CEN/TR 17603-60-10:2022

Space engineering - Control performance guidelines

This Handbook deals with control systems developed as part of a space project. It is applicable to all the elements of a space system, including the space segment, the ground segment and the launch service segment. It addresses the issue of control performance, in terms of definition, specification, verification and validation methods and processes. The handbook establishes a general framework for handling performance indicators, which applies to all disciplines involving control engineering, and which can be declined as well at different levels ranging from equipment to system level. It also focuses on the specific performance indicators applicable to the case of closed-loop control systems. Rules and guidelines are provided allowing to combine different error sources in order to build up a performance budget and to assess the compliance with a requirement. This version of the handbook does not cover control performance issues in the frame of launch systems.

Raumfahrttechnik - Richtlinien für Leistung von Regelung/Steuerung

Ingénierie spatiale - Lignes directrices des performances du contrôle

Vesoljska tehnika - Smernice za nadzor delovanja

Ta priročnik zajema nadzorne sisteme, razvite kot del vesoljskega projekta. Uporablja se za vse elemente vesoljskega sistema, vključno z vesoljskim delom, zemeljskim delom in lansirnimi storitvami. Obravnava nadzor delovanja v smislu opredelitve, določanja, preverjanja ter potrjevanja metod in postopkov. Priročnik vzpostavlja splošen okvir za obravnavanje kazalnikov uspešnosti, ki se uporablja za vse discipline, povezane s krmilnim inženiringom, in ki ga je mogoče zavrniti na različnih ravneh, od opreme do sistemov. Osredotoča se tudi na posebne kazalnike zmogljivosti, ki se uporabljajo v primeru krmilnih sistemov z zaprto zanko. Pravila in smernice so na voljo za kombiniranje različnih virov napak za namene priprave proračuna uspešnosti in ocenjevanja skladnosti z zahtevo. Ta različica priročnika ne zajema vprašanj nadzora delovanja v okviru sistemov za zagon.

General Information

• Status: Published
• Publication Date: 11-Jan-2022
• ICS: 49.140 - Space systems and operations
• Technical Committee: CEN/CLC/TC 5 - Space
• Drafting Committee: CEN/CLC/TC 5/WG 6 - Upstream standards
• Current Stage: 6060 - Definitive text made available (DAV) - Publishing
• Start Date: 12-Jan-2022
• Due Date: 29-Dec-2022
• Completion Date: 12-Jan-2022
• Mandate: M/496 - Mandate addressed to CEN, CENELEC and ETSI to develop standardisation regarding space industry (Phase 3 of the process)
• Ref Project: SIST-TP CEN/TR 17603-60-10:2022 - Space engineering - Control performance guidelines

Overview

CEN/TR 17603-60-10:2022 - Space engineering: Control performance guidelines is a CEN technical report adopted by SIST that provides a practical handbook for defining, specifying, verifying and validating control performance across space projects. Applicable to the space segment, ground segment and launch service segment elements of a space system, the handbook establishes a common framework for performance indicators, error budgeting and closed-loop control metrics. Note: this version does not address launch-system control performance in detail and is non‑normative (technical report).

Key topics and technical content

The report organizes guidance and methods useful to control and systems engineers, covering:

• General control performance process: structure from equipment to system level, requirements capture and dissemination.
• Extrinsic performance and error analysis: definition of error functions and error indices, statistical interpretations, measurement vs. knowledge errors.
• Performance budgeting: identifying error sources, statistical combination of terms, building an error budget to assess compliance with requirements.
• Closed-loop (intrinsic) performance indicators: stability, stability margins (SISO/MIMO concepts), robustness, time- and frequency-domain behaviour (transient responses, bandwidth, resonance rejection).
• Verification & validation methods: experimental tests, numerical simulations, Monte Carlo guidance for run counts, and guidance to choose appropriate verification strategies.
• Hierarchy and traceability: converting top-level mission requirements into intermediate and lower-level design rules without creating counterproductive or conflicting requirements.
• Case material and annexes: mathematical appendices, LTI system background, Monte Carlo statistics, and an AOCS (satellite attitude and orbit control system) case study illustrating requirements formulation and performance budgets.

