SPACEFLIGHT SAFETY HANDBOOK 

FOR SATELLITE OPERATORS

VERSION 1.5  AUGUST 2020

18 SPCS Processes for On-Orbit Conjunction Assessment & Collision Avoidance

18th Space Control Squadron

Combined Force Space Component Command 

Vandenberg Air Force Base, California, USA

Tel +1-805-605-3533

www.space-track.org
TABLE OF CONTENTS

Contents

Significant Revision History_____________________________________________________________________________ 1

USSPACECOM SSA Sharing Program____________________________________________________________________ 3

Program Description______________________________________________________________________________________________3

Space-Track.org____________________________________________________________________________________________________3

Conjunction Assessment Process _______________________________________________________________________ 4

On-Orbit Conjunction Assessment (CA) _________________________________________________________________________4

HAC vs HAC _____________________________________________________________________________________________________5

O/O Ephemeris vs HAC (O/O EPH vs HAC)___________________________________________________________________6

O/O Ephemeris vs O/O Ephemeris (O/O EPH vs O/O EPH) ________________________________________________6

On-Orbit Collision Avoidance (COLA)____________________________________________________________________________6

Basic Emergency CA and COLA Services ________________________________________________________________________7

Advanced CA and COLA Services_________________________________________________________________________________7

Screening Schedule _______________________________________________________________________________________________8

Screening Volumes ________________________________________________________________________________________________9

Reporting Criteria _______________________________________________________________________________________________ 10

Notifications _____________________________________________________________________________________________________ 11

Basic Emergency CA Notifications _____________________________________________________________________________ 12

Advanced CA Notifications _____________________________________________________________________________________ 12

Launch Conjunction Assessment_______________________________________________________________________________ 13

Early Orbit Conjunction Assessment __________________________________________________________________________ 13

End-of-Life/Disposal Support__________________________________________________________________________________ 14

Deorbit Support _________________________________________________________________________________________________ 14

Tasking___________________________________________________________________________________________________________ 14

Ephemeris Formats _____________________________________________________________________________________ 15

Overview_________________________________________________________________________________________________________ 15

NASA Ephemeris Format _______________________________________________________________________________________ 16

UTC Ephemeris Format_________________________________________________________________________________________ 17

Generic On-Orbit Ephemeris Format __________________________________________________________________________ 18
TABLE OF CONTENTS

Modified ITC Ephemeris Format_______________________________________________________________________________ 19

Orbital Ephemeris Message (OEM) Format ___________________________________________________________________ 20

How to Name Ephemeris Files _________________________________________________________________________ 23

Overview_________________________________________________________________________________________________________ 23

File Name Examples ____________________________________________________________________________________________ 24

How to Submit Ephemeris Files _______________________________________________________________________ 25

Overview_________________________________________________________________________________________________________ 25

Submitting EphEmeris Through Space-Track.org_____________________________________________________________ 26

Manual Upload of Ephemeris Files_____________________________________________________________________________ 26

API Upload of Ephemeris Files _________________________________________________________________________________ 27

Email _____________________________________________________________________________________________________________ 27

Guidance for High-Interest Events _____________________________________________________________________________ 27

Maneuver Notification Format_________________________________________________________________________ 28

Overview_________________________________________________________________________________________________________ 28

Format ___________________________________________________________________________________________________________ 28

How to Name Maneuver Notifications ________________________________________________________________ 33

Overview_________________________________________________________________________________________________________ 33

Examples_________________________________________________________________________________________________________ 33

How to Submit Maneuver Notifications _______________________________________________________________ 34

Overview_________________________________________________________________________________________________________ 34

Submitting MANEUVERS Notifications Through Space-Track.org___________________________________________ 35

Manual Upload of Maneuver Notifications ____________________________________________________________________ 35

API Upload of Maneuver Notifications_________________________________________________________________________ 36

Email _____________________________________________________________________________________________________________ 36

Contact Us _______________________________________________________________________________________________ 37

18 SPCS Conjunction Assessment Team _______________________________________________________________________ 37

18 SPCS SSA Sharing Team _____________________________________________________________________________________ 37

Space-Track.org Administration Team_________________________________________________________________________ 37

Annex A: 18 SPCS Probability of Collision ____________________________________________________________ 38

How 18 SPCS Calculates Probability of Collision _____________________________________________________________ 38
TABLE OF CONTENTS

Object Size _______________________________________________________________________________________________________ 39

Position and Velocity Vectors___________________________________________________________________________________ 40

Covariance and Error Ellipsoids _______________________________________________________________________________ 41

Assumptions_____________________________________________________________________________________________________ 42

Computation_____________________________________________________________________________________________________ 43

Notes _____________________________________________________________________________________________________________ 44

Bibliography _____________________________________________________________________________________________________ 44

Annex B: Frequently Asked Questions ________________________________________________________________ 46

1. How and when do I sign up for conjunction assessment (CA) alerts? ____________________________________ 46

2. Do I need to submit an Orbital Data Request for CA services? ____________________________________________ 46

3. Is there a charge for 18 SPCS spaceflight safety services? _________________________________________________ 46

4. My satellite has a predicted conjunction with another active satellite, how can I contact the other 

operator? ________________________________________________________________________________________________________ 46

5. I haven't received any conjunction data in a while, is something wrong? ________________________________ 46

6. Why is another group receiving conjunction data for our co-owned satellite when my group has not 

received any? ____________________________________________________________________________________________________ 46

7. How can I submit predicted ephemeris for screening? What about predicted maneuver information?

____________________________________________________________________________________________________________________ 46

8. I need to have larger than normal screenings for my satellite. What can 18 SPCS do? _________________ 47

9. Can I change the contacts in my Space-Track.org organization/CDM account?__________________________ 47

10. What does it mean when an object in the CDM has a number in the 80000’s? _________________________ 47

11. What does it mean when an object in the cDM has a number in the 799500000’s? ___________________ 47

12. Can the 18 SPCS analyze my data, make recommendations, or help me choose a plan of action? ____ 47

13. If I submit an Orbital Data Request for advanced CA services, can I pick which screening volume I 

would like?_______________________________________________________________________________________________________ 47

14. I have questions, is there someone I can talk to? _________________________________________________________ 47
TABLE OF CONTENTS
SIGNIFICANT REVISION HISTORY

Page 1

Significant Revision History

Version Description of Changes Date

1.1 1. Corrected NORAD_CAT_ID in Maneuver Notification format, pg. 25

2. Addition of contact information for Space-Track.org, pg. 32

12 Aug 16

1.2 1. Definitions of ellipsoid and covariance screenings, pg. 5-6 27 Aug 16

1.3 1. Screening schedule, pg. 4

2. Correction to Basic Reporting Criteria for LEO CDMs, pg. 4

3. Added decimal notation requirement to OEM, pg. 18

4. Added “USER_DEFINED_MAN_ID” to maneuver notification, pg. 27

5. Added additional values to “USER_DEFINED_MAN_PURPOSE,” pg. 

28

6. Changed “SOLVED” to “DETERMINED” for 

“USER_DEFINED_MAN_STATUS,” pg. 28

7. Under “How to Name Maneuver Notifications” update “File Names” 

and added “File Verification,” pg. 29

8. Updated “Submitting Maneuver Notifications” to explain that each 

organization has their own maneuver folder, pg. 31

9. Added “Updating Maneuver Notifications,” pg. 31

3 Jan 17

1.4 1. Replaced JSpOC and JFCC SPACE references with 18 SPCS, CSpOC or 

JFSCC, throughout the document as needed

2. Added SSA Sharing Program description, pg. 2

3. Added HAC vs HAC, O/O Ephemeris vs HAC, and O/O Ephemeris vs 

O/O Ephemeris descriptions, pg. 4-5

4. Added PDT/PST times to Screening Schedule, pg. 7

5. OEM state and covariance components must be in decimal notation, 

pg. 21

6. USER_DEFINED_MAN_ID added to maneuver notification, pg. 30

7. Added additional options to USER_DEFINED_MAN_PURPOSE, pg. 30

8. Added Annex A: How 18 SPCS Calculates Probability of Collision

Feb 19

1.5 1. Replaced all references to USSTRATCOM and JFSCC with 

USSPACECOM and CFSCC as needed

2. Replaced references to Orbital Safety Crew with Conjunction 

Assessment Team as needed

3. Changed ‘all active satellites’ to ‘all resident space objects’, pg. 4

4. Updated Conjunction Assessment Screening Process in Figure 1, 

pg.5

5. Added explanation of Concern List in HAC vs HAC screening 

process, pg. 5

6. Added explanation of different types of HAC screenings, pg. 6 

7. Added information on Concern List screening schedule, pg. 8

8. Updated screening schedule, pg. 8

Aug 20
SIGNIFICANT REVISION HISTORY

Page 2

9. Updated HAC screening volumes in Table 3, pg. 9

10. Updated basic reporting criteria in Table 5, pg. 10

11. Updated advanced reporting criteria in Table 6, pg. 10

12. Added information on 799500000 satellite number range, pg. 13

13. Removed ‘fewer than two sensors’ tasking criteria, pg. 14

14. Clarified requirements for covariance reference frames, pg. 15

15. Clarified requirements for covariance reference frames in OEM, 

pg. 21

16. Changed ‘CatalogNumber’ to ‘SatelliteNumber’ in ephemeris filenaming guidance, pg. 23

17. Added 9-digit temporary satellite numbers to Ephemeris File 

Name Convention in Table 8, pg. 23

18. Clarified that deviations from CCSDS OPM standard are indicated 

by ‘User Defined’ fields, pg. 28

19. Changed OBJECT_ID in Maneuver Notification OPM to not 

obligatory, pg. 29

20. Changed USER_DEFINED_NORAD_CAT_ID in Maneuver 

Notification OPM to obligatory, pg. 29

21. Added 9-digit option to USER_DEFINED_NORAD_CAT_ID in 

Maneuver Notification OPM, pg. 29

22. Added Frequently Asked Questions, Annex B 
USSPACECOM SSA SHARING PROGRAM

Page 3

USSPACECOM SSA Sharing Program

PROGRAM DESCRIPTION

The Unites States Space Command (USSPACECOM), as the U.S. government entity responsible for space 

domain awareness (SDA) and space situational awareness (SSA), is committed to promoting a safe, stable, 

sustainable and secure space environment through the sharing of SSA information. In 2009, the United 

States Strategic Command (USSTRATCOM) initiated the SSA Sharing Program to provide no-cost basic, 

emergency, and advanced SSA data and services that cover the full scope of support needed through the 

lifetime of a satellite from pre-launch preparation to end-of-life disposal. In 2019, the program was handed 

over to USSPACECOM.

