1 files changed, 41 insertions, 31 deletions

diff --git a/doc/documention.tex b/doc/documention.tex
index 5629beb..44038b7 100644
--- a/doc/documention.tex
+++ b/doc/documention.tex
@@ -13,51 +13,61 @@
 This thorn provides a routines for calculating
 the timestep for an evolution based on the spatial Cartesian grid spacing and
 a wave speed. 
-\section{Comments}
-
-There are currently two methods for calculating the timestep, either the
-simple courant method, or a dynamic courant method. The method is chosen
-using the keyword parameter {\tt time::method}
-The timestep,  is passed into the Cactus variable {\tt cctk\_delta\_time}.
-Both timesteps are based on the Courant condition, which states that for 
-numerical stability, the chosen timestep should satisfy
-$$
-\Delta t \le \mbox{min}(\Delta x^i)/\mbox{wave speed}/\sqrt(\mbox{dim})
-$$
 
-The two methods currently implemented are:
+\section{Description}
+
+Thorn {\tt Time} uses one of four methods to decide on the timestep
+to be used for the simulation. The method is chosen using the
+keyword parameter {\tt time::timestep\_method}. (Note: In releases Beta 8 and 
+earlier the parameter used was {\tt time::courant\_method}
 \begin{itemize}
 
-\item {\tt time::method = "standard"} (this is the default). The timestep is calculated once
-   at the start of the run in the {\tt BASEGRID} timebin, and is then held static
-   throughout the run. The algorithm, which uses the parameter {\tt time::dtfac} is
+\item{} {\tt time::timestep\_method = ``given''} The timestep is fixed to the
+	value of the parameter {\tt time::timestep}. 
+
+\item{} {\tt time::timestep\_method = ``courant\_static''} This is the default
+	method, which calculates the timestep once at the start of the
+	simulation, based on a simple courant type condition using 
+	the spatial gridsizes and the parameter {\tt time::dtfac}.
 $$
 \Delta t = \mbox{\tt dtfac} * \mbox{min}(\Delta x^i)
 $$
-	Note that the parameter {\tt dtfac} should take into account the dimension
-        of the space being used, and the wave speed.
-
-\item {\tt time::method = "courant"}. The timestep is calculated dynamically at the
-	start of each iteration in the {\tt PRESTEP} timebin. The algorithm is
+	Note that it is up to the user to custom {\tt dtfac} to take
+	into account the dimension of the space being used, and the wave speed.
+
+\item{} {\tt time::timestep\_method = ``courant\_speed''} This choice implements a 
+	dynamic courant type condition, the timestep being set before each
+	timestep using the spatial dimension of the grid, the spatial grid sizes, the 
+	parameter {\tt courant\_fac} and the grid variable {\tt courant\_wave\_speed}. 
+	The algorithm used is
 $$
-\Delta t = \mbox{\tt courant\_fac} * \mbox{min}(\Delta x^i)/\mbox{maximum wavespeed}/\sqrt(\mbox{dim})
+\Delta t = \mbox{\tt courant\_fac} * \mbox{min}(\Delta x^i)/\mbox{courant\_wave\_speed}/\sqrt(\mbox{dim})
 $$
+	For this algorithm to be successful, the variable {\tt courant\_wave\_speed}
+	must have been set by a thorn to the maximum wave speed on the grid.
 
-
-\item {\tt time::method = "courant\_time"}. The timestep is calculated dynamically at the
-    start of each iteration in the {\tt PRESTEP} timebin. The algorithm is
+\item{} {\tt time::timestep\_method = ``courant\_time''} This choice is similar to the
+	method {\tt courant\_speed} above, in implementing a dynamic timestep.
+	However the timestep is chosen using
 $$
-\Delta t = \mbox{\tt courant\_fac} * \mbox{minimum time to cross a zone}/\sqrt(\mbox{dim})
+\Delta t = \mbox{\tt courant\_fac} * \mbox{\tt courant\_min\_time}/\sqrt(\mbox{dim})
 $$
+        where the grid variable {\tt courant\_min\_time} must be set by a thorn to
+	the minimum time for a wave to cross a gridzone.
+
+\end{itemize}
 
-\section{Technical Details}
+In all cases, Thorn {\tt Time} sets the Cactus variable {\tt cctk\_delta\_time}
+which is passed as part of the macro {\tt CCTK\_ARGUMENTS} to thorns called 
+by the scheduler.
 
-If a dynamic {\tt courant} condition is selected, a thorn must set the protected variable
-{\tt courant\_wave\_speed} for the maximum wave speed before Time sets the timestep. 
+Note that for hyperbolic problems, the Courant condition gives a minimum 
+requirement for stability, namely that the numerical domain of dependency
+must encompass the physical domain of dependency, or
+$$
+\Delta t \le \mbox{min}(\Delta x^i)/\mbox{wave speed}/\sqrt(\mbox{dim})
+$$
 
-If a dynamic {\tt courant\_time} condition is selected, a thorn must set the protected variable
-{\tt courant\_time} for the minimum time for a wave to cross a zone
- before Time sets the timestep. 
 
 \end{itemize}