Keywords integrated: space engineering, control performance, error budget, closed-loop control, performance indicators, AOCS, verification and validation.

Practical applications

This handbook supports practical tasks such as:

• Writing and formalizing control performance requirements for spacecraft subsystems.
• Constructing pointing, stability and knowledge budgets for AOCS and payload pointing.
• Choosing and documenting verification approaches (test vs. simulation) and determining confidence levels.
• Assessing robustness and stability margins for flight control loops and MIMO interactions.
• Ensuring traceability from mission-level performance needs down to component test criteria.

Who should use this standard

• Space systems engineers and architects
• Control engineers and AOCS designers
• Verification & validation engineers and test teams
• Mission planners, payload engineers, and systems integrators
• Standards managers and quality/assurance personnel

Related standards

• ECSS and ECSS-HB style guidance (e.g., referenced ECSS-E-HB-60A in the handbook) and other CEN/CENELEC technical reports on space systems. Users should align handbook guidance with project-specific normative standards and contractual requirements.

Standards Content (Sample)

SLOVENSKI STANDARD
01-marec-2022
Vesoljska tehnika - Smernice za nadzor delovanja
Space engineering - Control performance guidelines
Raumfahrttechnik - Richtlinien für Leistung von Regelung/Steuerung
Ingénierie spatiale - Lignes directrices des performances du contrôle
Ta slovenski standard je istoveten z: CEN/TR 17603-60-10:2022
ICS:
49.140 Vesoljski sistemi in operacije Space systems and
operations
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

TECHNICAL REPORT CEN/TR 17603-60-10

RAPPORT TECHNIQUE
TECHNISCHER BERICHT
January 2022
ICS 49.140
English version
Space engineering - Control performance guidelines
Ingénierie spatiale - Lignes directrices des Raumfahrttechnik - Richtlinien für Leistung von
performances du contrôle Regelung/Steuerung

This Technical Report was approved by CEN on 29 November 2021. It has been drawn up by the Technical Committee
CEN/CLC/JTC 5.
CEN and CENELEC members are the national standards bodies and national electrotechnical committees of Austria, Belgium,
Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia,
Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom.

