The MarsCAT (Mars Array of ionospheric Research Satellites using CubeSats for Aeronomy and Tomography) Mission is a multi 6U CubeSat mission to study the ionosphere of Mars to be proposed for a future mission of opportunity. The mission will investigate the plasma and magnetic structure of the Martian ionosphere, including transient plasma structures, magnetic field structure and dynamics, and energetic particle activity. The transit plan calls for a piggy back ride on a major mission using a CAT burn for Mars Orbit Insertion (MOI), the first demonstration of CubeSat propulsion for interplanetary travel. MarsCAT will make correlated multipoint studies of the ionosphere and magnetic field of Mars. Specifically, the several spacecraft will make in situ observations of the plasma density, temperature, and convection in the ionosphere of Mars. Multipoint observations are crucial requirements for attaining all the major science goals. They will also make total electron content measurements along the line of sight between the various spacecraft and simultaneous 3-axis local magnetic field measurements in several locations. Additionally, MarsCAT will demonstrate the performance of new CubeSat telemetry antennas designed at the University of Houston that are designed to be low profile, rugged, and with a higher gain than conventional monopole (whip) antennas. The MarsCAT CubeSats will have five science instruments: a 3-axis DC magnetometer, a double-Langmuir probe, a Thermal Ion Analyzer, a solid state energetic particle detector, and interspacecraft total electron content radio occulation experiment. The MarsCAT spacecraft will be solar powered and equipped with a RFT thruster that can provide up to 4.8 km/s of Δv, which is sufficient to achieve Mars orbit using a major mission piggyback. They have an active attitude control system, using a sun sensor and flight-proven star tracker for attitude determination, and momentum wheels for 3-axis attitude control.