Space systems -- Simulation requirements for control system

This document establishes the requirements for simulation of the space control system, including the objective, architecture and procedure, etc. This document is applicable to four phases of control system development, including conceptual design, detailed design, prototype and integrated system. The control system referred to in this document is the flight control system for guidance, navigation and control (GNC) of space systems which include launch vehicle, satellite and spaceship, etc. This document establishes a minimum set of requirements for simulation of the flight control system, and provides guidance to engineers on what to simulate in each phase of control system development. The requirements are generic in nature because of their broad applicability to all types of simulations. Implementation details of the requirements are addressed in project-specific standards, requirements, and handbooks, etc.

Systèmes spatiaux -- Exigences de simulation pour le système de contrôle

General Information

Status
Published
Publication Date
14-Mar-2021
Current Stage
5060 - Close of voting Proof returned by Secretariat
Start Date
17-Feb-2021
Completion Date
17-Feb-2021
Ref Project

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INTERNATIONAL ISO
STANDARD 16781
Second edition
2021-03
Space systems — Simulation
requirements for control system
Systèmes spatiaux — Exigences de simulation pour le système de
contrôle
Reference number
ISO 16781:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 16781:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 16781:2021(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction ................................................................................................................................................................................................................................vi

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions and abbreviated terms ................................................................................................................................ 1

3.1 Terms and definitions ....................................................................................................................................................................... 1

3.2 Abbreviated terms ............................................................................................................................................................................... 3

4 Control system simulation ......................................................................................................................................................................... 3

4.1 Simulation system scheme of control system .............................................................................................................. 3

4.2 Objectives of control system simulation ........................................................................................................................... 4

4.3 Mathematical simulation and HITL simulation .......................................................................................................... 5

4.4 Simulation in different phases ................................................................................................................................................... 5

4.5 Simulation process .............................................................................................................................................................................. 8

5 General requirements ..................................................................................................................................................................................... 8

5.1 General ........................................................................................................................................................................................................... 8

5.2 Project level requirements ............................................................................................................................................................ 8

5.3 Simulation model requirements .............................................................................................................................................. 9

5.4 Simulation facility requirements ..........................................................................................................................................10

5.5 Simulation operation requirements ..................................................................................................................................10

5.6 Simulation result analysis requirements.......................................................................................................................11

5.7 Document requirements ..............................................................................................................................................................11

5.7.1 Design report of simulation system .............................................................................................................11

5.7.2 Simulation plan ..............................................................................................................................................................11

5.7.3 Simulation report .........................................................................................................................................................12

6 Requirements of conceptual design phase simulation ..............................................................................................12

6.1 General ........................................................................................................................................................................................................12

6.2 Objective ....................................................................................................................................................................................................13

6.3 Input ..............................................................................................................................................................................................................13

6.4 Output ..........................................................................................................................................................................................................13

6.5 Simulation model requirements ...........................................................................................................................................13

6.6 Simulation facility requirements ..........................................................................................................................................14

6.7 Simulation operation requirements ..................................................................................................................................14

7 Requirements of detailed design phase simulation .....................................................................................................14

7.1 General ........................................................................................................................................................................................................14

7.2 Objective ....................................................................................................................................................................................................14

7.3 Input ..............................................................................................................................................................................................................15

7.4 Output ..........................................................................................................................................................................................................15

7.5 Simulation model requirements ...........................................................................................................................................15

7.6 Simulation facility requirements ..........................................................................................................................................15

7.7 Simulation operation requirements ..................................................................................................................................15

8 Requirements of prototype phase simulation ....................................................................................................................16

8.1 General ........................................................................................................................................................................................................16

8.2 Objective ....................................................................................................................................................................................................16

8.3 Input ..............................................................................................................................................................................................................17

8.4 Output ..........................................................................................................................................................................................................17

8.5 Simulation model requirements ...........................................................................................................................................17

