A system can include a near-field instrument to be placed inside a chamber of a heart, a far-field instrument to be placed in a stable position in relation to the heart, and a control unit. The control unit is configured to identify a unique pattern in electrogram information received from the far-field instrument when the near-field instrument is in one or more positions within the heart. When the unique pattern is detected, the control unit is configured to receive electrogram information from the near-field instrument. While recording electrogram information from the near-field instrument, the control unit is also configured to record voltage and complex fractionated atrial electrogram (CFAE) characteristics of the tissue inside a heart chamber. This information combined with rotor information can be used to identify substrate versus non-substrate rotor characteristics.