Multiple CubeSat altimeters can work independently or corporately to form altimeter constellations. Different configurations of the constellations can acquire distinguished advantages: improved spatial/temporal sampling and high cross-track resolution, which will be helpful for observations of oceanic small-scale structures and weather forecasting. Compared to single conventional altimeters, CubeSat altimeter constellations may achieve better performances with lower costs. To fully understand these systems, this article focuses on the performance analysis and methodology for CubeSat altimeter constellations. Besides the typical analyses of the resolution, revisit, and absolute sea surface height (SSH) accuracy, the performance analysis was conducted by considering the characteristics of multiple measurements provided by CubeSat altimeter constellations. Local and global spatial sampling performances are investigated for various constellations and compared by sampling density and swath size. Moreover, relative SSH accuracy is introduced and evaluated based on the spatial structure functions of errors to effectively evaluate the measurement performance. Related system requirements on power, delta-v, etc., to achieve the performance are also discussed, which ensures that the analysis fits the boundary conditions of implementation. Finally, different concepts of the CubeSat altimeter constellations are compared, where their limitations and possible solutions are also discussed.

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.