Satellite altimetry is all about timing. Sea-surface height can be measured by the time it takes for radar pulses to hit the ocean surface and bounce back to the spacecraft. Laser altimetry works in a similar manner, as laser pulses bounce off land-ice and sea-ice surfaces. But this method requires precise knowledge of satellite altitude, as well as the terrestrial reference frame (see question on how NASA studies sea level change). Extensive calibration is necessary to ensure the accuracy of altimetry measurements.