- R2 — Role 2 Medical Treatment
- A forward medical facility providing resuscitation, damage-control surgery, and limited holding.
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- R3 — Role 3 Medical Treatment
- A theater hospital providing resuscitative surgery, specialty care, and inpatient capacity.
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- RAAN — Right Ascension of the Ascending Node
- The orbital element specifying the angle from the vernal equinox to the point where the orbit crosses the equatorial plane northward. Defines the orientation of the orbital plane in space.
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- RACI
- A responsibility assignment matrix documenting who is Responsible, Accountable, Consulted, and Informed for each task in a project. Clarifies ownership boundaries and reduces decision delays.
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- DiagramTry it: Team Leadership Guide
- RADAR — Radio Detection and Ranging
- A system that transmits radio waves and analyzes their reflections to detect and locate objects and measure their range and velocity.
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- Radar Imaging
- The class of radar modes that produce two-dimensional images of a scene — strip-map SAR, spotlight SAR, ISAR, real-aperture mapping. Resolution is set by aperture (cross-range) and bandwidth (range), not optical wavelength.
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- RADIAC — Radioactivity Detection, Indication, and Computation
- Military term for radiation detection and measurement instruments. RADIAC devices such as the AN/PDR-77 measure gamma and beta radiation dose and dose rate, enabling commanders to assess contamination levels.
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- 2D animationTry it: Cbrn Defense Guide
- Radiation
- Energy emitted as particles or electromagnetic waves. In the spacecraft context: trapped Van Allen, solar particle events, galactic cosmic rays. Drives shielding, part-selection, and TID/SEE radiation hardening requirements.
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- Radiation Hardening
- Design techniques to protect electronics from the space radiation environment (trapped particles, solar events, cosmic rays). Includes component selection, shielding, and error-correction.
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- DiagramTry it: Space Fundamentals GuideTry it: Space Environment Effects Guide
- Radiator (Spacecraft)
- A thermal control surface rejecting excess heat to space via thermal radiation. Sizing depends on heat load, operating temperature, and view factor to space.
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- RAIM — Receiver Autonomous Integrity Monitoring
- A GNSS receiver function that uses redundant satellite measurements to detect faulty signals. Required for aviation approach procedures and flagged by receivers when integrity falls below threshold.
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- DiagramTry it: Defense Spectrum MapTry it: Ew Guide
- RAIM / ARAIM — Receiver Autonomous Integrity Monitoring
- Algorithm that uses redundant GNSS measurements within a receiver to detect and exclude faulty satellites. ARAIM (Advanced) adds multi-constellation support and tighter civil aviation integrity bounds needed for LPV-200 approach service.
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- DiagramTry it: Defense Spectrum Map
- Rain Fade
- Attenuation of radio signals caused by absorption and scattering by rain. Increases with frequency; significant above ~10 GHz. Link budgets must include rain fade margin for high availability.
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- RAM — Reliability, Availability, Maintainability
- Engineering disciplines that ensure weapon systems perform their intended functions with minimal downtime. RAM requirements drive design trade-offs and lifecycle costs.
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- RAMS — Reliability, Availability, Maintainability and Safety
- Dependability attributes engineered and assured across a system.
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- Ransomware
- Malware that encrypts victim data and demands payment for decryption. Modern variants add double-extortion (also exfiltrate and threaten publication) and triple-extortion (notify customers/regulators). Targeted intrusions now average >US$1M ransom.
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- RAS — Replenishment at Sea
- The NATO term for transferring fuel and stores between vessels while steaming.
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- RAT — Ram Air Turbine
- A small turbine deployed into the airstream to provide emergency hydraulic or electrical power during total power loss.
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- Rayleigh Fading
- A statistical model for radio signal envelope amplitude in a multipath-rich, no-line-of-sight channel. Magnitude follows a Rayleigh distribution; phase is uniform. Drives diversity-combining and OFDM/MIMO architectures.
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- RBAC — Role-Based Access Control
- Permissions granted to roles rather than to individuals, so access follows what someone does rather than who they are.
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- RCE — Remote Code Execution
- Vulnerability class allowing an attacker to run arbitrary code on a target system over a network.
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- RCM — Reliability-Centered Maintenance
- A structured method for selecting maintenance tasks based on the consequences of each failure mode.
