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Space traffic management: Implementations and implications
Acta Astronautica 58 (2006) 279 – 286 www.elsevier.com/locate/actaastro
Space traffic management: Implementations and implications William H. Ailor∗ Center for Orbital and Reentry Debris Studies, The Aerospace Corporation, 2350 E. El Segundo Blvd, El Segundo, CA 90245, USA Received 30 June 2004; received in revised form 31 August 2005; accepted 13 December 2005 Available online 14 February 2006
Abstract Space traffic management means that space is no longer an open frontier; it means the space-faring nations have decided that there is a reason to bring order to near-Earth space. There are several ways space traffic management might be implemented, and “bringing order” raises numerous questions. This paper discusses justifications and goals for establishing space traffic management, triggers which might move the concept forward, current plans for piloting aspects for space traffic management, and questions that must be answered. The paper argues that effective space traffic management will become a reality when one of two things happens: a collision involving a major space asset, or interest by commercial satellite operators in such a service to protect their assets from collision. The paper includes a discussion of the likelihood of collision and how cost-effectiveness might be interpreted. © 2006 Elsevier Ltd. All rights reserved.
1. Introduction Ref. [1] defines space traffic management as follows: “Space traffic management encompasses all the phases of a space object’s life, from launch to disposal. It consists of activities intended to prevent damage in the near term (such as collision avoidance and coordination of reentry), as well as actions that must be taken to reduce the long-term potential for future damage (such as deorbiting or moving satellites into disposal orbits)”. It can be argued that space traffic management includes pre-mission planning to avoid placing satellites in orbits that increase the probability of damage or interference, and that the term “damage” in the statement above can be interpreted to include any type of manmade interference that might affect the normal operations of a satellite. This might include radio frequency
∗ Tel.: +1 310 336 1135; fax: +1 310 563 3004.
E-mail address: [email protected]. 0094-5765/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2005.12.002
interference (RFI), accidental impingement of directed energy on a satellite, or debris from an accident or experiment that could damage another satellite. In the broader sense, space traffic management might also include the information system and regulatory framework required to assure that all satellite operators abide by the same rules for disposing of space hardware. Someone must “keep book” on satellites to assure that satellite moves are properly coordinated, that satellites are deorbited as required, etc. At present, there is only limited space traffic management in place—that provided by the International Telecommunications Union (ITU), which regulates the orbital slots for communication satellites [1]. Interestingly, this management deals only with frequencies used by satellites, not physical locations of satellites. As a result, satellites broadcasting in different frequencies can occupy essentially the same physical space. Fig. 1 illustrates this point, showing that at some longitude positions, nearly 60 satellites in geosynchronous equatorial orbits (GEOs) share a common region of space.
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Fig. 1. The top figure shows the longitude motion of satellites in near-geosynchronous orbits; the bottom is a count of the number of vehicles that cross a given longitude.
Fortunately, while the probability of collision is increasing slowly with time as more objects are launched into space, the chances of collision remain very small. In fact, there has been only one documented collision of two tracked objects—the 1994 collision of the Cerise satellite with a piece of launch vehicle debris. The estimated average probability of collision for a satellite at GEO is about 1 in 20,000/year. Probability for a satellite at 75◦ East longitude, the area with the most satellites passing through in Fig. 1, is ten times higher, about 1 in 2000/year. Despite these relatively low numbers, there is increasing interest from operators in assuring to the best degree possible and practical that their spacecraft are not damaged or destroyed by a collision with a tracked object and in working together to resolve RFI and other issues. In addition, some high-value satellites like the Space Shuttle and Space Station are already being maneuvered away from threatening objects on a regular basis. These activities increase the perception that collision hazards exist and should be actively managed, even though the probability is small. Generally speaking, some commercial satellite operators recognize that space is no longer hazard-free and, where there is sufficient data, are working together to minimize potential liability. At the present time, however, there is relatively little direct government support for or involvement in some of these activities.
While some would argue that government involvement is not necessarily a good thing, all would likely recognize that government has resources, data, and enforcement capabilities that satellite operators or other commercial entities don’t have or shouldn’t have. For example, governments maintain an extensive satellite tracking capability and have the most complete catalog of orbiting objects. It might well be cost prohibitive for a commercial company to try to duplicate this infrastructure. Governments also bring regulatory and enforcement capabilities to the table, capabilities that will likely become increasingly important as efforts to formalize various aspects of space traffic management increase. Another consideration is proprietary information about the health of a satellite or constellation of satellites. Such information may be very sensitive to its owner, and for that reason, the owner would be very reluctant to provide this to a commercial entity that might have ties to a competitor, but might instead provide the same information to government entities or other organizations that might have, or be set up to have, desirable characteristics. From the government perspective, there has been growing interest in and support for limiting the growth of space debris. In fact, discussions and research coordinated by the Inter-Agency (Space) Debris Coordination Committee (IADC) have led to a number of measures
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designed to limit debris growth. These include venting propellant and pressurized tanks at the end of mission, discharging batteries, and moving hardware to disposal orbits. The IADC also says: “If reliable orbital data is available, avoidance maneuvers for spacecraft and coordination of launch windows may be considered if the collision risk is not considered negligible.”[2]. The development of active space traffic management is supported by results of two international workshops. The first [1] recommends: “A mechanism is needed to warn satellite operators when there will be close approach or potential collision and to provide real-time guidance regarding options.” The second [3] states: “Whereas the explosive growth in space traffic predicted just a few years ago appears to have been deferred into the future, this delayed growth provides us with the opportunity to develop the necessary framework of space-traffic-management practices, regulations, and cooperative processes in a relatively orderly manner, rather than in a crisis environment.” Clearly, government must play an active role in the development of a space traffic management system. The increasing concerns about the growth of space debris, possible on-orbit collisions and RF interference provide indications of how a space traffic management system might evolve. An active space traffic control system might be established to help prevent temporary or permanent loss of service by operational satellites, and this system combined with an IADC-like approach bringing together representatives of space faring nations to develop a regulatory framework might evolve into a more complete space traffic management system. Space traffic control requires excellent information on operating satellites (who owns them, when they will be launched, where they will operate, what communications frequency will they use, when will they move, what is the strategy for maintaining station, when will they be deorbited, etc.) and the evolution of the database and the mechanism for handling the information it contains may be a critical foundation for a formal space traffic management structure.