Services include Launch Conjunction Assessment (LCA), Launch and Early Orbit Phase (LEOP) support, OnOrbit Conjunction Assessment (CA), Collision Avoidance (COLA), anomaly support, end-of-life/disposal 

support, de-orbit support, and re-entry assessment. These services are available to all operators of active 

spacecraft at the basic emergency level, and at the advanced level for entities who have signed SSA Sharing

Agreements with USSPACECOM. Basic SSA data is disseminated to the world through the website 

www.space-track.org, which is accessible to anyone with a registered user account. USSPACECOM has 

delegated management of the SSA Sharing Program to the Combined Force Space Component Command 

(CFSCC). While CFSCC oversees the SSA Sharing program as a whole, the 18th Space Control Squadron (18 

SPCS) has been directed to execute day-to-day SSA data sharing and support, and act as the direct interface 

between satellite operators and the U.S. Department of Defense. The 18 SPCS performs the space 

surveillance mission for the U.S. Space Force, and provides foundational SSA for the U.S. government and 

global space partners through the SSA Sharing Program. This role was performed by the Joint Space 

Operations Center (JSpOC) until July 2016, at which time 18 SPCS was activated and assumed all space 

surveillance functions. In July 2018, the JSpOC became the Combined Space Operations Center (CSpOC).The 

18 SPCS is currently co-located with the CSpOC at Vandenberg Air Force Base, but now has the separate and 

distinct mission of providing frontline space defense, maintaining the space catalog, and providing 

foundational SSA to U.S. and globalpartners.

SPACE-TRACK.ORG

Space-Track.org (www.space-track.org) is a public website owned by USSPACECOM and CFSCC, and 

managed and populated by 18 SPCS. It is the primary method of sharing SSA information with the global 

space community at the basic, emergency, and advanced levels. All satellite operators should register for a 

user account in order to access the full scope of SSA data and services available through the SSA Sharing 

Program.
CONJUNCTION ASSESSMENT PROCESS

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Conjunction Assessment Process

ON-ORBIT CONJUNCTION ASSESSMENT (CA)

The 18 SPCS Conjunction Assessment (CA) process identifies close approaches between all resident space 

objects (RSOs) in the 18 SPCS catalog. It begins with satellite observations from the U.S. Space Surveillance 

Network (SSN), which includes a variety of sensors throughout the world that detect, track, catalog, and 

identify man-made objects orbiting Earth. SSN observations are input into the 18 SPCS mission system, 

which implements “Special Perturbations” (SP) orbit propagation theory. Orbit determination (OD) is 

performed automatically multiple times per day to determine the position and velocity of each object, 

which is then updated in the 18 SPCS High Accuracy Catalog (HAC). The 18 SPCS Conjunction Assessment 

Team uses the HAC to screen the orbital trajectories of all RSOs against all other RSOs, to include active 

satellites and non-active objects (debris, rocket bodies, dead payloads, etc). 

18 SPCS performs three types of screenings:

1. HAC vs. HAC: SP catalog data for all RSOs screened against SP catalog data for all other RSOs

2. O/O Ephemeris vs. HAC: Ephemeris provided by the satellite owner/operator (O/O) screened

against the SP catalog data for all RSOs

3. O/O Ephemeris vs. O/O Ephemeris: Ephemeris provided by the satellite O/O screened against

ephemeris provided by all other O/O’s
CONJUNCTION ASSESSMENT PROCESS

Page 5

18 SPCS performs screenings for every RSO and owner/operator (O/O) ephemeris a minimum of once 

every 24 hours in accordance with the process in the following diagram. The initial screening may begin 

with all near earth objects, all deep space objects, near earth concern lists, deep space concern lists, or 

O/O-provided ephemeris. 

Figure 1: Conjunction Assessment Screening Process

HAC vs HAC

The screening process starts with the update of orbital information for the entire catalog based on data 

from the SSN sensors and updated Dynamic Calibration Atmosphere (DCA) coefficients, which is then 

propagated 7 days for near-earth (NE) objects and 10 days for deep-space (DS) objects. These updates take 

place every 8 hours. Using this data, 18 SPCS performs an initial screening of RSOs against the rest of the 

catalog to identify conjunction candidates within screening criteria. Based on these results, 18 SPCS also 

generates Concern Lists to use in follow-on screenings. Concern Lists include all candidates that produce 

conjunctions with probability of collision (Pc) ≥ e^-7 in NE, and miss distance ≤ 5km in DS. These 

candidates are reevaluated by the Conjunction Assessment Team to ensure that all of the most current 

observations are incorporated into the OD. The 18 SPCS then conducts a refinement screening to update 

the conjunction estimates of all of the candidates identified in the initial screening. If the parameters of the 

conjunction fall within criteria that identify a close approach and the close approach involves an active 

satellite, the 18 SPCS will notify the O/O of the active satellite(s) in accordance with the methods listed in 

Reporting Criteria. Depending on the quality of the OD, 18 SPCS may increase sensor tasking on the 

primary or secondary objects. HAC vs HAC screenings include: 
CONJUNCTION ASSESSMENT PROCESS

Page 6

1. NE All HAC: All NE objects vs all NE objects, based on 18 SPCS SP data (NE is defined as Period ≤ 225 

minutes)

2. DS All HAC: All DS objects vs all DS objects, based on 18 SPCS SP data (DS is defined as period ≥ 225 

minutes)

3. NE Concern List HAC: Screening of all conjunction pairs identified in NE All HAC screening with Pc ≥ 

e^-7 and the time of closest approach (TCA) ≤ 5 days. Concern List objects will be screened through 

TCA.

4. DS Concern List HAC: Screening of all conjunction pairs identified in DS All HAC screening with overall 

miss distance ≤ 5km, and the time of closest approach (TCA) ≤ 10 days. Concern List objects will be 

screened through TCA.

O/O Ephemeris vs HAC (O/O EPH vs HAC)

All O/O’s may submit predictive ephemeris to the 18 SPCS for screening against the HAC. O/Os must submit 

ephemeris as Operational or Special. An Operational file represents where the satellite is planned to go 

and may or may not include maneuvers. These files typically incorporate normal station-keeps or represent 

a freefall orbit. If maneuvers are included, 18 SPCS assumes these will definitely occur; otherwise, the files 

should be submitted as Special. Operational files will be screened according to the schedule in Tables 1 and 

2. A Special file represents where the satellite is planned to go if a maneuver is being considered for a nonroutine event, such as orbit raising, relocations, or COLA. A Special file is used for planning purposes only, 

unless the maneuver becomes operational, in which case it must be resubmitted as Operational. All Special 

files will be treated as high-interest and screened as soon as possible after receipt by the 18 SPCS 

Conjunction Assessment Team. Multiple special files may be submitted for the same satellite, but only one 

operational file at a time can be submitted per satellite. Results will be provided in accordance with 

reporting criteria.

O/O Ephemeris vs O/O Ephemeris (O/O EPH vs O/O EPH)

All Operational O/O ephemeris will be screened against all other Operational O/O ephemeris on a 

scheduled basis. O/O ephemeris labeled as Special will not be screened against O/O ephemeris. Results 

will be provided in accordance with reporting criteria. Operational O/O-provided ephemeris is 

screened through its duration, meaning that 18 SPCS will retain it in the system and continually 

screen it every 8 hours until there is less than 8 hours of propagated ephemeris for NE objects or less than 

12 hours for DS objects remaining in the file. This is only applicable to O/O EPH vs. O/O EPH screenings.

ON-ORBIT COLLISION AVOIDANCE (COLA)

In response to a close approach, an O/O may consider performing collision avoidance (COLA), which is the 

process of planning and possibly executing a maneuver to mitigate the risk of collision. Based on 

information received in the CA notifications, or from other data sources, the O/O decides whether or not to 

perform collision avoidance by maneuvering their satellite. If the O/O decides to consider COLA actions, 

they may send 18 SPCS their predictive ephemeris data (in approved formats and coordinate systems) 

which 18 SPCS will then screen against the HAC and provide results within reporting criteria so that the 
CONJUNCTION ASSESSMENT PROCESS

Page 7

O/O may decide how to proceed. This exchange of data may continue until the time of closest approach 

(TCA), after which 18 SPCS will resume routine screening of the active satellite in accordance with the 

screening schedule. If they do not perform COLA, 18 SPCS will continue to monitor the conjunction and 

provide updates based on data from the SSN until the TCA has passed. 18 SPCS cannot provide maneuver 

recommendations.

BASIC EMERGENCY CA AND COLA SERVICES

18 SPCS performs CA and provides COLA support for all satellite operators of active spacecraft as a basic 

emergency service to ensure spaceflight safety. All operators are eligible for this basic emergency service, 

which includes the following using basic screening volumes and reporting criteria:

1. HAC vs HAC screenings

2. Operational O/O Ephemeris vs HAC screenings

3. Operational O/O Ephemeris vs Operational O/O Ephemeris screenings

4. Special O/O Ephemeris vs HAC screenings

4.1 O/O Ephemeris versus secondary object, or

4.2 O/O Ephemeris versus full catalog

5. HAC ‘1-versus-1’ requests: screening one primary object against one secondary using HAC data

ADVANCED CA AND COLA SERVICES

O/Os who have signed an SSA Sharing Agreement with USSPACECOM may request Advanced CA and COLA 

through the Orbital Data Request (ODR) process. (Visit https://www.space-track.org/documentation#/odr

for more information.) Advanced CA and COLA support includes the following using advanced screening 

volumes and reporting criteria. Advanced screening volumes will be assigned based on the orbital regime 

of the satellite, in accordance with Table 3.

1. HAC vs HAC screenings

2. Operational O/O Ephemeris vs HAC screenings

3. Operational O/O Ephemeris vs Operational O/O Ephemeris screenings

4. Special O/O Ephemeris vs HAC screenings

4.1 Ephemeris versus secondary object, or

4.2 Ephemeris versus full catalog

5. HAC ‘1-versus-1’ requests: screening one primary object against one secondary using HAC data

6. Access to propagated SP ephemeris (without covariance) for all RSOs in the unclassified space catalog

7. Access to SP state vectors (without covariance) for all RSOs in the unclassified catalog
CONJUNCTION ASSESSMENT PROCESS

Page 8

SCREENING SCHEDULE

18 SPCS performs HAC vs. HAC screenings for all NE and DS objects once every 24 hours, followed by 

Concern List screenings for each regime every 8 hours. (i.e., one full HAC vs HAC screening which includes a 

Concern List screening, followed by two Concern List screenings, 8 hours apart, every 24 hours.) For O/O 

Ephemeris screenings, the ephemeris file name dictates how 18 SPCS will process files. If the O/O 

designates it as Special, the 18 SPCS will process it as high-interest and screen it as soon as possible. If it is 

designated as Operational, the 18 SPCS will screen it according to the schedule in Table 1. Results for

Special ephemeris are provided within 4 hours of receiving the ephemeris, and Operational results are 

provided within 8 to 12 hours of the scheduled screening times, depending on when it is submitted by the 

O/O.

