The tremendous progress of small satellite technologies in recent years brings unique opportunities for satellite radar altimetry. Flying radar altimeters onboard a group of small satellites could provide better swath observation, an equivalent or even improved performance and coverage at a much lower price, while complementing large satellite missions. To this end, a feasibility study of a Ka-band altimeter CubeSat constellation for ocean monitoring, called AltiCube, is being carried out by the Delft University of Technology, under the support of the European Space Agency. This paper provides an overview of the AltiCube mission study. First, the potential observation products and performance of five constellation/formation concepts are analyzed. Then the requirements on the platform are discussed based on these concepts. The main focus is on the payload performance and the needed delta-V for the constellation/formation maintenance. Based on the assessment with respect to performance, platform, technological readiness level, cost and other factors, the along-track comb formation (consisting of five identical CubeSats) is selected. Further details of its system design is given in the paper. The selected CubeSat altimeter along-track comb formation shows the capability to map the ocean sub-mesoscale structures, which is extremely hard to monitor using traditional large spaceborne radar altimeters.