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The United States Congress has recognized the call for the types of services described and in its 2003 session is considering legislation [8] authorizing a 3-year pilot with the following language: “The Secretary of Defense shall carry out a pilot program to provide eligible entities outside the Federal Government with satellite tracking services using assets owned or controlled by the Department of Defense”. The services piloted will likely be related to satellite collision avoidance, space object identification and the like. The legislation allows for the government to be paid for services, but provides no specifics on charges. Finally, decisions about a space traffic management system, and government involvement in these decisions, might be accelerated should there be a major interference event or should it be shown that such a system would substantially reduce the possibility of collisions or other interference involving operational satellites AND would slow the growth of the debris population in general. In summary, rising desire for satellite collision and RF interference avoidance may lead to an active space traffic control capability, and active space traffic control might be the first major step toward a comprehensive space traffic management system. 2. Goals Assuming that efforts to develop a space traffic control system do move forward, what should goals for the system be? Goals may be considered for each of the major interested parties: satellite operators, governments, and the service provider(s).1 2.1. Goals from the operators’ perspective2 The keys to space traffic management/control from the satellite operators’ perspective appear to be:
1.1. Evolution of space traffic control
1. Minimal cost. Operators want to make as few moves as possible, and ideally would like to include hazard avoidance maneuvers as part of their normal station keeping activities. They want to move at a time when the propellant and mission impacts are minimal, so
Previous papers have provided an overview of space traffic control from the perspective of man-made objects that could pose a threat to other man-made objects [4], provided thoughts on how space traffic control might evolve from the organizational [5] or systems perspective [6], and addressed how the evolution might be determined by the availability of tracking data [7].
1 Ref. [5] argued that a single entity might be the provider of choice, while Ref. [1] concludes that existing entities can provide this service, provided the proper framework exists. 2 Over the past several years, The Aerospace Corporation provided a prototype collision avoidance service to as many as three commercial satellite operators, a service covering as many as 50 satellites at GEO and a large low earth orbit (LEO) constellation. The discussion is based on that experience.
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they need sufficient warning and information to plan the maneuvers carefully. At this point, the overall necessity of a space traffic management system has yet to be demonstrated— there has been only one collision in 40 years, so why worry? Operators are also working together to resolve RFI and other possible interference problems. Given this environment, the cost of collision avoidance or other traffic management service would be a significant factor in determining whether operators would participate. Since the service would be best if it included information on future maneuvers, etc., from as many operators as possible, the goal should be to set the cost at a level to encourage participation. This could mean that it would be free or at a very low cost to participants. This latter point might affect who provides the service in the short term. Commercial companies may not wish to make the up-front investment in required computers, tools, and manpower if the service would not pay for itself in the short term. Uncertainties about the future might also limit interest by commercial companies. Hence, a government-sponsored effort may be the only practical approach at present. It should be noted that some operators have expressed concerns about a free service. The concern is based on a belief that the service, if free, would not meet the operator’s needs and that the operator would be required to expend resources to establish and operate an internal capability. The preference expressed was for contract relationship allowing the operator to contract for a level of effort and specify requirements to be met by the service provider. These concerns may again affect the acceptance of a free service and should be included in planning. There also might be indirect cost implications that would encourage commercial participation. For example, insurance companies might lower rates for satellites taking active measures to avoid interference that might disrupt service. 2. Data quality sufficient to permit significant reduction in collision risk. Data quality is a significant issue. In fact, “current catalogs of debris objects are both incomplete and inaccurate. To enable effective collision avoidance, catalogs must be greatly improved” [1]. Some would argue that publicly-available catalog data may be adequate as a first filter for predictions involving GEO objects if combined with an
operator’s own data, but improvement of the accuracy of that data would be required. A recent study of the collision risk for a group of seven GEO satellites showed that significant reduction in collision risk is not feasible using publicly available catalog data [9]. The reason for this is related to the fact that very low collision probability thresholds are required to trigger maneuvers. Setting the threshold to induce a feasible maneuver rate (e.g., once per year for each satellite) has insignificant impact on the cumulative collision risk over mission lifetime. While the collision probability at each conjunction is small due to low data accuracy, the cumulative collision probability over the vast majority of conjunctions that do not trigger a maneuver remains almost the same as if no maneuvers were performed at all. The study of the group of seven GEO satellites showed that, to achieve a reduction in the cumulative collision risk over mission by a factor of 10, the threshold would have to be made low enough to force each satellite to perform a collision avoidance maneuver every 14 days on average. This maneuver rate would of course be unmanageable. A similar study [10] for satellites in LEO also showed that significant reduction in collision risk was not feasible using data of this same type. These results support the statement of [1] that better quality catalog data is required for significant improvements in collision avoidance. Thus, for the service to be accepted and utilized by operators, the predictions must be accurate enough to permit significant collision risk reduction at an acceptable maneuver rate, or they add very little value and, in fact, could lead to unnecessary moves. 3. Service protects operator assets from as many threatening objects as possible. At the present time, the resident space object catalog includes objects larger than 10 cm in LEO orbits and a meter or larger in GEO. Unfortunately, objects smaller than these can damage a satellite or a critical system and might be avoided if good data were available. In the short term, operators would likely take advantage of the service even if only larger objects are included, since some of these larger objects are other operating satellites and a collision could raise substantial legal, insurance and liability issues. As tracking systems are improved, smaller objects will be included in the catalogs. 4. Information on consequences of a mitigation action. Operators want to know that a maneuver planned
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7.
