ISO 21740:2025

International
Standard
ISO 21740
First edition
Space systems — Launch window
2025-07
estimation and collision avoidance
Systèmes spatiaux — Estimation de la fenêtre de lancement et
évitement des collisions
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative reference . 1
3 Terms and definitions . 1
4 Abbreviated terms . 3
5 Safety LCOLA requirements . 3
5.1 Conduct of a safety LCOLA .3
5.1.1 General .3
5.1.2 Launch vehicle stage(s) and deployed object(s) trajectories .4
5.1.3 Inhabitable vehicle ephemerides .4
5.2 Close encounter check for a safety LCOLA .4
5.3 Safety LCOLA launch hold criteria .5
5.4 Safety LCOLA screening timespan .5
5.5 LCOLA analysis run timing sequence .6
5.6 LCOLA standoff threshold .6
5.7 LCOLA standoff minimum threshold .6
6 LCOLA product . 7
Annex A (informative) LCOLA analysis challenges and methodologies . 8
Annex B (informative) Mission assurance LCOLA assessment .15
Annex C (informative) LCOLA survey . 17
Bibliography .20

iii
Foreword
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iv
Introduction
0.1 Overview
Human-inhabited and inhabitable space stations and space capsules are exposed to the risk of collision with
new launch vehicle stage(s) and spacecraft during their launch and early orbit phase. While such collision
threats also exist during on-orbit spacecraft operations, the launch and early orbit phase is unique in that
potential collisions with inhabitable space stations and space capsules can be avoided at minimal cost (i.e.
without the expenditure of on-orbit manoeuvring fuel) through the proper selection of suitable launch times.
[1]
Consistent with Clause B.5 of the United Nations Long Term Sustainability guidelines and to protect human
missions from the danger of collision with newly launched objects, the LCOLA approving agent may apply
launch collision avoidance (LCOLA) methods to assess either collision risk, close approach, or both. If this
assessment determines that launch at certain times would incur unacceptable risk to the human missions,
the LCOLA approving agent may delay the time of launch.
In addition to establishing requirements for the safety LCOLA process, Annex A provides details on
algorithms, processes, and screening criteria that may be used to conduct safety LCOLA assessments.
Annex A is intended to ensure the safety and integrity of human-inhabited or inhabitable space stations.
Annex B provides additional details on algorithms, processes, and screening criteria that may be used to
conduct mission assurance LCOLA assessments. Annex B is intended to ensure the safety and integrity of
both the newly launched space objects and any on-orbit active spacecraft that can potentially collide with
the launched objects for the initial LCOLA screening time period, and also to reduce the risk of unintended
fragmentation events caused by collision of the newly launched space objects with orbital debris during that
same initial screening period. A selection of general procedures for the determination of unacceptable risk
that result in the identification of all collision-safe launch opportunities is described.
0.2 Breakdown of space safety constituents across ISO standards
The space flight safety-relevant topics of space traffic coordination (STC), on-orbit collision avoidance, and
launch collision avoidance are closely related. To minimize duplication and maximize document consistency,
the various content that serve as the basis for these three disciplines has been divided up as shown in
Figure 1.
NOTE ISO 9490 and ISO 23705 are under development.
Figure 1 — Division of space safety operations content spanning several ISO standards

v
International Standard ISO 21740:2025(en)
Space systems — Launch window estimation and collision
avoidance
1 Scope
This document establishes the general safety launch collision avoidance (safety LCOLA) requirements for
the avoidance of collision between the collection of newly launched objects resulting from a space launch
[including launch vehicle stage(s) and payloads or released objects] and human-inhabited or human-
habitable space stations and space vehicles.
The document specifies the requirements for the analysis of launch times and procedures for identifying
safe launch opportunities. It also describes the assessment and constraints for collision avoidance risk
evaluation metrics (launch collision probability and standoff distance).
2 Normative reference
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
conjunction
event where the positional separation between two objects is at a local minimum and that minimum is either
closer than a specified minimum distance threshold, or the estimated probability of collision at this local
minimum exceeds a specified launch collision probability (3.5) threshold
3.2
integrated LCOLA
integrated launch collision avoidance
composite launch window (3.9) stemming from the combination of safety LCOLA (3.15) and mission assurance
LCOLA (3.13) analyses
3.3
miss distance
minimum approach distance between a launching body and an orbiting body at their closest approach point
3.4
launch collision avoidance
LCOLA
process of mitigating the risk of collision between the newly launched objects resulting from a space launch
and any space objects in their paths