8.6 Simulation facility requirements ..........................................................................................................................................17

8.6.1 Requirements of simulation devices ...........................................................................................................17

8.6.2 Requirements of simulation environment..............................................................................................18

8.7 Simulation Operation Requirements ................................................................................................................................18

9 Requirements of integrated system phase simulation ..............................................................................................18

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 16781:2021(E)

9.1 General ........................................................................................................................................................................................................18

9.2 Objective ....................................................................................................................................................................................................19

9.3 Input ..............................................................................................................................................................................................................19

9.4 Output ..........................................................................................................................................................................................................20

9.5 Simulation model requirements ...........................................................................................................................................20

9.6 Simulation facility requirements ..........................................................................................................................................20

9.7 Simulation operation requirements ..................................................................................................................................20

Annex A (informative) Phase comparison between ISO 14300-1 and this document ...................................21

Bibliography .............................................................................................................................................................................................................................22

iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 16781:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,

Subcommittee SC 14, Space systems and operations.

This second edition cancels and replaces the first edition (ISO 16781:2013), which has been technically

revised.
The main changes compared to the previous edition are as follows:

— the Introduction and the Scope have been revised; the scope is re-stated to concentrate on the

simulation requirements of the flight control system of space system;
— the definition of “control system” in 3.1.2 has been revised;

— the title of 4.1 and Figure 1 have been revised as “simulation system scheme of control system”;

— some statements have been added in 8.1 to explain the usage requirements of actual hardware

devices for prototype phase simulation;
— the previous Annex B has been deleted;
— the Bibliography has been added.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
© ISO 2021 – All rights reserved v
---------------------- Page: 5 ----------------------
ISO 16781:2021(E)
Introduction

Simulation is an important means to design, analyse and validate the space control system, and it is

widely used in each phase of control system development. The objective of simulation is to demonstrate

that the proposed or designed system will function as desired; and the simulation allows engineers and

technical decision makers to evaluate the feasibility, validity and rationality of the design scheme more

accurately.

This document provides space control system engineers, simulation engineers and customers with

guidance for using simulation to support their system engineering tasks. This document is intended to

help reduce the development time and cost of space control system design and also enhance its quality

and reliability. This document focuses on the requirements and recommendations during simulation. It

does not prescribe how the requirements are to be met, nor does it specify who the responsible team is

for conforming to the requirements.
vi © ISO 2021 – All rights reserved
---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 16781:2021(E)
Space systems — Simulation requirements for control
system
1 Scope

This document establishes the requirements for simulation of the space control system, including the

objective, architecture and procedure, etc. This document is applicable to four phases of control system

development, including conceptual design, detailed design, prototype and integrated system.

The control system referred to in this document is the flight control system for guidance, navigation

and control (GNC) of space systems which include launch vehicle, satellite and spaceship, etc. This

document establishes a minimum set of requirements for simulation of the flight control system, and

provides guidance to engineers on what to simulate in each phase of control system development. The

requirements are generic in nature because of their broad applicability to all types of simulations.

Implementation details of the requirements are addressed in project-specific standards, requirements,

and handbooks, etc.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1.1
accuracy
measure of how close a value is to the “true” value
[SOURCE: ISO 14952-1:2003, 2.1]
3.1.2
control system

system designed to give the controlled plant the specified control objectives, and including relevant

functions of controller, sensor and actuator

Note 1 to entry: In this document, the word “controller” is used to designate the flight control computer which

manages the flight dynamic behaviour of space system.
3.1.3
emulator

prototype of the flight equipment, which has the identical input/output interfaces as the flight

equipment and has similar operating behaviour
© ISO 2021 – All rights reserved 1
---------------------- Page: 7 ----------------------
ISO 16781:2021(E)
3.1.4
fidelity

degree to which a model or simulation (3.1.9) reproduces the state and behaviour of a real-world object

or the perception of a real-world object, feature, condition, or chosen standard in a measurable or