8. Fig. 2. Velocity increment required to lower collision probability below a 1.0 × 10−6 threshold for a GEO satellite.
to avoid a particular collision or other hazard will not worsen or cause an unacceptable encounter with another object within the predictable future. Thus, the service provider needs to look ahead and assure that these cases do not exist. Operators have considerable freedom in the timing of a maneuver (given sufficient warning), and this factor should be included in predictions. 5. Sufficient warning so that move can be planned, optimized, and verified prior to implementation. Generally speaking, the propellant usage and mission impacts increase as the warning time decreases. Fig. 2 shows the Delta-V required to lower the collision probability for a GEO satellite below a prescribed threshold, illustrating the time dependence of the maneuver [11]. In addition, many satellites operate within a stationkeeping “box” and perform regular stationkeeping burns. Predictions of upcoming close approaches should include the effects of planned stationkeeping burns, and it is possible to adjust these stationkeeping burns slightly for collision avoidance purposes, given sufficient warning. Similarly, if the encounter is with another operating satellite, its upcoming stationkeeping or maneuver burns should be included in the predictions. 6. Protection of sensitive and proprietary information about the health and operational characteristics of an operator’s satellites. Protection of information of this type, including accurate information on a satellite’s location, is of concern to all satellite operators. For example, a satellite operator might worry that a competitor will take advantage of a short-term problem or that a satellite communication link might be targeted for interference. Problems of this sort could affect the value of an operator’s stock. Hence, the
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10.
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service provider must be able to assure that this type of information is properly protected. Rules of engagement that are fair to all parties. Ref. [1] states that “enlightened self interest” is the present approach used for avoiding interference problems and recommends: “The International Academy of Astronautics should undertake a Cosmic Study to recommend rules of the road for space traffic management.” Operators want to be certain that rules of the road related to which satellite moves given a predicted interference are fair and apply to all. All operators held to the same requirements. Ref. [1] suggests incorporating the rules into national space regulation and licensing regimes as a way of increasing compliance with rules of the road. Under this approach, governments would oversee space operations and would help assure that operators comply with regulations. Of course, this implies that governments will have access to information that can be used to assess compliance. Improved coordination among operators. At present, satellite operators wishing to inform other operators of activities that might cause interference maintain a list of phone numbers and e-mail addresses of their counterparts at other companies. These lists become outdated; phone numbers change; they are not comprehensive; and some operators don’t respond. A space traffic management service could act as a facilitator to maintain current lists and bring affected operators together should an interference event be predicted. Service provider(s) responsive to operator needs. This last requirement is important, particularly in the early phases of development of a space traffic control system. Operators require ongoing, probably daily, information on interference issues and may require additional support to help resolve specific upcoming events. Service providers must plan to work closely with individual operators as they take advantage of the new service.
2.2. Goals from governments’ perspective Governments were once the primary users of space. While that distinction is no longer true, governments still have significant and critical assets in space and for this reason, they share commercial operators’ concerns about possible interference. Governments have a larger role: to assure that the space resource remains available to all. The following might be goals from government’s
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perspective:
2.3. Goals from the service provider’s perspective
1. Unimpeded access to space. Governments must assure and protect the rights of government and commercial operators to utilize space for national security, public benefit, and commercial activities. 2. Availability of space assets for commercial and nationally significant uses. Once in space, assets must be available for the job they were intended to do and not have mission compromising down times associated with interferences with other objects. Just as commercial operators, it is in governments’ best interest for satellites to be reliable and on-line. 3. A space operating environment that poses as few constraints as possible. Since the economic viability of commercial satellites and the robustness of government satellites can be adversely affected by government-imposed restrictions and constraints on satellite operations, governments will likely want to minimize these types of influences. 4. All satellite operators abiding by the same space debris mitigation and other internationally agreedupon rules. Governments will want to assure that rules of the road and other operational requirements are fairly applied. 5. Information to assess compliance of operators with space debris mitigation, hardware disposal, other requirements. While governments may not need specific information on day-to-day activities of space assets, they may want information by which they may assess compliance of spacecraft with international regulations. The first place an operator will likely go to lodge a complaint about unfair treatment or another operator not abiding by regulations is a governmental office, so governments must be able to get information they need to evaluate the complaint. 6. Minimization of the rate of growth of the population of space debris. Governments have been working together through the IADC, as discussed earlier, to evolve internationally agreed-upon measures to limit the growth of space debris. Governments are likely to be heavily involved should there be a collision of two operating satellites, particularly if the two involved are of different nationalities. 7. Protection of sensitive government data. Just as commercial satellite operators, governments protect information of a sensitive nature about their satellites and satellite locations, about tracking capabilities, about communication frequencies, and about other aspects of satellite operations. Protection of this type of information must continue in any implementation of space traffic control and management.
The third principal is the service provider. Likely goals/requirements include: 1. Accurate and reliable predictions provided in a timely manner. For the service provider to be relevant, it must provide information that is useful to operators. 2. Protection from consequences. While the service provider will do its best to provide accurate warnings, the current state and position accuracy limitations of the resident space object catalog mean that there is a probability that a potential collision may not be prevented. When position accuracy is low, the collision probability threshold cannot be set low enough to significantly reduce collision risk over a mission lifetime without inducing an unacceptable maneuver or “action” (additional sensor measurement or maneuver) rate [11,12]. Thus, the service provider must be protected from liability associated with such events. 3. Adequate and reliable information on operational characteristics (e.g., control boxes) and plans for upcoming maneuvers. Predictions of upcoming events require this type of information from as many operators as possible. Access to this type of data requires ongoing communications with satellite operators, and this level of communications means that the service provider could play a valuable role in bringing operators together to resolve common interference events. 4. Unimpeded access to tracking and/or resident space object catalog data of resolution and frequency sufficient for reliable and accurate predictions. The service will be based on tracking and other data. This data must be available at all times. 5. Ability to request and receive additional sensor measurements should a close approach warrant. Catalog data may be used to isolate potential interference events, but refinement of each event requires additional tracking data on both objects. The service provider will require reliable sources of this information, and the sources must be able to react quickly to service provider needs. One uncertainty is the quantity of requests for additional tracking that will be required. For LEO objects, the number of requests could be large, given the large number of objects in LEO and LEO-intersecting orbits and the greater aerodynamic and other forces which cause uncertainties in the position of orbiting objects to increase quickly as compared to GEO objects.
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6. Sufficient manpower, computer, and tracking resources. For the service provider to be relevant, it must be able to bring resources to bear to resolve predicted close approach events. Specialists in orbit mechanics, spacecraft dynamics and control, and probability estimation will likely be required to work with operators and develop specialized tools. As the number of tracked objects increases, the capabilities of interference prediction tools and computers must also increase.