CEN-CENELEC Management Centre:
Rue de la Science 23, B-1040 Brussels
© 2022 CEN/CENELEC All rights of exploitation in any form and by any means
Ref. No. CEN/TR 17603-60-10:2022 E
reserved worldwide for CEN national Members and for
CENELEC Members.
Table of contents
European Foreword . 7
Introduction . 8
1 Scope . 10
2 References . 11
3 Terms, definitions and abbreviated terms . 12
3.1 Terms from other documents . 12
3.2 Terms specific to the present handbook . 12
3.3 Abbreviated terms. 16
4 General outline for control performance process . 18
4.1 The general control structure . 18
4.1.1 Description of the general control structure – Extension to system
level . 18
4.1.2 General performance definitions . 19
4.1.3 Example – Earth observation satellite . 20
4.2 Review of generic performance specification elements . 21
4.2.1 General . 21
4.2.2 Preliminary remark on intrinsic and extrinsic performance properties . 21
4.2.3 Examples of high-level performance requirements . 23
4.2.4 Formalising requirements through performance indicators . 25
4.3 Overview on performance specification and verification process . 27
4.3.1 Introduction . 27
4.3.2 Requirements capture & dissemination . 28
4.3.3 Performance verification. 29
4.3.4 Control performance engineering tasks during development phases . 31
5 Extrinsic performance – error indices and analysis methods . 38
5.1 Introduction . 38
5.2 Performance and measurement error indices . 38
5.2.1 Definition of error function . 38
5.2.2 Definition of error indices. 39
5.2.3 Common performance error indices . 39
5.2.4 Common knowledge error indices . 41
5.3 Formulation of performance requirements using error indices. 42
5.3.1 Structure of a requirement . 42
5.3.2 Choice of error function . 42
5.3.3 Use of error indices . 43
5.3.4 Statistical interpretation of a requirement . 43
5.3.5 Formulation of Knowledge Requirements . 46
5.4 Assessing compliance with a performance requirement . 46
5.4.1 Overview . 46
5.4.2 Experimental approach . 47
5.4.3 Numerical simulations . 47
5.4.4 Use of an error budget . 49
5.5 Performance error budgeting . 50
5.5.1 Overview . 50
5.5.2 Identifying errors . 50
5.5.3 Statistics of contributing terms . 51
5.5.4 Combination of error terms . 52
5.5.5 Comparison with requirements . 53
5.5.6 Practical use of a budget (Synthesis) . 53
6 Intrinsic performance indicators for closed-loop controlled systems. 56
6.1 Overview . 56
6.2 Closed-loop controlled systems . 57
6.2.1 General closed-loop structure . 57
6.2.2 General definitions for closed-loop controlled systems . 57
6.3 Stability of a closed-loop controlled system. 59
6.4 Stability margins . 60
6.4.2 Stability margins for SISO LTI systems . 60
6.4.3 Stability margins for MIMO LTI system – S and T criteria . 62
6.4.4 Why specifying stability margins? . 64
6.5 Level of robustness of a closed-loop controlled system . 65
6.6 Time & Frequency domain behaviour of a closed-loop controlled system . 65
6.6.1 Overview . 65
6.6.2 Time domain indicators (transient) . 65
6.6.3 Frequency domain performance indicators . 67
6.7 Formulation of performance requirements for closed-loop controlled systems . 70
6.7.1 General . 70
6.7.2 Structure of a requirement . 70
6.7.3 Specification for general systems (possibly MIMO, coupled or nested
loops) . 71
6.7.4 Example of stability margins requirement . 71
6.8 Assessing compliance with performance requirements . 72
6.8.1 Guidelines for stability and stability margins verification . 72
6.8.2 Methods for (systematic) robustness assessment . 73
7 Hierarchy of control performance requirements . 74
7.1 Overview . 74
7.2 From top level requirements down to design rules . 74
7.2.1 General . 74
7.2.2 Top level requirements . 74
7.2.3 Intermediate level requirements . 75
7.2.4 Lower level requirements – Design rules . 75
7.3 The risks of counterproductive requirements . 76
7.3.1 An example of counterproductive requirement . 76
7.3.2 How to avoid counterproductive control performance requirements? . 76
Annex A LTI systems . 77
A.1 Overview . 77
A.2 General properties of LTI systems . 77
A.2.1 Simplified structure of a closed-loop controlled system . 77
A.2.2 Representation of LTI systems . 78
A.3 On stability margins of SISO and MIMO LTI systems . 80
A.3.1 Interpretation of stability margins . 80
A.3.2 Analysis of stability margins – some illustrations . 82
Annex B Appendices to clause 5: Guidelines and mathematical elements . 84
B.1 Error Indices with domains other than time . 84
B.2 Considerations regarding time intervals . 85
B.3 Relationship between error indices and physical quantities . 85
B.4 Statistics for Monte Carlo Minimum Number of Runs . 87
B.5 Determining the error PDFs . 88
B.5.1 Overview . 88
B.5.2 White noise . 88
B.5.3 Biases . 89
B.5.4 Uniform random errors . 90
B.5.5 Harmonic errors . 90
B.5.6 Drift Errors . 91
B.5.7 Transient Errors . 92
B.5.8 Others (General Analysis Methods) . 93
B.5.9 Distributions of Ensemble Parameters . 94
B.6 Mathematics of an Error Budget . 95
B.6.1 Probability distributions and the statistical interpretation . 95
B.6.2 Exact error combination methods . 96
B.6.3 Alternative approximation formulae . 97
Annex C Satellite AOCS case study . 98
C.1 Introduction . 98
C.2 Satellite AOCS architecture . 98
C.3 From Image quality to AOCS requirements. 98
C.4 Formulation of the requirements C.3a1 to C.3a4 using error indices . 101
C.4.1 General . 101
C.4.2 Choice of signal error function . 102
C.4.3 Choice of error indices and maximum values . 102
C.4.4 Assigning a probability density function (PDF) . 102
C.4.5 Choice of statistical interpretation (temporal, ensemble, mixed…) . 103
C.4.6 Requirements formulation . 103
C.5 Formulation of requirements C.3b1 and C.3b2 . 104
C.6 Control Performance verification principles . 104
C.6.1 Choice of verification method . 104
C.6.2 Compiling the error budget (requirements C.3a1 to C.3a4) . 105
C.6.3 Assessing compliance to control loop requirements C.3b1 and C.3b2 . 110
C.7 Performance budget examples . 111
C.7.1 Overview . 111
C.7.2 Pointing Knowledge Budget . 111
C.7.3 Pointing budget . 114
C.7.4 Pointing stability Budget (Requirements C.3a3 and C.3a4) . 116

