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Fractionated Space Systems: Decoupling Conflicting
Requirements and Isolating Requirement Change
Propagation
Alejandro Salado1 and Roshanak Nilchiani
2
Stevens Institute of Technology, Hoboken, NJ, 07030
The fractionated space systems vision can potentially provide various levels of flexibility
and adaptability that make a space system capable of overcoming a number of uncertain
scenarios during its development and operational lifetime. Significant research is being
performed on the benefits and drawbacks of such systems under operational scenarios, but
their capabilities during development phase remain rather unknown. This paper evaluates
how fractionated space systems can respond to requirements change during their
development with higher level of adaptability and, specifically, how fractionation can
facilitate the detection and resolution of conflicting requirements. We will also study how
fractionation helps in isolating the effects of change propagation in requirements during
system development. We suggest two frameworks for those purposes, which in addition
provide analytical capabilities to evaluate the difficulty induced by a set of requirements that
need to be fulfilled simultaneously and trade it against a variety of architectural design
choices.
I. Introduction
HE concept of Fractioanted Space Systems brings a new degree of responsiveness in the space industry1. The
idea of fractionation is simple: a monolithic satellite that carries all platform subsystems and instruments under
a common housing is split into several modules that fly freely as a cluster and communication wirelessly². Each
module provides one or more capability of the traditional monolithic satellite.
In short, the fractionation vision is to provide levels of flexibility and adaptability that make a space system
capable of overcoming a number of uncertain scenarios during its development and operational life. Significant
research has been performed on the benefits and drawbacks of such systems under operational scenarios1-4
. In
particular, the adaptability value of fractionated space systems has been calculated under uncertainty during
operation as basis for the calculation. These uncertainties include among others funding uncertainty, launche failure
uncertainty, collision uncertainty, abnormal radiation level uncertainty, or component failure. However, their
benefits and drawbacks during system development remain highly unexplored. In this paper, we study how
fractionated space systems react or are affected by two aspects of a system development in contrast to monolithic
satellites. The first aspect treats how fractionated space systems can decouple requirements that are conflicting when
satisfied by a monolithic satellite. For the second aspect, we study how fractionated systems are more robust against
changes in requirements during development by their inherent requirement isolation.
We consider the topics of conflicting requirements and requirement change propagation of primary importance
because an
rr r r ”5. As a matter of fact, requirements engineering is considered by some
researchers the cornerstone of the systems engineering process6 and it has been shown that the application of good
requirements engineering practices is correlated with the success or failure of the development of a system7-13
.
This paper is organized as follows. Section II provides a review of existing literature on the topics of fractionated
space systems, conflicting requirements, and change propagation. Section III presents two frameworks that enable
assessing the level of conflict level between a set of requirements for a given system development on one hand, and
1 Doctoral candidate, School of Systems and Enterprises, Castle Point on Hudson, Hoboken, NJ 07030. AIAA
Member. 2 Assistant Professor, School of Systems and Enterprises, Castle Point on Hudson, Hoboken, NJ 07030. AIAA
Associate Member.
T
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determining how the effects of changing a requirement propagate through the rest of the requirements within the
given set on the other hand. Both frameworks are applied to a case study in section IV to evaluate how a monolithic
satellite and a fractionated system react during their developments to the existence of conflicting requirements and
to changes in requirements. The paper concludes with a short summary of the conclusions and a proposal for future
work in section V.
II. Literature Review
A. Fractionated Space Systems
Fractionated satellite system is a future vision of spacecraft design suggested by DARPA (Defense Advanced
Research Project Agency) and encapsulated in F6 program ((Future Fast, Flexible, Fractionated, Free-Flying
Spacecraft united by Information exchange). A single monolithic spacecraft in physically separated into fractions.
The fractions or free flying modules operate and connected by wireless communication. Fractionation primarily has
various advantages over monolithic satellite including: Increasing upgradability (replacing the fraction), Reducing
technical risk, Increasing robustness and decreasing barrier to entry for small space companies1-4
.
