def compute_tt_mid(hdrs,debug=False):
'''Correct a UTC MJD start time to a TT MJD mid time. Also returns the
TT-UTC offset for later use.'''
UTC_mid_MJD = hdrs['MJD_UTC'] + (hdrs['EXPTIME'] / 86400.0 / 2.0)
dtt = S.sla_dtt(UTC_mid_MJD)
TT_mid_MJD = UTC_mid_MJD + dtt/86400.0
if debug: print S.sla_dtt(UTC_mid_MJD)
new_dict = { 'MJD_UTC_mid' : UTC_mid_MJD,
'MJD_TT_mid' : TT_mid_MJD,
'TT-UTC' : dtt }
hdrs.update(new_dict)
return hdrs
def datetime2mjd_tdb(date, obsvr_long, obsvr_lat, obsvr_hgt, dbg=False):
auinkm = 149597870.691
# Compute MJD_UTC from passed datetime
mjd_utc = datetime2mjd_utc(date)
if mjd_utc == None:
return None
# Compute MJD_TT
mjd_tt = mjd_utc2mjd_tt(mjd_utc, dbg)
# Compute TT->TDB
# Convert geodetic position to geocentric distance from spin axis (r) and from
# equatorial plane (z)
(r, z) = S.sla_geoc(obsvr_lat, obsvr_hgt)
ut1 = compute_ut1(mjd_utc, dbg)
if dbg:
print "UT1=", ut1
# Compute relativistic clock correction TDB->TT
tdb_tt = S.sla_rcc(mjd_tt, ut1, -obsvr_long, r * auinkm, z * auinkm)
if dbg:
print "(TDB-TT)=", tdb_tt
if dbg:
print "(CT-UT)=", S.sla_dtt(mjd_utc) + tdb_tt
mjd_tdb = mjd_tt + (tdb_tt / 86400.0)
return mjd_tdb
def mjd_utc2mjd_tt(mjd_utc, dbg=False):
"""Converts a MJD in UTC (MJD_UTC) to a MJD in TT (Terrestial Time) which is
needed for any position/ephemeris-based calculations.
UTC->TT consists of: UTC->TAI = 10s offset + 26 leapseconds (last one 2015 Jul 1.)
TAI->TT = 32.184s fixed offset"""
# UTC->TT offset
tt_utc = S.sla_dtt(mjd_utc)
if dbg:
print "TT-UTC(s)=", tt_utc
# Correct MJD to MJD(TT)
mjd_tt = mjd_utc + (tt_utc / 86400.0)
if dbg:
print "MJD(TT) = ", mjd_tt
return mjd_tt