perceivable manner
3.1.5
hardware-in-the-loop simulation
HITL simulation

kind of simulation (3.1.9), in which some simulation models (3.1.11) of the control system (3.1.2) are

implemented by real equipment
3.1.6
mathematical simulation

kind of simulation (3.1.9), in which all the simulation models (3.1.11) of the control system (3.1.2) are

implemented by software
3.1.7
real-time simulation

kind of simulation (3.1.9), in which the time scale of dynamic process in the simulation model (3.1.11)

strictly equals to that of the real system
3.1.8
reliability

ability of an item to perform a required function under given conditions for a given time interval

Note 1 to entry: It is generally assumed that the item is in a state to perform this required function at the

beginning of the time interval.

Note 2 to entry: Generally, reliability performance is quantified using appropriate measures. In some applications

these measures include an expression of reliability performance as a probability, which is also called reliability.

[SOURCE: ISO 10795:2019, 3.198]
3.1.9
simulation

use of a similar or equivalent system to imitate a real system, so that it behaves like or appears to be the

real system
3.1.10
control system simulation

complex progress of building simulation (3.1.9) system based on the mathematical model of the control

system (3.1.2), testing the model, solving the system dynamic equations, imitating dynamic behaviours

of the control system, and taking qualitative and quantitative analysis and research about the scheme,

structure, parameters, and performance of the control system
3.1.11
simulation model

equivalent model in the simulation (3.1.9) system, which is transformed from the mathematical model

of the control system (3.1.2) by means of simulation software or hardware
3.1.12
simulation plan

document in which the content, operate steps and implement method of all simulation (3.1.9) items are

specified
3.1.13
stability

ability of a system submitted to bound external disturbances to remain indefinitely in a bounded

domain around an equilibrium position or around an equilibrium trajectory
2 © ISO 2021 – All rights reserved
---------------------- Page: 8 ----------------------
ISO 16781:2021(E)
3.1.14
validation

confirmation, through the provision of objective evidence, that the requirements for a specific intended

use or application have been fulfilled

Note 1 to entry: The objective evidence needed for a validation is the result of a test or other form of determination

such as performing alternative calculations or reviewing documents.

Note 2 to entry: The word “validated” is used to designate the corresponding status.

Note 3 to entry: The use conditions for validation can be real or simulated.
[SOURCE: ISO 10795:2019, 3.243]
3.1.15
verification

confirmation, through the provision of objective evidence, that specified requirements have been

fulfilled

Note 1 to entry: The objective evidence needed for a verification can be the result of an inspection or of other

forms of determination such as performing alternative calculations or reviewing documents.

Note 2 to entry: The activities carried out for verification are sometimes called a qualification process.

Note 3 to entry: The word “verified” is used to designate the corresponding status.

[SOURCE: ISO 10795:2019, 3.244]
3.2 Abbreviated terms
For the purposes of this document, the abbreviated terms given in Table 1 apply.
Table 1 — Abbreviated terms
CM configuration management
HITL hardware-in-the-loop
M&S modelling and simulation
V&V verification and validation
4 Control system simulation
4.1 Simulation system scheme of control system

The control system is one of the most important systems of launch vehicle, satellite, spaceship, etc.

Generally, the simulation system scheme of the control system is illustrated in Figure 1.

© ISO 2021 – All rights reserved 3
---------------------- Page: 9 ----------------------
ISO 16781:2021(E)
Figure 1 — Simulation system scheme of control system

a) The flight environment includes atmosphere or space environment in which the spacecraft

exists. In terms of different kinds of spacecraft, the control system shall consider mechanical,

thermodynamic, optical and electromagnetic environment, etc.

b) Sensors are fixed on the spacecraft to measure the states, which are provided to the flight control

computer for control algorithm calculation.

c) The flight control computer receives and deals with the measured information from sensors, and

then control signals are gained by the control algorithm and sent to servos as commands.