B. Conflicting Requirements
Conflicting requirements can be defined as tho r q r rohibits
g r”14
. This definition can be generalized by considering conflicting requirements a continuous
variable instead of a discrete one. Under these terms, two or more requirements will have a certain level of conflict
or tension, being their extremes orthogonality, i.e., requirements are actually independent, and mutually exclusive, a
solution does not actually exist for some requirements.
Recent studies on cost growth in space systems have found that conflicting objectives or requirements correlate
with expected cost growth15-19
. Independent studies have also shown that the cost of correction increases as system
development advances20-21
. We therefore assert that early identification and decoupling of conflicting requirements
can facilitate reducing cost growth uncertainty.
Research in identification, representation, and analysis of conflicting requirements is however scarce in the
domain of space systems and, more generally, in large-scale systems. Some work has been done in the field of
software systems, where a variety of approaches have been proposed:
Concatenate logical statements to identify and analyze logical contradictions between scenarios, states,
and actions22-23
.
Use heuristics to assess difficulty to satisfy two or more requirements simultaneously24
.
Use patterns to identify types of requirements or scenarios that often result in the emergence of
conflict24
.
Use pair-wise comparisons supported in some cases by graph theory to evaluate the level of conflicts
between pairs of requirements25-26
.
Existing work addressing the identification of conflicts between requirements for hardware-intensive systems
seems to tend towards using pair-wise comarisons supported in some cases by heuristics to reduce computing
effort27-28
. Thefeore, conflicts between requirements have been majoritarily represented using two techniques:
Design Structure Matrices, where each element of the outer column and raw represents a requirement
and each inner cell represents the level of conflict.
Graphs, where each node represents a requirement and each link the level of conflict.
C. Uncertainty and Change Propagation
Change during system development is a natural state in current practice in the space industry and a result of
uncertainty surrounding the system development29
. It is widely known that changes at that stage have negative
consequences in terms of cost, schedule, and quality30
. As a result, a significant amount of research is addressing
how change propagates throughout the different elements of a system under development so that control
mechanisms can be put in place31-36
.
Given a set of requirements, various interdependencies may exist37-42
. Therefore, some authors have reconized
that changes in requirements can also propagate to other requirements across the system decomposition43
, but also
within a given system level36,42
.
Since flexible products seem to absorb more seamlessly such changes44
, we argue that Fractionated Space
Systems would also provide higher levels of adaptability during system development against changes in
requirements than traditional monolithic satellites.
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Change propagation throughout system components is generally visualized using one or more of the following
techniques45
:
Design Structure Matrix (DSM), where the elements of the outer column and row represent system
components and the inner cells propagation links.
Change Risk Plot, which is based on the DSM but include information about the criticality of the
propagation risk.
Propagation Networks, where each node represents a system component and each link represents a
propagation link.
Propagation Tree, which is based on the Propagation Networks, but nodes are repeated in order to
provide better visualization of clustering elements.
All four methods can also be used to visualize propagation of requirement changes, where requirements would
take the role of components36
.
III. Conflicting Requirements and Requirement Dependency Frameworks
In this section we present two frameworks that will support the case-study to showcase how fractionated space
systems are different with respect to monolithic satellites with regards the reaction to conflicting requirements and
requirement changes during their developments. Because the frameworks only serve a supporting purpose, they have
been kept simple yet effective in this paper.
A. Conflicting Requirements Framework
We propose a framework to assess the level of conflicts within a given set of requirements that is based on pair-
wise comparisons. We acknowledge that such framework may not provide complete information about the existence
of conflicts, i.e., the conflict space may not be exhaustively explored. However, we assert that:
(1) The results of the case-study in this paper are not affected by the level of exhaustiveness to which conflicts
are identified. Consequently, the important criterion is that both candidate solutions, i.e., the monolithic satellite
and the fractionated space system, face identical sets of conflicts.
(2) Its simplicity to visualize and calculate level of conflict seems adequate to convey the key messages of this
paper, namely how fractionated space systems can decouple conflicting requirements during their development.
