Spacecraft encounters glossary

A spacecraft encounter occurs when another spacecraft or nearby space object becomes operationally relevant to a mission.

Vimotek uses spacecraft encounters as the common operational challenge connecting two different contexts: Proximity Operations, where proximity is intentional and initiated by the operator, and Proximity Events, where proximity is not initiated or controlled by the operator.

The objective is to make each encounter safer, better understood and more predictable.

Proximity Operations are intentional operations in which a spacecraft operator initiates and manages proximity to another spacecraft, object or structure.

They include rendezvous, inspection, docking, servicing, logistics, assembly, debris handling, terminal events and robotics.

Rendezvous and Proximity Operations, or RPO, is the established market and technical term associated with this operational context.

Vimotek uses Proximity Events to describe situations in which relevant spacecraft proximity is not initiated or controlled by the operator.

Examples include unexpected or ambiguous approaches, anomalous manoeuvres, unplanned close approaches, interference and other nearby-object behaviour.

Managing a Proximity Event requires understanding what is happening, assessing its significance and preparing an informed response while preserving mission objectives.

Rendezvous and Proximity Operations (RPO) describe spacecraft operations involving controlled relative motion between spacecraft or other orbital objects.

RPO can include rendezvous, inspection, docking, servicing, assembly, logistics, debris handling and robotic operations.

At Vimotek, RPO is an important established market anchor within the broader operational context of Proximity Operations.

Space Situational Awareness (SSA) is the observation and understanding of objects, conditions and activity in the space environment.

For spacecraft encounters, SSA contributes to detecting and monitoring nearby objects, understanding their relative behaviour and assessing situations that may affect a mission or asset.

Vimotek uses SSA as an established technical and market anchor for aspects of Proximity Events, rather than as its top-level category.

Space Domain Awareness (SDA) extends beyond knowledge of objects and trajectories towards a broader understanding of activity, behaviour and context within the space domain.

SSA and SDA are particularly relevant to operators and organisations responsible for high-value or security-sensitive space assets.

Within Vimotek's framework, they support the broader operational challenge of understanding and managing Proximity Events.

Onboard Space Situational Awareness brings sensing, detection, monitoring and assessment capability directly onto the spacecraft.

Rather than relying solely on information produced elsewhere, onboard sensing can provide local observations of nearby objects and proximity behaviour.

Vimotek's Sentinel supports onboard awareness and assessment for Proximity Events.

Relative navigation determines the motion and geometry of one spacecraft relative to another spacecraft, target or object.

It is fundamental to proximity operations such as rendezvous, inspection, docking, servicing and robotics, where absolute orbital position alone is insufficient.

Onboard perception systems can provide observations and state estimates that support the spacecraft's navigation and GNC functions.

Relative state estimation is the process of estimating quantities such as relative position, velocity and orientation between a spacecraft and another object.

These estimates allow spacecraft systems to understand how the relative geometry of an encounter is changing over time.

Vimotek's Navcam supports relative state estimation for precise Proximity Operations; it does not itself manoeuvre or control the spacecraft.

Target acquisition is the process of initially detecting and establishing a usable observation of a target spacecraft or object.

It is an important early step in relative navigation and proximity operations, allowing subsequent tracking and characterisation to begin.

Target tracking is the continuous observation of a target over time to determine how its relative position and behaviour are changing.

Reliable tracking supports relative state estimation, navigation and operational understanding during spacecraft encounters.

Object characterisation is the process of determining relevant observable properties of a nearby spacecraft or object.

Depending on the mission and sensing configuration, this can support understanding of target geometry, orientation, behaviour and other characteristics relevant to the encounter.

Guidance, Navigation and Control (GNC) comprises the functions that determine where a spacecraft should go, estimate where it is and command the spacecraft to achieve the intended motion.

Vimotek contributes information and validation relevant to GNC through onboard perception and mission-specific digital twins.

Navcam does not control or manoeuvre the spacecraft.

A keep-out zone is a defined region around a spacecraft or target that an approaching vehicle should not enter except under specified conditions.

Keep-out zones form part of the safety logic and relative geometry used to plan and execute proximity operations.

An approach corridor is a defined path or volume through which a spacecraft is intended to approach a target during a proximity operation.

It helps constrain relative geometry and supports predictable, controlled approaches towards inspection, docking, servicing or other terminal operations.

A mission-specific digital twin is a digital representation of the spacecraft, sensors, mission logic, operational concepts and scenarios relevant to a particular mission.

It can be used to evaluate how systems and procedures behave under expected and unexpected conditions before and during operations.

Vimotek's Orbittwin enables teams to model spacecraft and sensors, validate mission logic, evaluate sensor and GNC behaviour, rehearse operations and assess alternatives and responses.

This makes the digital twin relevant to Mission Readiness, Mission Effectiveness and Mission Resilience — not only pre-flight preparation.

CONOPS, or Concept of Operations, describes how a mission or operational capability is intended to work.

It typically captures mission objectives, operational sequences, spacecraft behaviour, procedures, constraints and interactions between systems or operators.

Modelling a CONOPS digitally allows mission teams to test whether the intended operational logic works before committing it to flight.

Synthetic sensor data is simulated data designed to reproduce the outputs that a real sensor would generate under specified mission and environmental conditions.

It can be used to develop and evaluate perception algorithms, investigate sensor configurations and test how navigation or GNC functions respond under different encounter scenarios.

Orbittwin can generate mission-specific synthetic sensor data as part of simulation and validation.

Software-in-the-Loop (SIL) testing connects operational or flight software to a simulated environment so that its behaviour can be evaluated without requiring the physical flight hardware.

For spacecraft encounters, SIL can help teams validate mission logic, algorithms and interfaces across a range of nominal and off-nominal scenarios.

Hardware-in-the-Loop (HIL) testing introduces physical hardware into a simulated mission environment.

It enables engineers to evaluate how real hardware responds to simulated spacecraft, sensor and encounter conditions before relying on it in flight.

Mission Readiness is confidence that the mission, systems, procedures and possible responses are ready for the conditions they may encounter.

For spacecraft encounters, this includes validating mission logic, sensor and GNC behaviour, procedures and operational alternatives before they are needed.

Mission Effectiveness is the ability to achieve the intended operational outcome.

For Proximity Operations, this can mean carrying out rendezvous, inspection, docking, servicing or robotics precisely and repeatably.

Mission Resilience is the ability to preserve mission objectives when conditions depart from plan.

During spacecraft encounters, resilience can depend on earlier understanding of nearby-object behaviour, assessment of changing conditions and preparation of viable responses.

Mission assurance is confidence that planned operations will work as intended.

Simulation, validation, rehearsal and operational understanding can reduce development and integration risk while increasing confidence before committing to mission-critical actions.