Figures
Figure 4-1 General control structure, ECSS-E-HB-60A . 18
Figure 4-2 General control structure extended up to system level . 19
Figure 4-3 Example of requirements capture and dissemination for a typical AOCS
case . 30
Figure 4-4 Example of pointing performance verification, for a typical mission profile. 30
Figure 5-1 Illustration of the different ways of meeting a requirement. . 44
Figure 5-2 Statistics for the different statistical interpretations. L-R: temporal
interpretation, ensemble interpretation, mixed interpretation . 44
Figure 6-1 Simplified scheme for a closed-loop controlled system . 57
Figure 6-2 Example of gain and phase margins identification . 62
Figure 6-3 Illustration of the transient response indicators . 66
Figure 6-4 Bandwidth, cut-off frequency and rejection of resonances . 69

Tables
Table 3-1: Relationships of the definitions of the different kinds of performance,
performance knowledge and their corresponding errors . 16
Table 4-1 Example of a control structure breakdown for an Earth observation satellite . 21
Table 4-2 Example of AOCS extrinsic and intrinsic specifiable performances . 22
Table 4-3 General template for building extrinsic performance indicators . 26
Table 4-4 Summary of control performance engineering tasks . 28
Table 4-5 Summary of the control performance management activities during the
phases of mission development (guidelines only) . 31
Table 4-6 Control performance engineering inputs, tasks and outputs, Phase 0/A . 32
Table 4-7 Control performance engineering inputs, tasks and outputs, Phase B . 34
Table 4-8 Control performance engineering inputs, tasks and outputs, Phase C/D . 36
Table 4-9 Control performance engineering inputs, tasks and outputs, Phase E/F . 37
Table 5-1 Minimum number of simulation runs required to verify a requirement at
confidence level Pc to a verification confidence of 95 % . 49
Table 5-2 Example of a table used for a performance budget (APE for Euler angles) . 55
Table 6-1 Formulas for the usual SISO stability margins . 61