Under these premises, we define a framework under which the level of conflict between two requirements is
determined based on two variables:
(1) The Inherent Difficulty to fulfill each requirement as an independent element given the programmatic
constraints of the project, such as schedule and cost.
(2) The Social Difficulty, which represents the difficulty to fulfill r q r context in which other
requirements have to be fulfilled as well. This implies that only requirements at a given level of a system
decomposition contribute to the conflict level calculation.
The inherent difficulty r q r r r g r - Within the
context of this research a 1-to-5 scale where 1 represents an easy requirement and 5 r r a difficult
requirement. Specific valuation for each requirement is performed on this paper by engineering judgement
Representation and assessment of social difficulty requires a more sophisticated method. The present research
proposes to repr r r q r
Social difficultyInherent difficulty
Conflicting Requirements
Figure 1. Elements of the Conflicting Requirements Framework.
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g connectivity of requirements in this case is evaluating conflict, the
following definitions are used
Opposing force (1 to 5): combination of requirements reduces solution trade
Orthogonality (blank; no link shown): requirements are independent or dependent through a neutral
relation. In any case, the solution space is not affected by the inter
Pr g rk DSM’ r r to represent the relations between the different
requirements. The existence of a link and its level of influence have been determined by engineering judgment. Each
link is assigned a type of relation, as and the level of existing influence in a 1-to-5 scale where 1
represents a low conflict and 5 represents a sever conflict that could even lead to mutual exclusion r
rr r r r q r r
The resulting difficulty of fulfilling a set r q r r g following calculation,
where Ceq is the resulting conflict level, Dinh is the inherent difficulty, and Dsoc is the social difficulty:
∑ ∑ (1)
We acknowledge that using ordinal numbers for such type of calculation comes with significant mathematical
flaws. However, it is our opinion that these do not affect the main purpose of the paper, while provide adequate
simplicity to convey the key messages.
B. Requirements Dependency Framework
In this paper, we use a modified variant of the results of our previous work REF to determine and visualize
requirement dependencies.
Dependencies are determined by evaluating how the fulfillment of one requirement is affected by the fulfillment
of each other requirement. Those relationships are first visualized using propagation networks. Each link is assigned
either a (+) when improving the fulfillment of one requirement increases the chances of fulfilling another
requirement or a (-) when improving the fulfillment of one requirement decreases the chances of fulfilling another
requirement and vice versa.
Once dependencies have been determined, they are also displayed in a DSM.
IV. Notional Case-Study: Fractionated Spacecrafts
In this section, we present a notional case-study in which we explore how the developments of a monolithic
space system and of a fractionated space system are affected by the existence of conflicting requirements and by
changes in requirements they are expected to fulfill. First, the problem is presented through a set of stakeholder
needs that are used to originate system requirements, from which design parameters are specified. The sets are
incomplete, but present the necessary information for the purpose of this paper. Then, both candidate solutions, i.e.,
the notional monolithic space system and the notional fractionated space system, are presented. Finally, we evaluate
the impacts on development difficulty and the impacts of requirement changes for both solutions.
A. Problem Formulation
L ’ gr k r 1 k r
needs that will be the basis for the notional case-study used in this paper to showcase the application of how the
developments of fractionated space systems and monolithic space systems are affected or react to conflicting
requirements and requirements changes. In essence, stakeholders need a system that provides continuous high-
resolution imaging in a variety of spectral ranges, which include VNIR, SWIR, and X bands. Given the nature of
such bands, those needs imply imaging using passive optical instruments and active microwave instruments.
Table 2 lists a subset of system requirements that are derived from the stakeholder needs listed in Table 1. It is
important to note that they do not include any design dependent requirement and, consequently, could be fulfilled by
both a monolithic space system and a fractionated space system. In addition, because the objective of this paper is to
Table 1. Stakeholder needs for notional case-study.
ID Description
N1 Continuous high-resolution real-time image information in VNIR/SWIR spectral ranges (optical).
N2 Continuous high-resolution real-time image information in X spectral range (RADAR).
