Using data from an array of ground-based single beam riometers and the CRRES Medium Energy B (MEB) particle detector we examine the signature of dispersionless electron injections. We find that it is possible to distinguish a dispersionless electron injection by it's integrated (>30 keV) flux signature at in situ satellites with a simple rise time criteria. If the total electron flux >30 keV exhibits a dramatic rise within 3 minutes then the signature is guaranteed to correspond to a dispersionless injection. Since the Kennel-Petschek limit for strong pitch angle scattering is almost certainly met during substorm injections and riometers respond to the >30 keV energy range, this translates to an equivalent criteria for riometer absorption. We present results obtained using our selection criteria for the ground-based identification of dispersionless electron injections including MLT dependence and initial spatial extent of the electron injection region.