d) Servos receive commands from the flight control computer and drive actuators, which produce

forces and moments and affect the flight states of spacecraft, so that a closed loop is formed and the

objective of control is achieved.
e) The command input indicates the control command and binding parameter.
f) Vehicle dynamics indicates the dynamic behaviour of a plant.
g) The logger records the telemetry data and flight status.
4.2 Objectives of control system simulation

Control system design is an iterative process from design, test and validation to modification, retest,

and revalidation. Analytical method is not enough for research and design of the control system, so

simulation experiment is demanded.
The primary objectives of control system simulation are as follows:
a) verify and optimize the control system scheme;
b) verify and optimize the control system parameters;
c) analyse the stability and robustness of the control system;
d) emulate control system faults that can occur in flight;
e) predict the performance of the control system;
f) comprehensively verify functions of control system components;
g) minimize scheme design iteration;
h) shorten the development time;
4 © ISO 2021 – All rights reserved
---------------------- Page: 10 ----------------------
ISO 16781:2021(E)
i) minimize the development budget.
4.3 Mathematical simulation and HITL simulation

Compared to mathematical simulation, the structure of an HITL simulation system is more complex.

It can reflect the hardware/software characteristics of the control system, and verify the functions/

performances of the control system (e.g. interface matching properties). Generally, HITL simulation

should be done after mathematical simulation.

The corresponding relationship between simulation types and the practical control system is listed in

Table 2.
Table 2 — Relationship between simulation types and the practical control system
Parts of control Mathematical
HITL simulation
system simulation
Mathematical model and
Vehicle dynamics
motion simulator (turn table, robotic arm, air bearing)
Physical device (either flight hardware or engineering develop-
ment hardware)
Sensors
or equivalent mathematical model of sensors
Mathematical mod-

Flight control com- Physical device (either flight hardware, engineering hardware, or

els
puter emulator)
Equivalent servo/actuator mathematical model or
Servos and actuators
physical device (either flight hardware or engineering develop-
ment hardware)
Flight environment Emulator or mathematical model
4.4 Simulation in different phases

Design of a control system is not a simple iterative process. It can be divided into the conceptual design

phase, detailed design phase, prototype phase and integrated system phase. Simulation is demanded in

each phase in order to realize flight equivalent examples for the control system validation or equipment

verification. The relationship between each design phase and simulation is described in Figure 2.

© ISO 2021 – All rights reserved 5
---------------------- Page: 11 ----------------------
ISO 16781:2021(E)
Figure 2 — Relationship between each design phase and simulation

In the conceptual design phase simulation, mathematical simulation is used for control system

architecture and conceptual design studies. This pure software simulation environment supports

the identification of optional control system architectures/top level designs that meet both mission

performance requirements and stability robustness requirements. Low-order/low-fidelity models and

simple operational environment models are adopted for mathematical simulation. Multiple co-existing

models and simulation tools are managed by individual engineers.

In the detailed design phase simulation, mathematical simulation is used for system optimizations,

parameter sensitivity assessments, performance evaluations, stability robustness assessments,

etc. This simulation environment supports the identification of the final control system design that

matches mission performance and stability robustness requirements. High-order/high-fidelity,

possibly nonlinear models and detailed flight-equivalent operational environment models are adopted.

6 © ISO 2021 – All rights reserved
---------------------- Page: 12 ----------------------
ISO 16781:2021(E)

Some formal configuration management/control of models, parameter databases and simulations are

required in this simulation environment.

HITL simulation, which combines hardware and software, is often introduced in the prototype phase

simulation. The basic components of the control system are realized by prototype. In this phase, the

correctness of the control algorithm prototype and flight software and compatibility between various

interfaces are validated, in order to reduce the integrated risk of the control system and the entire

spacecraft. This simulation environment may allow substitution of control system sensors/actuators

models for hardware engineering units as needed. Simple software plant and environment models are

used to close the control system loop. Also, some formal configuration management/control of this

HITL simulation environment is required.

