ABSTRACT. The planning processes developed for the International Space Station (ISS) must recognize the fact that the ISS is an on-orbit facility which will operate continuously over its ten to fifteen year lifetime. In effect, the ISS is one "mission" with an extremely long duration. To date, much emphasis has been placed on subdividing the ISS mission into sequential time periods called "increments" in an attempt to apply the planning concepts used for discrete short duration missions to the ISS planning problem. An alternative approach, called "Continuous Operations Planning", has been developed which may provide a more robust and cost-effective method for planning in the continuous operations environment of the ISS. It separates ISS planning into two basic planning functions: 1) long-range planning for a fixed length planning horizon, which continually moves forward as ISS operations progress and emphasizes the preparation for operations; and 2) short-range planning, which takes a small segment of the long-range plan and develops the detailed operations schedules. This paper compares the continuous operations approach with that of the increment-based approach, describes the long and short-range planning functions, and summarizes the benefits and challenges of implementing a continuous operations planning approach for the ISS.

The International Space Station (ISS), once its assembly begins, is a continuously operating on-orbit facility which provides the systems, resources, and environment necessary to support scientific and commercial research goals. The ISS is visited periodically by various U.S., Russian, and European Earth-To-Orbit-Vehicles (ETOVs). These ETOV flights support the assembly of the station and provide for the transportation of crew, supplies, and payloads to and from the ISS. Onboard systems and payloads, once put in place, will operate for months or years. Over the lifetime of the station, new components will be added, onboard systems will be upgraded, payloads and crew will be changed out, ground facilities will be reconfigured, ETOVs will come and go, and onboard hardware will occasionally fail. The ongoing ISS operations will adapt to these changes and continue on.

The ISS planning processes should recognize the fact that the ISS is a continuously operating facility with a ten to fifteen year lifetime. In effect, the ISS is one "mission" with an extremely long duration. Therefore, planning techniques which are used in other continuous operations environments should be investigated for potential applicability to the ISS planning problem. Examples of other continuously operating space vehicles include the manned Russian MIR space station, and various unmanned space vehicles. However, unlike the ISS, the unmanned vehicles do not have to deal with periodic ETOV resupply/assembly flights or with the unique considerations of the onboard crew. Examples of non-space environments in which a facility operates continuously with periodic resupply and/or reconfiguration include: factories, retail establishments, naval operations (submarines, aircraft carriers, etc.), and hospitals. Some might argue that the planning processes for these non-space environments cannot be directly applied to space operations planning because the complex nature of space operations dictates some unique planning constraints and considerations. This is of course true to some extent. However, some of the basic concepts can and do apply; for example, the need to perform long-range, high level planning in support of operations preparation, as well as short-range, detailed planning in support of operations execution.

Because manned space operations have typically revolved around discrete, limited duration "missions", the space community has attempted from the beginning to force fit this "mission" paradigm into the ISS program rather than develop a new paradigm more appropriate for a continuously operating space station. There are several reasons for this. First, the processes for performing mission integration and preparation, crew training, and planning are well established and understood by the space community. Second, in the early years of the ISS program, most of the on-orbit activity is focused on the assembly of the station, which occurs primarily during ETOV flights to the ISS vehicle. Because of this early emphasis on the assembly flights, the current ISS processes for operations planning and preparation tend to be designed around individual "missions".

In order to create discrete ISS missions, current concepts call for ISS operations to be divided into sequential time periods called "increments". An increment is defined as the period of time between designated ETOV flights to the ISS, as illustrated in Figure 1. Typically, the particular ETOV flights which define a new increment are those which result in a changeout of the onboard crew. Therefore, it is possible that a single increment may include multiple ETOV flights, each of which can result in changes to the ISS configuration, system capabilities, and onboard payload complement. This division of station system and payload operations into segmented time periods based on the arrival of designated ETOV flights is called "Increment Operations". In effect, each increment is treated as a discrete mission for planning purposes.

