CL_op_locTime

Description

• Performs various conversions between quantities defined in the Earth equatorial plane.

  These quantities are: longitude, right ascension, true or mean local time. Additional quantities are computed: sidereal time and right ascension of (true or mean) Sun direction.

  Quick definitions:

  - The right ascension (ra) is the angle in the equator from the inertial reference axis (in the equatorial plane) chosen.

  - The longitude (lon) is the angle in the equator from the Greenwich meridian.

  - The sidereal time (sidt) is the angle between the inertial axis and the Greenwich meridian.

  - The true solar angle (tsa) is the angle from the projection of the Sun direction onto the equatorial plane.

  - The mean solar angle (msa) is the "mean" value of the true solar angle, that is the angle from a fictitious "mean" direction of the Sun that is rotating uniformly in the equatorial plane (i.e. unaffected by the equation of time).

• The relations between the different parameters are as follows:

  ra = lon + sidt

  tsa = ra - stra

  msa = ra - smra

  smra = sidt - 2*pi*decimal(t) + pi

  tlh = tsa * 12/pi + 12

  mlh = msa * 12/pi + 12

  where:

  - sidt: sidereal time.

  - stra: right ascension of the "true" Sun direction.

  - smra: right ascension of the "mean" Sun direction.

  - t is the number of days from some origin at 00h00 (UT);

  - decimal(t) = t - floor(t) (decimal part).

• The type and number of output arguments depend on "type_par_out":

  - If type_par_out is a (row) vector of strings, the number of output arguments must be equal to the size of type_par_out (and they are returned in the requested order).

  - If type_par_out is 'all', only one output argument is returned in a structure. The field names are: ra, lon, tsa, msa, tlh, mlh, sidt, stra, smra

• Important notes:

  - The function is only valid for the Earth.

  - The inertial axis (origin for right ascensions or sidereal time) is the x-axis of the ECI reference frame. (See Reference frames for more details)

  Remark: Some output parameters do not depend on all the inputs. Examples:

  lon = CL_op_locTime(cjd, 'lon', 0, 'lon') => cjd should not be necessary (but it is used in practice!)

  sidt = CL_op_locTime(cjd, 'lon', 0, 'sidt') => the result does not depend on 'lon'!)

Parameters

cjd:

Modified (1950.0) Julian date (Time scale: TREF) (1xN or 1x1)

type_par:

(string) Name of the input parameter: 'lon', 'ra', 'tsa', 'tlh', 'msa' or 'mlh' (1x1)

par:

Value of input parameter. Radians for lon, ra, msa, tsa and hours for mlh and tlh. (1xN or 1x1).

type_par_out:

(string) Name(s) of output parameter(s): 'lon', 'ra', 'tsa', 'tlh', 'msa', 'mlh', 'sidt', 'stra', 'smra', or a vector of any of these, or 'all'. (1xM)

ut1_tref:

(optional) UT1-TREF [seconds]. Default is %CL_UT1_TREF (1xN or 1x1)

tt_tref:

(optional) TT-TREF [seconds]. Default is %CL_TT_TREF. (1xN or 1x1)

par_out:

Value(s) of output parameter(s). See description for more details.

Authors

• CNES - DCT/SB

See also

• CL_mod_siderealTime

• CL_eph_sun

Examples

// Right ascension to local time: cjd = CL_dat_cal2cjd(2009,09,07,10,29,30.5); // September 7th 2009, 10:29:30.5 TREF ra = CL_deg2rad(10); mlh = CL_op_locTime(cjd, 'ra', ra, 'mlh') // => mean local time in hours tlh = CL_op_locTime(cjd, 'ra', ra, 'tlh') // => true local time in hours // Local time to right ascension: ra = CL_op_locTime(cjd, 'mlh', mlh, 'ra') // using mean local time in hours ra = CL_op_locTime(cjd, 'tlh', tlh, 'ra') // using true local time in hours // Vector of right ascensions: ra = CL_deg2rad(0:90:390); mlh = CL_op_locTime(cjd, 'ra', ra, 'mlh') tlh = CL_op_locTime(cjd, 'ra', ra, 'tlh') // List of output parameters: [tlh,mlh,sidt] = CL_op_locTime(cjd,'ra',ra,['tlh','mlh','sidt']) // All output parameters in a structure: par_out = CL_op_locTime(cjd, 'ra', ra, 'all'); par_out.tlh