The 18 SPCS screening schedule shifts one hour when the state of California (United States) transitions 

between Pacific Standard Time (PST) and Pacific Daylight (PDT) time, which generally occurs in March and 

November each year. Both schedules are provided below:

Table 1: PST Screening Schedule (24-hour cycle)

Table 2: PDT Screening Schedule (24-hour cycle)
CONJUNCTION ASSESSMENT PROCESS

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SCREENING VOLUMES

The following tables define the screening volumes and durations 18 SPCS uses for basic CA, advanced CA, 

and ephemeris screenings. 18 SPCS conducts two types of screenings: ellipsoid and covariance. Both 

screenings use covariance-based “uncertainty volumes” around the satellites and the physical size of the 

satellites themselves (referred to as “exclusion volumes”). The same basic mathematical approach is used 

for both ellipsoid and covariance options, although probability of collision (Pc) is only computed for the

covariance option. For the ellipsoid screening option, the uncertainty volume is static throughout the 

screening process, whereas covariance screenings employ a joint uncertainty volume that is based on timevarying covariance values for the primary and secondary objects at the time of closest approach. Please 

reference Annex A for more information on how 18 SPCS calculates probability of collision. 

Table 3: HAC Screening Volumes

Screening Orbit Regime Definition Propagation Radial In-Track Cross-Track

Deep Space

(GEO/HEO/MEO)

Ellipsoid

1300 min < Period < 1800 min 

Eccentricity < 0.25 & Inclination < 

35o

10 days 10 km 10 km 10 km

LEO 4

Covariance

1200 km < Perigee ≤ 2000 km 

Eccentricity < 0.25 5 days 0.4 km 2 km 2 km

LEO 3

Covariance

750 km < Perigee ≤ 1200 km 

Eccentricity < 0.25 5 days 0.4 km 12 km 12 km

LEO 2

Covariance

500 km < Perigee ≤ 750 km 

Eccentricity < 0.25 5 days 0.4 km 25 km 25 km

LEO 1

Covariance

Perigee ≤ 500 km 

Eccentricity < 0.25 5 days 0.4 km 44 km 51 km

Table 4: O/O Ephemeris Screening Volumes

Screening Orbit Regime Definition Propagation Radial In-Track Cross-Track

Deep Space

Ellipsoid Period > 225min 10 days 20 km 20 km 20 km

Near Earth

Covariance

Period < 225min 7 days 2 km 25 km 25 km
CONJUNCTION ASSESSMENT PROCESS

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REPORTING CRITERIA

The following tables define the reporting criteria used for basic and advanced CA and COLA. The advanced 

service provides more data further in advance of TCA and may be requested through the Orbital Data 

Request (ODR) process. These tables apply to both High Accuracy Catalog (HAC) and ephemeris screenings. 

“Deep space” includes Geosynchronous Earth Orbit (GEO), Highly Elliptical Orbit (HEO) and Medium Earth 

Orbit (MEO). “Near earth” includes all Low Earth Orbit (LEO) regimes. 

Table 5: Basic Reporting Criteria

Space-Track Criteria Emergency Criteria Emergency

Phone Call Criteria

Notification 

Method

Conjunction Data Message 

(CDM)

Conjunction Data Message 

(CDM) & Close Approach 

Notification (CAN) email

CDM, CAN email &

phone call

Deep Space

HAC

TCA ≤ 10 days and Overall 

miss ≤ 5km

TCA ≤ 3 days and Overall 

miss ≤ 5km

TCA ≤ 3 days and Overall 

miss ≤ 500m

Deep Space

O/O Ephemeris

TCA ≤ 10 days and Overall 

miss ≤ 5km

TCA ≤ 3 days and Overall 

miss ≤ 5km N/A

Near Earth

HAC

TCA ≤ 3 days and

Overall miss ≤ 1km

TCA ≤ 3 days &

Overall miss ≤ 1km &

Probability of Collision ≥ e^‐4

TCA ≤ 3 days and 

Overall miss ≤ 1km and

Probability of Collision ≥ e^‐2

Near Earth

O/O Ephemeris

TCA ≤ 3 days and

Overall miss ≤ 1km

TCA ≤ 3 days &

Overall miss ≤ 1km &

Probability of Collision ≥ e^‐4

N/A

Table 6: Advanced Reporting Criteria

Space-Track Criteria Emergency Criteria Emergency

Phone Call Criteria

Notification 

Method

Conjunction Data Message 

(CDM)

Conjunction Data Message 

(CDM) & Close Approach 

Notification (CAN) email

CDM, CAN email &

phone call

Deep Space

HAC

TCA ≤ 10 days & all results 

w/in 5km x 5km x 5km

TCA ≤ 3 days & 

Overall miss ≤ 5km

TCA ≤ 3 days & 

Overall miss ≤ 500 m

Deep Space

O/O Ephemeris

TCA ≤ 10 days & all results 

w/in 20km x 20km x 20km

TCA ≤ 3 days & 

Overall miss ≤ 5km N/A

Near Earth

(LEO 1-4)

HAC

TCA ≤ 5 days &

Probability of Collision ≥ e^‐7

w/in HAC screening volumes

TCA ≤ 3 days &

Overall miss ≤ 1km &

Probability of Collision ≥ e^‐4

TCA ≤ 3 days &

Overall miss ≤ 1km &

Probability of Collision ≥ e^‐2

Near Earth

(LEO 1-4)

O/O Ephemeris

TCA ≤ 7 days & all results 

w/in 2km x 25km x 25km

TCA ≤ 3 days and

Overall miss ≤ 1km &

Probability of Collision ≥ e^‐4

N/A
CONJUNCTION ASSESSMENT PROCESS

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NOTIFICATIONS

O/Os will receive a variety of email notifications depending on the screening performed and results 

produced. All of these emails are generated and transmitted by the Space-Track.org website based on files 

that the 18 SPCS Conjunction Assessment Team uploaded to the website. The messages are:

• Close Approach Notification (CAN) email: sent for every prediction that meets emergency reportable 

criteria. Includes primary object, secondary object, miss distances, TCA, and for near earth events, 

probability of collision (Pc). Only sent to email addresses that the O/O has designated to receive CANs. 

(Controlled by each organization’s Primary Representative or 18 SPCS through the Operator Panel on 

Space-Track.org. Contact 18SPCS.doo.customerservice@us.af.mil for assistance.)

• CDM email: notifies O/Os that new CDMs are available on Space-Track for their organization. All users 

with ‘CDM Notification’ permissions receive this email. (Controlled by each organization’s Primary 

Representative or 18 SPCS through the Operator Panel on Space-Track.org.)

• Negative results email: Confirms that a specific ephemeris file has been screened and states that there 

are no results within the ephemeris screening volume. All users with ‘CDM Notification’ permissions 

receive this email.

• Expanded results email: Confirms that a specific ephemeris file has been screened, and that there are 

results that are within the ephemeris screening volumes, but not within emergency reportable criteria.

O/Os who do not have the advanced CA service may receive this notification. All users with ‘CDM 

Notification’ permissions receive this email.

There are slight differences between the notifications sent for basic and advanced services. The following 

figures show which messages should be expected for different scenarios within the CA and COLA process.
CONJUNCTION ASSESSMENT PROCESS

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BASIC EMERGENCY CA NOTIFICATIONS

Figure 2: Basic Emergency CA Notifications

ADVANCED CA NOTIFICATIONS

Figure 3: Advanced CA Notifications

18 SPCS Screening

18 SPCS Screening
CONJUNCTION ASSESSMENT PROCESS

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LAUNCH CONJUNCTION ASSESSMENT

18 SPCS also provides launch conjunction assessment (CA), which identifies potential conjunctions that 

may result in a collision between launching objects and on-orbit objects. Using O/O-provided information, 

18 SPCS screens the launch vehicle against the space catalog and identifies periods during the launch 

window which may put the rocket and payload at increased risk for collision. For more information on this 

service, reference the Launch Conjunction Assessment Handbook, which is available on Space-Track’s Help 

Page. 

EARLY ORBIT CONJUNCTION ASSESSMENT

Early orbit CA includes the screening of O/O-provided ephemeris against the catalog to facilitate the safe 

maneuvering of a newly launched object into its final orbit. 18 SPCS will screen the ephemeris using early 

orbit screening volumes and report all results in accordance with advanced reporting criteria. The O/O 

may provide more than one ephemeris file for each maneuver to allow analysis of multiple scenarios, but 

they must be labeled Special. 18 SPCS may pre-assign temporary satellite numbers in the 799500000 

range to be used in the ephemeris file name. This will allow the O/O to receive screening results through 

the Space-Track CDM database. Once the newly launched satellites are cataloged in the 5-digit range, the 

O/O can seamlessly transition to the new catalog number. To receive early orbit CA support the O/O needs 

to:

• Pre-launch:

o Submit an ODR requesting early orbit CA, specifying the preferred screening volume

o Register for a user account on Space-Track.org

o Provide the early orbit maneuver plan

o Send in pre-launch ephemeris for testing, if desired

• Post-launch:

o Provide ephemeris in an approved format and using the correct file name format for each 

screening

o Provide an updated maneuver plan, as needed

Table 7: Early Orbit Screening Volumes

Screening Orbit Regime Definition Propagation Radial In-Track Cross-Track

Deep Space

Ellipsoid

Perigee > 2000 km & 

Eccentricity > 0.25 10 days 40 km 77 km 107 km

Near Earth 

Covariance

Perigee ≤ 2000 km 

Eccentricity < 0.25 7 days 2 km 44 km 51 km
CONJUNCTION ASSESSMENT PROCESS

Page 14

END-OF-LIFE/DISPOSAL SUPPORT

If an O/O decides to move a satellite to a less-populated orbit at the end of its lifetime, 18 SPCS will assist 

the O/O by screening maneuver ephemeris and providing results. All O/O-provided ephemeris will be 

screened using standard ephemeris screening volumes, and results will be provided in accordance with 

basic or advanced reporting criteria. To receive end-of-life/disposal CA the O/O needs to:

• Submit an Orbital Data Request (ODR) if all results within advanced reporting criteria are desired 

(no ODR is required for basic reporting criteria)

• Provide the maneuver plan

• Provide ephemeris in an approved format and using the correct file name format for each 

screening

DEORBIT SUPPORT

A deorbit is the controlled reentry of a satellite into the Earth’s atmosphere. If an O/O decides to deorbit a 

satellite or rocket stage through a series of maneuvers, 18 SPCS can provide CA screenings, as well as 

coordinate with NASA to ensure the deorbiting spacecraft safely descends through the International Space 

Station’s (ISS) orbit. After the spacecraft completes its maneuvers, 18 SPCS can confirm final reentry. To 

receive this service, the O/O needs to:

• Submit an ODR if all results within advanced reporting criteria are desired (no ODR is required for 

basic reporting criteria)

• Provide the maneuver plan

• Provide ephemeris in an approved format and using the correct file name format for each 

screening

TASKING

In certain situations, more observations on an object will improve the quality of the conjunction prediction. 

