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LITMSOL

LITMSOL

## Author

Craig B. Markwardt, NASA/GSFC Code 662, Greenbelt, MD 20770
craigm@lheamail.gsfc.nasa.gov
UPDATED VERSIONs can be found on my WEB PAGE:
http://cow.physics.wisc.edu/~craigm/idl/idl.html

## Purpose

Solve the light-time equation between two moving bodies

## Major Topics

Geometry, Physics

## Calling Sequence

LITMSOL, T1, X1, Y1, Z1, T2, INFO2, RAW2, OBJ2, INFOSUN, RAWSUN, \$
/RECEIVER, TBASE=, TOLERANCE=, POSUNITS=, MAXITER=, \$
/NO_SHAPIRO

## Description

The procedure LITMSOL solves the light time equation between two
moving bodies in the solar system. Given the time and position of
reception or transmission of a photon, this equation determines the
time of transmission or reception at the other solar system body.
Since both bodies may be moving, the equation must be solved
iteratively.
The trajectories of solar system bodies must be described by either
a JPL ephemeris, or by a JPL-like ephemeris generated by
JPLEPHMAKE. This routine calls JPLEPHINTERP.
The user specifies the known time and position of interaction as
T1, X1, Y1 and Z1, in units of POSUNITS. The time of interaction
at the other body -- the solution to the light time equation -- is
returned as T2. If the photon was *received* at time T1, then the
RECEIVER keyword should be set (in which case the transmission must
have occurred in the past).
Since the solution is iterative, the user may specify a solution
tolerance, and a maximum number of iterations.
If users wish to include the Shapiro time delay, which has a
maximum amplitude of approximately 250 usec, they must specify the
ephemeris of the Sun (INFOSUN, RAWSUN). The Shapiro delay is the
extra general relativistic delay caused by the Sun's potential.

## Inputs

T1 - epoch of interaction, in Julian days, in the TDB timescale.
(scalar or vector)
X1, Y1, Z1 - coordinates of interaction, referred to the solar
system barycenter, in J2000 coordinates. Units are
described by POSUNITS. (scalar or vector)
INFO2, RAW2 - ephemeris of other solar system body, returned by
INFOSUN, RAWSUN - ephemeris of at least the Sun, as returned by
JPLEPHREAD. Only used of NO_SHAPIRO is not set.

## Outputs

T2 - upon output, epoch of interaction at the second solar system
body, in Julian days, in the TDB timescale.

## Keyword Parameters

RECEIVER - if set, then the epoch T1 is a reception of a photon.
Otherwise T1 is the epoch of transmission of a photon.
VX1, VY1, VZ1 - upon input, the body velocity at time T1, in
VELUNITS units. This information is required only
if the SHAPIRO_DERIV is required.
X2, Y2, Z2 - upon return, the body position at time T2, in
POSUNITS units.
VX2, VY2, VZ2 - upon return, the body velocity at time T2, in
VELUNITS units.
TBASE - a fixed epoch time (Julian days) to be added to each value
of T1. Since subtraction of large numbers occurs with
TBASE first, the greatest precision is achieved when TBASE
is expressed as a nearby julian epoch, T1 is expressed
as a small offset from the fixed epoch.
Default: 0
POSUNITS - the units for positions, one of 'CM', 'KM', 'LT-S' or
'AU'.
Default: 'CM'
VELUNITS - the units for velocities (and Shapiro derivative).
Default: POSUNITS+'/S'
TOLERANCE - the solution tolerance, expressed in POSUNITS.
Default: 1000 CM
ERROR - upon return, a vector giving the estimated error in the
solution for each point, expressed in POSUNITS. This
quantity should be less than TOLERANCE unless the number
of iterations exceeded MAXITER.
MAXITER - maximum number of solution iterations to be taken.
Default: 5
NITER - upon return, contains the actual number of iterations used.
SHAPIRO_CALC - method of calculating Shapiro delay, a string with
one value of 'NONE', 'DELAY' or 'BOTH'. NONE means
do not calculate any Shapiro delay values. DELAY
means calculate Shapiro delay only. BOTH means
calculate the delay *and* its derivative with
respect to time. If SHAPIRO_CALC is set to
DELAY or BOTH, then INFOSUN and RAWSUN must be
specified. If BOTH, then VX1, VY1 and VZ1 must
also be specified. This keyword overrides
NO_SHAPIRO.
NO_SHAPIRO - if set, then the Shapiro delay will not be accounted
SHAPIRO_DELAY - upon return, contains the Shapiro delay in
seconds, if SHAPIRO_CALC is set to 'DELAY' or
'BOTH'.
SHAPIRO_DERIV - upon return, contains the derivative of the
Shapiro delay, in light seconds per time unit of
velocity (SHAPIRO_CALC must be set to 'BOTH' to
enable this calculation). Note that you must
supply VX1, VY1 and VZ1 to get the derivative
value.

## Modification History

Written, 6 May 2002, CM
Documented, 12 May 2002, CM
Added TGUESS keyword, 29 May 2002, CM
Added ERROR and X/Y/ZOFF keywords, 25 Sep 2002, CM