3.5
launch collision probability
quantification of the likelihood that a newly launched object would impact a designated space object during
a conjunction (3.1)
Note 1 to entry: Guidance, winds aloft, performance, estimated orbit propagation accuracy, and conjunction approach
geometry are examples of pre-launch launch vehicle (3.7) system errors.
3.6
launch period
selected range of dates when a launch is planned
3.7
launch vehicle
integrated system that is designed to carry payloads into and beyond its gravitational attracting body’s
atmosphere
Note 1 to entry: Such systems are typically composed of multiple stages.
3.8
launch vehicle stage
launch vehicle (3.7) component that has achieved flight above 150 km
3.9
launch window
range(s) of times on a day within the launch period (3.6) which adhere to all launch vehicle (3.7), payload, and
mission constraints, and during which the launch range is available and payloads and the launch vehicle are
ready for launch
3.10
launch window closure
portion of the launch window (3.9) which is unavailable (closed) due to a constraint violation
Note 1 to entry: Minimum LCOLA standoff distance (3.17) and launch collision probability (3.5) threshold are examples
of constraints.
3.11
launch window opportunity
portion of the launch window (3.9) which is available (open) because no launch range, launch vehicle (3.7),
payload or mission constraints are violated
3.12
LCOLA approving agent
entity who sets requirements for monitors and approves the procurement, management, oversight,
implementation, operations, performance evaluation, quality assurance and monitoring functions of the
LCOLA system under its authority
Note 1 to entry: The LCOLA approving agent’s responsibilities can be handled by a commercial, non-governmental,
governmental, or international individual or entity, as well as a mandated or delegated entity assigned by applicable
national regulations.
3.13
mission assurance LCOLA
mitigation of collision risk between the newly launched objects resulting from a space launch and any space
objects in their paths
Note 1 to entry: The assessment is conducted to ensure the safety and integrity of both sets of objects.

3.14
primary object
one out of the collection of newly launched objects resulting from a space launch to be protected from a
collision by the applied LCOLA analysis approach
Note 1 to entry: Launch vehicle stage(s) (3.8) and associated released space debris, mission payloads, platforms, and
deployment-related space debris are examples of primary objects.
3.15
safety LCOLA
assessment of collision risk between the newly launched objects resulting from a space launch and any
inhabited or inhabitable space objects in their paths
Note 1 to entry: The assessment is conducted to ensure the safety and integrity of both sets of objects.
3.16
secondary object
one out of a set of designated space objects to be protected from a collision by the applied LCOLA analysis
approach
Note 1 to entry: In the case of a safety LCOLA (3.15), this is an inhabited or inhabitable space objects.
3.17
standoff distance
screening distance
miss distance (3.3) at the time of closest approach between a newly launched object resulting from a space
launch and a space object
4 Abbreviated terms
CAM collision avoidance manoeuvre
CCSDS Consultative Committee for Space Data Systems
COLA collision avoidance
GEO geo-synchronous orbit
LEO low earth orbit
QC quality control
S/C spacecraft
SSA space situational awareness
TLE two-line element set
UTC Coordinated Universal Time
5 Safety LCOLA requirements
5.1 Conduct of a safety LCOLA
5.1.1 General
The launch service provider shall conduct or procure a safety LCOLA analysis product for the launch window
on a selected day within the launch period, to identify the launch window opportunities and complementary
launch window closures (due to collision risk being unacceptably high).

