Table of contents
CSPICE_LTIME computes the transmission (or reception) time of a signal at
a specified target, given the reception (or transmission) time at a
specified observer. This routine also returns the elapsed time between
transmission and reception.
Given:
etobs the epoch(s) expressed as ephemeris seconds past J2000 TDB.
[1,n] = size(etobs); double = class(etobs)
This is the time at which an electromagnetic signal is "at"
the observer.
obs the NAIF ID of some observer.
[1,1] = size(obs); int32 = class(obs)
dir the direction the signal travels.
[1,2] = size(dir); char = class(dir)
or
[1,1] = size(dir); cell = class(dir)
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 the NAIF ID of the target.
[1,1] = size(targ); int32 = class(targ)
the call:
[ettarg, elapsd] = cspice_ltime( etobs, obs, dir, targ )
returns:
ettarg the epoch(s) expressed as ephemeris seconds past J2000 TDB
at which the electromagnetic signal is "at" the target body.
[1,n] = size(ettarg); double = class(ettarg)
Note `ettarg' is computed using only Newtonian
assumptions about the propagation of light.
elapsd the number of ephemeris seconds (TDB) between transmission
and receipt of the signal.
[1,n] = size(elapsd); double = class(elapsd)
`elapsd' is computed as:
elapsd = abs( etobs - ettarg )
`ettarg' and `elapsd' return with the same vectorization
measure, N, as `etobs'.
None.
Any numerical results shown for this example may differ between
platforms as the results depend on the SPICE kernels used as input
and the machine specific arithmetic implementation.
1) Suppose a signal is transmitted from Earth towards the Jupiter
system barycenter on July 4, 2004.
signal traveling to Jupiter system barycenter
* -._.-._.-._.-._.-._.-._.-._.-._.-> *
Earth (399) Jupiter system barycenter (5)
Compute the time at which the signal arrives at Jupiter
and the time it took the signal to arrive there (propagation
time).
Suppose also that another signal is received at the Earth from
Jupiter system barycenter at the same time.
signal sent from Jupiter system barycenter
* <-._.-._.-._.-._.-._.-._.-._.-._.- *
Earth (399) Jupiter system barycenter (5)
Compute the time at which the signal was transmitted from Jupiter,
and its propagation time.
Use the meta-kernel shown below to load the required SPICE
kernels.
KPL/MK
File name: ltime_ex1.tm
This meta-kernel is intended to support operation of SPICE
example programs. The kernels shown here should not be
assumed to contain adequate or correct versions of data
required by SPICE-based user applications.
In order for an application to use this meta-kernel, the
kernels referenced here must be present in the user's
current working directory.
The names and contents of the kernels referenced
by this meta-kernel are as follows:
File name Contents
--------- --------
de421.bsp Planetary ephemeris
naif0012.tls Leapseconds
\begindata
KERNELS_TO_LOAD = ( 'de421.bsp',
'naif0012.tls' )
\begintext
End of meta-kernel
Example code begins here.
function ltime_ex1()
%
% Load an SPK and leapseconds kernel.
%
cspice_furnsh( 'ltime_ex1.tm' )
%
% Suppose a signal originates from Earth towards the
% Jupiter system barycenter. Define the NAIF IDs
% for the observer, Earth (399), the target, Jupiter
% barycenter (5), and time of interest.
%
OBS = 399;
TARGET = 5;
TIME_STR = 'July 4, 2004';
%
% Convert the transmission time to ET.
%
et = cspice_str2et( TIME_STR);
%
% Determine the arrival time and the time for propagation.
%
[arrive, lt] = cspice_ltime( et, OBS, '->', TARGET);
%
% Convert the arrival time (ET) to UTC.
%
arrive_utc = cspice_et2utc( arrive, 'C', 3 );
%
% Output the results.
%
txt = sprintf( 'Transmission at (UTC) : %s', TIME_STR );
disp(txt)
txt = sprintf( 'The signal arrived at (UTC) : %s', arrive_utc );
disp(txt)
txt = sprintf( 'Time for propagation (secs) : %16.4f', lt );
disp(txt)
disp( ' ' )
%
% Now assume the signal originated at Jupiter barycenter,
% received by Earth at TIME_STR. Determine the transmission
% time and the time for propagation.
%
[receive, lt] = cspice_ltime( et, OBS, '<-', TARGET);
%
% Convert the reception time (ET) to UTC.
%
receive_utc = cspice_et2utc( receive, 'C', 3 );
%
% Output the results.
%
txt = sprintf( 'Reception at (UTC) : %s', TIME_STR );
disp(txt)
txt = sprintf( 'The signal sent at (UTC) : %s', receive_utc );
disp(txt)
txt = sprintf( 'Time for propagation (secs) : %16.4f', lt );
disp(txt)
%
% It's always good form to unload kernels after use,
% particularly in Matlab due to data persistence.
%
cspice_kclear
When this program was executed on a Mac/Intel/Octave6.x/64-bit
platform, the output was:
Transmission at (UTC) : July 4, 2004
The signal arrived at (UTC) : 2004 JUL 04 00:48:38.717
Time for propagation (secs) : 2918.7171
Reception at (UTC) : July 4, 2004
The signal sent at (UTC) : 2004 JUL 03 23:11:21.248
Time for propagation (secs) : 2918.7525
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) If `dir' is not one of '->' or '<-', the error
SPICE(BADDIRECTION) is signaled by a routine in the call tree
of this routine. 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.
3) If any of the input arguments, `etobs', `obs', `dir' or
`targ', is undefined, an error is signaled by the Matlab error
handling system.
4) If any of the input arguments, `etobs', `obs', `dir' or
`targ', is not of the expected type, or it does not have the
expected dimensions and size, an error is signaled by the Mice
interface.
None.
None.
MICE.REQ
TIME.REQ
None.
J. Diaz del Rio (ODC Space)
E.D. Wright (JPL)
-Mice Version 1.1.0, 01-NOV-2021 (EDW) (JDR)
Edited the header to comply with NAIF standard. Added
example's problem statement and meta-kernel.
Added -Parameters, -Particulars, -Exceptions, -Files, -Restrictions,
-Literature_References and -Author_and_Institution sections.
Eliminated use of "lasterror" in rethrow.
Removed reference to the function's corresponding CSPICE header from
-Required_Reading section.
-Mice Version 1.0.1, 10-MAR-2015 (EDW)
Edited -I/O section to conform to NAIF standard for Mice
documentation.
-Mice Version 1.0.0, 22-JAN-2006 (EDW)
Compute uplink and downlink light time
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