilohertz to 300 gigahertz, but communications above 60 gigahertz are generally not practical because of high power needs and equipment costs. As a result, potential users must compete for a very limited slice of the electromagnetic spectrum.

International interactions are managed by the United Nations through the International Telecommunication Union (ITU). Every country has the sovereign right to manage spectrum use within its borders. The ITU generates rules and procedures that apply whenever communication signals go beyond one nation's borders, as is generally the case for space communication. The ITU has no policing powers, but member nations generally abide by the regulations, motivated both by international treaties and consideration of mutual benefit. Member nations can also deviate from the rules in matters of national defense.

To obtain a measure of formal protection for spectrum use, member nations apply for a "registration" from the ITU. Obtaining these registrations is critical because they establish the formal and legal rights of the recipient. These registrations are dispensed on a first-come, first-served basis and can only be obtained if the desired use is consistent with the Table of Allocations and does not create any unresolvable conflicts. Once an ITU registration is submitted, the registrant must bring the system into use within seven years or forfeit the protection.

Registration is a lengthy process. ITU submissions commonly take two years just to be entered into the computer database for processing, and on the national level, U.S. registrations often take years to complete. The Aerospace Corporation has briefed various government advisory boards on this matter and has suggested methods to reduce this backlog, predominantly through greater use of computer automation and human networking.

When Conflicts Arise

With so many parties seeking spectrum registrations, conflicts are inevitable (see sidebar, "Beachfront Property"). All parties adhering to ITU regulations are obligated to resolve any conflicts in good faith, and in practice, some accommodation can be reached. In resolving conflicts, three distinct factors are considered: actual spectrum use, including details of power and modulation; geometry, including satellite and terminal locations and antenna directionality; and time or operational limitations.

System designers must adhere to this allocation table in order to receive spectrum management approvals. The complexity of this chart clearly underscores that many different types of services must coexist, and can only do so by following the regulations set up by the ITU, NTIA, and FCC. A more legible version of this chart can be found at http://www.ntia.gov/osmhome/allochrt.html.

Spectrum use and geometry are the primary considerations in conflicts involving geosynchronous spacecraft. Systems operating in vastly different spectral bands, for example, present less risk of interference, as do spacecraft that remain on opposite sides of the globe. Problems arise when these systems want to use the same transmission bands and when their fields of view overlap.

Contention for the desired bandwidth gets resolved by detailed analysis of mutual radio-frequency interference and direct negotiations between the affected parties—first in terms of U.S. military interests, and then in terms of broader government concerns. Sometimes, U.S. spectrum-use policy requires favoring one mission over another to obtain high-priority concessions from other nations. Conflict resolution can take several years and numerous face-to-face meetings, and may even become a political issue. The length of time puts great emphasis on knowing which contending spacecraft are (or will be) registered and of those, which are actually viable.

Security is obviously an issue because any users of the spectrum must reveal basic mission information to other users. On the other hand, the full exploration of this information (needed to identify any potential or actual radio-frequency interference) can expose system vulnerabilities that should not be communicated outside of classified channels. For this reason, the spectrum-management community is moving toward more confidentiality, including the use of generic or nonidentifying names instead of actual program names for registration submissions. This separation will become more important in the future because it will facilitate the execution of a coordinated U.S. government-wide policy on frequency management instead of the current fragmented practice of pursuing individual programs, sometimes at the expense of other programs.

Geosynchronous Crowding

To avoid interference, geosynchronous space stations need to maintain an orbital separation of about two degrees for Ka-band systems and even greater for lower frequencies, with actual separations depending on how narrow the field of view is for the ground-terminal antennas. Two degrees out of a 360-degree orbital arc would seem to allow for an abundant number of spacecraft, but registrations in this area have gone from very few to oversaturation during the past 15 years. In the United States alone, for example, military systems will include Milstar, Advanced Extremely High Frequency, Defense Satellite Communications System, UHF Follow-On, Wideband Gapfiller satellites, Advanced Wideband System, Global Broadcasting Service, the future Mobile User Objective System, and Space-Based Infrared System (SBIRS)-High, all stationed in geosynchronous orbits and all making use of the military K band (20.2–21.2 gigahertz) for signal transmission.

Many U.S. military spacecraft use (or will use) the K band (20.2–21.2 gigahertz) for downlinking national security information to ground stations. These include Air Force MILSATCOM and surveillance systems and Navy tactical systems. This congestion demonstrates the need for tighter and more effective communications, both in planning and operations, between the various space-faring branches of the armed forces.

Compounding the problem, some geosynchronous satellites occasionally drift across the sky from one registered position to another to fulfill their military missions. The challenge is to coordinate all of these actions, given that each program considers its missions to be of the highest priority. Moreover, each program may fall under the management of a different agency, so relative priority adjudication is no simple matter. All of this is complicated by potential conflict with other U.S. government and international systems.