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evaluate the effect of conflicting requirements and requirement changes into their developments, programmatic
requirements (and eventual performance) such as cost or schedule have not been included. Again, the given subset is
incomplete, but it provides the necessary elements for the purpose of this paper. Finally, actual values have not been
allocated to the different requirements in order to provide generality to the case-study.
Fulfulling the requirements presented in Table 2 would require the fulfillment of various design parameter or
lower level requirements. A subset is presented in Table 3, followed by a traceability matrix in Table 4 between
those design parameters and the system requirements presented in Table 2. The subset is of course incomplete, but it
provides the necessary elements for the purpose of this paper. In addition, actual values have not been allocated to
the different design parameters in this case either in order to provide generality to the case-study.
B. Candidate Systems: Monolithic Versus Fractionated Satellites
As presented earlier, the developments of two candidate systems are evaluated in this paper. In particular, the
developments of a monolithic space system (or satellite specifically in this case) and of a fractionated space system
are compared.
The following assumptions are applicable to both systems:
(1) The system fulfills all its requirements.
Table 2. System requirements subset for notional case-study.
ID Description Justification
R1 The system shall image Earth surface in VNIR/SWIR
spectral range with performance A better than X units.
Satisfy N1.
R2 The system shall image Earth surface in X band (RADAR)
with performance B better than Y units.
Satisfy N2.
R3 The system shall transmit image data in less than Z s after
image acquisition.
Elicited for completeness of N1 and
N2.
R4 The system shall continuously image Earth surface. Elicited for completeness of N1 and
N2.
R5 The system shall image Earth surface in both spectral
ranges at accuracy of 0.1 s (synchronization).
Satisfy N1 and N2, elicited for their
completeness.
Table 3. Design parameters for notional case-study.
ID Description
TP1 Pointing accuracy optical sensor.
TP2 Pointing accuracy microwave sensor (RADAR).
TP3 Pointing accuracy transmitter antenna (data).
TP4 Power dissipation.
TP5 Data processing capability.
TP6 EIRP data transmitter.
TP7 Optical performance.
TP8 Radar performance.
TP9 Image synchronization.
Table 4. Traceability matrix for notional case-study.
TP1 TP2 TP3 TP4 TP5 TP6 TP7 TP8 TP9
R1 X X X
R2 X X
R3 X X X
R4 X X X
R5 X
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(2) Programmatic requirements such as cost and schedule are not part of the evaluation.
(3) Only the requirements and design parameters presented in the previous section are used for the study.
(4) The system is notional.
The notional monolithic space system is presented in Figure 2. A single satellite is used to ultimately satisfy all
stakeholder needs listed in Table 1. As a result, it incorporates two main payloads: (1) a passive optical instrument
that is used to image the Earth surface in VNIR and SWIR spectral bands and (2) a RADAR that is used to actively
image the Earth surface in X band.
The notional fractionated space system is presented in Figure 3. Three fractions form the fractionated space system
or cluster. One fraction provides traditional platform capabilities such as power generation and distribution,
command and control, attitude determination and orbit control, and communication to Earth. A second fraction
consists of a passive optical instrument that is used to image the Earth surface in VNIR and SWIR spectral bands.
The third fraction consists of a RADAR that is used to actively image the Earth surface in X band. Both payload-
type fractions could also incorporate some life-support elements such as inter-fraction communication systems or
internal power distribution systems.
Design parameters as presented in Table 3 can be allocated to the different systems as described in Table 5. It shall
be noted that TP4 has not been assigned to Fraction 2 because it is a limitation imposed by the performance of the
optical instrument into the boundaries of the RADAR instrument. In addition, the allocation of some design
parameters such as TP9 has been simplified in order to provide guarantee visibility of the case-study results.
Platform
Optical instrument
RADAR
Monolithic satelliteGro
un
d C
on
tro
l Seg
men
t
Eart
h s
urf
ace
Image dataVNIR/SWIR radiationX band transmission & reflection
Figure 2. Notional monolithic space system.
