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Standards & Units

SI Units

Astrodynamics.Pro uses SI units throughout.

Quantity Unit Symbol
Length Meters m
Mass Kilograms kg
Time Seconds s
Angle Radians rad
Velocity Meters per second m/s
Acceleration Meters per second squared m/s2
Gravitational parameter m3/s2 m3/s2
Temperature Celsius deg C
Pressure Kilopascals kPa
Density Kilograms per cubic meter kg/m3

Physical Constants

Defined in IO.Astrodynamics.Constants:

public const double G = 6.67430e-11;
public const double AU = 149597870700.0;
public const double C = 299792458.0;
public const double g0 = 9.80665;
public const double Deg2Rad = Math.PI / 180.0;
public const double Rad2Deg = 180.0 / Math.PI;

Reference Standards

  • Time: IERS conventions with leap seconds from NAIF LSK kernels
  • Ephemerides: JPL DE series
  • Frames: IAU, IAG, and IERS conventions
  • Gravity: EGM2008 spherical harmonics up to degree and order 70
  • Atmosphere: U.S. Standard Atmosphere 1976 and NRLMSISE-00
  • TLE propagation: SGP4 and SDP4 compatibility

Frame Transform Convention

Frame transforms in the managed API use a consistent source-to-destination convention.

  • frame.GetStateOrientationToICRF(epoch) returns a rotation frame -> ICRF
  • frame.ToFrame(targetFrame, epoch) returns a rotation frame -> targetFrame
  • applying the returned quaternion rotates a vector from the source frame into the destination frame

Example:

var so = Frames.CIRS.GetStateOrientationToICRF(epoch);
var positionIcrf = positionCirs.Rotate(so.Rotation);
var positionCirs = positionIcrf.Rotate(so.Rotation.Conjugate());

StateOrientation.ReferenceFrame identifies the source frame of the rotation, not the destination frame.

Earth-Centered Inertial And Intermediate Frames

Pro

Frames.GCRF, Frames.CIRS, and Frames.TIRS are Pro features.

  • ICRF: shared inertial frame used throughout the community and Pro APIs
  • GCRF: geocentric realization of ICRS, differing from SPICE J2000 by the IAU frame bias
  • CIRS: celestial intermediate frame using IAU 2006/2000 CIO-based precession-nutation
  • TIRS: terrestrial intermediate frame adding Earth rotation angle to CIRS

These frames are useful when you need an explicit IAU Earth-orientation chain instead of relying only on SPICE-defined built-in frames.

NAIF IDs

  • Sun: 10
  • Earth: 399
  • Moon: 301
  • Barycenters include 0 for the solar-system barycenter and 3 for the Earth-Moon barycenter

Be explicit about units when consuming external data, especially for OMM, OPM, and TLE-derived values where source conventions may differ from internal SI storage.

SSA And CDM Units (Pro)

Pro

Conjunction assessment and CDM support are Pro features.

Internally, the SSA workflow stays in SI units:

  • miss distance in meters
  • relative position in meters
  • relative velocity in meters per second
  • covariance in m², m²/s, and m²/s²
  • hard-body radius in meters

When you export a CCSDS CDM, the XML writer converts values to the units required by the schema:

  • X, Y, Z state-vector elements use km
  • X_DOT, Y_DOT, Z_DOT use km/s
  • MISS_DISTANCE and RELATIVE_POSITION_* use m
  • covariance terms use the CCSDS units defined by the CDM schema

That split is intentional: end users work in SI inside the managed API, while interchange files follow CCSDS rules.