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- RCS — Radar Cross Section
- A measure of a target's reflectivity to radar, expressed in square meters (often in dBsm). Depends on target size, shape, material, and radar frequency/polarization.
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- RCS-radar — Radar Cross Section
- A measure of how detectable an object is to radar, used to estimate the size of tracked space objects.
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- RCT — Regimental Combat Team
- A Marine Corps ground combat unit formed around an infantry regiment reinforced with supporting arms.
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- RDD — Radiological Dispersal Device
- A device that spreads radioactive material to cause contamination, injury, or disruption.
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- RDT&E — Research, Development, Test, and Evaluation
- The appropriation category funding the development and testing of new systems and technologies.
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- Reaction Wheel
- A momentum exchange device for fine attitude control, consisting of a flywheel spun by an electric motor. Provides precise pointing but can saturate, requiring desaturation via thrusters or magnetorquers.
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- Red Team
- A group that plays the adversary role in wargames and exercises, making realistic decisions based on enemy doctrine and capabilities. Also refers to teams conducting independent critical analysis of plans to identify vulnerabilities and assumptions.
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- Red Team / Blue Team
- Adversarial security assessment structure. Red Team simulates attacker tactics, techniques, and procedures to probe defenses; Blue Team defends and responds. Purple Teams integrate both to share TTP understanding in real-time.
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- Diagram
- Reference Frame
- A coordinate system defined by an origin and three basis vectors. In dynamics, frames are classified as inertial (Newton's laws apply directly) or non-inertial (need fictitious forces — Coriolis, centrifugal). Choice of frame can dramatically simplify a problem.
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- Relay Satellite
- A spacecraft providing communication links between other satellites and ground stations, extending coverage beyond direct line-of-sight. Examples: NASA TDRS, ESA EDRS.
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- Repeat Ground Track
- An orbit tuned so the satellite traces the same ground path after a fixed number of revolutions, ensuring consistent revisit geometry for Earth observation.
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- 2D animationTry it: Cycle Sync VisualizerTry it: Constellation Guide
- Replenishment Cadence
- Rate at which a constellation must launch replacement satellites to maintain capacity given finite mission life. Drives manufacturing + launch demand for megaconstellations more than peak fleet size.
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- DiagramTry it: Constellation Guide
- RER — Relative Edge Response
- An image-quality metric quantifying edge sharpness, a key input to the General Image Quality Equation.
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- RESA — Runway End Safety Area
- A cleared area beyond the runway end intended to reduce damage to aircraft that overrun or undershoot.
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- Resilience (Security)
- The ability of a system, community, or organization to absorb disruption, adapt to changing conditions, and rapidly recover functionality. Complements prevention by accepting that some threats will succeed and preparing for continued operation despite disruptions.
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- REST — Representational State Transfer
- A style of HTTP API in which URLs name things and the HTTP verb says what to do to them. Widely used to mean "a normal HTTP JSON API".
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- Retrograde Orbit
- An orbit with inclination >90 degrees, satellite moves opposite to Earth's rotation. Sun-synchronous orbits are slightly retrograde (~97-99 degrees inclination).
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- Review Gates
- Discrete decision points along a programme lifecycle (SRR, PDR, CDR, TRR, FRR, ORR) at which technical maturity is assessed and authority is granted to proceed. Standardised in NASA NPR 7120.5 and DoD 5000-series.
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- DiagramTry it: Waterfall Test GuideTry it: Space Standards Guide
- Revisit Time
- The time between consecutive observations of the same ground point by a satellite or constellation. Key performance metric for Earth observation missions.
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- RF — Radio Frequency
- The portion of the electromagnetic spectrum used for radio communications, typically 3 kHz to 300 GHz. Subdivided into HF, VHF, UHF, SHF, and EHF bands, each with distinct propagation characteristics and regulatory allocations.
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- RFA — Restrictive Fire Area
- An area in which specific restrictions are imposed and fires exceeding them require coordination.
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- RFI — Radio Frequency Interference
- Unwanted electromagnetic energy that degrades or interrupts radio system operation. A primary concern in spectrum management.
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- RFID — Radio-Frequency Identification
- A technology using radio tags to automatically identify and track assets and shipments.
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- RFL — Restrictive Fire Line
- A line established between converging friendly forces that prohibits fires across it without coordination.
