Procedure

     LTIME ( Light Time )

     SUBROUTINE LTIME ( ETOBS, OBS, DIR, TARG, ETTARG, ELAPSD )

Abstract

     Compute the transmission (or reception) time of a signal at a
     specified target, given the reception (or transmission) time at a
     specified observer. Also return the elapsed time between
     transmission and reception.

Required_Reading

     None.

Keywords

     SPK

Declarations

     IMPLICIT NONE

     DOUBLE PRECISION      ETOBS
     INTEGER               OBS
     CHARACTER*(2)         DIR
     INTEGER               TARG
     DOUBLE PRECISION      ETTARG
     DOUBLE PRECISION      ELAPSD

Brief_I/O

     VARIABLE  I/O  DESCRIPTION
     --------  ---  --------------------------------------------------
     ETOBS      I   Epoch of a signal at some observer.
     OBS        I   NAIF ID of some observer.
     DIR        I   Direction the signal travels ( '->' or '<-' ).
     TARG       I   NAIF ID of the target object.
     ETTARG     O   Epoch of the signal at the target.
     ELAPSD     O   Time between transmit and receipt of the signal.

Detailed_Input

     ETOBS    is an epoch expressed as ephemeris seconds past J2000
              TDB. This is the time at which an electromagnetic signal
              is "at" the observer.

     OBS      is the NAIF ID of some observer.

     DIR      is the direction the signal travels. The
              acceptable values are '->' and '<-'. When
              you read the calling sequence from left to
              right, the "arrow" given by DIR indicates
              which way the electromagnetic signal is traveling.

              If the argument list reads as below,

                 ..., OBS, '->', TARG, ...

              the signal is traveling from the observer to the
              target.

              If the argument reads as

                 ..., OBS, '<-', TARG

              the signal is traveling from the target to
              the observer.

     TARG     is the NAIF ID of the target.

Detailed_Output

     ETTARG   is the epoch expressed as ephemeris seconds past J2000
              TDB at which the electromagnetic signal is "at" the
              target body.

              Note ETTARG is computed using only Newtonian
              assumptions about the propagation of light.

     ELAPSD   is the number of ephemeris seconds (TDB) between
              transmission and receipt of the signal.

              ELAPSD is computed as:

                 ELAPSD = DABS( ETOBS - ETTARG )

Parameters

     None.

Exceptions

     1)  If DIR is not one of '->' or '<-', the error
         SPICE(BADDIRECTION) is signaled. In this case
         ETTARG and ELAPSD will not be altered from their
         input values.

     2)  If insufficient ephemeris information is available to
         compute the outputs ETTARG and ELAPSD, or if observer
         or target is not recognized, an error is signaled
         by a routine in the call tree of this routine.

         In this case, the value of ETTARG will be set to ETOBS
         and ELAPSD will be set to zero.

Files

     None.

Particulars

     Suppose a radio signal travels between two solar system
     objects. Given an ephemeris for the two objects, which way
     the signal is traveling, and the time when the signal is
     "at" at one of the objects (the observer OBS), this routine
     determines when the signal is "at" the other object (the
     target TARG). It also returns the elapsed time between
     transmission and receipt of the signal.

Examples

     Example 1.
     ----------
     Suppose a signal is transmitted at time ET from the Goldstone
     tracking site (id-code 399001) to a spacecraft whose id-code
     is -77.


           signal traveling to spacecraft
       *  -._.-._.-._.-._.-._.-._.-._.-._.->  *

       Goldstone (OBS=399001)            Spacecraft (TARG = -77)
       at epoch ETOBS(given)             at epoch ETTARG(unknown)

     Assuming that all of the required SPICE kernels have been
     loaded, the code fragment below shows how to compute the
     time (ARRIVE) at which the signal arrives at the spacecraft
     and how long (HOWLNG) it took the signal to reach the spacecraft.
     (Note that we display the arrival time as the number of seconds
     past J2000.)

        OBS   = 399001
        TARG  = -77
        ETOBS = ET

        CALL LTIME ( ETOBS, OBS, '->', TARG, ARRIVE, HOWLNG )
        CALL ETCAL

        WRITE (*,*) 'The signal arrived at time: ', ARRIVE
        WRITE (*,*) 'It took ', HOWLNG, ' seconds to get there.'


     Example 2.
     ----------
     Suppose a signal is received at the Goldstone tracking sight
     at epoch ET from the spacecraft of the previous example.

               signal sent from spacecraft
         *  <-._.-._.-._.-._.-._.-._.-._.-._.- *

       Goldstone (OBS=399001)               Spacecraft (TARG = -77)
       at epoch ETOBS(given)                at epoch ETTARG(unknown)

     Again assuming that all the required kernels have been loaded
     the code fragment below computes the epoch at which the
     signal was transmitted from the spacecraft.

        OBS   = 399001
        TARG  = -77
        ETOBS = ET

        CALL LTIME ( ETOBS, OBS, '<-', TARG, SENT, HOWLNG )
        CALL ETCAL

        WRITE (*,*) 'The signal was transmitted at: ', SENT
        WRITE (*,*) 'It took ', HOWLNG, ' seconds to get here.'

     EXAMPLE 3
     ---------
     Suppose there is a transponder on board the spacecraft of
     the previous examples that transmits a signal back to the
     sender exactly 1 microsecond after a signal arrives at
     the spacecraft. If we send a signal from Goldstone
     to the spacecraft and wait to receive it at Canberra.
     What will be the epoch at which the return signal arrives
     in Canberra? ( The id-code for Canberra is 399002 ).

     Again, assuming we've loaded all the necessary kernels,
     the fragment below will give us the answer.

        GSTONE = 399001
        SC     = -77
        CANBER = 399002
        ETGOLD = ET

        CALL LTIME ( ETGOLD, GSTONE, '->', SC, SCGET, LT1 )

     Account for the microsecond delay between receipt and transmit

        SCSEND = SCGET + 0.000001

        CALL LTIME ( SCSEND, SC, '->', CANBER, ETCANB, LT2 )

        RNDTRP = ETCANB - ETGOLD

        WRITE (*,*) 'The  signal arrives in Canberra at: ', ETCANB
        WRITE (*,*) 'Round trip time for the signal was: ', RNDTRP

Restrictions

     None.

Literature_References

     None.

Author_and_Institution

     J. Diaz del Rio    (ODC Space)
     W.L. Taber         (JPL)
     E.D. Wright        (JPL)

Version

    SPICELIB Version 1.1.3, 26-OCT-2021 (JDR)

        Edited the header to comply with NAIF standard.

    SPICELIB Version 1.1.2, 22-SEP-2004 (EDW)

        Placed Copyright after $Abstract.

    SPICELIB Version 1.1.1, 18-NOV-1996 (WLT)

        Errors in the $Examples section were corrected.

    SPICELIB Version 1.1.0, 10-JUL-1996 (WLT)

        Added Copyright Notice to the header.

    SPICELIB Version 1.0.0, 10-NOV-1995 (WLT)