5.1.2 Launch vehicle stage(s) and deployed object(s) trajectories
The launch service provider shall develop trajectories, inclusive of disposal manoeuvres, that enable LCOLA
analyses during all mission segments where conjunctions with secondary objects are possible, for the launch
vehicle and all pertinent deployed launch elements according to planned lift-off information.
For Earth-fixed missions, the launch ascent ephemeris should be specified in the Earth-relative frame such
that only one launch trajectory is required to completely specify the launch grid for boosters that do not
perform in-flight retargeting.
NOTE 1 While such trajectories typically exist as required by the launch vehicle flight control analysts and launch
ranges, the above requirement is intended to ensure that such trajectories are obtained and fit for the purpose of
LCOLA analysis.
[8,9]
NOTE 2 CCSDS Orbit Data Message (ODM) standards can be used as the exchange format .
5.1.3 Inhabitable vehicle ephemerides
The launch service provider shall collect orbital information for all inhabited or inhabitable space systems.
This can include on-orbit ephemerides obtained or derived from data obtained from either inhabitable space
[2]
system operators or government or commercial SSA systems, or both.
Semi-analytical orbit states (such as the TLE used with the SGP4 propagator) shall not be used to represent
human-inhabited or human-inhabitable space stations due to their generally reduced accuracy as compared
with numerically integrated products.
NOTE Authoritative ephemerides incorporating planned manoeuvres is publicly available for at least some
inhabitable space stations.
5.2 Close encounter check for a safety LCOLA
For each launch time, the launch vehicle elements and deployed objects ephemerides shall be compared with
ephemerides for all inhabitable space stations and space vehicles to ascertain whether a collision threat
exists. If a close encounter is determined to be too risky, then that launch time shall not be utilized.
NOTE The main elements of the overall process used to determine launch windows are shown in Figure 2.

Figure 2 — Safety LCOLA launch window screening process
5.3 Safety LCOLA launch hold criteria
For safety LCOLA analyses, the LCOLA metrics, risk criteria, and associated threshold settings shall be
selected by the LCOLA approving agent to make a conjunction with a human-inhabited or habitable object a
virtual impossibility.
This would dictate that either:
a) the standoff distance criteria represent the sums of the maximum covariance-based ellipsoidal
error dimensions (major eigenvalues) for both launching and human-inhabited or habitable asset
incorporating a high margin of safety; or
b) the relative motion of the N-sigma error ellipsoids for both launching and human-inhabited or habitable
assets cannot contact each other, where N is a large number (e.g. 3).
As human spaceflight safety is of paramount importance, a probability-based approach is to use the
maximum probability formulation (Reference [6]) to select a ‘keep-out zone’ constraint which guarantees
that the probability of collision for any conjunctions with standoff distances greater than that keep-out zone
can never exceed the selected safety criterion.
5.4 Safety LCOLA screening timespan
Collision between the launch vehicle elements, including the injected spacecraft, and inhabitable systems
shall be assessed considering the dispersion of the trajectories assessed over the LCOLA approving agent’s
mandated timespan after launch.
NOTE The selection of launch time intervals for a space mission is dependent upon many factors, which include
the predicted safe passage for all objects of the launch vehicle and its deployed cargo and any released debris. This safe
passage ideally spans from launch up to the point when tracking, orbit determination, on-orbit conjunction screening, and
operational control are established, coupled with sufficient time to plan, and execute, avoidance manoeuvres by operators
of inhabitable space stations and/or space capsules and SSA systems (i.e. from 12 h for LEO to 32 h for GEO cases).
However, the large degree of error extant in pre-launch positional predictions makes it impractical for a
space launch service provider to protect its launching elements for such a long span of time. As a result,

