Chief Ray

Photogrammetric and Mapping Software

Support for stellar catalogs

• Accepts data and units that are standard for FK5 and Hipparcos star catalogs

  • Right ascension, declination

  • Proper motion in right ascension and declination

  • Parallax

• Conversion between Hipparcos epoch (J1991.25) and FK5 epoch (J2000.0)

Conversion from star catalog mean place to apparent place, topocentric place, and local horizon coordinates

• Apparent place: Position of star at some date and time

  • In the coordinates system defined by the true equator and equinox of date

  • Ignoring atmospheric refraction

  • As seen from the Earth's center of mass

• Topocentric place:  Position of star at some date and time

  • In the coordinate system defined by the true equator and equinox of date

  • Ignoring atmospheric refraction

  • As seen from a specific location on Earth

• Local horizon coordinates: Position of star at some date and time

  • In local azimuth and elevation coordinates

  • Including atmospheric refraction (optional)

  • As seen from a specific location on Earth

Time conversions among UT1, UTC, TAI, TDT, TDB

• UT1 – Time corresponding to Earth rotation

• UTC – Universal coordinated time (Civil time)

• TAI – Atomic time

• TDT – Terrestrial dynamic time (TAI + 32.184 sec)

• TDB – Barycentric dynamic time (removes general relativistic tidal effects from TDT)

• General conversion chain:      TDB ↔ TDT ↔ TAI ↔ UTC ↔ UT1

ECF ↔ ECI conversion using quaternions

• X_ECI = q X_ECF q^*

  • q = q_P^* q_N^* q_S q_W  expresses that attitude of the ECF system relative to the ECI system

  • q_P  (precession) accounts for precession of the mean celestial system

  • q_N  (nutation) accounts for the nutation correction from the mean celestial system of date

  • q_S  (spin) accounts for earth rotation

  • q_W  (wobble) accounts for polar motion

Earth Orientation Parameters

• Precision offsets and corrections that must be measured, not computed

  • Polar Motion (x,y)

  • UTI – UTC

  • Nutation offsets (Dpsi, Deps)

• Interpolated from files containing the IERS CO4 solution (daily values from 1962 to present)

For more information, contact Chief Ray .

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