{"dataset":{"slug":"communication-technologies","title":"Communication Technologies","description":"Communication and timing systems — optical relays, TDRS, telemetry/tracking/command, and time standards.","version":"1.0.0","lastGenerated":"2026-06-29","license":"CC BY-SA 4.0","entityCount":15,"sources":["nasa","jpl"]},"entities":[{"id":"time_standard:apparent-solar-time","name":"Apparent Solar Time","type":"time_standard","domain":"science","description":"Time kept by the real Sun, as a sundial shows it — noon is the moment the Sun crosses the meridian. Because the Earth's orbit is elliptical and tilted, the Sun runs fast or slow through the year, so clocks keep mean solar time instead, differing by up to about a quarter of an hour.","entryPath":"/celestial-mechanics/apparent-solar-time"},{"id":"time_standard:barycentric-dynamical-time","name":"Barycentric Dynamical Time","type":"time_standard","domain":"science","description":"The time coordinate of the Solar System barycentre, used as the independent variable of the planetary ephemerides. It differs from Terrestrial Time only by small periodic terms arising from the Earth's motion around the Sun, never drifting by more than a couple of milliseconds.","entryPath":"/celestial-mechanics/barycentric-dynamical-time"},{"id":"time_standard:utc","name":"Coordinated Universal Time (UTC)","type":"time_standard","domain":"science","description":"The civil time standard, derived from International Atomic Time but kept within a second of the Earth's rotation by occasional leap seconds. Deep-space tracking tags its measurements in UTC.","entryPath":""},{"id":"communication_system:dsoc","name":"Deep Space Optical Communications (DSOC)","type":"communication_system","domain":"science","description":"NASA's technology demonstration of laser communication from deep space, flying as a rider on the Psyche spacecraft. It transmitted data over tens of millions of kilometres at rates far beyond radio, proving optical links for future missions.","entryPath":""},{"id":"time_standard:gps-time","name":"GPS Time","type":"time_standard","domain":"science","description":"The continuous timescale of the Global Positioning System, offset from TAI by a fixed number of seconds and widely used to synchronise ground equipment.","entryPath":""},{"id":"time_standard:tai","name":"International Atomic Time (TAI)","type":"time_standard","domain":"science","description":"A continuous atomic timescale kept by hundreds of atomic clocks worldwide — the stable foundation from which civil time is derived. TAI never inserts leap seconds, making it ideal for measuring precise intervals.","entryPath":""},{"id":"time_standard:julian-date","name":"Julian Date","type":"time_standard","domain":"science","description":"A continuous count of days (and fractions) since noon Universal Time on 1 January 4713 BC in the proleptic Julian calendar, used throughout astronomy to timestamp observations without the awkwardness of calendar months and leap years. The Modified Julian Date (MJD = JD − 2400000.5) shifts the origin to a recent midnight for convenience.","entryPath":"/reference-systems/julian-date"},{"id":"communication_system:lcrd","name":"Laser Communications Relay Demonstration (LCRD)","type":"communication_system","domain":"science","description":"A NASA optical-communication relay in geostationary orbit that demonstrates two-way laser links between ground stations and spacecraft, a stepping stone toward operational optical relays in the Near Space Network.","entryPath":""},{"id":"time_standard:sidereal-time","name":"Sidereal Time","type":"time_standard","domain":"science","description":"Time kept by the stars rather than the Sun — it measures the Earth's rotation relative to the fixed sky. A sidereal day is about four minutes shorter than a solar day, because the Earth must turn a little further each day to face the Sun again as it moves along its orbit.","entryPath":"/celestial-mechanics/sidereal-time"},{"id":"time_standard:spacecraft-clock","name":"Spacecraft Clock (SCLK)","type":"time_standard","domain":"science","description":"The onboard counter that timestamps a spacecraft's data. Because it drifts relative to ground time, mission teams maintain a correlation (SCLK-to-ground time) using the tracking network so that events can be placed on a common timescale.","entryPath":""},{"id":"communication_system:telemetry-tracking-command","name":"Telemetry, Tracking & Command (TT&C)","type":"communication_system","domain":"science","description":"The end-to-end function every mission depends on: downlinking telemetry (spacecraft health and science), measuring the signal for tracking and navigation, and uplinking commands. The deep-space and near-Earth networks exist to provide TT&C.","entryPath":""},{"id":"time_standard:terrestrial-time","name":"Terrestrial Time","type":"time_standard","domain":"science","description":"The idealised time on the Earth's surface used as the time axis for astronomical events and geocentric ephemerides. It runs at the same rate as International Atomic Time but is offset ahead of it by a fixed 32.184 seconds, for continuity with the older ephemeris time.","entryPath":"/celestial-mechanics/terrestrial-time"},{"id":"time_standard:leap-second","name":"The Leap Second","type":"time_standard","domain":"science","description":"The extra second occasionally inserted into Coordinated Universal Time to keep it within 0.9 seconds of UT1, so that civil clocks stay in step with the turning of the Earth. As atomic time and Earth rotation drift apart, leap seconds bridge the gap — a practice now being reconsidered.","entryPath":"/celestial-mechanics/leap-second"},{"id":"communication_system:tdrs","name":"Tracking and Data Relay Satellite System (TDRS)","type":"communication_system","domain":"science","description":"A fleet of geostationary relay satellites that give near-Earth spacecraft — including the ISS and Hubble — near-continuous contact with the ground, instead of only during the few minutes of a ground-station pass. TDRS is the space-based half of NASA's Near Space Network.","entryPath":""},{"id":"time_standard:ut1","name":"Universal Time (UT1)","type":"time_standard","domain":"science","description":"The form of Universal Time tied to the actual rotation of the Earth, measured from the positions of distant sources. Because the Earth's spin is slightly irregular and slowly slowing, UT1 drifts against the steady beat of atomic time.","entryPath":"/celestial-mechanics/ut1"}]}