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Standards & Units

All tools follow the conventions of the underlying IO.Astrodynamics framework. This page collects the things you need to know to interpret inputs and outputs.

Units

Quantity Unit
Length / position meters (m)
Velocity meters per second (m/s)
Mass kilograms (kg)
Gravitational parameter (GM) m³/s²
Time seconds (s) for durations; ISO 8601 strings for instants
Angles radians unless stated otherwise
Frequency / mean motion radians per second
Energy (specific) J/kg
Angular momentum (specific) m²/s

The dedicated unit-conversion tools (DegreesToArcseconds, MetersToAstronomicalUnits, etc.) are available for callers that prefer to work in degrees, parsecs, AU, light-years and arcseconds. See Unit conversions.

Time

Date/time values are passed as ISO 8601 strings. The server understands the standard time scales:

Scale Suffix Description
UTC none (default) Coordinated Universal Time
TDB TDB Barycentric Dynamical Time
TAI TAI International Atomic Time
TDT TDT Terrestrial Dynamical Time
GPS GPS GPS time
Local Local Local civil time

If no suffix is present, UTC is assumed. Suffixes are separated by a single space, e.g. 2024-01-01T00:00:00 TDB.

The ConvertDateTime, ConvertToJulianDate, ConvertToModifiedJulianDate, ConvertToEphemerisTime and AddDuration tools cover the common transformations. Ephemeris Time is TDB seconds since the J2000 epoch (2000-01-01T12:00:00 TDB).

Reference frames

Frames are passed as enumerated values. The most common ones:

  • ICRF — International Celestial Reference Frame, the inertial frame the server uses as default
  • J2000 — J2000 mean equator and equinox
  • ECLIPJ2000 — J2000 ecliptic
  • IAU_EARTH, IAU_MOON, IAU_MARS, … — body-fixed frames for each major body
  • ITRF93 — IERS Terrestrial Reference Frame (high-precision Earth-fixed)
  • TEME — True Equator Mean Equinox (the native frame of SGP4 / TLE)

The full set is enumerated by the FramesEnum type exposed in tool schemas. Topocentric frames for arbitrary sites follow the IO.Astrodynamics naming <SITE_NAME>_TOPO (see GetSiteFrameName).

Center of motion

State vectors and orbital elements carry an explicit center of motion (the gravitating body the orbit is referenced to), specified by NAIF ID via the CelestialItemsEnum / PlanetsMoonsEnum enums. Periodicity, period, mean motion, apsides, and energy computations all use the GM of that body.

Aberration

Where appropriate, tools take an aberration parameter:

  • None — geometric (instantaneous) positions
  • LT — one-way light-time corrected
  • LT+S — light-time plus stellar aberration
  • CN, CN+S, XLT, XLT+S — converged and transmission-side variants

These map onto the standard SPICE aberration codes.

NAIF IDs and enumerations

Bodies, barycenters, Lagrange points and Deep Space Network stations are addressed by name through enumerations whose underlying integer values are the NAIF IDs:

  • CelestialItemsEnum — every SPICE-known item (planets, moons, barycenters, Lagrange points, DSN stations, the Sun, etc.)
  • PlanetsMoonsEnum — restricted set of bodies that have a geophysical ellipsoid (used when an ellipsoid is needed, e.g. planetodetic conversions)
  • GroundStationsEnum — Deep Space Network ground stations (DSS_14, DSS_43, DSS_63, …)
  • FramesEnum — SPICE-known reference frames

Tool schemas exposed to MCP clients always declare these as closed enumerations, so a well-behaved client will only let you pick valid values.

Sign conventions

  • Range rate is positive when receding, negative when approaching
  • Phase angle is the illuminator-target-observer angle measured at the target. cos(phaseAngle) drives illuminated fraction; phase angle of 0 means fully lit
  • Elevation is measured from the local horizon (0 = horizon, π/2 = zenith). Negative values are below the horizon
  • Delta-V magnitudes returned by the Compute*DeltaV tools are always positive (the sign of the burn direction depends on whether you are raising or lowering; the magnitude is what the budget needs)
  • Inclination, RAAN, AOP follow the standard celestial-mechanics convention used by IO.Astrodynamics

Floating-point edge cases

Astrodynamics results legitimately contain NaN and Infinity (parabolic orbits, degenerate geometries). The JSON serializer accepts named floating-point literals in both directions. Plan your client code to tolerate them — don't reject responses out of hand.

Reference

For the underlying physics, sign conventions, and algorithm details, refer to the IO.Astrodynamics documentation. Every tool on this server is a thin wrapper over that library.