Platform
Optical instrument
Fractionated satellite
Gro
un
d C
on
tro
l Seg
men
t
Eart
h s
urf
ace
Fraction 1
Image dataVNIR/SWIR radiationX band transmission & reflection
RADAR
Fraction 3
Fraction 2
Life support system
Life support system
Figure 3. Notional fractionated space system.
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C. Decoupling Conflicting Requirements
The study on conflicting requirements is performed at the system requirements level. Level of conflict is
evaluated for each candidate solution independently. Assessments of inherent and social difficulties are performed
using engineering judgement. It should be noted that the allocation of those levels is qualitative and for illustrative
purposes only.
Figure 4 shows two graphs that represent the inherent and social complexities for the given set of requirements
when allocated to a monolithic satellite and to a fractionated space system. Moreover, they include the calculation of
the total conflict level in both cases. Table 6 discusses the differences in inherent and social difficulties between
both solutions.
Table 5. Design parameter allocation.
Design
parameter
Monolithic
satellite
Fractionated Space System
Fraction 1 Fraction 2 Fraction 3
TP1 X X
TP2 X X
TP3 X X
TP4 X X X
TP5 X X
TP6 X X
TP7 X X
TP8 X X
TP9 X X
Figure 4. Conflicting requirements in both candidate solutions.
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As can be seen from the assessment, even though the total inherent difficulty increases when the system is
fractionated, the total conflict level becomes lower than for the monolithic satellite. This finding is very interesting,
as it conveys two main messages:
(1) Fractionation facilitates reduction of social difficulty, at the expense of increasing the difficulty of inherent
difficulties.
(2) A decision on whether to use a monolithic solution or a fractionated one is not trivial. Instead, rigorous
assessment and allocation of difficulty levels is necessary in order to deliver sound decisions.
D. Isolating Requirements Change Propagation
Since both candidate solutions are expected to fulfill all system requirements, change propagation is studied on
the design parameters. Figure 5 shows the different interdepdencies between the design parameters for the
monolithic satellite. Table 7 summarizes them in a DSM form and Table 8 justifies why the interdependencies exist.
It should be noted that some relationships have been kept at a very simple level, for example linking optical and
radar performances to single parameters. This is not the case in actual systems, but it provides sufficient rigor to
convey the main messages of this paper.
Table 6. Justifications for differences between monolithic and fractionated solutions.
Element Justification for differences
R5 Synchronization to a great accuracy is significantly more difficult to achieve using wirelessly
connected components that hardwired connections.
R1-R2 Active RADAR in X band and passive optical imaging in SWIR require different operating
envirnoments to achieve high performance. Whereas RADAR require high power, which results in
high power dissipation, optical imaging in SWIR spectral range demands low temperature
distortions. Consequently, achieving the required performance when both instruments share housing
is considered very difficult (monolithic solution). In contrast, both instruments are split into
different fractions in the fractionated solution and, consequently, both requirements become
orthogonal.
R1-R3 Similarly to the previous case, transmission of data may induce thermal distortions that affect the
performance of passive optical equipment in the SWIR spectral range. Fractionation helps in the
sense that half of the data is transmitted through an orthogonal path.
TP2Point. Acc.
RADAR
TP4Power
dissipation
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L4
L5L6
L7
L8
L9 (-)
(+)
(-)
(-)(-) (+)
(+)
(+)
Monolithic
Figure 5. Interdepdencies of the monolithic satellite.
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Breaking the monolithic satellite in various fractions to form the fractionated space system automatically breaks
dependencies between design parameters, as shown in Figure 6. We acknowledge that new design parameters may
emerge, but they are considered irrelevant for the study in the present paper. Dependencies are summarized in DSM
form in Table 9, Table 10, and Table 11, for Fraction 1, Fraction 2, and Fraction 3, respectively. As can be seen,
fractionation has automativally removed L4 and L5. In addition, TP4 no longer exists. Since this design parameter
was defined to ensure proper interface to the optical instrument, it is no longer needed in any of the fractions. As a
result, L6, L8, and L9 are also automatically removed. Overall, fractionation has reduced the amount of
interdependencies between requirements from 9 to 4.