European Foreword
This document (CEN/TR 17603-60-10:2022) has been prepared by Technical Committee
CEN/CLC/JTC 5 “Space”, the secretariat of which is held by DIN.
It is highlighted that this technical report does not contain any requirement but only collection of data
or descriptions and guidelines about how to organize and perform the work in support of EN 16603-
60.
This Technical report (CEN/TR 17603-60-10:2022) originates from ECSS-E-HB-60-10A.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document has been prepared under a mandate given to CEN by the European Commission and
the European Free Trade Association.
This document has been developed to cover specifically space systems and has therefore precedence
over any TR covering the same scope but with a wider domain of applicability (e.g.: aerospace).
Introduction
This document focuses on the specific issues raised by managing all performance aspects of control
systems in the frame of space projects. It provides a set of practical definitions, engineering rules,
recommendations and guidelines to be used when specifying or verifying the performance of a
general control system; attention was paid by the authors to keep the application field as open as
possible, and not to restrict to a specific domain – such as spacecraft attitude control for example.
It is not intended to substitute to textbook material on automatic control theory. The readers and the
users are assumed to possess general knowledge of control system engineering and its applications to
space missions. Nevertheless when required – to avoid any risks of ambiguity for example, or for the
clearness of the presentation – some basic definitions and rules are provided in dedicated annexes.
This document was originally intended to focus on the specific case of pointing systems and AOCS,
starting from an existing ESA handbook [Pointing Error Handbook, ESA-NCR-502], to be updated,
completed, and extended to be built up as an applicable ECSS document. But after reviewing the
scope, this approach appeared somewhat restrictive:
• restricting performance concepts to “pointing” does not allow to deal with problems such as
thermal control, position control (robotics), or more generally any other type of control systems,
even though these problems share the same theoretical framework;
• AOCS is one major contributor to the overall system pointing performance, yet not the only
one: misalignments, thermoelastic effects, payload behaviour, etc. all contribute to the final
performance. This remark can be extended to general systems, considering that the controlled
part is but one of the contributors.
Accounting for these remarks led to extending the initial scope of this document. The upgraded
objective is to set up a generalised framework introducing performance definitions, performance
indices and budget calculations. “Generalised” is understood here in two directions:
• transversally, so as to be applicable independently on the physical nature of the control system
(not only pointing),
• and vertically, in the sense that in many practical situations the proposed definitions and
techniques can also apply to any part of the system (basically to the controlled part, but not
restrictively). This should assure consistency between the performances indices (error budgets)
of the complete system and of the controlled system part. Motivation is also that dedicated but
generic methods for budget breakdown can be applied on different levels i.e. on system level
and on controlled system level.
NOTE 1 The idea of defining a general framework applying from
equipment level to system level is driven by a concern for technical
and conceptual consistency. In a later phase, relevant system
aspects can be transferred or copied to the appropriate System
Engineering standard – if found more convenient.
NOTE 2 The general control structure from the Control Engineering
handbook [ECSS-E-HB-60A, Figure 4-1] has been extended in
support, showing also the system performance in the output
(Figure 4-2 of this handbook)
NOTE 3 The objective of this document is not to cover the high level system
or mission performance aspects, which clearly belong to a different
category.
In addition to this will for general and generic concepts, a clause of this document covers the
performance issues which are more specific for the controlled systems themselves (mainly involving
feedback loops in practice) or which are based on well-known control methods. The need for this
clause arises as such systems call for particular technical know-how and feature specific performance
indicators that require additional insight. For example: stability and robustness properties, transient
responses (settling time, response time etc.) and frequency domain indicators.
Although this document is designed to be as general as possible, clearly in practice pointing and
AOCS issues are the most demanding space engineering disciplines in terms of control systems. They
are covered by an informative annex of the document which declines the general concepts and
illustrates how pointing issues can be managed as a special case of vector-type data on a high
resolution Earth observation mission.
Driven by a similar concern for illustration on space engineering applications of practical interest,
another annex of the document shows how to decline the general concepts to deal with the control
performance issue arisen by robotics applications.

