Altimeter bias errors (the ability to link overlapping missions)
Reference frame origin error (affects the satellite orbits)
Systematic vertical motion error (affects the altimeter calibration)
Total error (root-sum-squared)
models (see recommendations of the 2009 OCEANOBS workshop; Cazenave et al., 2010). All of the space-based techniques rely on an accurate reference frame, so maintaining and improving the accuracy of the terrestrial reference frame is of paramount importance for the study and for understanding global sea level rise. Ongoing research on altimeter drift and bias errors can be expected to reduce those uncertainties. The reference frame errors, however, are outside of the control of the altimeter data analysts and must be addressed by the geodetic community through improvements in the geodetic networks and the analysis of the data provided by those networks.
One of the most dramatic effects of global change is the melting of ice from continental glaciers and polar ice sheets. Mountain glaciers around the globe have been in fast retreat for the past few decades, and observations indicate that the Greenland and Antarctic ice sheets are beginning to lose mass at alarming rates (Lemke et al., 2007). The acceleration of ice loss in Greenland and Antarctica was not widely anticipated before it was observed, and explanations to account for this acceleration are still incomplete. Only through careful monitoring of the ice sheets—using techniques that rely heavily on geodetic infrastructure—was the acceleration noticed and quantified. The continued application of current and future geodetic techniques is required for the scientific community to be able to monitor the ice sheet mass balance at the accuracy needed to understand what is happening today and to develop models for predicting future ice sheet mass changes. Of particular importance is the systematic application of geodetic imaging techniques that use radar and LiDAR to produce images of the Earth’s surface wherein the location each pixel is known with geodetic precision, so that the difference between two pictures of the same area can be interpreted geologically and physically.
Information about the mass balance of the ice sheets is based on four types of remote sensing and ground techniques: (1) elevation change of the ice sheet, as measured by laser altimetry (for example, IceSAT), satellite radar altimetry (for example, ERS, EnviSat), and ground-based GNSS/GPS receivers; (2) measurements of horizontal velocities near the grounding line (where the ice starts to float free of its bed) of outlet glaciers using either on-ice GNSS/GPS receivers or satellite-based InSAR (for example, ERS, Radarsat, Envisat, ALOS satellites); (3) changes in the gravity field above the ice sheet, as measured by space-based gravimetry; and (4) geodetic measurements (for example, GNSS/GPS, gravity) of vertical motions in deglaciated areas. Monitoring changes in ice sheet elevation provides a direct estimate of changes in ice sheet volume, from which mass change is deduced based on the density through the snow/ice column. However, an accurate orbit is needed to achieve these calculations, which in turn depends on the accuracy of the reference frame. The GRACE mission (see Box 3.1) uses satellite-to-satellite tracking, as well as measurements from onboard GPS receivers and accelerometers, to determine global, monthly gravity solutions. These solutions can be used to map monthly changes in the distribu-