To collect more observations, 18 SPCS can increase the tasking priority and revisit frequency assigned to 

sensors. 18 SPCS has embedded this in the routine screening cycle, as seen in Figure 1. If an object meets 

any of the following criteria, its tasking will be automatically increased:

• Not tracked within 72 hours, or

• Less than 20 observations in current Differential Correction (DC), or

• At least one component of covariance over 100,000 m2

For emergency reportable events, the O/O may contact 18 SPCS to confirm that both objects in the 

conjunction are tasked appropriately by emailing 18SPCS.doo.orbitalsafety@us.af.mil.
EPHEMERIS FORMATS

Page 15

Ephemeris Formats

OVERVIEW

Satellite operators may provide ephemeris in various formats, which will be converted into formats that

can be processed by 18 SPCS’ ASW (Astrodynamics Support Workstation). Using the Super Computation of 

Miss Between Orbits (COMBO) software, 18 SPCS screens O/O ephemeris against the HAC or other O/O 

ephemeris to identify potential conjunctions and generate conjunction data messages. Operators may elect 

to include covariance data in order to accommodate probability of collision calculations. 

All ephemeris formats must be in the mean equator/mean equinox (MEME) J2000.0 frame with positional 

values (x, y, z) given in kilometer units and velocity values (dX, dY, dZ) given in units of kilometers/second. 

If covariance values are provided, they must be in decimal notation and in the same reference frame as the 

positional values unless otherwise specified in the ephemeris file (see OEM format CCSDS Orbit Data 

Message (ODM) Blue Book). 

All ephemeris formats may include informational header lines prior to the data lines which provide date, 

time, and state vector (position and velocity) values. Each format differs in the number and content of 

header lines, format of the date and time values, and different precision (number of decimal places) for the 

state vector components.

18 SPCS’ conversion utility software converts all ephemeris formats to true equator/mean equinox (TEME) 

of Date for epoch J2000. The most recent time constants data file within the 18 SPCS mission system 

corrects for leap seconds. 

The NASA and UTC formats include position and velocity only. 18 SPCS accepts three formats with 

covariance:

• General On-Orbit (GOO) may include 3 x 3 (6 lower triangular matrix) position covariance data values,

• Modified ITC may include 6 x 6 (21 lower triangular matrix) position and velocity covariance data 

values

• Orbital Ephemeris Message (OEM) format, both with and without covariance data, which includes a 

capability for processing 6 x 6 (21 lower triangular matrix) position and velocity covariance data 

values. Visit the Consultative Committee for Space Data Systems (CCSDS) website for more 

information on the OEM. 
EPHEMERIS FORMATS

Page 16

NASA EPHEMERIS FORMAT

The NASA ephemeris data file format contains no descriptive header lines preceding the ephemeris data. 

Each data line contains a date formatted as YYDOY (last two digits of year and Day of Year (DOY) in place of 

month and day), a time stamp formatted as HHMMSS.SSS, and the state vector position (x, y, z) and velocity 

(dx, dy, dy) values.

Figure 4: NASA Ephemeris Format Example

18 SPCS’ conversion utility separates the entries on each ephemeris data line wherever one or more space 

(i.e. “ “) characters are found. For each data line this processing results in a combined date/time field (i.e. 

“yyDOYhhmmss.sss”) and each state vector component (i.e. position (x, y, z) and velocity (i.e. dx, dy, dz))

value. This ephemeris format does not include covariance values.
EPHEMERIS FORMATS

Page 17

UTC EPHEMERIS FORMAT

The UTC ephemeris data file format contains 21 descriptive and header lines preceding the ephemeris 

data. Each data line contains a date formatted as YYYY/MM/DD, a time stamp formatted as HH:MM:SS.SSS, 

and the state vector position (x, y, z) and velocity (dx, dy, dy) values.

Figure 5: UTC Ephemeris Format Example

The conversion utility ignores the 21 header and descriptive lines and separates the entries on each 

ephemeris data line wherever one or more space (i.e. “ “) characters are found. For each data line this 

processing results in a date field (i.e. “yyyy/mm/dd”), a time field (i.e. “hh:mm:ss.sss”) and each state 

vector component (i.e. position (x, y, z) and velocity (i.e. dx, dy, dz)) value.

The date field is separated into month, day, and year values wherever a forward slash (i.e. “/”) character is 

found; and the time field is separated into hour, minute, and second values wherever a colon (i.e. “:”) 

character is found. The Day of Year (DOY) value is calculated from the month and day, taking into account 

any adjustment needed for a leap year. 

This ephemeris format does not include covariance values. 
EPHEMERIS FORMATS

Page 18

GENERIC ON-ORBIT EPHEMERIS FORMAT

The Generic On-Orbit ephemeris data file format is the only format that is required to contain position 

covariance data in addition to ephemeris data. It contains four descriptive and header lines, where the 

fourth indicates the orbital reference frame of the covariance data. For each ephemeris data point, the data 

section includes one line which contains the epoch date/time and state vector, followed by one line holding 

the six values of the 3x3 lower triangular position covariance matrix.

Figure 6: Generic On-Orbit Data Example

The conversion utility makes a rudimentary check that the correct number of data lines are present. The 

number of header lines is subtracted from the number of lines in the ephemeris data file; if the result is not 

divisible by 2 (a state vector line plus a covariance data line) a warning message is output, but the 

conversion continues.

The conversion utility ignores the header and descriptive lines except for the covariance orbit frame 

reference value. (Figure 6 depicts ‘UVW’ which is the same as ‘RTN.’) It then reads the data lines in groups 

of two and separates the entries on each data line wherever one or more space (i.e. “ “) characters are 

found. For each group of two data lines this processing results in a date/time field (i.e. 

“yyyDOYhhmmss.sss”), each state vector component (i.e. position (x, y, z) and velocity (i.e. dx, dy, dz)) value 

and 6 covariance values.

The combined date/time field is separated into year, Day of Year, hour, minute, and second values. 
EPHEMERIS FORMATS

Page 19

MODIFIED ITC EPHEMERIS FORMAT

The Modified ITC ephemeris data file format is the only format that is required to contain both positional 

and velocity covariance data in addition to ephemeris data. It contains four descriptive and header lines, 

where the fourth indicates the orbital reference frame of the covariance data. For each ephemeris point, 

the data section includes one line which contains the epoch date and time and state vector, followed by 

three lines holding the twenty-one values of the 6 x 6 lower triangular position and velocity covariance 

matrix.

Figure 7: Modified ITC Data Example

The conversion utility makes a rudimentary check that the correct number of data lines are present. The 

number of header lines is subtracted from the number of lines in the ephemeris data file; if the result is not 

divisible by 2 (a state vector line plus a covariance data line) a warning message is output, but the 

conversion continues.

The preprocessing script ignores the header and descriptive lines except for the covariance orbit frame 

reference value. It then reads the data lines in groups of four and separates the entries on each data line 

wherever one or more space (i.e. “ “) characters are found. For each group of four data lines this results in a 

date/time field (i.e. “yyyDOYhhmmss.sss”), each state vector component (i.e. position (x, y, z) and velocity 

(i.e. dx, dy, dz)) value and 21 covariance values.

The combined date/time field is separated into year, Day of Year, hour, minute, and second values. 
EPHEMERIS FORMATS

Page 20

ORBITAL EPHEMERIS MESSAGE (OEM) FORMAT

The OEM ephemeris data file format contains a variable number of optional descriptive lines interspersed 

with blocks of ephemeris and optionally covariance data. Note that blocks of covariance data may contain 

more than one set of covariance data separated by EPOCH keywords.

For a complete discussion of this ephemeris data format refer to please visit the Consultative Committee 

for Space Data Systems (CCSDS) website and reference the ‘Orbit Data Messages’ Blue Book.

Figure 8: OEM (without covariance) Example
EPHEMERIS FORMATS

Page 21

Figure 9: OEM (with covariance) Example

The conversion utility removes all lines that are blank or contain the keywords (defined in the format 

document) CCSDS_OEM_VERS, CENTER_NAME, COMMENT, COV_REF_FRAME, CREATION_DATE, 

INTERPOLATION, INTERPOLATION_DEGREE, META_START, META_STOP, OBJECT_ID, OBJECT_NAME, 

ORIGINATOR, REF_FRAME, START_TIME, STOP_TIME, TIME_SYSTEM, USEABLE_START_TIME, or 

USEABLE_STOP_TIME. This reduces the data file to lines containing blocks of ephemeris data and optional 

blocks of covariance data. The covariance data blocks are delimited by lines containing the keywords 

COVARIANCE_START and COVARIANCE_STOP. The reference frame must be annotated for each covariance 
EPHEMERIS FORMATS

Page 22

data block throughout the OEM file (COV_REF_FRAME, Figure 9). If no annotation is provided, the 18 SPCS 

system defaults to the reference frame annotated in the header of the file (REF_FRAME, Figure 9). There 

can be more than one section of covariance data within a block, each delimited by an EPOCH line. The 

date/time values of the optional covariance data on the EPOCH lines are not guaranteed by the format 

standard to match any ephemeris data date/time values.

The values on each ephemeris data line are separated out wherever one or more space (i.e. “ “) characters 

are found. 

For each ephemeris data line, further processing results in date/time fields (i.e. “yyyy-mm-dd” and 

“hh:mm:ss.sss”) and each state vector component (i.e. position (x, y, z) and velocity (i.e. dx, dy, dz)) value.

For each optional block of covariance data, processing results in date/time fields (i.e. “yyyy-mm-dd” and 

“hh:mm:ss.sss”) and the 21 covariance values. Note that state vector and covariance components must 

be in decimal notation as seen in Figure 9. 

The date field is separated into month, day, and year, values wherever a dash (i.e. “-”) character is found; 

and the time field is separated into hour, minute, and second values wherever a colon (i.e. “:”) character is 

found. The Day of Year (DOY) value is calculated from the month and day, taking into account any 

adjustment needed for a leap year.

The conversion utility then matches the covariance data, if it exists, by date/time to a line of ephemeris 

data. If the date/time exactly matches, the covariance data is appended to the ephemeris state vector for 

that date/time. The covariance data is discarded if the date/time does not match for a section of ephemeris

data. Ephemeris data lines that do not have date and time matches of covariance data will have twenty-one 

zero (i.e. “0.0”) values following the state vector.
HOW TO NAME EPHEMERIS FILES

Page 23

How to Name Ephemeris Files

OVERVIEW

18 SPCS requires that satellite operators name their files following a specific format, which dictates how 

the ephemeris file will be processed. When the O/O submits their files through the website SpaceTrack.org, Space-Track will verify the file is named correctly and return error messages if it is not. The file 

name format is:

<DataType>_<SatelliteNumber#>_<CommonName>_<DayTimeGroup>_<Operational/Special>_<Met

aData>_<Classification>.<FileExtension>

Table 8: Ephemeris File Name Convention

Data Field Description Normative Value (N) or 

Example (E) Obligatory

DataType The type of data being provided. (N) MEME (unless

otherwise directed by 18 

SPCS) This entry must be 

in uppercase letters

Yes

Catalog# The 5-digit designator for the object as defined in 

USSPACECOM’s satellite catalog https://www.spacetrack.org/#/catalog

If the object is not cataloged, use:

• UCN## for near-earth objects or UCD## for deepspace objects (Substitute ## with 01, 02, 03 etc. up 

to the total number of files submitted), or

• The 9-digit number assigned to you by 18 SPCS 

(Contact 18 SPCS SSA Sharing for details.)

