Two traverses have been conducted for validation of the NASA Ice, Cloud, and land Satellite 2 on the flat interiors of the Antarctic and Greenland ice sheets. GNSS data collected on three separate 88S Traverses intersect 20% of the ICESat-2 reference ground tracks and have height errors of 5.6 and 8.0 cm. GNSS data from the monthly Summit Traverse in Greenland have 0.9 cm of bias and 5.7 cm precision. Data from these traverses were used to assess heights from ICESat-2, CryoSat-2, and Airborne Topographic Mapper (ATM). ICESat-2 heights have better than ±2.9 cm bias and better than ±6.7 cm precision. ATM heights have better than 9.3 cm bias and better than ±9.6 cm precision. CryoSat-2 heights have more than 20 cm of bias and more than ±40 cm precision. These best-case results are from the ice-sheet interiors but provide a characterization of the quality of satellite and airborne altimetry.