This study proposes a novel cross-validation method to assess the instrumental stability in sea level trends. The method involves implementing a second tandem flight phase between two successive altimeter missions a few years after the first phase. The trend in systematic instrumental differences made during the two tandem phases can be estimated below ± 0.1 mm yr^-1 (16–84 % confidence level) on a global scale for time intervals between the tandem phases of 4 years or more.