Testing of the control system flight software nominal functionality as well as failure mode functions is

accomplished by HITL simulation in the integrated system phase simulation. The basic components of

the control system are realized by real devices, at least including a flight processor hosting the control

algorithm as well as other relevant flight software elements. Consistency of all parts of control system

is certified to ensure that requirements of space system are satisfied. This M&S environment is often

maintained after space system launch to allow:

a) model validation via comparing the operational performance of the actual in-flight control system

with pre-launch M&S results;

b) the checkout and verification of control system flight software modifications (e.g. a flight software

“patch”) prior to implementing the change to the on-board space syste
...

INTERNATIONAL ISO
STANDARD 16781
Second edition
Space systems — Simulation
requirements for control system
Systèmes spatiaux — Exigences de simulation pour le système de
contrôle
PROOF/ÉPREUVE
Reference number
ISO 16781:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 16781:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 16781:2021(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction ................................................................................................................................................................................................................................vi

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms, definitions and abbreviated terms ................................................................................................................................ 1

3.1 Terms and definitions ....................................................................................................................................................................... 1

3.2 Abbreviated terms ............................................................................................................................................................................... 3

4 Control system simulation ......................................................................................................................................................................... 3

4.1 Simulation system scheme of control system .............................................................................................................. 3

4.2 Objectives of control system simulation ........................................................................................................................... 4

4.3 Mathematical simulation and HITL simulation .......................................................................................................... 5

4.4 Simulation in different phases ................................................................................................................................................... 5

4.5 Simulation process .............................................................................................................................................................................. 8

5 General requirements ..................................................................................................................................................................................... 8

5.1 General ........................................................................................................................................................................................................... 8

5.2 Project level requirements ............................................................................................................................................................ 8

5.3 Simulation model requirements .............................................................................................................................................. 9

5.4 Simulation facility requirements ..........................................................................................................................................10

5.5 Simulation operation requirements ..................................................................................................................................10

5.6 Simulation result analysis requirements.......................................................................................................................11

5.7 Document requirements ..............................................................................................................................................................11

5.7.1 Design report of simulation system .............................................................................................................11

5.7.2 Simulation plan ..............................................................................................................................................................11

5.7.3 Simulation report .........................................................................................................................................................12

6 Requirements of conceptual design phase simulation ..............................................................................................12

6.1 General ........................................................................................................................................................................................................12

6.2 Objective ....................................................................................................................................................................................................13

6.3 Input ..............................................................................................................................................................................................................13

6.4 Output ..........................................................................................................................................................................................................13

6.5 Simulation model requirements ...........................................................................................................................................13

6.6 Simulation facility requirements ..........................................................................................................................................14

6.7 Simulation operation requirements ..................................................................................................................................14

7 Requirements of detailed design phase simulation .....................................................................................................14

7.1 General ........................................................................................................................................................................................................14

7.2 Objective ....................................................................................................................................................................................................14

7.3 Input ..............................................................................................................................................................................................................15

7.4 Output ..........................................................................................................................................................................................................15

7.5 Simulation model requirements ...........................................................................................................................................15

7.6 Simulation facility requirements ..........................................................................................................................................15

7.7 Simulation operation requirements ..................................................................................................................................15

8 Requirements of prototype phase simulation ....................................................................................................................16

8.1 General ........................................................................................................................................................................................................16

8.2 Objective ....................................................................................................................................................................................................16

8.3 Input ..............................................................................................................................................................................................................17

8.4 Output ..........................................................................................................................................................................................................17

8.5 Simulation model requirements ...........................................................................................................................................17

8.6 Simulation facility requirements ..........................................................................................................................................17

8.6.1 Requirement of simulation devices ..............................................................................................................17

8.6.2 Requirements of simulation environment..............................................................................................18

8.7 Simulation Operation Requirements ................................................................................................................................18