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- RFP — Request for Proposal
- A formal solicitation document issued to potential suppliers describing requirements, evaluation criteria, and terms for a competitive procurement.
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- RFQ — Request for Quotation
- A solicitation asking suppliers to provide pricing for specified goods or services.
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- RGPO — Range Gate Pull-Off
- Deception jamming technique that walks a radar range gate off the true target to break lock.
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- RHIB — Rigid-Hull Inflatable Boat
- A high-speed small boat with a solid hull and inflatable collar used for boarding and SAR.
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- RHU — Radioisotope Heater Unit
- A small device using radioactive decay heat to keep spacecraft components warm without electrical power.
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- RIC — Radial In-track Cross-track
- A relative coordinate frame decomposing position and covariance into radial, along-track, and cross-track components.
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- RIP — Routing Information Protocol
- A distance-vector interior routing protocol using hop count as its routing metric.
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- Risk Management
- The systematic process of identifying, analysing, treating, and monitoring risks across a programme or portfolio. ISO 31000 sets the international standard; defence and aerospace programmes overlay this with FMEA, FTA, and bow-tie analyses.
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- DiagramTry it: Cultural Transformation SimulatorTry it: Aerodef Standards GuideTry it: Risk Management Guide+1 more
- Risk Matrix
- A 5×5 (or 4×4) grid plotting likelihood against consequence to rank risks. Outputs a heat-mapped priority list. Workhorse of programme risk reviews despite well-known limitations (compressed scales, double-counting). Pair with quantitative analysis for major risks.
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- DiagramTry it: Risk Management GuideTry it: Aerodef Standards GuideTry it: Homeland Security Fundamentals
- RL — Reverberation Level
- The intensity of unwanted scattered returns that mask targets in active sonar.
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- RMF — Risk Management Framework
- NIST process for integrating security and risk management into the system development life cycle.
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- RMS — Root Mean Square
- A statistical measure of a varying quantity’s magnitude, widely used for noise and error characterization.
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- RNP — Required Navigation Performance
- RNAV with onboard performance monitoring and alerting.
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- ROC — Rehearsal of Concept Drill
- A collaborative rehearsal in which leaders walk through an operation to synchronize the plan.
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- ROC_R — Range Resolution
- The minimum separation in range at which a radar can distinguish two targets, set by signal bandwidth.
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- ROE — Rules of Engagement
- Directives governing the use of force by military forces. ROE define the circumstances, conditions, and manner under which force may be applied, balancing military necessity with legal and policy constraints.
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- ROM — Refuel on the Move
- A technique to quickly refuel vehicles during movement to extend operational reach.
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- Root Cause Analysis
- A class of methods for identifying the underlying cause of a problem rather than treating symptoms. Common tools: 5 Whys (depth), fishbone / Ishikawa (breadth), fault tree (probability), bow-tie (controls). Foundational to modern incident management.
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- ROP — Reorder Point
- The inventory level at which a replenishment order is triggered to avoid a stockout.
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- ROS — Robot Operating System
- Open-source middleware (now ROS 2 on DDS) providing message-passing, hardware abstraction, tooling, and a package ecosystem for robotics R&D. Heavy in academia and prototyping; ROS 2 is reaching deployable maturity in industrial and defence applications.
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- Diagram
- Route Clearance
- Specialised operations to detect and neutralise IEDs, mines, and obstacles along main supply routes. Uses armoured vehicles, mine rollers, robots, and EOD teams to clear passage for follow-on traffic.
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- ROV — Remotely Operated Vehicle
- A tethered underwater robot controlled from the surface for inspection and mine disposal.
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- ROVER — Remotely Operated Video Enhanced Receiver
- A portable terminal that lets ground users receive real-time video and data downlinked from ISR platforms.
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- ROZ — Restricted Operations Zone
- Airspace of defined dimensions reserved for specific activities and restricted to other airspace users.
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- RP — Release Point
- A location on a route where units revert to control of their own commanders after a movement.
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- RP-1 — Rocket Propellant-1
- Refined kerosene fuel burned with LOX in many first stages.
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- RPA — Remotely Piloted Aircraft
- An unmanned aircraft flown by a pilot at a ground control station rather than from onboard.
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- RPAS — Remotely Piloted Aircraft System
- ICAO term for a UAS flown by a remote pilot.