Scope
This Handbook deals with control systems developed as part of a space project. It is applicable to all
the elements of a space system, including the space segment, the ground segment and the launch
service segment.
It addresses the issue of control performance, in terms of definition, specification, verification and
validation methods and processes.
The handbook establishes a general framework for handling performance indicators, which applies to
all disciplines involving control engineering, and which can be declined as well at different levels
ranging from equipment to system level. It also focuses on the specific performance indicators
applicable to the case of closed-loop control systems.
Rules and guidelines are provided allowing to combine different error sources in order to build up a
performance budget and to assess the compliance with a requirement.
This version of the handbook does not cover control performance issues in the frame of launch
systems.
References
EN Reference Reference in text Title
EN 16601-00-01 ECSS-S-ST-00-01 ECSS System - Glossary of terms
EN 16603-10 ECSS-E-ST-10 Space engineering – System engineering general
requirements
EN 16603-60-10 ECSS-E-ST-60-10 Space engineering – Control performance
EN 16603-60-20 ECSS-E-ST-60-20 Space engineering – Stars sensors terminology and
performance specifications
TR 16703-60 ECSS-E-HB-60 Space engineering – Control engineering handbook
EN 16601-40 ECSS-M-ST-40 Space project management – Configuration and
information management
Terms, definitions and abbreviated terms
3.1 Terms from other documents
For the purpose of this document, the terms and definitions from ECSS-S-ST-00-01 apply.
3.2 Terms specific to the present handbook
3.2.1 control performance (state)
quantified output of a controlled system
NOTE 1 Depending on the context, the control performance is realised
either as signal performance or as control loop performance.
NOTE 2 Can also be applied to a control system.
3.2.2 control (performance) knowledge (state)
estimated control performance after measurement and processing, if any
NOTE The control performance knowledge is not necessarily the best
available knowledge of the control performance. The achieved
accuracy and the allowed deviation (control performance
knowledge error) depends on the application.
3.2.3 control reference (state)
ideal reference input, desired state or reference state of controlled part of the plant
3.2.4 domain variable
independent variable which can be used to put some dependent quantity into a certain order
NOTE This comprises continuous time, discrete time, N-dimensional
space, etc.
3.2.5 ergodicity
property of a stochastic process such that its ensemble and time statistical properties are identical.
Ergodicity allows to transfer the statistical results of a single realisation of a stochastic process to the
whole ensemble
NOTE (Weak) stationarity is prerequisite for (weak) ergodicity.
3.2.6 error index
parameter isolating a particular aspect of the time variation of a performance error or knowledge error
3.2.7 extrinsic performance
element of performance related to the response of the system to its interaction with the outer world
(control reference signal, error sources and other disturbances)
NOTE 1 for example the pointing error of a satellite is relevant to this
category of extrinsic performance (it depends on the disturbing
torques and on the measurement noises)
NOTE 2 can also be defined in opposition to intrinsic performance
3.2.8 intrinsic performance
element of performance related to the intrinsic properties of the system, independently on its
interaction with the outer world (control reference, the nature and the amplitude of the error sources
and other disturbances)
NOTE 1 for example the stability of a closed-loop controlled system is
relevant to this category of intrinsic performances
NOTE 2 can also be defined in opposition to extrinsic performance
NOTE 3 “I have some of my properties purely in virtue of the way I am.
(My mass is an example.) I have other properties in virtue of the
way I interact with the world. (My weight is an example.) The
former are the intrinsic properties, the latter are the extrinsic
properties” [Weatherson, Brian, "Intrinsic vs. Extrinsic Properties",
The Stanford Encyclopedia of Philosophy (Fall 2004 Edition),
Edward N. Zalta (ed.)]
3.2.9 individual error source
elementary physical characteristic or process originating from a well-defined source which contributes
to a performance error or a performance knowledge error
NOTE For example sensor noise, sensor bias, actuator noise, actuator bias,
disturbance forces/torques (e.g. micro-vibrations, manoeuvres,
external or internal subsystem motions), friction force/torque,
misalignments, thermal distortions, assembly distortions, digital
quantization, control law performance (steady state error), jitter,
etc.
3.2.10 performance error (state difference)
deviation of a performance from its reference; realised as control (signal or control loop) performance
error or system performance error, depending on the context
3.2.11 performance error indicator (state difference)
any quantity suitable to define the performance error or performance knowledge error of a
controlled system or one of its parts. Examples are signal error functions, signal error indices or
control loop performance indicators
3.2.12 performance knowledge error (state difference)
deviation of a performance from its performance knowledge; realised as control (signal or control
loop) knowledge error or system performance knowledge error, depending on the context
3.2.13 robustness
ability of a controlled system to maintain some performance or stability characteristics in the presence
of plant, sensors, actuators and/or environmental uncertainties
