%                              veciecef.m
%  Scope:   This MATLAB macro performs the transformation from ECI (Earth 
%           Centered Inertial) to ECEF coordinates for a given position vector
%           and time elapsed since reference time; WGS-84 earth's rotation 
%           rate constant is used.
%  Usage:   pecef = veciecef(peci,dtime)
%  Description of parameters:
%           peci  -  input, ECI position vector (with components in meters)
%           dtime -  input, time elapsed since reference time in seconds
%           pecef -  output, ECEF position vector, with components in 
%                    same units as the input ECI position vector
%  Remarks: The ECI to ECEF transformation takes into account only Earth's 
%           rotation, i.e. no Earth's precession and mutation effects.
%  External Matlab macros used:  wgs84con
%  Last update:  04/05/00
%  Copyright (C) 1997-00 by LL Consulting. All Rights Reserved.

function   pecef = veciecef(peci,dtime)

wgs84con;
% global constants used: rot_rate (WGS-84 earth's rotation rate, in rad/sec)

swie = sin(rot_rate*dtime);
cwie = cos(rot_rate*dtime);

pecef = zeros(3,1);
pecef(1) = peci(1) * cwie + peci(2) * swie;
pecef(2) = peci(1) * swie - peci(2) * cwie;
pecef(3) = peci(3);