(E) 25544 

(E) 00900

(E) UCN01

(E) UCD02

(E) 799500000

(E) 799599999

Yes

CommonName The spacecraft name for the object. Recommend 

using the common name listed in USSPACECOM’s 

satellite catalog https://www.spacetrack.org/#/catalog

If the object is not cataloged, use the common name 

designated by the satellite operator

(E) ISS (ZARYA)

(E) CALSHERE 1

Yes

DayTimeGroup Start time of the ephemeris, using three-digit day of 

year and HH:MM in UTC, where HH is the hour in the 

24-hour system

(E) 3001224 = Oct 26, 

2020 at 12:24pm UTC

Yes

Continued on next page
HOW TO NAME EPHEMERIS FILES

Page 24

Table 8: Ephemeris File Name Convention, cont.

Data Field Description Normative Value (N) or 

Example (E) Obligatory

Operational/

Special

An Operational file represents where the satellite is 

planned to go and may or may not include maneuvers

These files typically incorporate normal stationkeeps, or represent a freefall orbit

If maneuvers are included, 18 SPCS assumes these 

will definitely occur. Otherwise, the files should be 

submitted as Special.

A Special file represents where the satellite is 

planned to go if a special maneuver is executed

A Special file is used for planning purposes only, 

unless the maneuver becomes operational, in which 

case it must be resubmitted as Operational.

All Special files will be treated as high-interest.

*Multiple Special files may be submitted for the same 

satellite, but only one Operational file at a time can be 

submitted per satellite*

(N) Operational

(N) Special

Yes

MetaData Additional information provided at the discretion of 

the satellite operator. Often used to indicate 

maneuver status, or maneuver option plan. If there 

is no additional information to provide there 

must be a double underscore between 

Operational/Special and Classification

(E) nomnvr

(E) stationkeepingmnvr

(E) burn02

(E) __

Yes

Classification The security classification of the data. Only 

unclassified files will be accepted through 

www.space-track.org 

(E) unclassified

(E) UNCLASSIFIED

(E) Unclassified 

Yes

File Extension Indicates the type of file (N) .txt (unless

otherwise directed by 18 

SPCS)

Yes

FILE NAME EXAMPLES

• MEME_25544_ISS_1651200_oper__unclassified.txt

• MEME_25544_ISS(ZARYA)_1651200_operational_nomnvr_UNCLASSIFIED.txt

• MEME_25544_ISS_1651200_special_mnvr01_Unclassified.txt

• MEME_UCN01_SPACESAT1_1651200_special_burn01_Unclassified.txt

• MEME_799500234_Sat1_1651200_special_separation_unclassified.txt
HOW TO SUBMIT EPHEMERIS FILES

Page 25

How to Submit Ephemeris Files

OVERVIEW

Ephemeris files may be submitted in three ways: 

1. Through the website Space-Track.org (preferred):

1.1 Manually, or

1.2 Using Application Programming Interface (API)

2. By email to the 18 SPCS Orbital Safety Crew at 18SPCS.doo.orbitalsafety@us.af.mil. This method should 

only be used if the satellite operator cannot access Space-Track.

Space-Track.org is the most efficient and secure method of submitting ephemeris files on a recurring basis. 

To set up access, contact the 18 SPCS SSA Sharing Team at 18SPCS.doo.customerservice@us.af.mil or +1-

805-605-5219. The SSA Sharing Team will request the following information:

1. The title of your CDM organization (the organization name under which you access CDMs on the 

Operator Panel)

2. The Space-Track user names of the personnel who will upload ephemeris for your satellite 

constellation

The SSA Sharing Team will create a permissions-based ephemeris folder for your organization that will 

only be accessed by 18 SPCS and the personnel you authorize. You will have upload, download, and delete 

permissions to the folder, which you will access through the Files Panel:

Figure 10: Files Panel on Space-Track.org
HOW TO SUBMIT EPHEMERIS FILES

Page 26

SUBMITTING EPHEMERIS THROUGH SPACE-TRACK.ORG

Once you have an ephemeris folder on the Files Panel, you will also have access to the ‘Download’ and 

‘Upload’ tabs. On the ‘Upload’ tab you can upload a variety of files to the folders you have permissions to. 

All of these folders will display in the ‘Destination’ dropdown menu:

Figure 11: Files Panel Upload Tab - Ephemeris

MANUAL UPLOAD OF EPHEMERIS FILES

1. Log in to Space-Track.org, click on ‘Files’ at the top of the page, and then click on the ‘Upload’ tab

2. Click ‘Choose Files’ and select your ephemeris files

2.1 Maximum number of files in a single upload is 100

2.2 Maximum total size per upload is 60 MB

2.3 Files must be named in accordance with guidelines in ‘How to Name Ephemeris Files’

3. Using the ‘Destination’ dropdown menu, select the ‘/Ephemeris/’ folder for your organization 

4. Click ‘Upload’ to upload the files

5. After a successful upload, the files will display in ‘My Uploads (Last 30 Days)’ Table
HOW TO SUBMIT EPHEMERIS FILES

Page 27

6. The 18 SPCS Orbital Safety Crew will receive a notification that you have uploaded new ephemeris files 

for screening. However, if the ephemeris is Special or in response to a high-interest event, please 

phone or email the Conjunction Assessment Team to notify them that you need an immediate 

response: +1-805-605-3533 or 18SPCS.doo.orbitalsafety@us.af.mil.

API UPLOAD OF EPHEMERIS FILES

Please visit Space-Track.org’s ‘API’ and ‘How To’ pages for the most up-to-date guidance (including usage 

limitations) on API capabilities: https://www.space-track.org/documentation#/api and 

https://www.space-track.org/documentation#/howto. Similar to manual uploads:

1. Files must be named in accordance with guidelines in ‘How to Name Ephemeris Files’

2. After a successful upload, the files will display in ‘My Uploads (Last 30 Days)’ Table

3. The 18 SPCS Orbital Safety Crew will receive a notification that you have uploaded new ephemeris files 

for screening. However, if the ephemeris is Special or in response to a high-interest event, please 

phone or email the Conjunction Assessment Team to notify them that you need an immediate 

response: +1-805-605-3533 or 18SPCS.doo.orbitalsafety@us.af.mil.

EMAIL

If your organization does not have an ephemeris folder on the Files Panel, you may send your ephemeris by 

email to the 18 SPCS Conjunction Assessment Team at 18SPCS.doo.orbitalsafety@us.af.mil. Please:

1. Name files in accordance with guidelines in ‘How to Name Ephemeris Files’

2. Send only .txt files - .zip files cannot be accepted

GUIDANCE FOR HIGH-INTEREST EVENTS

After you submit your ephemeris, please send an email or phone the 18 SPCS Conjunction Assessment 

Team (18SPCS.doo.orbitalsafety@us.af.mil or +1-805-605-3533) to provide the following information 

regarding the high-interest request:

1. The conjunction event that you are concerned about (provide secondary object, CDM ID number, 

and/or time of closest approach)

2. The object(s) you need your ephemeris screened against (for example, one object, a short list of 

objects, or the whole catalog)

3. Reference your approved Orbital Data Request, if applicable

4. Confirmation that you do or do not have an SSA Sharing Agreement with USSPACECOM (For more 

information on SSA Sharing Agreements, please visit https://www.spacetrack.org/documentation#/odr) 
MANEUVER NOTIFICATION FORMAT

Page 28

Maneuver Notification Format

OVERVIEW

18 SPCS respectfully requests that satellite operators notify 18 SPCS of planned, confirmed, and cancelled 

maneuvers. This allows 18 SPCS to maintain a more accurate catalog, anticipate spaceflight safety risks, and 

provide more relevant conjunction notifications. 18 SPCS has chosen the Orbital Parameter Message (OPM) 

as the most appropriate method of exchanging maneuver information. However, to make it applicable to 18 

SPCS operations and reporting procedures, 18 SPCS has made modifications to the message, which are 

detailed in the following Maneuver Notification Format. Guidelines that add to or deviate from the 

direction provided in the CCSDS Orbit Data Message Blue Book are indicated by “USER_DEFINED” in the 

keyword. To access the Blue Book visit the CCSDS website. 

FORMAT

The OPM shall be represented as a combination of the following: (1) a header, (2) metadata (data about 

data), (3) data, and (4) optional comments. Space-Track only accepts KVN (Key/Value Notation).

The following tables specify for each section of the OPM: (1) the keyword to be used, (2) a short description 

of the item, (3) examples of allowed values or the units of the time, when applicable, and (4) whether the 

item is obligatory or optional.

Table 9: OPM Header

Keyword Description Example Obligatory

CCSDS_OPM_VERS Format version in the form of 

‘x.y’, where ‘y’ is incremented 

for corrections and minor 

changes, and ‘x’ is 

incremented for major 

changes.

2.0 Yes

COMMENT Reference ODM Blue Book 

6.7

This is a comment No

CREATION_DATE File creation date/time in 

UTC

2015-07-

04T12:00:00

Yes

ORIGINATOR Creating agency or operator. 

Must be the same title as the 

CDM account on SpaceTrack.org

CNES, JAXA, ESA, 

IRIDIUM

Yes

USER_DEFINED_RELEASABILITY Acceptable values: PUBLIC, 

PRIVATE

PUBLIC: Operator agrees to 

share file with all other 

satellite operators who have 

access to Space-Track’s 

Operator Panel

PUBLIC No. If not 

provided, 

will default 

to PRIVATE
MANEUVER NOTIFICATION FORMAT

Page 29

PRIVATE: Operators agrees 

to share file with 18 SPCS 

only

USER_DEFINED_CLASSIFICATION Acceptable value: 

UNCLASSIFIED

18 SPCS will only accept 

unclassified files.

UNCLASSIFIED Yes

TABLE 10: OPM METADATA

Keyword Description Example Obligatory

COMMENT Reference Orbital Data Message Blue 

Book 6.7

This is a comment No

OBJECT_NAME Spacecraft name for which the orbit 

state or maneuver information is 

provided. Recommend using 

common names in the Satellite 

Catalog on www.space-track.org

ISS (ZARYA) Yes

OBJECT_ID Object identifier of the object for 

which the orbit state is provided. 

Use international designator as 

published on Space-Track.org using 

format YYYY-NNNP{PP}, where:

YYYY = Year of launch. 

NNN = Three digit serial number of 

launch in year YYYY (with leading 

zeros).

P{PP} = At least one capital letter for 

the identification of the part brought 

into space by the launch.