NOTE 1 Performance robustness is the ability to maintain performance in
the presence of defined bounded uncertainties.
NOTE 2 Stability robustness is the ability to maintain stability in the
presence of defined bounded uncertainties.
3.2.14 signal performance (state)
characteristic output signal of the plant; either a control performance or a system performance
3.2.15 stability
property that defines the specified static and dynamics limits of a system
[ECSS-E-HB-60A]
3.2.16 signal stability
variations of a signal over a given time frame
NOTE The signal stability belongs to the category of extrinsic
performances.
3.2.17 system stability
ability of a system submitted to small external disturbances to remain indefinitely in a bounded
domain around an equilibrium position or around an equilibrium trajectory
NOTE 1 This property is essential for closed-loop control design. But it also
applies to uncontrolled systems; for example a free body spinning
about an intermediate axis of inertia is unstable.
NOTE 2 As clearly stated signal stability and system stability are different
properties which apply in different contexts. The risk for confusion
is minor in practice. It is proposed to keep the wording “stability”
unchanged in the frame of this standard, clarifying the current
meaning should any ambiguity occur.
3.2.18 stability margin
maximum excursion of the parameters describing a given system for which the system remains stable
NOTE 1 Stability margins belong to the category of intrinsic performances
(they do not depend on the system inputs and disturbances).
NOTE 2 The most frequent stability margins defined in classical control
design are the gain margin, the phase margin, the modulus
margin, and – less frequently – the delay margins (see Clause 6 of
this document).
3.2.19 stationarity
property of a stochastic process such that its statistical behaviour is time independent
NOTE Weak stationarity comprises only the time independence of the
first two statistical moments (mean and variance).
3.2.20 statistical ensemble
set of all physically possible combinations of values of parameters which describe a controlled system
3.2.21 steady state
situation where all internal and external parameters of a system (states, control reference,
environment, disturbances) vary slowly compared to the intrinsic time constants of the system
NOTE 1 steady state can also be defined by opposition to transient events
NOTE 2 “steady” does not mean that all parameters are invariant. For
example a controlled system can be in steady state although its
control reference is moving (tracking systems).
3.2.22 stochastic process
function defining a random variable for each time instance (discrete or continuous) and each
realisation of a statistical ensemble
3.2.23 system performance (state)
quantified output of the sum of the controlled and uncontrolled parts of the plant. In most cases, the
system performance is realised as a signal performance
3.2.24 system (performance) knowledge (state)
estimated system performance after measurement and processing, if any. If no additional open-loop
sensor is available, the system performance knowledge is identical to the control performance
knowledge
NOTE The system performance knowledge is not necessarily the best
available knowledge of the system performance. The achieved
accuracy and the allowed deviation (system performance
knowledge error) depends on the application.
3.2.25 system reference (state)
desired state or reference state of the sum of the controlled and uncontrolled parts of the plant
3.2.26 transient event
situation where one at least of the internal or external parameters of a system (control reference,
environment, disturbances) exhibits a stiff variation compared to the intrinsic time constants of the
system
NOTE Can also be defined by opposition to steady state.
3.2.27 tracking system
control system requested to follow a given reference profile
NOTE Table 3-1 summarizes the main relationships for the definitions of
the different kinds of performance, performance knowledge and
their corresponding errors. Quantities not defining some kind of
state like settling times can only be addressed in terms of control
loop performance.
Table 3-1: Relationships of the definitions of the different kinds of performance,
performance knowledge and their corresponding errors
Performance Performance error Performance knowledge
Performance
(states and non-states) (states difference) error (states difference)
Form in which the
Signal Control loop Signal Control loop
quantity Signal Control loop
performance performance performance performance
contributes to the performance performance
error error error error
performance
System (states)
•   Reference  X
X
•   Performance X n/a X n/a X n/a
•   Performance
X
knowledge
Control (states)
•   Reference  X X
•   Performance X X X X X X
•   Performance
X X
knowledge
Others
n/a X n/a n/a n/a n/a
(non-states)
3.3 Abbreviated terms
The following abbreviated terms are defined and used within this document:
Abbreviation Meaning
absolute knowledge error
AKE
attitude and orbit control system
AOCS
absolute performance error
APE
beginning of life
BOL
expected knowledge error
EKE
end of life
EOL
expected performance error
EPE
guidance, navigation and control
GNC
global positioning system
GPS
hardware
HW
image navigation and registration
INR
in-orbit tests
IOT
Quantity
knowledge reproducibility error
KRE
linear time invariant
LTI
measurement drift error
MDE
multiple input multiple output
MIMO
mean knowledge error
MKE
middle of life
MOL
mean performance error
MPE
performance drift error
PDE
probability density function
PDF
proportional integral derivative
PID
performance reproducibility error
PRE
power spectral density
PSD
relative knowledge error
RKE
root mean square
RMS