This paper offers for consideration an alternative planning approach, designed around the fact that the ISS is one continuous, ongoing mission, not a series of consecutive, independent missions (increments). This non-segmented approach to the planning and operation of systems and payloads onboard the ISS is known as "Continuous Operations", and has the potential of greatly simplifying the planning process and reducing operations costs during the mature phase of the ISS program. The remainder of this paper compares the concepts of increment-based planning and continuous operations planning, and discusses the benefits and challenges associated with implementing a continuous operations planning approach for the International Space Station.

In the ISS program, on-orbit operations planning must be performed both for the ISS vehicle and for the various earth-to-orbit vehicles which visit the ISS. This results in several independent, yet interrelated planning functions:

ETOV planning addresses the operations on the earth-to-orbit vehicle during the periods en route to/from the ISS as well as the ISS-attached period. Each nation's ETOV program already has its own unique, well-established planning processes and templates.

ISS planning addresses the ongoing operations onboard the ISS. It is performed by the ISS planning organizations and will follow the independent ISS planning templates and processes.

Joint operations planning addresses the time periods when an ETOV is docked at the space station. This requires significant coordination between the ISS and ETOV planning organizations.

2.1 ISS INCREMENT-BASED PLANNING TEMPLATE

With the increment-based planning approach, ISS on-orbit operations plans are produced long in advance for each and every defined increment. The increment operations planning template for an individual increment, as depicted in Figure 2, begins 18 months prior to the start of the increment (I-18). The Preliminary, Basic, and Final versions of the Increment Operations Plan (IOP) are developed and released at I-12, I-6, and I-2 months, respectively. The IOP contains high level plans for the entire increment, along with detailed operations schedules for the ETOV-attached periods, showing planned activities onboard both the ISS and the ETOV. Detailed schedule development for ISS operational periods between the ETOV flights is performed on a weekly basis during the increment; that is, the schedule for a given week of ISS operations is generated only one week prior to its execution. In addition to developing the next week's detailed schedule, the weekly planning process also maintains and updates the high level plan describing the operations to occur during the remainder of the increment.

2.2 PROBLEMS INHERENT IN THE INCREMENT-BASED APPROACH

With a typical increment duration of from one to three months, it is obvious that there will be significant overlap of the increment planning templates, with planning for multiple increments being performed simultaneously. This overlap of templates introduces several complications. First and foremost, planning personnel must be assigned to support each increment's planning template. This may not be desirable or even possible in today's tight budget environment. Second, there is the possibility of discrepancies between the Increment Operations Plans for the various increments. Because the specific operations planned for one increment may significantly affect the planning for all subsequent increments, and because each increment is planned independently (and concurrently, given the overlapping templates), the probability of inconsistencies being introduced between the multiple increment plans is quite high. The process is further complicated by the fact that ISS operations continue during the two month gap between the release of the Final IOP and the start of the increment.

Also, in order to reflect in the IOP the planned joint operations during the ETOV-attached phase, the planning for the joint ISS/ETOV operations must occur at appropriate points within the ISS planning template. This joint operations planning requires the support of both the ISS and ETOV planning organizations. However, there is no guarantee that the ETOV planning templates will line up sufficiently with the ISS templates to allow this to happen, as is effectively illustrated in Figure 2. Because each ETOV flight may have a different planning template, tied to its specific launch date, and because there may be multiple ETOV flights within a particular increment, there will likely be major disconnects between the ETOV planning templates and the ISS increment planning template. In fact, many of these disconnects are becoming readily apparent as we get closer to the actual start of ISS operations, not only in the operations planning templates, but also in the templates for crew training, simulations, increment and ETOV flight reviews, etc.

The continuous operations planning approach, on the other hand, is designed around the fact that the International Space Station is one continuous ongoing mission, not a series of consecutive, independent missions (increments). Because no planning is performed for individual "increments", many of the problems inherent in the increment-based, segmented planning approach are avoided.

