summary - spacenav · summary ca is not an easy task: risk evaluation + recommendation. focus on...

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JACConjunction Assessment

[email protected]

François LAPORTEOperational Flight Dynamics

CNES Toulouse

SSA Operators’ WorkshopDenver, Colorado

November 3-5, 2016

SSA Operators’ Workshop - November 3-5, 2016 – Denver, Colorado 2

SUMMARY

� CA is not an easy task: Risk evaluation + Recommendation.

� Focus on the determination of the level of the risk => PoC computation + Covariance Assessment

�CNES developed JAC to help Owners/Operators.

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C D M

Space Surveillance Network

JSpOC

Precise and complete catalog

Screening – Information Messages generation

The current situation:Since 2009 JSpOC distributes CM to O/O



What is a JSpOC’s CDM:� The best available data to avoid collision in space:

� Takes benefit of the US SP catalog;� Is distributed to all O/O;

� A description of a forecasted conjunction :� TCA: Time of Closest Approach;� Orbit information of the 2 objects:

o Position / Velocity at TCA;o Covariance;o Orbit Determination characteristics;

� Information on the size of the object;� Generated with geometric criteria (Miss distance & Radial separation):

� Emergency criteria, up to 3 days before TCA:o LEO: 1 km / 200 m;o GEO: 10 km / 5 km;

� Large criteria (95% capture screening):o LEO: 50 km / 2 km (maximum value), up to 7 days before TCA;o GEO: 360 km / 12 km, up to 14 days before TCA;

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But a CDM

� is NOT a conjunction ALERT:� No need to maneuver for each CDM received:

o Emergency criteria: ~30 CM/Year/Satellite;o Large criteria: ~3000 CM/Year/Satellite.

� Need to “evaluate” each CDM to detect a HIE according to O/O criteria;

� is NOT an avoidance maneuver recommendation:� The real need is for an asset, on average:

o LEO: ~1 maneuver per Year;o GEO: ~1 maneuver every 10 Years.


I. CDM automatic acquisition and check ;

II. Analysis of incoming CDMs to detect HIE ;

III. Determination of the avoidance action.

Acquisition / Monitoring functionCovariance matrices validityRe-computation of relative geometryCloser approaches detection around TCA


An Operational Conjunction Assessment process is:

Dedicated maneuver analysis windows:• to size the avoidance maneuver;• to evaluate the effect on all other future conjunctions.

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CDM Analysis:Three issues

Analysis of incoming CDMs to detect HIEimplies to evaluate:

I. Position & Velocity of the two objects at the TCA

II. Covariance of the two objects at the TCA

III. Radius of the englobing sphere of each object at th e TCA

Detect maneuverability of objects Use of JSpOC’s LARGE CRITERIA to take into account O/O SKM

Automatic evaluation process of the radius from CM and user dataUser customization of the process


�Covariance is almost never perfectly representative of reality;

�Primary’s and/or secondary’s covariance can be:

�Too pessimistic;

�Or, too optimistic.

�PoC is very sensitive to covariance matrix C:

CDM Analysis:Covariance at the TCA: not a deterministic value

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OD 1Pos1Vit1

Cov1

OD 2Pos2

Vit2Cov2

OD 3Pos3

Vit3Cov3

P1

P2t3t2t1

time

3 Orbit Determination Updates (with Cov. Matrix), of the position at T

P3

P1P2P3

P1

P2

P3

Expected evolution in Local Orbital Frame of OD 3

Too pessimistic covariance Too optimistic covariance

CDM Analysis:Covariance at the TCA: not a deterministic value

T


CDM Analysis:Covariance at the TCA: PoC* for a PoC analysis

� Definition of the PoC* (expanded PoC):

� PoC(Kp, Ks) gives the PoC as a function of scale coefficients:

» with C = Kp Cp + Ks Cs;

» Kp for the Primary and Ks for the Secondary, are independent scale factors applied to covariance;

� PoC* is the Maximum value of PoC(Kp, Ks) with Kp and Ks in a given interval.

� PoC analysis:

� analysis of the function PoC(Kp, Ks), with Kp and Ks in a given interval;

�determination of the realistic values of Kp and Ks, knowing the OD parameters:

» Number of observations, residuals, weighted root mean squared, energy dissipation rate, …

» Evolution of the OD from updated CDM.