1998-067A No

USER_DEFINED_NORAD_CAT_ID • 5-digit satellite catalog number 

assigned by USSTRATCOM, or

• 9-digit satellite number assigned 

by 18 SPCS

25544

799500234

Yes

CENTER_NAME Origin of reference frame Earth Yes

REF_FRAME Name of reference frame in which 

the state vector and optional 

Keplerian element data are given

MEME2000

International Terrestrial 

Reference Frame (ITRF)

ICRF

TOD

TDR

Yes, if state 

vector 

included

REF_FRAME_EPOCH Epoch of reference frame, if not 

intrinsic to the definition of the 

reference frame.

2001-11-06T11:17:33 No

TIME_SYSTEM Time system used for state vector, 

maneuver, and covariance data. 

Must be UTC.

UTC Yes
MANEUVER NOTIFICATION FORMAT

Page 30

TABLE 11: OPM DATA – STATE VECTOR COMPONENTS IN SPECIFIED COORDINATE SYSTEM

Keyword Description Units Obligatory

COMMENT Reference Orbital Data Message Blue Book 6.7 N/A No

EPOCH Epoch of state vector in UTC N/A No

X Position vector X-component km No

Y Position vector Y-component km No

Z Position vector Z-component km No

X_DOT Velocity vector X-component km/s No

Y_DOT Velocity vector Y-component km/s No

Z_DOT Velocity vector Z-component km/s No

TABLE 12: OPM DATA – Osculating Keplerian Elements in Specified Reference Frame

Keyword Description Units Obligatory

COMMENT Reference Orbital Data Message Blue Book 6.7 n/a No

SEMI_MAJOR_AXIS Semi-major axis km No

ECCENTRICITY Eccentricity n/a No

INCLINATION Inclination deg No

RA_OF_ASC_NODE Right ascension of the ascending node deg No

ARG_OF_PERICENTER Argument of pericenter deg No

MEAN_ANOMALY Mean anomaly deg No

GM Gravitational coefficient (Gravitational Constant x Central 

Mass)

km**3/s**2 No

TABLE 13: OPM DATA – SPACECRAFT PARAMETERS

Keyword Description Units Obligatory

COMMENT Reference Orbital Data Message Blue Book 6.7 n/a No

MASS Spacecraft mass kg No

SOLAR_RAD_AREA Solar Radiation Pressure Area (AR) m**2 No

SOLAR_RAD_COEFF Solar Radiation Pressure Coefficient (CR) n/a No

DRAG_AREA Drag Area (AD) m**2 No

DRAG_COEFF Drag Coefficient (CD) n/a No

TABLE 14: OPM DATA – POSITION/VELOCITY COVARIANCE MATRIX

Keyword Description Units Obligatory

COMMENT Reference Orbital Data Message Blue Book 6.7 n/a No

COV_REF_FRAME Coordinate system for covariance matrix n/a No

CX_X Covariance matrix [1,1] km**2 No

CY_X Covariance matrix [2,1] km**2 No

CY_Y Covariance matrix [2,2] km**2 No

CZ_X Covariance matrix [3,1] km**2 No

CZ_Y Covariance matrix [3,2] km**2 No

CZ_Z Covariance matrix [3,3] km**2 No
MANEUVER NOTIFICATION FORMAT

Page 31

CX_DOT_X Covariance matrix [4,1] km**2/s No

CX_DOT_Y Covariance matrix [4,2] km**2/s No

CX_DOT_Z Covariance matrix [4,3] km**2/s No

CX_DOT_X_DOT Covariance matrix [4,4] km**2/s**2 No

CY_DOT_X Covariance matrix [5,1] km**2/s No

CY_DOT_Y Covariance matrix [5,2] km**2/s No

CY_DOT_Z Covariance matrix [5,3] km**2/s No

CY_DOT_X_DOT Covariance matrix [5,4] km**2/s**2 No

CY_DOT_Y_DOT Covariance matrix [5,5] km**2/s**2 No

CZ_DOT_X Covariance matrix [6,1] km**2/s No

CZ_DOT_Y Covariance matrix [6,2] km**2/s No

CZ_DOT_Z Covariance matrix [6,3] km**2/s No

CZ_DOT_X_DOT Covariance matrix [6,4] km**2/s**2 No

CZ_DOT_Y_DOT Covariance matrix [6,5] km**2/s**2 No

CZ_DOT_Z_DOT Covariance matrix [6,6] km**2/s**2 No

TABLE 15: OPM DATA – MANEUVER PARAMETERS

Keyword Description Units Obligatory

COMMENT Reference ODM Blue Book 6.7 n/a No

MAN_EPOCH_IGNITION Epoch of ignition. Reference ODM Blue Book 6.5.9 UTC Yes

MAN_DURATION Maneuver duration (If = 0, impulsive maneuver) s Yes

MAN_DELTA_MASS Mass change during maneuver (value is < 0) kg No

MAN_REF_FRAME Coordinate system for velocity increment vector (value 

must be selected from OPM Blue Book annex A)

n/a Yes

MAN_DV_1 1st component of the velocity increment km/s Yes

MAN_DV_2 2nd component of the velocity increment km/s Yes

MAN_DV_3 3rd component of the velocity increment km/s Yes

USER_DEFINED_MAN_ID A unique ID applied to the specific maneuver by the 

operator. Unique IDs may be 1-30 alphanumeric 

characters, dashes, or underscores

n/a No

USER_DEFINED_DV_

OVERALL

If this field is not populated, Space-Track will calculate it 

based on component DVs

km/s No

USER_DEFINED_EPHEM Name of the corresponding ephemeris file, if the operator 

has submitted ephemeris to 18 SPCS for screening

n/a No

USER_DEFINED_MAN_

PURPOSE

Specifies the purpose of the maneuver. Acceptable values: 

AEROBRAKE (Aerobraking)

ATT_ADJUST (Attitude adjust)

COLA (Collision avoidance)

DEORBIT (Deorbit)

DISPOSAL (Disposal)

FLYBY_TARG (Flyby targeting)

LEOP (Launch & Early Orbit)

MNVR_CLEANUP (Maneuver cleanup)

MASS_ADJUST (Mass adjust)

TRIM (Orbit trim) 

n/a Yes
MANEUVER NOTIFICATION FORMAT

Page 32

OTHER (Other)

PER_RED (Period reduction)

RELOCATION (Relocation)

SCI_OBJ (Science objective)

SPIN_RATE_ADJUST (Spin rate adjust)

SK (Station-keeping)

TRAJ_CORR (Trajectory correction)

USER_DEFINED_TCA For COLA maneuvers, provide the Time of Closest 

Approach (TCA) in UTC of the corresponding conjunction 

event. Recommend using the TCA in the Conjunction Data 

Message.

UTC Yes for 

COLA 

maneuver, 

No for all 

others

USER_DEFINED_MAN_

STATUS

Acceptable values: PREDICTED, DETERMINED, 

CANCELLED.

PREDICTED = planned maneuver

DETERMINED = confirmed maneuver

CANCELLED = cancelled maneuver

n/a Yes
HOW TO NAME MANEUVER NOTIFICATIONS

Page 33

How to Name Maneuver Notifications

OVERVIEW

To simplify the process for O/Os, Space-Track will generate the file name for each maneuver notification 

using information provided within the OPM, specifically the RELEASABILITY, USER_DEFINED_NORAD_CAT_ID, 

ORIGINATOR, and CLASSIFICATION fields. In the file name, the O/O only needs to include their organization 

name (must be the same as their CDM account on Space-Track) and a unique identifier, such as a sequence 

number or timestamp, followed by the .opm extension:

<CDM Organization>_<unique identifier>_<metadata>.opm

EXAMPLES

As long as the obligatory fields are correctly included within the OPM, the operator may name their file 

anything they want as long as the first two fields are the CDM Organization and a unique identifier:

• NASA_1234_ISS.opm

• NASA_1235_25544.opm

• NASA_165120000.opm

• NASA_165120001_25544COLA.opm
HOW TO SUBMIT MANEUVER NOTIFICATIONS

Page 34

How to Submit Maneuver Notifications

OVERVIEW

Maneuver notifications may be submitted in three ways: 

1. Through the website Space-Track.org (preferred)

1.1 Manually, or

1.2 Using Application Programming Interface (API)

2. By email to the 18 SPCS Conjunction Assessment Team at 18SPCS.doo.orbitalsafety@us.af.mil. (This 

method should only be used in the event the satellite operator cannot access Space-Track.)

Space-Track.org is the most efficient and secure method of submitting maneuver notifications on a 

recurring basis. All operators who have access to the Operator Panel will automatically be given 

permissions to upload maneuver notifications for the satellites assigned to their CDM account. To set up a 

CDM account or a Maneuver folder, contact the 18 SPCS SSA Sharing Team at 

18spcs.doo.customerservice@us.af.mil or +1-805-605-5219. 

Once an active CDM account is established, the operator(s) assigned to the account will have access to the 

Operator and Files Panels:

Figure 12: Files Panel on Space-Track.org
HOW TO SUBMIT MANEUVER NOTIFICATIONS

Page 35

SUBMITTING MANEUVERS NOTIFICATIONS THROUGH SPACE-TRACK.ORG

On the Files Panel, you will have access to the ‘Download’ and ‘Upload’ tabs. On the ‘Upload’ tab you can 

upload a variety of files to the folders you have permissions to. All of these folders will display in the 

‘Destination’ dropdown menu:

Figure 13: Files Panel Upload Tab - Maneuvers

MANUAL UPLOAD OF MANEUVER NOTIFICATIONS

1. Log in to Space-Track.org, click on ‘Files’ at the top of the page, and then click on the ‘Upload’ tab

2. Click ‘Choose Files’ and select your OPM maneuver notification files

2.1 Maximum number of files in a single upload is 100

2.2 Maximum total size per upload is 60 MB

2.3 Files must be formatted in accordance with ‘Maneuver Notification Format’

3. Using the ‘Destination’ dropdown menu, select the ‘/Maneuvers/YourOrganization’ folder. (Note: To set 

up a maneuver folder, contact the 18 SPCS SSA Sharing Team at 18SPCS.doo.customerservice@us.af.mil

or +1-805-605-5219.)
HOW TO SUBMIT MANEUVER NOTIFICATIONS

Page 36

4. Click ‘Upload’ to upload the files. 

5. After a successful upload, the files will display in ‘My Uploads (Last 30 Days)’ Table

6. The 18 SPCS Orbital Safety Crew will receive a notification that you have uploaded new maneuver 

information.

API UPLOAD OF MANEUVER NOTIFICATIONS

Please visit Space-Track.org’s ‘API’ and ‘How To’ pages for the most up-to-date guidance (including usage 

limitations) on API capabilities: https://www.space-track.org/documentation#/api and 

https://www.spacetrack.org/documentation#/howto. Similar to manual uploads:

1. Files must be formatted in accordance with ‘Maneuver Notification Format’

2. After a successful upload, the files will display in ‘My Uploads (Last 30 Days)’ Table

3. The 18 SPCS Conjunction Assessment Team will receive a notification that you have provided new 

maneuver information. However, if the maneuver requires attention as a high-interest event, please 

phone or email the Conjunction Assessment Team at: +1-805-605-3533 or

18SPCS.doo.orbitalsafety@us.af.mil.