9 Requirements of integrated system phase simulation ..............................................................................................18

© ISO 2021 – All rights reserved PROOF/ÉPREUVE iii
---------------------- Page: 3 ----------------------
ISO 16781:2021(E)

9.1 General ........................................................................................................................................................................................................18

9.2 Objective ....................................................................................................................................................................................................19

9.3 Input ..............................................................................................................................................................................................................19

9.4 Output ..........................................................................................................................................................................................................20

9.5 Simulation model requirements ...........................................................................................................................................20

9.6 Simulation facility requirements ..........................................................................................................................................20

9.7 Simulation operation requirements ..................................................................................................................................20

Annex A (informative) Phase comparison between ISO 14300-1 and this document ...................................21

Bibliography .............................................................................................................................................................................................................................22

iv PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
ISO 16781:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 20, Aircraft and space vehicles,

Subcommittee SC 14, Space systems and operations.

This second edition cancels and replaces the first edition (ISO 16781:2013), which has been technically

revised.
The main changes compared to the previous edition are as follows:

— the Introduction and the Scope have been revised; the scope is re-stated to concentrates on the

simulation requirements of the flight control system of space system;
— the definition of “control system” in 3.1.2 has been revised;

— the title of 4.1 and Figure 1 have been revised as “simulation system scheme of control system”;

— some statements have been added in 8.1 to explain the usage requirements of actual hardware

devices for prototype phase simulation;
— the previous Annex B has been deleted;
— the Bibliography has been added.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
© ISO 2021 – All rights reserved PROOF/ÉPREUVE v
---------------------- Page: 5 ----------------------
ISO 16781:2021(E)
Introduction

Simulation is an important means to design, analyse and validate the space control system, and it is

widely used in each phase of control system development. The objective of simulation is to demonstrate

that the proposed or designed system will function as desired; and the simulation allows engineers and

technical decision makers to evaluate the feasibility, validity and rationality of the design scheme more

accurately.

This document provides space control system engineers, simulation engineers and customers with

guidance for using simulation to support their system engineering tasks. This document is intended to

help reduce the development time and cost of space control system design and also enhance its quality

and reliability. This document focuses on the requirements and recommendations during simulation. It

does not prescribe how the requirements are to be met, nor does it specify who the responsible team is

for conforming to the requirements.
vi PROOF/ÉPREUVE © ISO 2021 – All rights reserved
---------------------- Page: 6 ----------------------
INTERNATIONAL STANDARD ISO 16781:2021(E)
Space systems — Simulation requirements for control
system
1 Scope

This document establishes the requirements for simulation of space control systems, including the

objective, architecture and procedure, etc. This document is applicable to four phases of control system

development, including conceptual design, detailed design, prototype and integrated system.

The control system referred to in this document is the flight control system for guidance, navigation

and control (GNC) of space systems which include launch vehicle, satellite and spaceship, etc. This

document establishes a minimum set of requirements for simulation of flight control systems, and

provides guidance to engineers on what to simulate in each phase of control system development. The

requirements are generic in nature because of their broad applicability to all types of simulations.

Implementation details of the requirements are addressed in project-specific standards, requirements,

and handbooks, etc.
2 Normative references
There are no normative references in this document.
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
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3.1.1
accuracy
measure of how close a value is to the “true” value
[SOURCE: ISO 14952-1:2003, 2.1]
3.1.2
control system

system designed to give the controlled plant the specified control objectives, and including relevant

functions of controller, sensor and actuator

Note 1 to entry: In this document, the word “controller” is used to designate the flight control computer which

manages the flight dynamic behaviour of space system.
3.1.3
emulator

prototype of the flight equipment, which has the identical input/output interfaces as the flight

equipment and has similar operating behaviour
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3.1.4
fidelity

degree to which a model or simulation (3.1.9) reproduces the state and behaviour of a real-world object

or the perception of a real-world object, feature, condition, or chosen standard in a measurable or