The continuous operations approach recognizes the need to perform: (1) long-range, high level planning in support of execution preparation, as well as (2) short-range, detailed planning (schedule development) in support of realtime operations execution. Long-range planning is required to provide some reasonable assurance that the long term goals of the ISS payloads and systems can be satisfied over time within the known constraints. It also provides a feasible prediction of expected onboard activities so that payload users, ground controllers, program management, and others can make the necessary preparations to support them. This approach also recognizes the fact that in a continuous operations environment, detailed operations schedules should not be developed far in advance because unexpected events, updated orbital predictions and resource availabilities, and changes in scheduling requirements resulting from the ongoing systems and science operations need to be factored into the scheduling process. (See Reference #1 for a more in-depth discussion of long-range vs. short-range planning.)

3.1 ISS CONTINUOUS OPERATIONS PLANNING TEMPLATE

With the increment-based planning approach, long-range, high level planning is performed primarily during the 18 month IOP development process, and detailed schedule development for the ongoing ISS operations is performed primarily during the weekly planning process. In contrast, with the continuous operations planning approach, both functions are performed exclusively as part of the weekly planning process.

With this approach, long-range planning is performed for a fixed length planning horizon, which continually moves forward as ISS operations progress. This planning horizon can be of any duration, from a few weeks to several months, as deemed appropriate to meet the needs of the ISS program. The long-range plan basically defines those payload and system activities which will likely be scheduled during the upcoming weeks of ISS operations in order to satisfy the long term station goals. A big advantage of the continuous operations approach is that instead of having multiple long-range plans (one for each increment) in development at any point in time, there is a single, consistent plan for use by all participants in the ISS program.

The short-range planning function develops the detailed schedule for a specific one-week segment of the long-range plan. As in the increment-based approach, this detailed schedule is generated only one week prior to its execution. As each given week is scheduled and executed, it will be dropped from the long-range plan, and a new week will be added to the end of the plan. Refer to Figure 3 for a graphical depiction of this continuous operations planning template.

3.2 ETOV AND JOINT OPERATIONS PLANNING

To eliminate the problems caused by disconnects between the ISS and ETOV planning templates, the ISS planning must be decoupled from the ETOV planning processes to the maximum extent possible. This does not mean that there will be no coordination between the ETOV and ISS planning organizations for development of the attached-phase joint operations plans. Rather, it means that primary responsibility for the planning of the joint ISS/ETOV operations must be assigned to either the ISS planning function or the ETOV planning function. For several reasons, it is more appropriate to plan the joint operations according to the ETOV planning processes and templates. First, most joint activities are either assembly or resupply operations which are heavily focused around the ETOV vehicle. Second, the ETOV planning will occur much earlier than the ISS planning if following the ISS continuous operations planning template. The operational goals outlined in the ISS long-range plan can be considered in the development of the detailed ISS/ETOV joint operations plans. These ISS/ETOV joint operations plans, once established, can then be folded into the ongoing ISS planning through the long-range and short-range planning functions described above.

A continuous operations planning approach would provide a number of valuable benefits if adopted for the International Space Station program. It would do the following:

• Streamline the ISS planning process, eliminating overlapping planning cycles.
• Prevent discrepancies between multiple, independently-developed increment planning products by providing a single, consistent long-range plan for use across the ISS program.
• Reduce the manpower/resources required to plan ISS operations. Note that this is a significant savings for the National Aeronautics and Space Administration, the other ISS International Partners, and the individual payload users since all are actively involved in the distributed planning process for the ISS (see Reference #2).
• Focus the attention of ISS operations personnel on the ongoing on-orbit operations, where it ought to be, instead of on concurrent planning for a multitude of future increments.
• Allow the payload users to concentrate on the realtime execution and near term planning of their experiments rather than on the planning for increments which are months or years away. In the long run, this may result in better science return.
• Decouple ISS planning from ETOV planning to the extent possible. This would alleviate the planning template disconnects between ISS and ETOV flights caused by the inclusion of multiple ETOV flights within a single increment.

