  We show that the free distance, as a function on a space parameterizing a
family of convolutional codes, is a lower-semicontinuous function and that,
therefore, the property of being Maximum Distance Separable (MDS) is an open
condition. For a class of convolutional codes, an algorithm is offered to
compute the free distance. The behaviour of the free distance by enlargements
of the alphabet and by increasing the length is also studied. As an
application, the algebraic equations characterizing the subfamily of MDS codes
is explicitly computed for families of 1-dimensional convolutional Goppa codes
(CGC).
