I. What VTD is for?
-------------------

  VTD is a library of routines that compute very long baseline 
interferometry (VLBI) time delay or Doppler frequency shift with 
the highest accuracy. The accuracy of time delay is limited by 
accuracy of a priori data. Had the a priori data been known 
perfectly, the errors of path delay were at a level of 10^{-13}.
Realistic errors of VLBI time delay prediction are at a
level of 40--400 picoseconds of observations at frequencies 
higher than 8 GHz, and the errors may reach tens nanoseconds 
for observations at frequencies at or below 1.6 GHz due to 
uncertainties in the a priori ionosphere path delay.

  VTD is not an executable, but a library. This design provides 
you a maximal flexibility in developing your application that
uses library VTD.


II. Linking.
------------

  VTD depends on a) library petools ; b) library BLAS (basic 
linear algebra subroutines) and some extra system library;
c) [optionally] cfitsio library. For your convenience, definitions 
of environment libraries are gathered in ${prefix}/bin/vtd_vars 
(csh and tcsh) and ${prefix}/bin/vtd_vars.sh (bash and sh shells).

  When you sourced ${prefix}/bin/vtd_vars (i.e. execute it under
C-shell as "source ${prefix}/bin/vtd_vars") or ${prefix}/bin/vtd_vars.sh
the definitions of environment variables used by VTD become available. 
Include files related to vtd are in $VTD_INC directory, vtd library can 
be found in $VTD_LIB/libvtd.a or $VTD_LIB/libvtd.so, data files are in 
$VTD_DATA. When you compile  your application against vtd, you should 
specify these libraries in the makefile:

LIBS 	=				\
          (... your libraries ...)      \
	-L $(MY_LIBS) -lvtd  -lfitslib -lcfitsio \
        -L $(PETOOLS_ROOT) -lpetools    \
        $(SOLVE_LIB_VEC)		\
	$(SOLVE_LIB_BLAS) 		\
	$(SOLVE_EXTRA_LIB) 

  If you compiled VTD without support of cfitsio, the lines 
-lfitslib -lcfitsio should be dropped.

  NB: some compilers by default insert trailing underscores to global
variables. VTD is compiled **without trailing underscores**. You should
disable inserting trailing underscores when you compile routines that
call VTD.

  Recommended lines in makefiles for compilation of VTD-compatible 
applications:

.f.o:
	$(MK5_F95) -I$(VTD_INC) -c -o $*.o $*.f  
or
	$(MK5_F95_OPT) -I$(VTD_INC) -c -o $*.o $*.f  
.c.o:
	$(MK5_C) -Wstrict-prototypes -I$(VTD_ROOT)/include -c -o $*.o $*.c  

  The environment variables above are defined in file 
${prefix}/bin/vtd_vars (or ${prefix}/bin/vtd_vars.sh) where ${prefix} 
is the name of the directory where VTD is installed.

Refer to example/makefile


III. C-bindings:
----------------


  Although VTD is written in Fortran, nothing prevents its using from C.
Several tips for C-programmers:
a) All arguments are passed to VTD routines by reference.
b) Strings in Fortran may or may not have trailing zeros.
c) When a string is passed as an argument to Fortran routine,
   then it should by accompanied by a hidden argument string length
   that follows the last actual argument. String length is of type
   int and it is passed by value. If more than one string is passed
   to a Fortran routine, than hidden arguments follow the last
   actual argument in the order of string arguments.

The header file vtd.h is for compiling applications that call VTD routines.
Refer to example/vtd_testware_05.c . That example is re-write of 
example/vtd_testware_01.f in C. The give identical results. 


IV. Routine description

    See user guide in doc/vtd_01.txt