new file mode 100644
--- /dev/null
+++ b/scripts/quaternion/qderivmat.m
@@ -0,0 +1,23 @@
+function Dmat = qderivmat(Omega)
+# function Dmat = qderivmat(Omega)
+# Derivative of a quaternion.
+#   Let Q be a quaternion to transform a vector from a fixed frame to
+#      a rotating frame.  If the rotating frame is rotating about the 
+#      [x,y,z] axes at angular rates [wx, wy, wz], then the derivative
+#      of Q is given by
+#   Q' = qderivmat(Omega)*Q
+#
+#   If the passive convention is used (rotate the frame, not the vector),
+#   then Q' = -qderivmat(Omega)*Q; see the accompanying document
+#   quaternion.ps for details.
+
+Omega = vec(Omega);
+if(length(Omega) != 3)
+   error("qderivmat: Omega must be a length 3 vector.");
+endif
+
+Dmat = 0.5*[ 0.0,  Omega(3), -Omega(2),  Omega(1); ...
+ 	  -Omega(3),      0.0,  Omega(1),  Omega(2); ...
+  	  Omega(2), -Omega(1),      0.0,  Omega(3); ...
+ 	  -Omega(1), -Omega(2), -Omega(3),      0.0 ];
+endfunction