Pc Dilution
In certain high-uncertainty conjunctions, nominal probability of collision (Pc) values may underrepresent the actual risk. These dilution region events arise when one or both object covariances are large enough to suppress computed Pc, despite close geometry. Satcat detects these events and reports an additional metric, Maximum Pc, computed by scaling covariance to estimate the worst-case likelihood.
Nominal Pc values may significantly understate collision risk in dilution region events. Operators should use Max Pc as a fallback metric and consider remediation methods, such as additional sensor tasking, to resolve large covariances.
What it Means
Pc Dilution describes a region in conjunction assessment where nominal Pc values decrease as covariance size increases despite geometric proximity suggesting high probability of collision risk.
- A Dilution region arises when the orbital covariance grows large relative to the nominal miss vector magnitude. In this zone, increasing covariance initially increases Pc up to a peak, then drives it downward as the probability density spreads beyond the conjunction sphere.
- The Maximum Pc construct applies scaled-down covariance (preserving aspect ratio and nominal miss distance) to compute the largest possible Pc under assumed additional tracking data. Max Pc is reached by iteratively shrinking covariance until Pc peaks.
Indicators & Symptoms
- Symptom: nominal Pc values remain low even when the miss distance is small or geometries appear convergent.
- Indicator: Mahalanobis distance is low (i.e. state means appear close), but combined uncertainty volumes are large. This mismatch signals dilution.
- Diluted flag: Satcat marks cases where Pc falls within the dilution regime.
Satcat computes Max Pc by scaling down the covariance while preserving its shape and nominal miss vector. The peak of the resulting Pc curve defines the Max Pc value, creating an estimate of the highest plausible collision probability given improved tracking.
Operational Context
Satcat automatically identifies dilution region events where Nominal Pc underestimates the collision likelihood due to oversized covariances. The Max Pc metric can be used in the following ways:
- If Max Pc exceeds remediation threshold (e.g., 1e-4), the event should be monitored, and remediation actions should be considered.
- If Max Pc is below threshold, the event may be considered "safe" unless updated tracking data prompts reevaluation.
Operators can remediate dilution region events by uploading refined ephemerides and/or requesting Sensor Tasking, which is particularly useful for poorly tracked secondaries.