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Another possible trigger might be international recognition that the growth of space debris represents a serious challenge to future space activities and a finding that space traffic management would provide a means for controlling this growth or limiting the probability of damage to an operating satellite. Present studies on the long-term debris population generally do not consider the effect of space traffic control and collision avoidance.
4. Cost effectiveness 3. Triggers for action As has been noted, the world space faring community is recognizing the need for space traffic management and space traffic control. The U.S. Congress may approve legislation authorizing a 3-year pilot of a collision avoidance service. Some operators have taken advantage of pilot services already offered and some satellites have been moved to avoid predicted close approaches. Operators are working together to resolve RFI issues. The ITU currently regulates positions of satellites to avoid RF interference problems. Governments are working together at IADC to minimize growth of space debris. Based on these activities, some would argue that we are already on a path that will lead to space traffic management. But these essentially independent activities do not define a system or overall structure for space traffic management. So what triggers can we expect that will cause convergence on and formalization of a structure? A significant trigger must always be the loss of a critical asset attributable to a collision or major interference. Such an event would likely cause a public review, perhaps involving political and policy-making offices of multiple governments depending on the seriousness of the incident, and this process would ultimately bring the space faring governments and satellite operators together in an effort to prevent future events. A second trigger would be a call for help by several major international commercial satellite operators. This group is currently establishing a Satellite Users Interference Reduction Group (SUIRG) [13] to help commercial satellite operators coordinate and deal with RF interference issues. SUIRG or another group might decide to favor certain regulations as a way of insuring all operators are playing by the same rules. It should be noted that a similar call for help by operators of government satellites would not be likely have the same effect, since they would represent only a subset of operators and might be viewed as having ulterior motives.
Several factors must be included in discussions of cost effectiveness of a space traffic management system, and these should be included in a more detailed cost/benefit analysis as planning proceeds. First, will such a system increase costs to satellite manufacturers, launchers, and operators? The answer is probably yes, since satellite operators may have increasing need for maneuver capability, both to avoid interference and to dispose of satellites at end of mission in accordance with emerging requirements. Adding or increasing these capabilities could increase spacecraft weight and complexity, increasing the cost. In addition, there may be some costs to operators for the service itself. Second, will such a system increase cost to governments? Again, the answer is probably yes. Governments currently maintain the tracking networks and maintain databases of tracked objects. Calls for improved data quality and for special taskings of sensors to resolve interferences will increase costs to governments. It is unlikely that government will pass along much of this cost. Given that costs will likely increase, what are the benefits? First, loss of a high-value satellite would be very expensive from many perspectives. The probability of economic or other loss might be substantially reduced by a space traffic management system utilizing data on all threatening objects. Second, governments would help assure that nearEarth space remains open with minimal constraints. Collisions of satellites could lead to increases in the debris populations that would increase the possibility of collision, meaning that satellite designs and operations must increase in complexity to maintain capability. A space traffic management system would provide information governments require to assure that operators are following international requirements for debris reduction.
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Third, for operators, such a system would enable them to design and manage constellations with much improved information on the current and future environment. Just as the ITU process minimizes radio frequency interference, a space traffic management system would provide similar benefits for selection of orbit altitudes and other characteristics of new services. As a final thought, if better catalogs are available, if catalogs include orbits of smaller objects, and if operators do take advantage of this capability to avoid oncoming threats and threat objects, how does the probability of damaging an operating satellite or serious interference change in the future? How does the cost of operating spacecraft change (e.g., how many satellite moves are required to keep the probability of collision below a specified threshold)? The answer to these questions and a better characterization of the costs and benefits noted above could lead to motion toward a space traffic management system.
5. Summary The first steps toward management of space traffic have already been taken in an uncoordinated fashion, and emerging requirements for minimization of space debris and a U.S. government sanctioned pilot operation of a collision avoidance service (if it approved by Congress) may be the beginning of the next phase. Such a service must meet a number of operator, government, and service provider goals to be successful. All steps from this point will involve more sensitive issues and data for both governments and satellite operators and will require substantial cooperation by all parties. It is likely that progress toward a true space traffic management system will remain slow and steady unless a major interference or satellite collision event
occurs or unless commercial satellite operators begin to support and lobby for institution of more space traffic management capabilities and regulations. References [1] International Space Cooperation, Addressing challenges of the new millennium, Report of an AIAA, UN/OOSA, CEAS, CASI Workshop, March 2001. [2] Inter-Agency Space Debris Coordination, Committee space debris mitigation guidelines, United Nations document AtAC.lOS/C.l/L.260, November 29, 2002. [3] International Space Cooperation, Solving global problems, Report of an AIAA, UN/OOSA, CEAS, CASI Workshop, April 1999. [4] W.H. Ailor, Controlling space traffic, Aerospace America, November 1999. [5] W.H. Ailor, Space traffic control: a view of the future, Space Policy 18 (2002) 99–105. [6] W.O. Glascoe, The Earth outer space traffic control system constellation, AIAA 2001-4808. [7] W.H. Ailor, Space traffic control: data access defines the future, presentation to the IISL–ECSL Symposium on Prospects for Space Traffic Management, Vienna, Austria, April 2, 2002. [8] H.R.1588, National Defense Authorization Act for FY2004 (Public Print); SEC. 914. Pilot Program to Provide Space Surveillance Network Services to Entities Outside the United States Government. [9] A.B. Jenkin, G.E. Peterson, Collision risk management in geosynchronous orbit, PEDAS1-B1.4-0049-02, 34th COSPAR Scientific Assembly, Houston, TX, October 10–19, 2002. [10] A.B. Jenkin, Effect of orbit data quality on the operational cost of collision risk management, AIAA Paper No. 2002-1810, SatMax 2002, Arlington, Virginia, April 22, 2002. [11] R.P. Patera, G.E. Peterson, Space vehicle maneuver method to lower collision risk to an acceptable level, Journal of Guidance, Control, and Dynamics 25(2), 233–237. [12] A.B. Jenkin, G.E. Peterson, Collision risk management for geosynchronous spacecraft, COSPAR02-A-00286, 34th COSPAR Scientific Assembly, World Space Congress, Houston, TX, October 2002. [13] R. Ames, SUIRG: what is it? where is it going? Presentation to Forum on RFI Issues in Space Operations, March 18–19, 2003.
