The "SAR-altimetry" or "delay-Doppler altimetry" is a recent technique to process altimetry data. It was first operationally used for the CryoSat-2 mission, launched in 2010. In a nutshell, the technique uses the same concept as Synthetic Aperture Radar imagers by exploiting the Doppler effect caused by the satellite movement in the along-track direction. This provides a strong spatial resolution improvement in that direction, while across-track resolution remains the same than in conventional altimetry. 

SAR-altimetry is for a large part a question of processing. Compared to conventional altimetry (or “LRM”: Low Resolution Mode) radar waves (or pulses) are emitted at a higher rate  (~18 000 pulses per second instead of about 2000). This ensures that the phase of the signals remains relatively constant along consecutives pulses, a characteristic very useful in "unfocused" processing (see below). Since the satellite is in motion, ground reflectors backscatter the emitted signals at different frequencies depending on their along-track locations (i.e. in front, just below, or behind). This is the Doppler effect on which the SAR-altimetry rely. A Fast Fourier Transform applied to a pack (or burst) of consecutive reflected pulses (time dimension) discriminates the surface sampled on different Doppler strips on ground (frequency dimension). In practice, SAR altimetry unfocused processing is usually applied to bursts of 64 pulses. The radar wave footprint is thus split in 64 Doppler strips (or cells) about 300-meter wide in the along-track direction.