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- RPC — Rational Polynomial Coefficients
- A sensor model expressing image-to-ground geometry as ratios of polynomials for orthorectifying imagery.
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- RPN — Risk Priority Number
- The product of Severity, Occurrence, and Detection ratings in FMEA (each 1-10). Higher RPN indicates failure modes requiring priority corrective action.
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- RPO — Rendezvous and Proximity Operations
- Controlled approach and station-keeping between two spacecraft.
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- RSA
- Asymmetric encryption algorithm based on the computational difficulty of factoring the product of two large prime numbers. Used for key exchange, digital signatures, and encryption. Named after Rivest, Shamir, and Adleman (1977).
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- RSO — Resident Space Object
- Any trackable object in Earth orbit — operational satellite, rocket body, or debris fragment — catalogued in SATCAT. Tracked via radar (LEO) and optical sensors (GEO).
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- RSOI — Reception, Staging, Onward Movement, and Integration
- The process of receiving deploying forces in theater and integrating them into operations.
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- RSRP — Reference Signal Received Power
- An LTE and 5G measurement of the average power of reference signals in a cell.
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- RSRQ — Reference Signal Received Quality
- An LTE and 5G quality metric combining reference signal power with total received power.
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- RSSI — Received Signal Strength Indicator
- A measure of the power level a receiver detects from a received radio signal.
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- RTCA — Radio Technical Commission for Aeronautics
- US not-for-profit standards body for civil aviation electronics, working in coordination with EUROCAE in Europe. Author of DO-178 (software), DO-254 (hardware), DO-160 (environmental), DO-260 (ADS-B), and other safety-critical avionics standards.
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- DiagramTry it: Aerodef Standards GuideTry it: Waterfall Test GuideTry it: Software Testing Guide
- RTCP — RTP Control Protocol
- A companion protocol to RTP that reports on media stream quality and participant statistics.
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- RTG — Radioisotope Thermoelectric Generator
- A power source converting radioactive decay heat (typically Pu-238) into electricity. Used for deep-space missions where solar power is impractical (Voyager, Curiosity).
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- DiagramTry it: Interplanetary G2g OpsTry it: Space Fundamentals GuideTry it: Space Systems Engineering+1 more
- RTI — Run-Time Infrastructure
- The middleware implementing High Level Architecture services — the component through which federated simulations exchange data and synchronise time.
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- RTK-GPS — Real-Time Kinematic GPS
- Differential GNSS technique providing centimetre-level positioning by streaming carrier-phase corrections from a base station (or NTRIP service) to a rover. Standard for survey-grade UAS photogrammetry, agricultural robotics, and precision landing.
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- Diagram
- RTL — Right-to-Left
- Text direction for Arabic and Hebrew scripts.
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- RTLS — Return To Launch Site
- A recovery profile in which a reusable booster flies back to land near its launch site.
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- RTN — Radial Tangential Normal
- An orbit-local frame equivalent to RIC, used to express relative state and maneuver directions.
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- RTO — Rejected Take-Off
- The decision and procedure to abort a takeoff and bring the aircraft to a stop on the runway.
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- RTOS — Real-Time Operating System
- Operating system providing deterministic scheduling and bounded interrupt latency for real-time tasks. Safety-critical RTOSes (VxWorks 653, PikeOS, INTEGRITY-178, LynxOS-178) are certified to DO-178C DAL A. Implements ARINC 653 partitioning for IMA.
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- Diagram
- RTP — Real-time Transport Protocol
- A protocol for delivering audio and video streams with timing and sequencing over IP.
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- RTPS — Real-Time Publish-Subscribe
- The wire protocol underneath the Data Distribution Service, which is what makes implementations from different vendors interoperate.
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- RTT — Round-Trip Time
- The time for a packet to travel to a destination and for its acknowledgment to return.
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- RUF — Rules for the Use of Force
- Directives governing the use of force by military forces during domestic and certain non-combat operations.
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- RVSM — Reduced Vertical Separation Minimum
- A standard reducing vertical separation between aircraft to 1,000 feet between FL290 and FL410 to increase capacity.
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- RWA — Reaction Wheel Assembly
- A spinning flywheel actuator that stores and exchanges angular momentum to control spacecraft attitude.
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- RWR — Radar Warning Receiver
- A passive EW system detecting, identifying, and displaying radar emissions directed at the aircraft, providing threat awareness to the crew.
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