In addition, if the continuous operations planning approach is implemented for ISS, the concept of "increments" could potentially go away altogether since they would no longer be needed for planning purposes. The elimination of increments could reduce the complexity and costs of the ISS program by eliminating the need for increment-specific planning, requirements, documentation, and reviews. Other processes which are currently being driven to support increment preparation schedules and reviews could instead be linked to more natural and appropriate events. For example, since new payloads, onboard systems, and ground facilities may be introduced by an ETOV mission in the middle of an increment, templates for crew training, ISS hardware delivery/integration/return, simulations, and ground control facility additions/upgrades should, in reality, be tied to specific ETOV launches rather than to the start of an ISS increment.

This paper proposes an alternative planning approach which makes sense in the continuous operations environment of the International Space Station and has tangible benefits to the ISS program and its customers. There are, however, several obstacles which must be overcome to successfully implement the continuous operations planning approach.

5.1 SELLING THE IDEA

The most difficult challenge will be in overcoming the "mission" mindset which is inherent within the space operations community. "Increments", which represent discrete ISS missions to many people, have been around for years, and are firmly ingrained in the ISS program, even though the definition of "increment" has changed significantly as the ISS program has matured. To overcome this mindset, the advantages of switching to a continuous operations paradigm will have to be clear cut, technically and financially quantifiable, and overwhelming. A full commitment by all affected parties is required to effectively implement the concept.

5.2 RE-ENGINEERING THE ISS PROGRAM

To some extent, the ISS program must be re-engineered to implement continuous operations. Current ISS processes, templates, program requirements documents, and even organizational structures, are designed to support increment operations. New processes must be developed, negotiated, and documented for the continuous operations planning approach. Resistance to change, inertia, and political forces could make this re-engineering effort quite difficult.

The program must decide early on if the concept of "increments" must be retained for purposes other than operations planning, or if the complete benefits of the continuous operations approach can be realized by doing away with increments altogether. For example, increments are currently being used as a time period over which resources are allocated and requirements are applied. If increments were to go away, other schemes would have to be developed for allocating resources over time and for reporting the satisfaction of ISS requirements and goals. One possibility might be to borrow a technique from industry and track ISS resources and goals on a yearly/quarterly basis; for example, allocate resources on a yearly basis, and report resource usage and goals attainment on a quarterly basis (i.e., quarterly reports).

A partial implementation of the continuous operations approach might also be considered in which increments are retained for program management purposes (allocations, goals), but not for operations planning purposes. Operations planning would follow the templates outlined in Section 3 of this paper. This hybrid approach would yield many of the benefits of continuous operations planning, but might also be a constant source of confusion.

5.3 PROVIDING FOR A SMOOTH TRANSITION

Today, the "Increment Operations" paradigm is firmly established in the ISS program, and processes and personnel are already being set in place to support increment-specific planning for the early increments. Even if the "Continuous Operations" paradigm were to be adopted for ISS, it would be some time before the specific planning processes could be developed, negotiated, and implemented across the ISS program. Therefore, it is likely that planning would have to begin using an increment-based approach and transition at some point to the continuous operations approach.

A transition plan would have to be established early, and formalized in the official ISS program requirements documents, to facilitate a smooth transition from one planning mode to the other. Of course, there is a real danger that once the ISS program begins operations in the increment mode, some participants will be resistant to making the final transition to continuous operations. Because of this, the transition should occur as early as possible. In addition, to help mitigate this risk, selected portions of the concept might be gradually incorporated into the increment-based planning approach. This would provide for a smoother transition, and would help to practically demonstrate the benefits of adopting the continuous operations planning approach for the International Space Station.

1. Maxwell, T. and E. Howell, "Planning as a Precursor to Scheduling for Space Station Payload Operations", American Institute of Aeronautics and Astronautics (AIAA) 1995 Space Programs and Technologies Conference, Huntsville, Alabama, USA, September 26-28, 1995.

2. Hagopian, J. and T. Maxwell, "An Approach for Implementing Distributed Planning for Space Station Payload Operations", AIAA 1995 Space Programs and Technologies Conference, Huntsville, Alabama, USA, September 26-28, 1995.