To this mix, one must also add the low Earth orbiting SBIRS-Low system, whose spacecraft will pass through the downlink beams of these other systems. With nongeosynchronous spacecraft such as SBIRS-Low, some radio-frequency interference is inevitable if spectrum used by the various parties overlaps. The issue is how much interference can be expected and for how long. Moreover, nongeosynchronous spacecraft are given lower priority than geosynchronous systems in the spectrum-management world, and they therefore bear all the burden of preventing or mitigating such interference. To avoid generating radio-frequency interference, SBIRS-Low is considering using satellite-to-satellite crosslinks with alternate routing from several different spacecraft to a given ground station or stations.

Spectrum and Acquisition

Given the lengthy domestic and international registration process, it's clearly important for spectrum-management support people to work closely with the acquisition community. A thorough consideration of the opportunities, limitations, and general realities of spectrum use is critical to cost-effective space-system conceptualization, acquisition, and overall operation. The

Department of Defense (DOD) Instruction on the Operation of the Defense Acquisition System already requires program offices to submit registration paperwork before beginning the system demonstration and production phases of the acquisition cycle. This paperwork in turn triggers a formal review by the applicable military agency and the NTIA. Unfortunately, even at these stages, the system architecture may already be established in such a way that puts unreasonable demands upon the available spectrum. Aerospace is working with the government to require an increased level of attention to spectrum issues well before the system acquisition phase so that critical issues such as sensor-data generation, communication-channel requirements, and realistically available spectrum can all contribute harmoniously to an overall integrated system architecture.

Growing Demand at 7–8 Gigahertz	Growing Demand at 20 Gigahertz
On the international level, spectrum allocation is managed by the International Telecommunication Union (ITU). Requests for registered slots within the 7–8-gigahertz range—commonly used by geosynchronous satellites—increased significantly in the last two decades and have remained high for the past several years.	The United States and NATO have agreed to restrict the 20-gigahertz band to military purposes, but that has not stopped many commercial parties from joining the fray. Years ago, spectrum analysts thought that programs might "escape" to this band through the use of advanced technology, thereby avoiding contention for spectrum at lower bandwidths.

For example, at the Air Force Space and Missile Systems Center, Aerospace helped draft a Commander's Instruction that requires the various programs to develop a plan addressing spectrum-use viability in the conceptual stages. This Instruction further dictates that all programs must identify any deviations from spectrum regulations so that they can be addressed early in the acquisition stages.

IMT-2000

It must be emphasized that the military's communication needs are fundamentally distinct from commercial needs. Military spectrum requirements are based upon the need for high-volume communications and sensing 100 percent of the time when fighting a war. Because the United States is not fighting a war most of the time, much of the military's allocated spectrum goes unused (except for training exercises) for long periods of time. This sporadic use leads to the unfortunate misconception that the military is an inefficient user of spectrum. As a result, the Defense Department sometimes has a difficult job defending its allocations.

A prime example of this difficulty can be found in the International Mobile Telephone 2000 (IMT-2000) initiative, which has been occupying spectrum analysts since October 13, 2000, when President Clinton directed the NTIA and charged the FCC to help select the spectrum to be used by the third-generation mobile wireless (cell-phone) service, known as 3G.

As part of this effort, NTIA studied the possibility of using a bandwidth of 1755–1850 megahertz and FCC looked at 2500–2690 megahertz. Most of the 1755–1850-megahertz band is utilized by the Air Force Satellite Control Network (AFSCN) for tracking, telemetry, and command and also for other government services such as Air Combat Training. This band uses the Space Ground Link Subsystem for implementation and is commonly referred to as the SGLS band. Most of the 2500–2690-megahertz band is used for educational television, Internet access, and similar services. Clearly, the military would prefer that the 2500–2690-megahertz band be selected for 3G use, but so far, that has not been the indicated course of action.

AFSCN ground stations around the world use large antennas (10–18 meters in diameter) and high power levels (100–10,000 watts) to support about 120 expensive and mission-critical spacecraft. To ensure successful communication, particularly during anomalous conditions,

AFSCN generally uses high-power transmissions. Aerospace has worked to analyze the impact of using the SGLS-band for the proposed 3G implementation, particularly in terms of possible spectrum sharing and mitigation of potential radio-frequency interference. The effort considered in great detail and high fidelity the 3G system parameters and the characteristics of the DOD space-system orbits and equipment. The study indicated that the 3G system of base stations can seriously disrupt the reception of SGLS uplink commands under many circumstances because all the radio-frequency emissions from 3G users within the field of view of a spacecraft antenna combine to cause a significant amount of interference. This study was cited in a report by the General Accounting Office, which indicated the need for further study.