Space traffic management: Implementations and implications William H. Ailor∗ Center for Orbital and Reentry Debris Studies, The Aerospace Corporation, 2350 E. El Segundo Blvd, El Segundo, CA 90245, USA Received 30 June 2004; received in revised form 31 August 2005; accepted 13 December 2005 Available online 14 February 2006
Abstract Space traffic management means that space is no longer an open frontier; it means the space-faring nations have decided that there is a reason to bring order to near-Earth space. There are several ways space traffic management might be implemented, and “bringing order” raises numerous questions. This paper discusses justifications and goals for establishing space traffic management, triggers which might move the concept forward, current plans for piloting aspects for space traffic management, and questions that must be answered. The paper argues that effective space traffic management will become a reality when one of two things happens: a collision involving a major space asset, or interest by commercial satellite operators in such a service to protect their assets from collision. The paper includes a discussion of the likelihood of collision and how cost-effectiveness might be interpreted. © 2006 Elsevier Ltd. All rights reserved.
1. Introduction Ref. [1] defines space traffic management as follows: “Space traffic management encompasses all the phases of a space object’s life, from launch to disposal. It consists of activities intended to prevent damage in the near term (such as collision avoidance and coordination of reentry), as well as actions that must be taken to reduce the long-term potential for future damage (such as deorbiting or moving satellites into disposal orbits)”. It can be argued that space traffic management includes pre-mission planning to avoid placing satellites in orbits that increase the probability of damage or interference, and that the term “damage” in the statement above can be interpreted to include any type of manmade interference that might affect the normal operations of a satellite. This might include radio frequency
∗ Tel.: +1 310 336 1135; fax: +1 310 563 3004.
E-mail address: [email protected]. 0094-5765/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.actaastro.2005.12.002
interference (RFI), accidental impingement of directed energy on a satellite, or debris from an accident or experiment that could damage another satellite. In the broader sense, space traffic management might also include the information system and regulatory framework required to assure that all satellite operators abide by the same rules for disposing of space hardware. Someone must “keep book” on satellites to assure that satellite moves are properly coordinated, that satellites are deorbited as required, etc. At present, there is only limited space traffic management in place—that provided by the International Telecommunications Union (ITU), which regulates the orbital slots for communication satellites [1]. Interestingly, this management deals only with frequencies used by satellites, not physical locations of satellites. As a result, satellites broadcasting in different frequencies can occupy essentially the same physical space. Fig. 1 illustrates this point, showing that at some longitude positions, nearly 60 satellites in geosynchronous equatorial orbits (GEOs) share a common region of space.
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Fig. 1. The top figure shows the longitude motion of satellites in near-geosynchronous orbits; the bottom is a count of the number of vehicles that cross a given longitude.
Fortunately, while the probability of collision is increasing slowly with time as more objects are launched into space, the chances of collision remain very small. In fact, there has been only one documented collision of two tracked objects—the 1994 collision of the Cerise satellite with a piece of launch vehicle debris. The estimated average probability of collision for a satellite at GEO is about 1 in 20,000/year. Probability for a satellite at 75◦ East longitude, the area with the most satellites passing through in Fig. 1, is ten times higher, about 1 in 2000/year. Despite these relatively low numbers, there is increasing interest from operators in assuring to the best degree possible and practical that their spacecraft are not damaged or destroyed by a collision with a tracked object and in working together to resolve RFI and other issues. In addition, some high-value satellites like the Space Shuttle and Space Station are already being maneuvered away from threatening objects on a regular basis. These activities increase the perception that collision hazards exist and should be actively managed, even though the probability is small. Generally speaking, some commercial satellite operators recognize that space is no longer hazard-free and, where there is sufficient data, are working together to minimize potential liability. At the present time, however, there is relatively little direct government support for or involvement in some of these activities.
While some would argue that government involvement is not necessarily a good thing, all would likely recognize that government has resources, data, and enforcement capabilities that satellite operators or other commercial entities don’t have or shouldn’t have. For example, governments maintain an extensive satellite tracking capability and have the most complete catalog of orbiting objects. It might well be cost prohibitive for a commercial company to try to duplicate this infrastructure. Governments also bring regulatory and enforcement capabilities to the table, capabilities that will likely become increasingly important as efforts to formalize various aspects of space traffic management increase. Another consideration is proprietary information about the health of a satellite or constellation of satellites. Such information may be very sensitive to its owner, and for that reason, the owner would be very reluctant to provide this to a commercial entity that might have ties to a competitor, but might instead provide the same information to government entities or other organizations that might have, or be set up to have, desirable characteristics. From the government perspective, there has been growing interest in and support for limiting the growth of space debris. In fact, discussions and research coordinated by the Inter-Agency (Space) Debris Coordination Committee (IADC) have led to a number of measures
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designed to limit debris growth. These include venting propellant and pressurized tanks at the end of mission, discharging batteries, and moving hardware to disposal orbits. The IADC also says: “If reliable orbital data is available, avoidance maneuvers for spacecraft and coordination of launch windows may be considered if the collision risk is not considered negligible.”[2]. The development of active space traffic management is supported by results of two international workshops. The first [1] recommends: “A mechanism is needed to warn satellite operators when there will be close approach or potential collision and to provide real-time guidance regarding options.” The second [3] states: “Whereas the explosive growth in space traffic predicted just a few years ago appears to have been deferred into the future, this delayed growth provides us with the opportunity to develop the necessary framework of space-traffic-management practices, regulations, and cooperative processes in a relatively orderly manner, rather than in a crisis environment.” Clearly, government must play an active role in the development of a space traffic management system. The increasing concerns about the growth of space debris, possible on-orbit collisions and RF interference provide indications of how a space traffic management system might evolve. An active space traffic control system might be established to help prevent temporary or permanent loss of service by operational satellites, and this system combined with an IADC-like approach bringing together representatives of space faring nations to develop a regulatory framework might evolve into a more complete space traffic management system. Space traffic control requires excellent information on operating satellites (who owns them, when they will be launched, where they will operate, what communications frequency will they use, when will they move, what is the strategy for maintaining station, when will they be deorbited, etc.) and the evolution of the database and the mechanism for handling the information it contains may be a critical foundation for a formal space traffic management structure.