EMAIL

If your organization doesn’t have access to the Operator and Files Panels, you may send your maneuver 

notifications by email to the 18 SPCS Conjunction Assessment Team at 18SPCS.doo.orbitalsafety@us.af.mil. 

Please:

1. Format the files in accordance with ‘Maneuver Notification Format’ 

2. Send only .opm files - .zip files cannot be accepted. Or,

3. If it’s not possible to use the OPM format, maneuver notification may be provided in the body of an 

email.
CONTACT US

Page 37

Contact Us

18 SPCS CONJUNCTION ASSESSMENT TEAM

For questions on conjunction assessment and collision avoidance support, please contact the 18 SPCS

Orbital Safety Crew by email at 18SPCS.doo.orbitalsafety@us.af.mil or by phone at +1-805-605-3533. 

Examples of support include:

• Questions about screening results

• Screenings for high-interest events

• Missing CDMs

18 SPCS SSA SHARING TEAM

For questions about specialized SSA support or data products, please contact the 18 SPCS SSA Sharing 

Team by email at 18SPCS.doo.customerservice@us.af.mil or by phone at +1-805-605-5219. Examples 

include:

• Submitting an Orbital Data Request for advanced services or support

• Questions about data formats 

• Space-Track access and permissions

• Registering your satellite to receive spaceflight safety support

• Access to Space-Track.org’s for-testing-only website

SPACE-TRACK.ORG ADMINISTRATION TEAM

For questions about the Space-Track.org website, please contact admin@space-track.org. Examples include:

• API queries

• Automation

• Website performance
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 38

Annex A: 18 SPCS Probability of Collision

Figure X: Geometry of a Conjunction

HOW 18 SPCS CALCULATES PROBABILITY OF COLLISION

For a conjunction1 between two objects in earth orbit, the 18th Space Control Squadron (18 SPCS) can 

compute and report a value commonly referred to as probability of collision (Pc2

). This value can be 

used by a satellite owner/operator (O/O) as a warning and may motivate further study of the 

encounter on their part3

. 

 

1 Also referred to as a “close approach”

2 Also PoC, POC

3 When an assumption is made in the calculation of Pc, it’s made on the side of overestimating the Pc, in other 

words, being conservative. In this way O/O’s are made aware of all potential encounters. The first place where 

this conservatism appears is in what is actually calculated. What is computed is not probability of collision but 

rather the probability that two objects are less than a specified distance apart at their time of closest approach 

(TCA). This is due to several factors, 18 SPCS not knowing the exact size, shape and orientation of both objects 

at TCA being one of the major ones. So the approach is to determine the longest distance the two satellite’s 

centers of mass can be apart and still have the two satellites touch. This defines the “specified distance apart” 

that is used in the Pc calculation. Note that if the two satellites are not spheres, then a simple change in

orientation means that they may not touch and no collision would occur. So the Pc value calculated always has 

this factor of conservatism built in.
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 39

The data at the time of closest approach (TCA) required to compute Pc includes:

• The size of the primary object

• The size of the secondary object

• The inertial position and velocity vectors of the primary object

• The inertial position and velocity vectors of the secondary object

• The 3-dimensional position covariance of the primary object

• The 3-dimensional position covariance of the secondary object

If available and reliable these data items are provided in the Conjunction Data Message (CDM).

OBJECT SIZE

The Pc calculation requires an estimate of the sizes of the primary and secondary objects. Because of 

a dimensionality reduction of the problem (to be described subsequently), the combined sizes of the 

two objects ultimately will be represented as an area in a projected plane—the “conjunction plane.” 

The typical procedure is to circumscribe the primary and secondary objects each by a sphere, add the 

two sphere radii to generate a supervening sphere that can contain both circumscribing spheres, and 

project this supervening sphere into the conjunction plane as a circle.

The size of the objects4 is called AREA_PC in the CDM and given in square meters. The CDM Blue 

Book5 states:

AREA PC: The actual area of the object (m**2). The area could be known by the 

owner/operator of the satellite or defined by using a Radar Cross Section (RCS) as in the case 

of debris. If the value of the area is unknown or not available, “0.0” may be displayed. This 

parameter can be useful for calculation collision probability.

SuperCOMBO6 is used to identify conjunctions in 18 SPCS. It has three modes: standoff radius, 

ellipsoid, and covariance. Pc is computed in covariance mode. It requires that the sizes of both objects 

be input by the operator. Size is specified (in meters) by the Radius of Exclusion Volume; as stated 

 

4 The primary and secondary objects are referred to as OBJECT1 and OBJECT2 in the CDM.

5 Conjunction Data Message Recommended Standard CCSDS 508.0-B-1 BLUE BOOK, June 2013

6 Super Computation of Miss Between Orbits. SuperCOMBO is an application program in the Astrodynamics 

Support Workstation (ASW). It is based on the Air Force Space Command (AFSPC) Astrodynamics Standards

program COMBO.
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 40

above, it represents the minimum radius of a sphere that can contain the object7 8. SuperCOMBO can 

also assign specific values or use an RCS catalog to establish size. 

Caution is advised if RCS is used to establish size. For many reasons RCS may not represent the 

physical size of an object, and certainly not its shape or orientation at TCA.

POSITION AND VELOCITY VECTORS

The position and velocity vectors9 of the primary and secondary objects at TCA are given in the CDM 

in the fields X, Y, Z, X_DOT, Y_DOT, and Z_DOT. Position units are km and velocity units are km/sec. 

They are normally referenced to the International Terrestrial Reference Frame (ITRF), but other 

coordinate frames may be specified in accordance with the CDM Blue Book. The velocity vectors are 

not explicitly used in the computation of Pc. However, they are required in order to establish the point 

and time of closest approach, to perform needed coordinate transformations/rotations to prepare the 

data properly for the Pc calculation, and to determine encounter geometry at TCA.

In 18 SPCS the position and velocity of the objects at TCA are normally computed by interpolating 

between ephemeris points10. The ephemeris points are obtained by propagating forward in time the 

object’s special perturbations (SP) state vector at epoch11. The state vector at epoch is computed 

using an orbit determination method based on minimum variance differential correction (DC).

In some cases ephemeris points of an object are provided to 18 SPCS by the satellite 

owner/operator12. If so, these are used in lieu of, or in addition to, internal ephemeris.

While not explicitly input to compute Pc, the position and velocity of the secondary relative to the 

primary at TCA are included in the CDM. They are given in Radial, Transverse, and Normal (RTN) 

 

7 If an O/O reports object size to 18 SPCS in m**2 then that should be the area of a circle that completely 

“covers” the object when viewed from any angle. 18 SPCS would then compute the Radius of Exclusion Volume 

from this area as (O/O-reported-size divided by π)**0.5.

8 Pre-assigned default values for payloads and platforms (5 meters), rocket bodies and unknown objects (3 

meters) and debris (1 meter) were determined through a study of sizes of objects in the space object catalog 

and are normally used. Plans are being developed to allow O/O’s to provide a more accurate value of this 

parameter for use in CA screening in the future.

9 Together, often referred to as the state vector

10 An ephemeris is a time ordered listing of an object’s position and velocity vectors (that may include 

covariance).

11 Or general perturbations (GP) element set (ELSET) in the absence of an SP state vector

12 In J2000 mean equator mean equinox coordinates 
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 41

coordinates of the primary object13. Also provided are overall MISS_DISTANCE and RELATIVE_SPEED. 

Relative position and velocity offer a more intuitive view of the conjunction14 and tell the user if the 

secondary is moving fast or slow relative to the primary at TCA.

COVARIANCE AND ERROR ELLIPSOIDS

The covariance matrix, provided separately for the primary and secondary object (as described in 

Annex C of CCSDS 508.0-B-1), is routinely furnished in 6 x 6 form, even though only the position 

portion is needed for the Pc calculation as described in this memorandum. A covariance matrix 

characterizes the uncertainty in a satellite state vector, similar to the way that variance, the square of 

the standard deviation, is a metric for assessing the spread of student test scores about the mean. An 

example of a position covariance, using the notation of the CCSDS standard, is given as the symmetric 

matrix below. The diagonal elements represent the variance in each of the components (R, T, and N), 

and the off-diagonal terms give the covariance between the two named components (the product of 

the two components’ standard deviations and their correlation coefficient):





















=

CN R CN T CN N

CT R CT T CN T

CR R CT R CN R

Cov

_ _ _

_ _ _

_ _ _

The covariance matrix for each object is referenced to its own RTN coordinate frame. For each object 

matrix components are computed using 5-point Lagrange interpolation of the covariance in the 

ephemeris file produced by the 18 SPCS15. 

An owner/operator may also send 18 SPCS an ephemeris that includes covariance16. In these cases the 

covariance values in the CDM are interpolated from the ephemeris (with covariance) provided by the 

owner/operator.

 

13 Also called the Radial-Intrack-Crosstrack (RIC) coordinate frame or the UVW system

14 For example, at TCA the secondary relative to the primary is at “eleven o’clock high.”

15 Covariance at TCA quantifies uncertainty in the state vector at TCA. Covariance at epoch quantifies uncertainty 

at epoch. Covariance at epoch in essence maps the observed-minus-computed residuals in the observations 

used in the DC to the uncertainty in the state vector produced by the DC.

16 Owner/operators provide ephemeris with covariance in three formats: (1) the Generic On-Orbit (GOO) (2) 

Modified ITC, or Orbital Ephemeris Message (OEM)
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 42

The one-standard-deviation17 three dimensional (3D) error ellipsoid may be used to visualize the 

“size” of the position uncertainty at TCA. Diagonalization of the 3 x 3 covariance matrix yields the size, 

shape, and orientation of the error ellipsoid18. Often, because of drag, the largest uncertainty will be 

in the direction of the satellite’s motion relative to the atmosphere19. 

ASSUMPTIONS

When computing Pc at 18 SPCS several assumptions are made. These include:

• Object sizes are known or can otherwise be assigned an upper bound.

• The conjunction is “hyperkinetic, meaning that the conjunction duration is very short; this 

allows the additional simplifying assumption that the relative motion between objects is 

rectilinear throughout the encounter.

• Gaussian theory and statistics apply.

• Covariance for both objects is known and constant throughout the encounter.

• Primary and secondary errors are independent allowing “combined” covariance to be the 

simple sum of the individual covariances (in a common frame), resulting in the “joint” 

covariance.

• The covariance is not “too large.”

• The covariance is not “too small20.”

Technical papers have been written to address the computation and/or validity of Pc when one or 

more of these assumptions come into question. If at TCA (1) covariance is too small, (2) covariance is 

too large, or (3) the relative speed of the secondary relative to the primary is too small, then Pc is not 

computed in SuperCOMBO. The values for covariance “too large” and relative speed “too small” are 

user-settable.