perceivable manner
3.1.5
hardware-in-the-loop simulation
HITL simulation

kind of simulation (3.1.9), in which some simulation models (3.1.11) of the control system (3.1.2) are

implemented by real equipment
3.1.6
mathematical simulation

kind of simulation (3.1.9), in which all the simulation models (3.1.11) of the control system (3.1.2) are

implemented by software
3.1.7
real-time simulation

kind of simulation (3.1.9), in which the time scale of dynamic process in the simulation model (3.1.11)

strictly equals to that of the real system
3.1.8
reliability

ability of an item to perform a required function under given conditions for a given time interval

Note 1 to entry: It is generally assumed that the item is in a state to perform this required function at the

beginning of the time interval.

Note 2 to entry: Generally, reliability performance is quantified using appropriate measures. In some applications

these measures include an expression of reliability performance as a probability, which is also called reliability.

[SOURCE: ISO 10795:2019, 3.198]
3.1.9
simulation

use of a similar or equivalent system to imitate a real system, so that it behaves like or appears to be the

real system
3.1.10
control system simulation

complex progress of building simulation (3.1.9) system based on the mathematical model of the control

system (3.1.2), testing the model, solving the system dynamic equations, imitating dynamic behaviours

of the control system, and taking qualitative and quantitative analysis and research about the scheme,

structure, parameters, and performance of the control system
3.1.11
simulation model

equivalent model in the simulation (3.1.9) system, which is transformed from the mathematical model

of the control system (3.1.2) by means of simulation software or hardware
3.1.12
simulation plan

document in which the content, operate steps and implement method of all simulation (3.1.9) items are

specified
3.1.13
stability

ability of a system submitted to bound external disturbances to remain indefinitely in a bounded

domain around an equilibrium position or around an equilibrium trajectory
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3.1.14
validation

confirmation, through the provision of objective evidence, that the requirements for a specific intended

use or application have been fulfilled

Note 1 to entry: The objective evidence needed for a validation is the result of a test or other form of determination

such as performing alternative calculations or reviewing documents.

Note 2 to entry: The word “validated” is used to designate the corresponding status.

Note 3 to entry: The use conditions for validation can be real or simulated.
[SOURCE: ISO 10795:2019, 3.243]
3.1.15
verification

confirmation, through the provision of objective evidence, that specified requirements have been

fulfilled

Note 1 to entry: The objective evidence needed for a verification can be the result of an inspection or of other

forms of determination such as performing alternative calculations or reviewing documents.

Note 2 to entry: The activities carried out for verification are sometimes called a qualification process.

Note 3 to entry: The word “verified” is used to designate the corresponding status.

[SOURCE: ISO 10795:2019, 3.244]
3.2 Abbreviated terms
For the purposes of this document, the abbreviated terms given in Table 1 apply.
Table 1 — Abbreviated terms
CM configuration management
HITL hardware-in-the-loop
M&S modelling and simulation
V&V verification and validation
4 Control system simulation
4.1 Simulation system scheme of control system

The control system is one of the most important systems of launch vehicle, satellite, spaceship, etc.

Generally, the simulation system scheme of the control system is illustrated in Figure 1.

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Figure 1 — Simulation system scheme of control system

a) The flight environment includes atmosphere or space environment in which the spacecraft

exists. In terms of different kinds of spacecraft, the control system shall consider mechanical,

thermodynamic, optical and electromagnetic environment, etc.

b) Sensors are fixed on the spacecraft to measure the states, which are provided to the flight control

computer for control algorithm calculation.

c) The flight control computer receives and deals with the measured information from sensors, and

then control signals are gained by the control algorithm and sent to servos as commands.

d) Servos receive commands from the flight control computer and drive actuators, which produce

forces and moments and affect the flight states of spacecraft, so that a closed loop is formed and the

objective of control is achieved.
e) The command input indicates the control command and binding parameter.
f) Vehicle dynamics indicates the dynamic behaviour of a plant.
g) The logger records the telemetry data and flight status.
4.2 Objectives of control system simulation

