NASA studies changes in sea-level primarily by using orbiting spacecraft. These satellites capture the changes with altimeters – instruments that rely on radar or laser pulses. These altimetry measurements can be combined with data from coastal tide gauges and from Argo floats, a global network of mobile ocean sensors that move up and down the water column, to provide a fuller picture of sea-level rise. To ensure the accuracy of such measurements, NASA helps maintain another global network of ground-based and orbital instruments that track the position and drift of geographic points – creating the International Terrestrial Reference Frame.
NASA also computes components of sea level using measurements by ship and plane, through campaigns like “Oceans Melting Greenland (OMG)” and Operation IceBridge. The idea is to measure a variety of contributors to sea-level rise, including melting land ice and other factors, to see how they all “add up” to observed sea-level rise.