TOPSAR Processing - Sentinel-1 SAR Technical Guide - Sentinel Online
The Terrain Observation with Progressive Scans SAR (TOPSAR) technique is a form of ScanSAR imaging, where data is acquired in bursts by cyclically switching the antenna beam between multiple adjacent sub-swaths. TOPSAR acquisitions can provide large swath widths and enhanced radiometric performance by reducing the scalloping effect. TOPSAR is used in SENTINEL-1's Interferometric Wide swath and Extra Wide swath modes.
With the TOPSAR technique, in addition to steering the beam in range as in ScanSAR, the beam is also electronically steered from backward to forward in the azimuth direction for each burst, avoiding scalloping and resulting in homogeneous image quality throughout the swath. 
TOPSAR mode is intended to replace the conventional ScanSAR mode, achieving the same coverage and resolution as ScanSAR, but with a nearly uniform Signal-to-Noise Ratio (SNR) and Distributed Target Ambiguity Ratio (DTAR).
Azimuth resolution is reduced compared to SM due to the shorter target illumination time of the burst. Using the sweeping azimuth pattern, each target is seen under the same antenna pattern, independently from its azimuth position in the burst image. By shrinking the azimuth antenna pattern, as seen by a target on the ground, scalloping effects on the image can be reduced. Bursts are synchronised from pass to pass to ensure the alignment of interferometric pairs.
For TOPSAR, the processing must handle the antenna steering rate and the DC rate due to the steering. The azimuth pre and post-processing of the data must include de-ramping of the data prior to base-band DC estimation, azimuth ambiguity estimation and GRD azimuth processing. Please see the technical note COPE-GSEG-EOPG-TN-14-0025 for details on how deramping is performed by the IPF.
TOPSAR azimuth antenna sweeping causes Doppler centroid variations of approximately 5.5 kHz introducing an azimuth phase ramp (azimuth fringes) for small co-registration errors. To correct this, azimuth co-registration is required to be better than 0.001 samples in order to obtain phase error less than 3°.
To be useful for generating interferograms, TOPSAR bursts are synchronized between repeat-pass data takes. A burst synchronization of <5 ms is required.
 De Zan, F., & Guarnieri, A. M. (2006). TOPSAR: Terrain Observation by Progressive Scans. Geoscience and Remote Sensing, IEEE Transactions on, 44(9), 2352-2360. doi:10.1109/TGRS.2006.873853