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CDM Analysis:Covariance at the TCA: PoC* for a PoC analysis

Covariance sensitivity analysis on PoC(Kp, Ks)

PoC scale: from 10-0 to 10-10

If Primary’s or Secondary’s covariance is optimistic the risk is under-estimated If Primary’s and

Secondary’s covariance are pessimistic the risk is over-estimated

Example of display: Kp in [0.5 ; 3.] and Ks in [0.5 ; 3.]

Kp

Ks

(1 , 1)

3.0

3.00.5

0.2


CDM Analysis:Covariance at the TCA: PoC analysis - Real example

Time tagged received CDMs:GNOSE = O/O for Primary; JSpOC for secondary

Notice = # hours before TCA

PoC always < CRITERIA = 5. 10-4

Decision to perform an avoidance maneuver because … it is a risky conjunction.

This is the default values considered at CNES

CM Analysis Main Window

Example of a dangerous conjunction identified thank s to the expanded PoC analysis.

A click here, opens the dedicated PoC analysis window.

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PoC scale: from 10-0 to 10-10

� As soon as Kp & Ks <1, PoC > CRITERIA;

� If Kp and Ks < 0.6 then PoC > 10-3

The analyst must answer :Kp < 0,9 and Ks < 0,8 : realistic ?

Let’s have a look at the evolution of Primary’s and Secondary’s covariance …

PoC Analysis Window White when cell’s PoC > 5. 10-4

Kp=0.9 Ks=0.8 => PoC = 5.4 10-4

Kp

Ks

(1 , 1)

Standard PoC = 3.7 10-4

Kp=0.7 Ks=0.5 => PoC = 1. 10-3

4.0

4.00.2

0.2



Primary dispersions evolution visualization


Secondary dispersions evolution visualization

The covariance of both objects are pessimistic: the PoC can realistically be greater than the CRITERIA.

⇒ This is a risky conjunction⇒ A classical analysis would have miss this risk

Kp

Ks

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CDM Analysis:Covariance at the TCA: PoC forecast - Real example

The high risk have been anticipated thanks to the P oC analysis window

CDM #6: PoC still below blue PoC* is orange.

When the orange area if bottom/left, the risk usually increases when the geometry is steady:� Because dispersions reduce.

Very useful to anticipate operational activities.


-III- CDM Analysis:Covariance Determination

Covariance determination

� It can be an output of the Orbit Determination process:

o It must include in the Orbit Determination process

o very often it is an under-estimation of the reality.

� Post analysis of the O/O ephemeris: from historical set of ephemerides

A

B

C

D

E

F

1 day extrapolation: A, B, C

2 days extrapolation: D, E

3 days extrapolation: F

Day 1

Day 2

Day 4

Day 3

⇒ provides statistics of dispersion in the RIN local orbital frame;

⇒ from this statistics we can create a “variance abacus”.

Determinated orbit Extrapolated orbit

Day 1 Day 4Day 3Day 2

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-III- CDM Analysis:Covariance Determination

Variance abacus: basic function

gives the evolution of variance (in meters) in the threedirections (Radial, In-track and Normal) of orbital localframe as a function of extrapolation duration (in days).

For a given extrapolation duration, such function (the green orthe red one in the above example) gives the evolution ofvariance (in meters) in the three directions (Radial, In-trackand Normal) of orbital local frame as a function of On orbitPosition (in degree).

⇒Takes into account the evolution of dispersions along theorbit due to the non-uniformity of distribution of sensorsproviding the measurements for the OD.

⇒This lead to a more realistic computation and reduc e the uncertainties on some orbit positions.

Variance abacus: as a function of On Orbit Position


Conclusion

Evolution of CA Process:� In the past: Miss distance & relative geometry

� Its minimum does not always represent the highest risk;� Doesn’t take into account Position uncertainties:

⇒ Not a valid criteria⇒ Need to take into account position uncertainties (i.e. covariance)

� Now: PoC � Takes into account Positions uncertainties: very good improvement;� But relies on the realism of the covariance …

⇒ Can hide dangerous situations / Can lead to undersize avoidance action ⇒ Need to take into account covariance uncertainties

� Next step: PoC + Covariance Assessment� Takes into account covariance uncertainties

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Conclusion

� CA a fully automatic process … not yet: � Can miss some risks / undersize avoidance action;� Can lead to too many avoidance events:

⇒ The final analysis must be a manual analysis.

� JAC can help O/O to perform this manual analysis

� JAC is distributed by CNES in 2 levels: � Basic (“to be aware of the situation”) for free;� Expert (“to take and validate a decision”) for an annual fee.

[email protected]

summary - spacenav · summary ca is not an easy task: risk evaluation + recommendation. focus on...

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