Satellite Safety

CARA : Conjunction Assessment Risk Analysis

Process overview

This is a three-step process between the Combined Space Operations Center (CSpOC), the CARA team at NASA GSFC, and the mission Owner/Operators (O/O).

  • identification of close approaches (i.e. Conjunction Assessment)
  • screening to determine the risk posed by a given conjunction event (i.e. Conjunction Assessment Risk Analysis), and
  • planning and execution of any risk mitigation strategies (i.e. Collision Avoidance).

Mission data provided to CARA

For each mission that receives services from the CARA team, an ICD is formulated and signed between the CARA team and the specific mission. The purpose of the ICD is to define any data and information exchanges between to the two teams, the mechanism(s) for that exchange, and the cadence / frequency of exchange. Each ICD is tailored to each mission and includes any mission-specific requirements. In general, the CARA-Mission ICD includes, but is not limited to:

  • Mission Overview, including nominal orbit and orbit maintenance plans
  • Mission data provided to CARA (e.g. predicted ephemeris, covariance, &c.)
  • CARA data and services provides to the mission (e.g. CARA Summary Report, High Interest Event notifications & information, etc.)

Products needed (ephemeris, covariance, maneuver notification)

Each mission routinely provides an ephemeris, or file containing the spacecraft state and specific time increments, to CARA. This ephemeris is transferred to the CSpOC for screening, or identification of close approaches. The ephemeris format, duration, time-step, and delivery frequency are mission-specific and specified in the ICD. Typically, these files are delivered daily and span seven to ten days in duration. CARA can accept many native formats and convert to the format required by the CSpOC. If a single ephemeris is provided routinely then it should include any planned maneuvers within the duration of the ephemeris. For some missions or during periods of high interest, multiple ephemerides can be provided containing multiple maneuvers, no burn, or potential variations due to modeled maneuver performance.

Similar to the ephemeris data product, CARA also requires missions to provide a predictive covariance file. The purpose of this product is to allow CARA to compute a collision probability based on the Owner/Operator ephemeris and covariance data. The format details are to be defined in the ICD, but it is desired that the predictive covariance file have the same time-step cadence as the ephemeris. Moreover, the covariance should be provided from the same orbit determination process that produces the epoch state and the same propagation that produced the ephemeris.

Although the CARA software has a native capability to detect maneuvers in an ephemeris, with the diverse set of missions and maneuver capabilities supported, that capability is intended for quality assurance purposes only. Therefore, CARA prefers that each mission notify the CARA Operations Team as soon as a maneuver time is known or, at minimum, when it is modeled in the ephemeris. For each maneuver notification, CARA would like the maneuver time, size, and type to be reported.

What does CARA provide?

Operationally, CARA provides conjunction risk analysis services for NASA's non-human spaceflight missions through all phases of the mission lifecycle. Examples of CARA support include, but are not limited to:

  • Tailored, mission-unique operations concept development
  • Single interface to CSpOC
  • Dedicated Orbital Safety Analyst support
  • Event trending & automated reporting to missions for situation awareness
  • High interest event notification
  • Pc computation
  • Pc sensitivity analysis
  • Conjunction geometry analysis
  • Covariance & Orbit Determination solution assessment
  • Collision avoidance maneuver planning & assessment
  • Collocation / repeat geometry strategy assistance
  • Critical event support: launch & early orbit, maneuvers, decommissioning / end-of-mission
  • Historical data exploitation & statistical analysis

During nominal on-orbit operations, CARA provides routine and high-interest conjunction risk analysis. The routine support consists of daily processing of close approach data and delivery of a daily summary report. This report is intended for situational awareness of conjunction events as it provides high-level information regarding any active conjunctions (i.e. conjunctions for which the time of closest approach (TCA) has not yet passed). The second primary conjunction risk analysis support provided is for high-interest conjunction events. A high-interest event is any conjunction that the CARA team deems to be a potential risk to the asset. An email is sent to the mission notifying them of the situation and additional analysis beyond the routine automation is performed. Sometimes these events require briefing to mission Owner/Operators and stakeholders and may ultimately result in collision avoidance maneuver planning and execution.


There are three policies that pertain to the CARA process. The most recent versions of these documents are available on NASA Online Directives information System (NODIS):

  • NASA Procedural Requirement (NPR) 8715.6b
  • Goddard Policy Directive (GPD) 7120.1.1a: Goddard Space Flight Center (GSFC) Space Asset Protection Policy
  • Launch collision avoidance (COLA) GPR 8000.1