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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.
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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 (TDB) past the epoch of the j2000_c reference system 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 )
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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 );
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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.
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N.J. Bachman (JPL) W.L. Taber (JPL)
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-CSPICE Version 1.0.0, 29-MAY-1999 (WLT) (NJB)
Compute uplink and downlink light time