Model Based Systems Engineering Approach for Geocentric Satellite Formations
By: LTC Stacy Godshall
The future of military Space Operations may benefit from leveraging three significant opportunities: 1) the greater feasibility of using CubeSats and other small satellites for a wide variety of missions, 2) the use of clusters, formations, or swarms of satellites to achieve specific operational objectives, and 3) the use of Model Based System Engineering (MBSE) for military space missions to hasten design, development, optimization, and procurement processes. For the remainder of this article, the term “formation” will be used for any grouping of satellites that could be characterized as a swarm, cluster, or constellation, except when directly quoting/citing another author that uses any term other than “formation.” This paper will briefly outline the first two opportunities and elaborate on how they will be implemented more efficiently via an MBSE approach.
Use of CubeSats for a Wide Variety of Missions
Igor Levchenko and Michael Keidar, in their 2018 Nature article, argued that there are three ways in which space technology needs to advance: 1) reduce costs 2) reduce satellite size while improving satellite maneuverability, and 3) satellites operating in formations. They suggest that thousands of small satellites could operate in formations and therefore as a network. To produce this large number of small satellites, there needs to be a standardized design to hasten development, production, and deployment and to reduce overall costs .
The benefit of having these networks of small satellites is that they can function as if they were a much larger platform because of their interconnectivity, spatial separation, and temporal separation. Instead of building one very large satellite to perform a specific mission, the idea is to build and deploy a formation of CubeSats to perform a variety of tasks as if they were a larger platform. The premise is that these standardized CubeSats would be inexpensive compared to the larger flagship satellites and have a much shorter production to deployment cycle timeframe; Graph 1 below shows some of the economic aspects of these trends .
Graph 1: Economy and launches as related to small satellites. (Reference 1: Igor Levchenko, et al. “Explore space using swarms of tiny satellites”)
Another benefit to this idea of formations of CubeSats is that many of them “could be released from a large central satellite in orbit” . This is a technique that is already being used in limited situations to include the future deployment of 13 CubeSats from NASA’s Artemis 1 mission which will be the first non-crew, integrated test of the Orion spacecraft and Space Launch System rocket (see https://www.nasa.gov/image-feature/artemis-i-map).
Satellite Formations to Achieve Operational Objectives
Satellite formations, as mentioned, meet unique mission requirements of temporal and spatial dispersion which in most cases cannot be achieved from a single “flagship” satellite. In 2015, a team from University of Illinois at Urbana-Champaign along with one person from California Institute of Technology’s Jet Propulsion Laboratory presented a fantastic review of thirty-nine completed, current, and future missions that were comprised of formations in one form or another . Figures 1 and 2 below are from that review article and show the state of small satellite formation missions and the current trends .
Figure 1: Categorization of 39 multi-satellite missions based on mission type, formation type and number of satellites. (Reference: S.B. et al, “A Review of Impending Small Satellite Formation Flying Missions”)
Figure 2: Categorization of 39 multi-satellite missions based on mission status, leading organization and funding source. The key for these missions is given in Table 1 in the reference article cited. (Reference: S.B. et al, “A Review of Impending Small Satellite Formation Flying Missions”)
A NASA Goddard Space Flight Center (GSFC) team pointed out some shortfalls in existing “flagship” Earth observation missions conducted by a single satellite. One of those shortfalls is the fact that a single satellite can only conduct observations along a restrictive plane. For this reason, small satellite formations are a logical way to augment existing single satellite missions and possibly replace them in the future. These formations fill the gap of observations between a single aircraft at a low altitude with a narrow field of view and a single satellite at high altitude with a wide field of view; with these small satellite formations having the ability to scan many angles within the field of view. Since these formations are a system of systems, the design stage needed to include a way to thoroughly vet the tradeoffs and interdependencies, thus the authors of the GSFC paper use MBSE tools coupled with science evaluation tools to demonstrate the feasibility of such an augmentation.
MBSE enables the design team to include those tradeoffs and interdependencies; trade space analysis when possibly conflicting design variables exist is critical. This trade space analysis should include relevant modules such as: “Signal-to-noise ratio (SNR), commercial-off-the-shelf availability of technology required for the payload, GNC/ADC, communications, onboard processing and propulsion subsystems and their corresponding technology readiness levels” .
In 2018, a team from North Carolina Agricultural and Technical State University developed and proposed an application of MBSE for inter-satellite communications (ISC) for small satellite formations . Small satellite ISC is a new and emerging technology, and a needed capability for many future missions.
The formation they proposed is comprised of two 6U-Cubesats in LEO and one small satellite in a sun-synchronous orbit at a slightly greater altitude than that of the CubeSats. This formation is modeled in Systems Tool Kit (STK) and then the information from STK can then be imported into the MBSE model.
The MBSE analysis of ISC and/or Intelligence, Surveillance, and Reconnaissance (ISR) formations of CubeSats could be perfect starting points for incorporation of MBSE into geocentric military mission planning, design, and development since both mission sets could be achieved via possible future satellite formations.
Department of Defense (DoD) Use of MBSE
In 2014, the Office of the Deputy Assistant Secretary of Defense for Systems Engineering (DASD(SE)) conducted a review of MBSE practices and recommended future directions for said practices within the DoD . The stated mission of the DASD(SE) is to “develop and grow the Systems Engineering capability of the Department of Defense – through engineering policy, continuous engagement with component Systems Engineering organizations and through substantive technical engagement throughout the acquisition life cycle with major and selected acquisition programs” .
