Ephemeris Comparison
Ephemeris comparison evaluates the conjunction risk of several ephemerides for one RSO side by side, against the conjunction data currently in effect for that RSO. Where CDM comparison on an event page examines a single close approach, ephemeris comparison covers every close approach an ephemeris produces, so an operator can identify the mitigation plan that lowers risk overall without introducing a new close approach or moving an existing one later in time.
Comparison is available from Comparison on the Conjunction Events dashboard, and requires a Pro or Enterprise subscription with on-orbit screening enabled. To learn more, see Service Tiers.
Getting Started
- Select the Primary RSO to compare.
- Use Manage ephemeris to select the ephemerides to compare, or upload a new one.
- Choose the CDM selection method that determines which CDM represents each conjunction event.
- Compare aggregate Pc, minimum miss distance, and risk totals across the ephemerides.
- Upload the selected plan's ephemeris with an
OPERATIONALdesignation to put it into effect.
Primary RSO
Comparison covers one RSO at a time, selected under Primary RSO. Organization and constellation narrow the RSOs available for selection. Changing the primary RSO clears the ephemerides selected under the previous one.
Selecting an RSO loads its Current operational data as the first row of the comparison. This row is built from the conjunction events already held for the RSO rather than from a single file, so it reflects CDMs from every source contributing to those events. It is listed above the other ephemerides and is not screened, uploaded, or removed from the comparison.
Active Risk Window
The Active risk window is the span covered by the current operational conjunction events, from the earliest TCA to the latest. It sets the period the whole comparison is measured over, and it determines which ephemerides are eligible: comparison uses ephemerides whose usable time spans the active risk window.
Selecting Ephemerides
Manage ephemeris lists the ephemerides available for the selected RSO with their designation, context, screening state, and creation time. Up to 25 ephemerides can be compared at once. Use Filter by filename to narrow a long list, and Update ephemerides to apply the selection.
An ephemeris whose usable time ends within 24 hours, or has already ended, is marked in the list.
CDM Derived Ephemerides
Show CDM derived ephemeris files includes ephemerides assembled from operational CDMs that Satcat matched to a file uploaded to Space-Track, EU SST, or Starlink. These are already screened and carry the originators that produced them. A CDM derived ephemeris reports a CDM window rather than a usable window, because its span comes from the CDMs rather than from an uploaded file.
Uploading a New Ephemeris
An ephemeris that is not yet in Satcat can be uploaded from Manage ephemeris. Uploaded files are saved to Catalog Data for the organization and screened against the active catalog.
CDM Selection Method
CDM selection method determines which of the latest CDMs is used to compute aggregate risk metrics when a conjunction has been reported by more than one source. It applies identically to every ephemeris in the comparison.
| Method | Selects |
|---|---|
| Best overall | The Best CDM for each conjunction event |
| Highest Pc | The CDM reporting the highest Pc for each conjunction event |
Changing the selection method recalculates every value in the comparison, including Current operational.
Reading a Comparison
Each ephemeris reports the following, calculated across its own conjunctions. Every metric in the comparison is calculated over the conjunctions the current filters display, not over the ephemeris's full set.
- #: the stable row number used to match the ephemeris to its timeline row
- Created: when the ephemeris was added
- Aggregate Pc: the combined collision probability across those conjunctions
- Min. MD: the smallest miss distance among them
- Risk totals: the count of conjunctions at each risk level
Current operational reports the same values for the conjunction data in effect, which is the reference the other rows are read against. An ephemeris that has not been screened reports no metrics until a screening job completes for it.
The Timeline comparison places every conjunction at its TCA on a shared time axis, one row per ephemeris. Its y-axis labels match the table's # column. Comparing rows down the axis shows which close approaches an ephemeris removes, which it leaves in place, and which it moves to a different time. Any ephemeris can be hidden from the timeline without removing it from the comparison.
Screening an Ephemeris
An ephemeris that has not been screened can still be added to the comparison. Screen opens Start screening job for that ephemeris without leaving the comparison, and its metrics populate once the job completes. To learn more about screening jobs, see On-Orbit Screening.
Ephemeris Detail
Each row carries the detail behind its results, covering both the file and the screening job that produced them.
| Detail | Description |
|---|---|
| Usable window start and end | The span the ephemeris is valid for. A CDM derived ephemeris reports a CDM window instead |
| Context | The purpose of the ephemeris, and the mitigation intent it signals |
| Filtered / total events | How many of the ephemeris's conjunctions the current filters display |
| Secondary catalog(s) | The catalogs the ephemeris was screened against |
| Job status, completed, and source | The state of the screening job, when it finished, and where the results came from |
Designation, solution epoch, hard body radius, and whether covariance is present are also shown.
Pc requires covariance for both the primary and the secondary state at TCA. An ephemeris without covariance reports its close approaches, and Aggregate Pc reads No CDMs with Pc where no filtered conjunction carries one.
To learn more about context values and the mitigation intent each one signals, see Ephemeris Metadata. To learn more about how HBR is sourced, see Risk Metrics.
Filtering a Comparison
Risk level, Pc, miss distance, originator, and TCA filters change which conjunctions are displayed and recalculate every metric in the comparison. Filtered / total events reports how many of an ephemeris's conjunctions the current filters display, both for the comparison as a whole and for each ephemeris.
The risk level filter selects a single risk level, so it shows that level alone rather than that level and above. Reading across risk levels means clearing the filter and using the risk totals. To learn more, see Risk Levels.
Intra-org conjunctions and auto-maneuver secondary conjunctions can each be hidden.
Putting a Mitigation Plan into Effect
Upload the selected mitigation plan's ephemeris with an OPERATIONAL designation and a COLA context. Satcat then generates new CDMs from the uploaded ephemeris, updates the risk metrics on each affected conjunction event, and signals the RSO's intent to mitigate to other operators.
The comparison itself does not change what is in effect. Until the ephemeris is uploaded with an OPERATIONAL designation, Current operational continues to reflect the conjunction data already in place.
To learn more about what an OPERATIONAL upload triggers, see Operational Data. To learn more about designing a collision avoidance maneuver, see Collision Avoidance.