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The United States Congress has recognized the call for the types of services described and in its 2003 session is considering legislation [8] authorizing a 3-year pilot with the following language: “The Secretary of Defense shall carry out a pilot program to provide eligible entities outside the Federal Government with satellite tracking services using assets owned or controlled by the Department of Defense”. The services piloted will likely be related to satellite collision avoidance, space object identification and the like. The legislation allows for the government to be paid for services, but provides no specifics on charges. Finally, decisions about a space traffic management system, and government involvement in these decisions, might be accelerated should there be a major interference event or should it be shown that such a system would substantially reduce the possibility of collisions or other interference involving operational satellites AND would slow the growth of the debris population in general. In summary, rising desire for satellite collision and RF interference avoidance may lead to an active space traffic control capability, and active space traffic control might be the first major step toward a comprehensive space traffic management system. 2. Goals Assuming that efforts to develop a space traffic control system do move forward, what should goals for the system be? Goals may be considered for each of the major interested parties: satellite operators, governments, and the service provider(s).1 2.1. Goals from the operators’ perspective2 The keys to space traffic management/control from the satellite operators’ perspective appear to be:
1.1. Evolution of space traffic control
1. Minimal cost. Operators want to make as few moves as possible, and ideally would like to include hazard avoidance maneuvers as part of their normal station keeping activities. They want to move at a time when the propellant and mission impacts are minimal, so
Previous papers have provided an overview of space traffic control from the perspective of man-made objects that could pose a threat to other man-made objects [4], provided thoughts on how space traffic control might evolve from the organizational [5] or systems perspective [6], and addressed how the evolution might be determined by the availability of tracking data [7].
1 Ref. [5] argued that a single entity might be the provider of choice, while Ref. [1] concludes that existing entities can provide this service, provided the proper framework exists. 2 Over the past several years, The Aerospace Corporation provided a prototype collision avoidance service to as many as three commercial satellite operators, a service covering as many as 50 satellites at GEO and a large low earth orbit (LEO) constellation. The discussion is based on that experience.
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they need sufficient warning and information to plan the maneuvers carefully. At this point, the overall necessity of a space traffic management system has yet to be demonstrated— there has been only one collision in 40 years, so why worry? Operators are also working together to resolve RFI and other possible interference problems. Given this environment, the cost of collision avoidance or other traffic management service would be a significant factor in determining whether operators would participate. Since the service would be best if it included information on future maneuvers, etc., from as many operators as possible, the goal should be to set the cost at a level to encourage participation. This could mean that it would be free or at a very low cost to participants. This latter point might affect who provides the service in the short term. Commercial companies may not wish to make the up-front investment in required computers, tools, and manpower if the service would not pay for itself in the short term. Uncertainties about the future might also limit interest by commercial companies. Hence, a government-sponsored effort may be the only practical approach at present. It should be noted that some operators have expressed concerns about a free service. The concern is based on a belief that the service, if free, would not meet the operator’s needs and that the operator would be required to expend resources to establish and operate an internal capability. The preference expressed was for contract relationship allowing the operator to contract for a level of effort and specify requirements to be met by the service provider. These concerns may again affect the acceptance of a free service and should be included in planning. There also might be indirect cost implications that would encourage commercial participation. For example, insurance companies might lower rates for satellites taking active measures to avoid interference that might disrupt service. 2. Data quality sufficient to permit significant reduction in collision risk. Data quality is a significant issue. In fact, “current catalogs of debris objects are both incomplete and inaccurate. To enable effective collision avoidance, catalogs must be greatly improved” [1]. Some would argue that publicly-available catalog data may be adequate as a first filter for predictions involving GEO objects if combined with an
operator’s own data, but improvement of the accuracy of that data would be required. A recent study of the collision risk for a group of seven GEO satellites showed that significant reduction in collision risk is not feasible using publicly available catalog data [9]. The reason for this is related to the fact that very low collision probability thresholds are required to trigger maneuvers. Setting the threshold to induce a feasible maneuver rate (e.g., once per year for each satellite) has insignificant impact on the cumulative collision risk over mission lifetime. While the collision probability at each conjunction is small due to low data accuracy, the cumulative collision probability over the vast majority of conjunctions that do not trigger a maneuver remains almost the same as if no maneuvers were performed at all. The study of the group of seven GEO satellites showed that, to achieve a reduction in the cumulative collision risk over mission by a factor of 10, the threshold would have to be made low enough to force each satellite to perform a collision avoidance maneuver every 14 days on average. This maneuver rate would of course be unmanageable. A similar study [10] for satellites in LEO also showed that significant reduction in collision risk was not feasible using data of this same type. These results support the statement of [1] that better quality catalog data is required for significant improvements in collision avoidance. Thus, for the service to be accepted and utilized by operators, the predictions must be accurate enough to permit significant collision risk reduction at an acceptable maneuver rate, or they add very little value and, in fact, could lead to unnecessary moves. 3. Service protects operator assets from as many threatening objects as possible. At the present time, the resident space object catalog includes objects larger than 10 cm in LEO orbits and a meter or larger in GEO. Unfortunately, objects smaller than these can damage a satellite or a critical system and might be avoided if good data were available. In the short term, operators would likely take advantage of the service even if only larger objects are included, since some of these larger objects are other operating satellites and a collision could raise substantial legal, insurance and liability issues. As tracking systems are improved, smaller objects will be included in the catalogs. 4. Information on consequences of a mitigation action. Operators want to know that a maneuver planned
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7.