the ability to ‘protect’ both launching and on-orbit space objects becomes degraded to the point that the
collision risk of an identified conjunction is significantly lower than the average collision risk posed by the
background debris population.
The paradox of conjunction assessment is that as the relative motion predictions extend farther into the
future, the number of possible conjunctions increases (due to growth in the error volume size about the
nominal predictions) whereas the risk of those conjunctions resulting in a hard-body collision drops.
5.5 LCOLA analysis run timing sequence
The LCOLA analyst shall ensure that safety LCOLA products are generated on a timeline set by the LCOLA
approving agent, with results being available at least 2 h prior to the launch window open time.
NOTE LCOLA analysts have found it useful to conduct LCOLA analyses:
a) several weeks prior to the first anticipated launch opportunity to get a feel for the anticipated quantity and
density of launch closures;
b) at two and one days prior to launch day, and approximately 6 h prior to launch.
5.6 LCOLA standoff threshold
The LCOLA analyst shall ensure that identified LCOLA standoff distance thresholds and allowable launch
collision probability thresholds set by the safety LCOLA approving agent are consistent with the overall level
of positional knowledge uncertainty present in the launch vehicle, its deployed objects and on-orbit objects.
5.7 LCOLA standoff minimum threshold
The LCOLA analyst and safety LCOLA approving agent shall ensure that the safety LCOLA screening protects
all inhabitable space stations from all launch vehicle stages, jettisoned components, and deployed payloads
exceeding 150 km altitude with a minimum ellipsoid-shaped screening volume of ±200 km in-track by
±50 km cross-track and ±50 km radially (total ellipsoid dimensions of 400 km × 100 km × 100 km).
NOTE 1 This is a minimum consensus set of thresholds; spherical 200 km is more conservative and often easier to
implement for this important category of protecting human spaceflight.
NOTE 2 Instantaneous collision probability thresholds of one in one million are also commonly used to augment
the above minimum safety LCOLA threshold.
NOTE 3 Large uncertainties in these quantities can stem from many factors to include the following:
a) day-of-launch conditions, to include temperature and winds aloft;
b) engine performance and booster event sensor variations;
c) either inertial guidance or in-flight retargeting variations, or both;
d) booster trajectory modelling errors;
e) uncertainties in lift-off time;
f) the inability to perfectly determine orbits and propagate their resultant orbits for both on-orbit and post-
deployment launch vehicle elements to specific times.
For the launching objects, factors a) through e) are typically captured in guidance accuracy assessments
by the booster contractor through accumulation of error statistics via Monte Carlo methods. Factor f) is
typically addressed by covariance estimation and subsequent propagation methods.
NOTE 4 Sample safety and mission assurance LCOLA metrics, thresholds, durations, and timings for major launch
countries are provided in an anonymized form in Annex C.

6 LCOLA product
The LCOLA process shall generate LCOLA results and products that clearly identify when the mission is able
to launch and when it is not (launch window closures).
NOTE There are multiple ways to characterize and share this information, including in tabular format (e.g. launch
window holds presented in tabular format as in Table 1) or graphical products.
Table 1 — Sample safety LCOLA launch window closure table
Launch window closure entry Closest standoff Highest proba- Launch window closure exit
Closure ID # [UTC] distance [km] bility of collision [UTC]
BBBBBBBXB 12/15/2023 04:02:42.980 35,387 7.935e-7 12/15/2023 04:06:18.791
BBBBBBBXC 12/15/2023 04:20:01.345 182,472 3.019e-6 12/15/2023 04:20:57.922

Annex A
(informative)
LCOLA analysis challenges and methodologies
A.1 LCOLA overview
The LCOLA process is very efficient because this launch time dependency provides the space operator with
an easy method to avoid collisions: simply delay lift-off by seconds or minutes, and the potential collision
can be eliminated.
The trajectory resulting from a lift-off event is very dependent upon the launch time achieved. Lift-off time
determines the position of the launch pad at launch, therefore modifying the booster trajectory starting point
in inertial space as shown in Figure A.1. Also, there is typically a one-to-one relationship between launch
time delay and progress of the rocket along its intended path. The resulting mesh of launch trajectories
represents a ‘grid’ that can be interpolated upon to determine if space objects penetrate this mesh at any
point throughout the launch window at a mission elapsed time when the launching object(s) are present at
that part of the mesh. But in the digitized LCOLA approach introduced in A.5.2, such interpolation does not
occur, meaning that potential collision risks can be missed.
Figure A.1 — Mesh of launch trajectories extending into the GEO regime

A.2 Standoff distance and launch collision probability launch windows
A.2.1 General
There are two basic launch window closure methods used to ensure safe passage for newly launched obje
...