Spaceborne radar altimeters have proven to be effective tools for measuring global water levels. In the coastal zone and over inland water bodies, increased cross-track resolution, better spatiotemporal coverage, and increased altitude accuracy are essential. Radar interferometry can be used to obtain sub-kilometer resolution over a swath at the expense of additional transmit power and a sufficiently long baseline to accommodate at least two antennas. A good example is the NASA’s SWOT, a satellite with two 5-meters radar altimeter antennas on a 10-meters long boom, paid with 15 years’ development time and more than one billion dollars cost. As a consequence, it is unlikely that oceanographers will ever get a much wanted high spatiotemporal revisit rate. However, if the cost of a single altimeter can be significantly reduced, then launching multiple systems to obtain a constellation with good spatiotemporal coverage could overcome the problem. Instead of a distributed interferometric swarm requiring extremely high accurate time/attitude synchronizations and centimeter level formation maintenance, hereby we propose AltiCube+, a low-cost long fixed-baseline radar altimeter solution using an aggregated CubeSat swarm. Based on this solution, various altimeter concepts including off-nadir SARIn and a Multiple-Input and Multiple-Output (MIMO) will be explored, aiming at sub-kilometer level cross-track resolution. The uniqueness of AltiCube+ is on the potential scientific opportunities brought by the fixed long baseline (10 m and above) and the sustainability due to its significantly low cost (~20 MEuros) and short development lifecycle (3 years for IOD, In Orbit Demonstration). If budget allows, multiple AltiCube+ systems with same or different altimetry capabilities can form a constellation to dramatically reduce the revisit time and, therefore, provide much better spatiotemporal coverage.