One of the documents that highlights the need for MBSE analysis for DoD systems is Interim DoDI 5000.02, Operations of the Defense Acquisition System, which “requires the integration of modeling/simulation activities into program planning and engineering efforts” . The Joint Capabilities Integration and Development System (JCIDS) has benefitted from modeling and simulation, but “not consistently across the acquisition lifecycle” .
The DASD(SE) identified several significant challenges currently hindering the JCIDS process: 1) Linear acquisition process, 2) Lack of change adaptation, 3)Stove-piped workforce and data sources, 4) Information shared via static documents, and 5) Limited reuse. These in turn cause Lifecycle costs to be fixed too soon (at Milestone A) in the JCIDS timeline, requirements mismatching, unstable designs underperforming, cost and schedule overruns, substandard quality, and operational non-suitability .
Part of the solution to many of these challenges and associated negative consequences is the application of MBSE. “MBSE is part of a long-term trend toward model-centric approaches (in Systems Engineering) adopted by other engineering disciplines, including mechanical, electrical and software” . MBSE has the ability to reduce the time, cost and risk to develop, deliver, and sustain systems, through the use of models as a fundamental element of program information” . The DASD(SE) suggests efforts to implement MBSE as follows: “1) methodologies, tools, languages, and standards, 2) organizing engineering data into a model for use across a program, and 3) implementation within research” . These implementation efforts will yield a “digital thread (DT) / System Model (SM) concept”  for the JCIDS process.
The emphasis on MBSE analysis for DoD systems and the JCIDS process will be a valuable improvement to the military system development, acquisition, and integration process. With this emphasis on MBSE, and with a significant future need for more CubeSat formations for DoD and research missions, there is significant value in establishing a reusable MBSE reference model for CubeSats.
MBSE for Small Satellites
There has been significant work since 2011 in developing an MBSE approach to developing small satellite systems. In 2011, the International Council of Systems Engineering (INCOSE) sent out a challenge in which they desired a formalized application of modeling to support system requirements, design, analysis, verification, and validation activities. From this challenge the INCOSE Space Systems Working Group (SSWG) set out to create a CubeSat model to achieve the goals of the INCOSE challenge. The SSWG wanted, and needed, to leverage Systems Modeling Language (SysML) as well as other Commercial off-the-shelf (COTS) analysis tools such as STK, MagicDraw, Cameo Simulation Tool Kit, and MATLAB.
A team from Boeing and MSFC cite a report showing that SysML was a powerful tool in the development of the US Air Force’s TACSAT-3 system . Likewise, another team successfully applied an MBSE approach to a 1U CubeSat called SwampSat which had an objective of demonstrating rapid retargeting and precision pointing which in turn would “support Tier-3 objectives of the U.S. Department of Defense’s (DoD’s) Operationally Responsive Space (ORS) program” .
These examples demonstrate that early adoption of this MBSE focused multi-mission operations system will be more repeatable from mission to mission, be more cost effective, and yield a higher quality design and requirements achievement. This approach is therefore very consistent with efforts of both NASA and DoD in finding organizational-wide efficiencies via a scalable model and methodologies.
MBSE for Small Satellites via the CubeSat Reference Model (CRM)
To answer the 2011 INCOSE challenge, members of the SSWG established a “hypothetical FireSat satellite system to evaluate the suitability of SysML for describing space systems” . That SSWG then applied that model, to be known as the CubeSat Reference Model (CRM), “to an actual CubeSat mission, the Radio Aurora Explorer (RAX) mission” . “The purpose of the CRM is to provide a logical architecture, which serves as a guide and provides the building blocks for any CubeSat mission. The goal is to provide an object-oriented architecture framework so that teams can easily compose their CubeSat system and mission from the elements and objects found in the reference model” . The members of the SSWG and others continue to refine the CRM and are in the process of partnering with several universities to apply the CRM to current and future CubeSat missions .
There are significant future opportunities for space missions abound with many challenges associated with the design, development, and execution of each. This paper outlined, to the extent possible, three of those possible opportunities.
Consideration should be given for an MBSE approach to enhance the design, development, optimization, procurement, and operations of formations of small satellites being used for future missions such as satellite communications, remote sensing, terrestrial and space weather monitoring, and missile warning. The information gathered this article was very instructional in determining where the future work will need to focus in this regard of cross pollinating the MBSE analysis techniques to military applications.
As discussed and cited, there has been much progress made in this regard as manifested by the CubeSat Reference Model developed by members of the SSWG. However, the CRM developed by the SSWG needs to be extrapolated into a standardized reference model for a formation of CubeSats so that organizations such as the DoD, companies, and academic teams will have a good starting point to develop and design many formation missions that are undoubtably on the horizon of space activities of the near future. Perhaps the DoD should consider partnering with institutions that are applying the CRM and leverage the knowledge gained from those partnerships to meet the intent of the DASD(SE) in showing the benefits of MBSE to the JCIDS process.
Lastly, Table 1 below shows a summary of the major findings and conclusions, identifying a few common metrics that describe each item. The goal of this summary table is to serve as a semi-quantitative evaluation matrix to show what has been done and perhaps what needs to be done in the future.
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