Control system design is an iterative process from design, test and validation to modification, retest,

and revalidation. Analytical method is not enough for research and design of the control system, so

simulation experiment is demanded.
The primary objectives of control system simulation are as follows:
a) verify and optimize the control system scheme;
b) verify and optimize the control system parameters;
c) analyse the stability and robustness of the control system;
d) emulate control system faults that can occur in flight;
e) predict the performance of the control system;
f) comprehensively verify functions of control system components;
g) minimize scheme design iteration;
h) shorten the development time;
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i) minimize the development budget.
4.3 Mathematical simulation and HITL simulation

Compared to mathematical simulation, the structure of an HITL simulation system is more complex.

It can reflect the hardware/software characteristics of the control system, and verify the functions/

performances of the control system (e.g. interface matching properties). Generally, HITL simulation

should be done after mathematical simulation.

The corresponding relationship between simulation types and the practical control system is listed in

Table 2.
Table 2 — Relationship between simulation types and the practical control system
Parts of control Mathematical
HITL simulation
system simulation
Mathematical model and
Vehicle dynamics
motion simulator (turn table, robotic arm, air bearing)
Physical device (either flight hardware or engineering develop-
ment hardware)
Sensors
or equivalent mathematical model of sensors
Mathematical mod-

Flight control com- Physical device (either flight hardware, engineering hardware, or

els
puter emulator)
Equivalent servo/actuator mathematical model or
Servos and actuators
physical device (either flight hardware or engineering develop-
ment hardware)
Flight environment Emulator or mathematical model
4.4 Simulation in different phases

Design of a control system is not a simple iterative process. It can be divided into the conceptual design

phase, detailed design phase, prototype phase and integrated system phase. Simulation is demanded in

each phase in order to realize flight equivalent examples for the control system validation or equipment

verification. The relationship between each design phase and simulation is described in Figure 2.

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Figure 2 — Relationship between each design phase and simulation

In the conceptual design phase simulation, mathematical simulation is used for control system

architecture and conceptual design studies. This pure software simulation environment supports

the identification of optional control system architectures/top level designs that meet both mission

performance requirements and stability robustness requirements. Low-order/low-fidelity models and

simple operational environment models are adopted for mathematical simulation. Multiple co-existing

models and simulation tools are managed by individual engineers.

In the detailed design phase simulation, mathematical simulation is used for system optimizations,

parameter sensitivity assessments, performance evaluations, stability robustness assessments,

etc. This simulation environment supports the identification of the final control system design that

matches mission performance and stability robustness requirements. High-order/high-fidelity,

possibly nonlinear models and detailed flight-equivalent operational environment models are adopted.

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Some formal configuration management/control of models, parameter databases and simulations are

required in this simulation environment.

HITL simulation, which combines hardware and software, is often introduced in the prototype phase

simulation. The basic components of the control system are realized by prototype. In this phase, the

correctness of the control algorithm prototype and flight software and compatibility between various

interfaces are validated, in order to reduce the integrated risk of the control system and the entire

spacecraft. This simulation environment may allow substitution of control system sensors/actuators

models for hardware engineering units as needed. Simple software plant and environment models are

used to close the control system loop. Also, some formal configuration management/control of this

HITL simulation environment is required.

Testing of the control system flight software nominal functionality as well as failure mode functions is

accomplished by HITL simulation in the integrated system phase simulation. The basic components of

the control system are realized by real devices, at least including a flight processor hosting the control

algorithm as well as other relevant flight software elements. Consistency of all parts of control system

is certified to ensure that requirements of space system are satisfied. This M&S environment is often

maintained after space system launch to allow:

a) model validation via comparing the operational performance of the actual in-flight control system

with pre-launch M&S results;
b) the checkout and verificat
...

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