# Ocean Surface Topography Altimetry by Large Baseline Cross-Interferometry from Satellite Formation
###### Abstract  
Imaging Radar Altimeter (IRA) is the current development tendency for ocean surface topography (OST) altimetry, which utilizes Synthetic Aperture Radar (SAR) and interferometry to improve the spatial resolution of OST to several kilometers or even better. Meanwhile, centimetric altimetry accuracy should be guaranteed for applications such as geostrophic currents or marine gravity anomaly inversion. However, the baseline length of IRA which determines the altimetric sensitivity is confined by the satellite platform, in consideration of baseline vibration and payload capability. Therefore, the baseline length from a single satellite can extend to only tens of meters, making it difficult to achieve centimetric accuracy. Referring to the successful experience from TerraSAR-X/TanDEM-X, satellite formation can easily extend the baseline length to hundreds or thousands of meters, depending on the helix orbit. Therefore, we propose the large baseline IRA (LB-IRA) from satellite formation for OST altimetry: the carrier frequency shift (CFS) is brought in to compensate for the severe baseline decorrelation, and the helix orbit is carefully selected to prevent severe time decorrelation from along-track baseline. The numerical results indicate that the LB-IRA, whose cross-track baseline ranges between 629-1000 m and along-tack baseline ranges between 0-40 m, can achieve \(\sim\)1 cm relative accuracy at 1 km resolution. satellite formation; carrier frequency shift (CFS); ocean surface topography (OST) altimetry; cross-interferometry 2020 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Vol. 2, No. 2, pp. 2020-2020  
altimetry, however, none of them can be directly used for OST altimetry due to the coarse accuracy, which is no better than a few meters. Therefore, the concept of Imaging Radar Altimeter (IRA) was first proposed by [PERSON] (2016), which can be considered as the special version of InSAR, dedicated for OST altimetry with an altimetric accuracy that should be at least comparative to traditional radar altimeters ([PERSON] _et al._, 2017). Another distinguishable characteristic of IRA is its wide swath, which resides along one side of the nadir track and extends to tens of kilometers wide. After aperture synthesis and pulse compression ([PERSON] _et al._, 2017), the intrinsic resolution of IRA is only a couple of meters. The most prominent IRA plan is the Surface Water and Ocean Topography (SWOT) mission ([PERSON] _et al._, 2017; [PERSON] _et al._, 2017). The primary payload of SWOT satellite is the Ka-band Radar Interferometer (KaRIn), whose dual transmit/receive antennas are connected by a deployable rigid mast. The interferometric baseline, which is the distance between the phase centers of two antennas, is about 10 m ([PERSON] _et al._, 2017). The baseline length of KaRIn is much shorter than the 60 m baseline of Shuttle Radar Topography Mission (SRTM), which was dedicated for terrain topography mapping ([PERSON] _et al._, 2017). To a certain extent, longer baseline results in higher altimetric sensitivity, and furthermore, higher altimetric accuracy. However, SRTM suffers from serious baseline vibration ([PERSON] _et al._, 2017), which should be strictly constrained by KaRIn as well. To compensate for the insufficient baseline length, the radar look angle of KaRIn is set to be 0.7-4\({}^{\circ}\). SWOT is expected to achieve \(\sim\)2.5 cm relative accuracy at 1 km resolution (under 1\(\sigma\) demand, a swath averaged result among the 50 km swath width ([PERSON] _et al._, 2017)). To fulfill the rigorous accuracy and resolution goal, the SWOT team are facing several technical and engineering problems, and hence the launching date has been postponed several times; recently, SWOT has been scheduled to launch in 2022. On 15 September 2016, China launched the Tiangong-2 space laboratory, and the Interferometric Imaging Radar Altimeter (InIRA) was one of the experimental payloads on it.