void ltime_c ( SpiceDouble etobs,
SpiceInt obs,
ConstSpiceChar * dir,
SpiceInt targ,
SpiceDouble * ettarg,
SpiceDouble * elapsd )
This routine computes the transmit (or receive) time
of a signal at a specified target, given the receive
(or transmit) time at a specified observer. The elapsed
time between transmit and receive is also returned.
None.
SPK
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
etobs is an epoch expressed in ephemeris seconds (TDB)
past the epoch of the J2000 reference system.
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.
ettarg is the epoch, expressed in 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 = fabs( etobs - ettarg )
None.
1) If dir is not one of "->" or "<-" the error SPICE(BADDIRECTION)
will be signalled. In this case ettarg and elapsd will not be
modified.
2)
If insufficient ephemeris information is available to compute the
outputs ettarg and elapsd, or if observer or target are not
recognized, the problem is diagnosed 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.
None.
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.
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.)
#include <stdio.h>
#include "SpiceUsr.h"
.
.
.
#define OBS 399001
#define TARG -77
#define LENOUT 81
#define OBSUTC "1999 May 25"
#define LSK "leapseconds.ker"
SpiceChar timestr [ LENOUT ];
SpiceDouble arrive;
SpiceDouble howlng;
SpiceDouble etobs;
SpiceDouble sent;
[ load kernels ]
str2et_c ( OBSUTC, &etobs );
ltime_c ( etobs, OBS, "->", TARG, &arrive, &howlng );
etcal_c ( arrive, LENOUT, timestr );
printf ( "The signal arrived at time: %s\n", timestr );
printf ( "It took %15.6f seconds to get there.\n", howlng );
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.
ltime_c ( etobs, OBS, "<-", TARG, &sent, &howlng );
etcal_c ( sent, LENOUT, timestr );
printf ( "The signal was transmitted at: %s\n", timestr );
printf ( "It took %15.6f seconds to get there.\n", howlng );
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.
#define GSTONE 399001
#define SC -77
#define CANBER 399002
str2et_c ( OBSUTC, &etgold );
ltime_c ( etgold, GSTONE, "->", SC, &scget, <1 );
/.
Account for the microsecond delay between receipt and
transmission.
./
scsend = scget + 0.000001;
ltime_c ( scsend, SC, "->", CANBER, &etcanb, <2 );
rndtrp = etcanb - etgold;
printf ( "The signal arrives in Canberra at ET: %15.6f\n"
"Round trip time for the signal was: %15.6f\n",
etcanb,
rndtrp );
None.
None.
N.J. Bachman (JPL)
W.L. Taber (JPL)
-CSPICE Version 1.0.1, 09-NOV-2006 (NJB)
Corrected a reference to the function j2000_c; this had been
erroneously changed from the name J2000 to j2000_c
during translation from Fortran.
Re-ordered header sections to conform to standard.
-CSPICE Version 1.0.0, 29-MAY-1999 (WLT) (NJB)
Compute uplink and downlink light time
Link to routine ltime_c source file ltime_c.c
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