Table 7. Dependency matrix between design parameters for notional monolithic
system.
Monolithic TP1 TP2 TP3 TP4 TP5 TP6 TP7 TP8 TP9
TP1 L1
TP2 L2
TP3 L3
TP4 L4 L5
TP5 L6 L7
TP6 L8
TP7
TP8 L9
TP9
Table 8. Requirement interdependencies justifications.
Interdependency Justification
L1 For the given mission, optical performance depends positively on the
pointing accuracy.
L2 For the given mission, RADAR performance depends positively on the
pointing accuracy.
L3 The necessary EIRP to achieve the given Bit Error Rate depends negatively
on the pointing accuracy of the transmitter. The more accurate pointing is,
the less EIRP is needed.
L4 Because pointing accuracy depends on themoelastic deformation, the higher
the temperature the higher the risk of achieving proper pointing accuracy.
Moreover, the more power that is dissipated, the higher the temperature,
including gradients if dissipated power is pulsed (which is the case for
RADAR’ ) H r x g r en power
dissipation and pointing accuracy.
L5 The optical performance in VNIR and in particular SWIR depends
negatively on the surrounding temperature. Using the same rationale as in
L4, there exists a negative relationships between power dissipation and
optical performance.
L6 The more processing power that is needed, the higher the power
consumption and hence dissipation of electronic equipment.
L7 The more onboard data processing that is available, the less data needs to be
transmitted to ground and, as a result, lower EIRP is needed. And vice versa.
L8 The higher the EIRP, the higher the power dissipation.
L9 Radar performance depends positively on the ouput power of the RADAR
antenna, i.e., higher power levels improve performance. Using the same
rationale as in L8, higher RADAR performance implies higher power
dissipation.
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In the following, three cases in which a requirement change is needed are studied. The impact of the requirement
change is evaluated on both candidate solutions: the monolithic satellite and the fractionated space system.
Case 1. Higher RADAR performance.
In this case, it is assumed that higher performance for the RADAR instrument is required. Therefore, TP8 needs
to be more stringent. Using the propagation network for the monolithic satellite, it is easy to see that higher RADAR
performance implies higher power dissipation and, as a result, fulfillments of TP1 and, in particular of TP7, are
TP2Point. Acc.
RADAR
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L7
(-)
(+)
(-)
Fraction 3
Fraction 1Fraction 2
Figure 6. Interdepdencies of the fractionated space system.
Table 9. Dependency matrix between
design parameters for Fraction 1 of the
notional fractionated system.
Fraction 1 TP3 TP5 TP6 TP9
TP3 L3
TP5 L7
TP6
TP9
Table 10. Dependency
matrix between design
parameters for Fraction 2
of the notional fractionated
system.
Fraction 8 TP1 TP7
TP1 L1
TP7
Table 11. Dependency
matrix between design
parameters for Fraction 1
of the notional fractionated
system.
Fraction 3 TP2 TP8
TP2 L2
TP8
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endangered (see Figure 7). Consequently, accepting a change of TP8 would result in additional rework, potentially
on the optical instrument, to fulfill TP1 and TP7 as well because of the change propagation.
In the fractionated case, since L4 and L5 no longer exist, the change of TP8 does not propagate outside Fraction
3. Consequently, the change can be accepted without compromising the development of Fraction 1 and Fraction 2
(see Figure 8).
Case 2. Higher Optical performance.
In this case, it is assumed that higher performance for the optical instrument is required. Therefore, TP7 needs to
be more stringent. Using the propagation network for the monolithic satellite, it is easy to see that higher optical
performance requires lower pointing accuracy (TP1) and/or lower power dissipation (TP4). As a result, fulfillment
TP2Point. Acc.
RADAR
TP4Power
dissipation
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L4
L5L6
L7
L8
L9 (-)
(+)
(-)
(-)(-) (+)
(+)
(+)
Monolithic
Figure 7. Case 1 – Change propagation in monolithic satellite.