8. Fig. 2. Velocity increment required to lower collision probability below a 1.0 × 10−6 threshold for a GEO satellite.
to avoid a particular collision or other hazard will not worsen or cause an unacceptable encounter with another object within the predictable future. Thus, the service provider needs to look ahead and assure that these cases do not exist. Operators have considerable freedom in the timing of a maneuver (given sufficient warning), and this factor should be included in predictions. 5. Sufficient warning so that move can be planned, optimized, and verified prior to implementation. Generally speaking, the propellant usage and mission impacts increase as the warning time decreases. Fig. 2 shows the Delta-V required to lower the collision probability for a GEO satellite below a prescribed threshold, illustrating the time dependence of the maneuver [11]. In addition, many satellites operate within a stationkeeping “box” and perform regular stationkeeping burns. Predictions of upcoming close approaches should include the effects of planned stationkeeping burns, and it is possible to adjust these stationkeeping burns slightly for collision avoidance purposes, given sufficient warning. Similarly, if the encounter is with another operating satellite, its upcoming stationkeeping or maneuver burns should be included in the predictions. 6. Protection of sensitive and proprietary information about the health and operational characteristics of an operator’s satellites. Protection of information of this type, including accurate information on a satellite’s location, is of concern to all satellite operators. For example, a satellite operator might worry that a competitor will take advantage of a short-term problem or that a satellite communication link might be targeted for interference. Problems of this sort could affect the value of an operator’s stock. Hence, the
9.
10.
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service provider must be able to assure that this type of information is properly protected. Rules of engagement that are fair to all parties. Ref. [1] states that “enlightened self interest” is the present approach used for avoiding interference problems and recommends: “The International Academy of Astronautics should undertake a Cosmic Study to recommend rules of the road for space traffic management.” Operators want to be certain that rules of the road related to which satellite moves given a predicted interference are fair and apply to all. All operators held to the same requirements. Ref. [1] suggests incorporating the rules into national space regulation and licensing regimes as a way of increasing compliance with rules of the road. Under this approach, governments would oversee space operations and would help assure that operators comply with regulations. Of course, this implies that governments will have access to information that can be used to assess compliance. Improved coordination among operators. At present, satellite operators wishing to inform other operators of activities that might cause interference maintain a list of phone numbers and e-mail addresses of their counterparts at other companies. These lists become outdated; phone numbers change; they are not comprehensive; and some operators don’t respond. A space traffic management service could act as a facilitator to maintain current lists and bring affected operators together should an interference event be predicted. Service provider(s) responsive to operator needs. This last requirement is important, particularly in the early phases of development of a space traffic control system. Operators require ongoing, probably daily, information on interference issues and may require additional support to help resolve specific upcoming events. Service providers must plan to work closely with individual operators as they take advantage of the new service.
2.2. Goals from governments’ perspective Governments were once the primary users of space. While that distinction is no longer true, governments still have significant and critical assets in space and for this reason, they share commercial operators’ concerns about possible interference. Governments have a larger role: to assure that the space resource remains available to all. The following might be goals from government’s
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perspective:
2.3. Goals from the service provider’s perspective
1. Unimpeded access to space. Governments must assure and protect the rights of government and commercial operators to utilize space for national security, public benefit, and commercial activities. 2. Availability of space assets for commercial and nationally significant uses. Once in space, assets must be available for the job they were intended to do and not have mission compromising down times associated with interferences with other objects. Just as commercial operators, it is in governments’ best interest for satellites to be reliable and on-line. 3. A space operating environment that poses as few constraints as possible. Since the economic viability of commercial satellites and the robustness of government satellites can be adversely affected by government-imposed restrictions and constraints on satellite operations, governments will likely want to minimize these types of influences. 4. All satellite operators abiding by the same space debris mitigation and other internationally agreedupon rules. Governments will want to assure that rules of the road and other operational requirements are fairly applied. 5. Information to assess compliance of operators with space debris mitigation, hardware disposal, other requirements. While governments may not need specific information on day-to-day activities of space assets, they may want information by which they may assess compliance of spacecraft with international regulations. The first place an operator will likely go to lodge a complaint about unfair treatment or another operator not abiding by regulations is a governmental office, so governments must be able to get information they need to evaluate the complaint. 6. Minimization of the rate of growth of the population of space debris. Governments have been working together through the IADC, as discussed earlier, to evolve internationally agreed-upon measures to limit the growth of space debris. Governments are likely to be heavily involved should there be a collision of two operating satellites, particularly if the two involved are of different nationalities. 7. Protection of sensitive government data. Just as commercial satellite operators, governments protect information of a sensitive nature about their satellites and satellite locations, about tracking capabilities, about communication frequencies, and about other aspects of satellite operations. Protection of this type of information must continue in any implementation of space traffic control and management.
The third principal is the service provider. Likely goals/requirements include: 1. Accurate and reliable predictions provided in a timely manner. For the service provider to be relevant, it must provide information that is useful to operators. 2. Protection from consequences. While the service provider will do its best to provide accurate warnings, the current state and position accuracy limitations of the resident space object catalog mean that there is a probability that a potential collision may not be prevented. When position accuracy is low, the collision probability threshold cannot be set low enough to significantly reduce collision risk over a mission lifetime without inducing an unacceptable maneuver or “action” (additional sensor measurement or maneuver) rate [11,12]. Thus, the service provider must be protected from liability associated with such events. 3. Adequate and reliable information on operational characteristics (e.g., control boxes) and plans for upcoming maneuvers. Predictions of upcoming events require this type of information from as many operators as possible. Access to this type of data requires ongoing communications with satellite operators, and this level of communications means that the service provider could play a valuable role in bringing operators together to resolve common interference events. 4. Unimpeded access to tracking and/or resident space object catalog data of resolution and frequency sufficient for reliable and accurate predictions. The service will be based on tracking and other data. This data must be available at all times. 5. Ability to request and receive additional sensor measurements should a close approach warrant. Catalog data may be used to isolate potential interference events, but refinement of each event requires additional tracking data on both objects. The service provider will require reliable sources of this information, and the sources must be able to react quickly to service provider needs. One uncertainty is the quantity of requests for additional tracking that will be required. For LEO objects, the number of requests could be large, given the large number of objects in LEO and LEO-intersecting orbits and the greater aerodynamic and other forces which cause uncertainties in the position of orbiting objects to increase quickly as compared to GEO objects.
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6. Sufficient manpower, computer, and tracking resources. For the service provider to be relevant, it must be able to bring resources to bear to resolve predicted close approach events. Specialists in orbit mechanics, spacecraft dynamics and control, and probability estimation will likely be required to work with operators and develop specialized tools. As the number of tracked objects increases, the capabilities of interference prediction tools and computers must also increase.