The other side of this issue is the impact that the high-power AFSCN ground stations will have on the 3G systems. Initial analyses by Aerospace and the DOD's Joint Spectrum Center (an office chartered to ensure the military's effective and efficient use of the electromagnetic spectrum) have shown disruption to 3G users as far away as 320 kilometers. Aerospace analyzed the possibility of mitigating these effects. Filtering of the AFSCN output signals was shown to have great benefit for 3G use within a few megahertz of the frequencies used, provided that certain innovative and potentially costly techniques are used. For overlapping spectral use, Aerospace devised a novel technique called dynamic reallocation, which would essentially warn the 3G system of an intended AFSCN radiation burst. The 3G system could then adjust its spectrum use to avoid AFSCN interference. This would reduce the 3G service by a moderate amount in the region around the AFSCN station, but only while commands are being sent to military spacecraft. This concept has security implications and is just one of the many alternatives being considered by the joint government team seeking a consensus that would support U.S. commercial interests while ensuring defense mission satisfaction.

Conclusion

Given the exponential growth of spectrum demands during the last 10 years, it would not be an exaggeration to say that, after cost and mission objectives, spectrum availability is the primary driver in space system architectural design. Consequently, organizations such as Aerospace that support DOD must study not only military mission requirements but also global economic trends. While it is true that Aerospace is chartered to support national security space and not commercial interests such as cell-phone sales, the demand for spectrum will inevitably result in these two forces colliding. An appreciation of the commercial world enables the military spectrum-management community to anticipate and understand the technical issues associated with bandwidth sharing and thus support the overall objective of an economically strong and secure nation.

National security spacecraft are designed for extreme longevity, and their basic spectrum use cannot be changed while in orbit. Therefore, system designers must anticipate future needs for spectrum use. Aerospace has thus written long-term spectrum-use plans and white papers for its various customers to facilitate this conversation. The focus and decision-making dialogue generated by such an activity validates the need for conceptual planning in spectrum use. More work on detailed long-range spectrum-use plans for DOD or for the government overall is clearly needed.

A national spectrum-management plan must acknowledge the critical need to foster cooperative relationships with international policymakers. Within the spectrum-management world, relationships are perhaps even more important than rules and regulations because rules are always subject to interpretation by key people. Trusted relationships are also critical because spectrum-use planning requires the exchange of database tools, mission goals, trend evaluations, and various potentially sensitive mission details.

Finally, emerging technologies such as lasers and new modulation techniques may foster more-efficient use of the available spectrum and improve the ability of users to share this limited resource. But as always, technology must be leveraged by sound policies, clear foresight, and constructive engagement within the spectrum-management community. A combination of vision, leadership, and synergism is critical for achieving harmonious, efficient, and effective use of this increasingly limited resource.

Further Reading

1. I. Brown, A. Kavetsky, M. J. Riccio, M. Weiskopf, "Spectrum Management and International Filing from the Acquisition Program Manager's Perspective: Current Process and Recent Changes," Proceedings of MILCOM 2000, Vol.1, pp. 1–7 (October 2000).
2. "Connecting the Globe—A Regulator's Guide to Building a Global Information Community," Federal Communications Commission (June 16, 1999).
3. Defense Information Systems Agency, Office of Spectrum Analysis and Management, http://www.disa.mil/d3/depdirops/spectrum/, accessed November 20, 2001.
4. Federal Communications Commission, 2003 World Radiocommunications Conference, http://www.fcc.gov/wrc-03/, accessed November 20, 2001.
5. Federal Communications Commission, Radio Spectrum Home Page, http://www.fcc.gov/oet/spectrum/, accessed November 20, 2001.
6. International Telecommunication Union, http://www.itu.int/ITU-R/index.html, accessed November 20, 2001.
7. Joint Spectrum Center Home Page, http://www.jsc.mil/, accessed November 20, 2001.
8. B. Z. Kobb, Wireless Spectrum Finder: Telecommunications, Government and Scientific Radio Frequency Allocations in the US 30 MHz–300 GHz, McGraw-Hill Professional Publishing (March 2001).
9. Naval Electromagnetic Spectrum Center, http://www.navemscen.navy.mil/, accessed November 20, 2001.
10. NTIA Manual of Regulations & Procedures for Federal Radio Frequency Management, January 2000 Edition, May/Sept 2000 Revisions, U.S. Department of Commerce, National Telecommunication and Information Administration, http://www.ntia.doc.gov/osmhome/redbook/redbook.html, accessed November 21, 2001.
11. NTIA Office of Spectrum Management, http://www.ntia.doc.gov/osmhome/osmhome.html, accessed November 20, 2001.
12. P. C. Roosa Jr., NTIA Special Publication 91-25, Federal Spectrum Management: A Guide to the NTIA Process, U.S. Department of Commerce, National Telecommunication and Information Administration (August 1992).
13. Radio Regulations, Volumes 1–4, International Telecommunication Union (2001).
14. Field Manual No. 24-2, Spectrum Management, Department of the Army, Washington, DC (August 1991).
15. U.S. Army Spectrum Management and Communications, Publications, http://www.army.mil/spectrum/library/regulations.htm, accessed November 21, 2001,
16. D. J. Withers, ed., Radio Spectrum Management: Management of the Spectrum and Regulation of Radio Services (IEEE Telecommunications Series, 45) (January 2000).

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