TP2Point. Acc.
RADAR
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L7
(-)
(+)
(-)
Fraction 3
Fraction 1Fraction 2
Figure 8. Case 1 – Change propagation in fractionated space system.
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of TP1, and in particular TP4, are endangered. Consequently, accepting a change of TP7 would result in additional
rework, potentially on the RADAR and platform parts, to fulfill TP1 and TP4 as well because of the change
propagation (see figure 9).
In the fractionated case, since L4 and L5 no longer exist, the change of TP7 does not propagate outside Fraction
2. Consequently, the change can be accepted without compromising the development of Fraction 1 and Fraction 3
(see figure 10).
Case 3. Lower processing capability.
TP2Point. Acc.
RADAR
TP4Power
dissipation
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L4
L5L6
L7
L8
L9 (-)
(+)
(-)
(-)(-) (+)
(+)
(+)
Monolithic
Figure 9. Case 2 – Change propagation in monolithic satellite.
TP2Point. Acc.
RADAR
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L7
(-)
(+)
(-)
Fraction 3
Fraction 1Fraction 2
Figure 10. Case 2 – Change propagation in fractionated space system.
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In this case, it is assumed that the initial expected processing power cannot be achieved. Therefore, TP5 needs to
be relaxed at the expense of inreaseing EIRP (TP6). Using the propagation network for the monolithic satellite, it is
easy to see that fulfillments of TP1 and, in particular of TP7, are endangered because of the higher power dissipation
due to the increase in EIRP. Consequently, accepting a change of TP5 would result in additional rework to fulfill
TP1 and TP7 as well because of the change propagation (see Figure 11).
In the fractionated case, since L4 and L5 no longer exist, the change of TP5 does not propagate outside Fraction
1. Consequently, the change can be accepted without compromising the development of Fraction 2 and Fraction 3
(see Figure 12).
TP2Point. Acc.
RADAR
TP4Power
dissipation
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L4
L5L6
L7
L8
L9 (-)
(+)
(-)
(-)(-) (+)
(+)
(+)
Monolithic
Figure 11. Case 3 – Change propagation in monolithic satellite.
TP2Point. Acc.
RADAR
TP3Point. Acc. TX
TP1Point. Acc.
Optical
TP5Data proc.
TP6EIRP TX
TP7Optical perf.
TP8RADAR perf.
TP9Image sync.
(+)
L1
L2
L3
L7
(-)
(+)
(-)
Fraction 3
Fraction 1Fraction 2
Figure 12. Case 3 – Change propagation in fractionated space system.
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V. Conclusion
In this paper, we looked at the concept of identifying the requirement conflict and change propagation of
requirement for space system with a particular attention to the concept of fractionated space systems. We proposed
two intial frameworks: the first framework suggests a methodology to identify the level of conflict and difficulty of
fulfillment of the requirements at hand; and the second framework suggests a basis to map the uncertainty and
change propagation of a requirement and how it affects the design. We presented a case study that focused on a
different design paradigm, fractionated space systems, in ccomparison with the traditional monolithic space system.
The two frameworks were applied to the monolithic satellite as well as a case of fractionated space systems under
three different emphasis (higher radar performance, higher radar performance, and lower processing capability). The
finding of the case study shows that the fractionated space systems paradigm facilitates identification of the
conflicting requirements and therefore by fractionating, the decoupling and minimizing the conflicts becomes a
practical option. The case study also shows that due to the nature of fractionation, the change in requirements in
certain cases can be localized and limited to a single fraction (or more fractions) rather than being a change that
propagates and affects the whole space system.
Acknowledgments
The authors thank Dr. Owen Brown for proposing the research questions leading to the research presented in this
paper and providing us with the sample requirements for the case study, and Dr. Owen Brown, Dr. Dinesh Verma,
Dr. Wiley J. Larson, and Dr. Yu Tao for their comments and recommendations on the elements presented in this
paper.
This work was supported in part by the DARPA/NASA Ames Contract Number NNA11AB35C on the
Fractionated Space Systems F6 project.
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