relative performance error
RPE
root sum of squares
RSS
solar array drive mechanism
SADM
spacecraft
SC
single input single output
SISO
software
SW
to be confirmed
TBC
to be defined
TBD
General outline for control performance
process
4.1 The general control structure
4.1.1 Description of the general control structure – Extension to
system level
A general control structure is introduced and described in [ECSS-E-HB-60A, Clause 4.1.1]. The
controlled system is defined as the control relevant part of a system to achieve the specified control
objectives. As shown on Figure 4-1 hereafter, it includes the control system (consisting of all relevant
functional behaviour of controllers, sensors and actuators) and the controlled plant.
Interaction with
environment
Controlled
System
Control
Control
objectives Control
performance
commands
Controller Actuators
Controlled
Plant
Control
feedback
Sensors
Control
system
Figure 4-1 General control structure, ECSS-E-HB-60A
In the most general situation, this controlled system itself is embedded in a higher level system layer
which also includes additional elements that contribute to the final system performance, but that are
not directly monitored by the control commands. This configuration is illustrated on Figure 4-2
hereafter; on this figure the plant is split into two sub-elements:
• the part of the plant which is inside the control loop, G1. The states of interest associated to this
part can be identified from the sensors outputs, either by direct measurement or by a dedicated
processing. They also can be driven so as to match the control reference by the closed-loop
actions applied by the actuators.
• the part of the plant which is outside the control loop, G2. The states associated to this part are
not fully known by the control system, and they cannot be fully driven by the control
commands.
Interaction with
environment
Controlled
System
Control
reference Control
System
commands
performance
Controller Actuators
G G
1 2
Control
knowledge
Control
Control
performance
feedback
Sensors
Controlled plant
Control system
System
knowledge
Additional
Processing
Sensors
Figure 4-2 General control structure extended up to system level
An example is proposed in clause 4.1.2.3 (based on a typical Earth observation satellite) to illustrate
the physical meaning of the different elements involved in this general, extended control structure in
terms of hardware, software and functions.
4.1.2 General performance definitions
4.1.2.1 General
For completeness and to avoid ambiguity this extended control system requires some additional
definitions, presented in 4.1.2.2.1 to 4.1.2.2.4 (it is recommended to refer to the example proposed in
clause 4.1.2.3).
4.1.2.2 Definitions
4.1.2.2.1 control performance
quantified capabilities of a controlled system – refer to [ECSS-E-HB-60A, 3.2.11]:
NOTE 1 More precisely according to [ECSS-E-HB-60A, 3.2.11, NOTE 1] this
is the quantified output of the part of the controlled plant which is
directly monitored by the control system.
NOTE 2 On Figure 4-2 this part of the plant corresponds to the block G1.
4.1.2.2.2 system performance
quantified output of the overall controlled plant, including its extension (if any)
NOTE 1 It can be or not directly monitored by the control system.
NOTE 2 On Figure 4-2 this part of the plant corresponds to the block G2.
4.1.2.2.3 control knowledge
knowledge of the system behaviour, restricted to the part of the controlled plant which is directly
monitored by the control system, gained by processing the information provided by the sensors of the
control system
4.1.2.2.4 system knowledge
knowledge of the system behaviour, including its extension (if any), gained by gathering all the
information available inside and outside the control system
NOTE 1 It can be or not directly monitored by the control system.
NOTE 2 If no additional observable is available, the control knowledge is
the best knowledge that can be obtained on the behaviour of the
system. However in some cases it can happen that additional
sensors are available outside the internal control loop, which allow
for improved (complete or partial) observation of the system
performance, possibly requiring a dedicated processing.
4.1.2.3 Discussion
The diagram of Figure 4-2 and these definitions show that there is no qualitative difference between
control and system performance – nor between control and system knowledge. Both can be handled
by a common formalism, to be presented in the following of this document.
4.1.3 Example – Earth observation satellite
As a typical illustration consider an Earth observation spacecraft, featuring a three-axis stabilised
platform and an imaging payload.
The platform is controlled by the AOCS to follow a given reference pointing profile (Nadir pointing,
slow slew motion for example). A set of sensors monitors the attitude of the platform (gyros, star
sensors for example) and feeds the on-board navigation (GPS for example); the AOCS control loops
generate the appropriate commands for the actuators (reaction wheels, control moment gyros,
magnetic torque rods.) so as to maintain the platform attitude close to the reference profile.
Meanwhile the instrument is submitted to effects that cannot be controlled by the AOCS loops – such
as misalignments, thermoelastic, microvibrations, payload distortions, etc. – that also affect the final
system performance, and which can be in part identified and corrected on ground using additional
information (such as image processing and landmarks identification).
Table 4-1 maps this system to the extended structure of Figure 4-2:
Table 4-1 Example of a control structure breakdown for an Earth observation
satellite
Controlled Controller AOCS control loops, on board navigation functions,
part attitude estimation
...