Another concern is covariance realism: does the covariance used in the computation of Pc truly reflect 

the uncertainty in the state vectors at TCA? Because quantitative studies have shown that covariance 

is often underestimated, empirical techniques have been devised to scale or otherwise inflate the 

covariance – make it larger – to compensate for the underestimation.

 

17 1 sigma or 1σ

18 The square root of each of the eigenvalues of the covariance matrix gives the sizes of the ellipsoid axes. The 

eigenvectors provide the orientation of the ellipsoid axes relative to the object’s RTN coordinate frame.

19 A very rough estimate of the sizes of the three axes of the error ellipsoid can be obtained by taking the square 

root of each of the diagonal elements CR_R, CT_T, and CN_N.

20 This test is needed to trap zero covariance.
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 43

COMPUTATION

Computation of Pc takes place in the collision plane21. This is the plane perpendicular to the relative 

velocity vector at TCA. This reduces the mathematics from 3D to 2D, and the kinematics from dynamic 

to static. In 2D the equation used to compute Pc is: 

∫∫

+ ≤

− 



 



 = − − − 2 2 2

( ) ( ) 2

1

exp 2 | ( ) |

1 1

1/ 2

x y d

S/P

T

S/P C dxdy Det C

PoC r r r r

π

In this double integral equation:

• π = 3.141592653589793.

• C is the 2X2 projection of the combined 3X3 covariance at TCA onto the collision plane22.

• Det(C) is the determinant of C.

• d is the sum of the two object sizes23.

• “exp” is the exponential function, i.e., e to the power in parentheses with e = 

2.718281828459045.

• r = (x, y)T is any point in the collision plane such that x2 + y2 ≤ d2

.

• rS/P = (rS/P, 0)T is the position of the secondary relative to the primary along the x-axis in the 

collision plane.

• C-1 is the inverse of C.

This equation integrates the 2D Gaussian probability density function (centered on the secondary 

object) over the circle of radius d (centered on the primary object24).

 

21 Also called the encounter plane

22 The combined covariance is the sum of the covariance matrices corresponding to each object (both 

referenced to the same coordinate frame). The applicability of Gaussian statistics allows for combining two 

covariance matrices into one using term-by-term summation.

23 The sum of the radii of exclusion volumes for the two objects

24 With the primary object located at the origin of a 2D coordinate frame in the collision plane
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 44

Methodologies, coordinate transformations, assumptions, combining Gaussian distributions, 

limitations, and techniques used to facilitate computation of Pc are discussed at length in technical 

papers listed in the bibliography25. 

NOTES

18 SPCS uses error functions (ERF) for computing the double integration in the Pc equation. In 

addition 18 SPCS performs integration over a square circumscribing the circle of radius d. This square 

is aligned with the axes of the combined 2D probability density function in the collision plane. This 

simplifies the computation of Pc but gives a very slightly larger – but safe – value.

For these and other reasons agencies may compute slightly different values for Pc for the same 

conjunction. If the difference is large however further investigation is warranted26.

BIBLIOGRAPHY

Several excellent references for understanding the computation of Pc are listed at the Space Assigned 

Number Authority (SANA) registry at http://sanaregistry.org/r/cdm_cpm/cdm_cpm.html.

In addition the following references are valuable because in the 1990s Doctor Foster matured much of 

the theory and methodology used by NASA to compute Pc for Space Shuttle and International Space 

Station (ISS) safety of flight analysis. A version of the NASA algorithm for computing Pc was integrated 

into the ASW program SuperCOMBO in 2000. Certain theoretical issues were modified and expanded 

 

25 The computation of Pc is a dynamic 3D problem: the two objects are moving in 3-dimensions. However, it can 

be simplified by reducing the dynamic 3D problem to a static 2D problem. This is accomplished by performing 

calculations in the 2D collision plane. Consider the two error ellipsoids of the primary and the secondary objects 

at the instant of TCA as they “punch holes” in the collision plane that’s perpendicular to the relative velocity 

vector. The problem of computing Pc immediately collapses to a static 2D problem in the collision plane. The 

ellipsoids become ellipses in the collision plane representing the 1σ “contour lines” of the two-dimensional 

Gaussian probability density functions (pdf) of the objects. One can think of the 2D Gaussian (“bell-shaped”) 

probability density functions for the primary and the secondary as being “little hills” sitting over their respective 

satellite positions at TCA in the collision plane. In effect, the ellipse for one object is but one of many nested 

“Russian Doll” contour lines going out to infinity in the 2D collision plane for the Gaussian pdf, just as in 3D the 

error ellipsoid is but one of an infinite number of nested Russian dolls. These two “little hills” combine into one 

hill – one pdf – centered over the secondary for the actual computation of the Pc.

26 In 2000 AFSPC/A9 numerically validated the computation of Pc in SuperCOMBO.
ANNEX A: 18 SPCS PROBABILITY OF COLLISION

Page 45

by F.C. Chan in his important monograph Spacecraft Collision Probability (El Segundo, CA: Aerospace 

Press, 2008).

Foster, J.L., and Frisbee, J.H., Comparison of the Exclusion Volume and Probability Threshold Methods 

for Debris Avoidance for the STS Orbiter and International Space Station. NASA /TP-2007-214751. 

Houston, Texas: NASA Johnson Space Flight Center, May 2007 (written in October 1999).

Foster, J.L., and Wortham, M.B, ISS Debris Avoidance Maneuver Threshold Analysis. NASA /TP-2007-

214752. Houston, Texas: NASA Johnson Space Flight Center, May 2007 (written in October 1999).

Foster, J.L., The Analytic Basis for Debris Avoidance Operations for the International Space Station. 

Proceedings of the Third European Conference on Space Debris, p. 441-445, Darmstadt, Germany, 19 -

21 March 2001.
ANNEX B: FREQUENTLY ASKED QUESTIONS

Page 46

Annex B: Frequently Asked Questions

1. HOW AND WHEN DO I SIGN UP FOR CONJUNCTION ASSESSMENT (CA) ALERTS? 

First, register for a user account on Space-Track.org. Then fill out a satellite registration form and send it to 

18SPCS.DOO.CustomerService@us.af.mil. We will set up a CDM account for your organization that helps us 

route alerts to the right people. You can do this at any time, whether you have a satellite on orbit, or are

waiting for the satellite to be launched. We recommend contacting us a few weeks prior to launch.

2. DO I NEED TO SUBMIT AN ORBITAL DATA REQUEST FOR CA SERVICES?

You do not need to submit an ODR for basic CA services. ODRs are only required for advanced services.

3. IS THERE A CHARGE FOR 18 SPCS SPACEFLIGHT SAFETY SERVICES? 

All of our services are provided at no cost to the owners and operators (O/Os) of active spacecraft.

4. MY SATELLITE HAS A PREDICTED CONJUNCTION WITH ANOTHER ACTIVE SATELLITE, HOW 

CAN I CONTACT THE OTHER OPERATOR?

If the other O/O has chosen to share their contact information with other O/Os on Space-Track.org, you can 

find it on the Directory Tab of the Operator Panel. 18 SPCS highly encourages all O/Os to share their 

contact information with other O/Os. If it's not available there, you can contact 18 SPCS, but it will likely 

take extra time as we will ask to receive their permission before we give you their contact information. 

5. I HAVEN'T RECEIVED ANY CONJUNCTION DATA IN A WHILE, IS SOMETHING WRONG? 

This likely indicates that 18 SPCS has not predicted any conjunction events that meet reportable criteria. 

(Reference Tables 5 and 6.) You may contact the Conjunction Assessment Team at 

18SPCS.doo.orbitalsafety@us.af.mil to confirm. 

6. WHY IS ANOTHER GROUP RECEIVING CONJUNCTION DATA FOR OUR CO-OWNED SATELLITE 

WHEN MY GROUP HAS NOT RECEIVED ANY? 

The other group likely has advanced CA services while yours are at the basic level, so they will have access 

to more data. To confirm, contact the 18 SPCS SSA Sharing Team at 18SPCS.doo.customerservice@us.af.mil.

7. HOW CAN I SUBMIT PREDICTED EPHEMERIS FOR SCREENING? WHAT ABOUT PREDICTED 

MANEUVER INFORMATION?

Contact 18SPCS.DOO.CustomerService@us.af.mil, and we'll set up folders on Space-Track.org where you 

can upload your predicted ephemeris and maneuver files. You can specify who should have access to the 

folder. 
ANNEX B: FREQUENTLY ASKED QUESTIONS

Page 47

8. I NEED TO HAVE LARGER THAN NORMAL SCREENINGS FOR MY SATELLITE. WHAT CAN 18 

SPCS DO?

If you have a valid SSA Sharing Agreement with USSPACECOM, you may submit an Orbital Data Request 

(ODR) form to request advanced CA services. For more information, on pursuing an SSA Sharing Agreement 

contact stratcom.offutt.j51.mbx.j513@mail.mil. For more information on how to submit an ODR contact 

18SPCS.doo.ssasharing@us.af.mil. 

9. CAN I CHANGE THE CONTACTS IN MY SPACE-TRACK.ORG ORGANIZATION/CDM ACCOUNT?

Primary Representatives (PR) for the account can make the changes. If you don't have a PR or need to 

add/change one, contact us at 18SPCS.DOO.CustomerService@us.af.mil for assistance. 

10. WHAT DOES IT MEAN WHEN AN OBJECT IN THE CDM HAS A NUMBER IN THE 80000’S? 

A satellite number from 80,000-89,999 indicates that it is an "analyst object." These are on-orbit objects 

that are tracked by the U.S. Space Surveillance Network (SSN) but with insufficient fidelity for publication 

in the public satellite catalog (SATCAT).

11. WHAT DOES IT MEAN WHEN AN OBJECT IN THE CDM HAS A NUMBER IN THE 

799500000’S?

A satellite number from 799500000-799999999 is a temporary number that 18 SPCS has assigned to a 

newly launched satellite. This allows the O/O of the satellite to submit ephemeris and receive CDM results 

prior to the satellite being entered into the public catalog and assigned a 5-digit number. 

12. Can the 18 SPCS analyze my data, make recommenda�ons, or help me choose a plan of 

ac�on? 

We are legally prohibited from performing analysis or making recommendations, but we will provide you 

with the best possible data to assist in your decision making.

13. IF I SUBMIT AN ORBITAL DATA REQUEST FOR ADVANCED CA SERVICES, CAN I PICK WHICH 

SCREENING VOLUME I WOULD LIKE? 

No, you will receive the screening volume for the orbital regime that your satellite is operating in, in 

accordance with Table 3. If this changes due to orbit raising or deorbiting, 18 SPCS may adjust the 

screening volume. 

14. I HAVE QUESTIONS, IS THERE SOMEONE I CAN TALK TO?

Contact the 18 SPCS SSA Sharing Team at 18SPCS.doo.customerservice@us.af.mil or +1-805-605-5219.