  In the last decade there has been a great interest in extending results for
codes equipped with the Hamming metric to analogous results for codes endowed
with the rank metric. This work follows this thread of research and studies the
characterization of systematic generator matrices (encoders) of codes with
maximum rank distance. In the context of Hamming distance these codes are the
so-called Maximum Distance Separable (MDS) codes and systematic encoders have
been fully investigated. In this paper we investigate the algebraic properties
and representation of encoders in systematic form of Maximum Rank Distance
(MRD) codes and Maximum Sum Rank Distance (MSRD) codes. We address both block
codes and convolutional codes separately and present necessary and sufficient
conditions for an encoder in systematic form to generate a code with maximum
(sum) rank distance. These characterizations are given in terms of certain
matrices that must be superregular in a extension field and that preserve
superregularity after some transformations performed over the base field. We
conclude the work presenting some examples of Maximum Sum Rank convolutional
codes over small fields. For the given parameters the examples obtained are
over smaller fields than the examples obtained by other authors.
