%                              uercor.m
%  Scope:   This MATLAB macro computes user earth rotation correction vector as per
%           ICD-GPS-200C [1]. WGS-84 constants are used.
%  Usage:   xercor = uercor(upos,dt)
%  Description of parameters:
%           upos   - input, user ECEF position, the components are in meters
%           dt     - input, time delay of signal propagation from satellite to user,
%                    in seconds
%           xercor - output, user earth rotation correction vector, ECEF components are 
%                    in meters
%  External Matlab macros used:  wgs84con
%  References:  [1] Anon., Navstar GPS Space Segment/Navigation User Interfaces, 
%                   ICD-GPS-200C, October 10, 1993
%               [2] DiEsposti, R., Time-dependency and coordinate system issue in GPS
%                   measurement models. ION GPS-2000, Salt Lake City, UT, September
%                   19-22, 2000
%  Last update:  09/26/00
%  Copyright (C) 1999-00 by LL Consulting. All Rights Reserved.

function    xercor = uercor(upos,dt)

wgs84con;
% global constants used:  rot_rate

xercor = zeros(3,1);
temp = rot_rate*dt;
xercor(1) = - temp*upos(2);
xercor(2) =   temp*upos(1);
xercor(3) =   0.;