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Another possible trigger might be international recognition that the growth of space debris represents a serious challenge to future space activities and a finding that space traffic management would provide a means for controlling this growth or limiting the probability of damage to an operating satellite. Present studies on the long-term debris population generally do not consider the effect of space traffic control and collision avoidance.
4. Cost effectiveness 3. Triggers for action As has been noted, the world space faring community is recognizing the need for space traffic management and space traffic control. The U.S. Congress may approve legislation authorizing a 3-year pilot of a collision avoidance service. Some operators have taken advantage of pilot services already offered and some satellites have been moved to avoid predicted close approaches. Operators are working together to resolve RFI issues. The ITU currently regulates positions of satellites to avoid RF interference problems. Governments are working together at IADC to minimize growth of space debris. Based on these activities, some would argue that we are already on a path that will lead to space traffic management. But these essentially independent activities do not define a system or overall structure for space traffic management. So what triggers can we expect that will cause convergence on and formalization of a structure? A significant trigger must always be the loss of a critical asset attributable to a collision or major interference. Such an event would likely cause a public review, perhaps involving political and policy-making offices of multiple governments depending on the seriousness of the incident, and this process would ultimately bring the space faring governments and satellite operators together in an effort to prevent future events. A second trigger would be a call for help by several major international commercial satellite operators. This group is currently establishing a Satellite Users Interference Reduction Group (SUIRG) [13] to help commercial satellite operators coordinate and deal with RF interference issues. SUIRG or another group might decide to favor certain regulations as a way of insuring all operators are playing by the same rules. It should be noted that a similar call for help by operators of government satellites would not be likely have the same effect, since they would represent only a subset of operators and might be viewed as having ulterior motives.
Several factors must be included in discussions of cost effectiveness of a space traffic management system, and these should be included in a more detailed cost/benefit analysis as planning proceeds. First, will such a system increase costs to satellite manufacturers, launchers, and operators? The answer is probably yes, since satellite operators may have increasing need for maneuver capability, both to avoid interference and to dispose of satellites at end of mission in accordance with emerging requirements. Adding or increasing these capabilities could increase spacecraft weight and complexity, increasing the cost. In addition, there may be some costs to operators for the service itself. Second, will such a system increase cost to governments? Again, the answer is probably yes. Governments currently maintain the tracking networks and maintain databases of tracked objects. Calls for improved data quality and for special taskings of sensors to resolve interferences will increase costs to governments. It is unlikely that government will pass along much of this cost. Given that costs will likely increase, what are the benefits? First, loss of a high-value satellite would be very expensive from many perspectives. The probability of economic or other loss might be substantially reduced by a space traffic management system utilizing data on all threatening objects. Second, governments would help assure that nearEarth space remains open with minimal constraints. Collisions of satellites could lead to increases in the debris populations that would increase the possibility of collision, meaning that satellite designs and operations must increase in complexity to maintain capability. A space traffic management system would provide information governments require to assure that operators are following international requirements for debris reduction.
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Third, for operators, such a system would enable them to design and manage constellations with much improved information on the current and future environment. Just as the ITU process minimizes radio frequency interference, a space traffic management system would provide similar benefits for selection of orbit altitudes and other characteristics of new services. As a final thought, if better catalogs are available, if catalogs include orbits of smaller objects, and if operators do take advantage of this capability to avoid oncoming threats and threat objects, how does the probability of damaging an operating satellite or serious interference change in the future? How does the cost of operating spacecraft change (e.g., how many satellite moves are required to keep the probability of collision below a specified threshold)? The answer to these questions and a better characterization of the costs and benefits noted above could lead to motion toward a space traffic management system.
5. Summary The first steps toward management of space traffic have already been taken in an uncoordinated fashion, and emerging requirements for minimization of space debris and a U.S. government sanctioned pilot operation of a collision avoidance service (if it approved by Congress) may be the beginning of the next phase. Such a service must meet a number of operator, government, and service provider goals to be successful. All steps from this point will involve more sensitive issues and data for both governments and satellite operators and will require substantial cooperation by all parties. It is likely that progress toward a true space traffic management system will remain slow and steady unless a major interference or satellite collision event
occurs or unless commercial satellite operators begin to support and lobby for institution of more space traffic management capabilities and regulations. References [1] International Space Cooperation, Addressing challenges of the new millennium, Report of an AIAA, UN/OOSA, CEAS, CASI Workshop, March 2001. [2] Inter-Agency Space Debris Coordination, Committee space debris mitigation guidelines, United Nations document AtAC.lOS/C.l/L.260, November 29, 2002. [3] International Space Cooperation, Solving global problems, Report of an AIAA, UN/OOSA, CEAS, CASI Workshop, April 1999. [4] W.H. Ailor, Controlling space traffic, Aerospace America, November 1999. [5] W.H. Ailor, Space traffic control: a view of the future, Space Policy 18 (2002) 99–105. [6] W.O. Glascoe, The Earth outer space traffic control system constellation, AIAA 2001-4808. [7] W.H. Ailor, Space traffic control: data access defines the future, presentation to the IISL–ECSL Symposium on Prospects for Space Traffic Management, Vienna, Austria, April 2, 2002. [8] H.R.1588, National Defense Authorization Act for FY2004 (Public Print); SEC. 914. Pilot Program to Provide Space Surveillance Network Services to Entities Outside the United States Government. [9] A.B. Jenkin, G.E. Peterson, Collision risk management in geosynchronous orbit, PEDAS1-B1.4-0049-02, 34th COSPAR Scientific Assembly, Houston, TX, October 10–19, 2002. [10] A.B. Jenkin, Effect of orbit data quality on the operational cost of collision risk management, AIAA Paper No. 2002-1810, SatMax 2002, Arlington, Virginia, April 22, 2002. [11] R.P. Patera, G.E. Peterson, Space vehicle maneuver method to lower collision risk to an acceptable level, Journal of Guidance, Control, and Dynamics 25(2), 233–237. [12] A.B. Jenkin, G.E. Peterson, Collision risk management for geosynchronous spacecraft, COSPAR02-A-00286, 34th COSPAR Scientific Assembly, World Space Congress, Houston, TX, October 2002. [13] R. Ames, SUIRG: what is it? where is it going? Presentation to Forum on RFI Issues in Space Operations, March 18–19, 2003.