# Performance Analysis of Ocean Surface Topography
## 2 Altimetry Simulation of OST  
### System Parameters of InIRA  
All system parameters of InIRA are designed in order to increase its altimetric precision and swath width; however, limited by the capsule size of Tiangong II and the compatible requirements with other experiment payloads, some of the system parameters are restricted.  
The altimetric precision of InSAR is given by [10]:  
\[\Delta{t}=\frac{\lambda{{\tau}_{1}}\sin{\theta}}{2\pi{B}\cos(\theta-\alpha)} \Delta{\phi} \tag{1}\]  
where \(B\) is the physical baseline length, \(\lambda\) is the radar wavelength, \(\theta\) is the incidence, \(\alpha\) is the baseline roll angle, and \(\Delta{\phi}\) is the phase error. Therefore, a longer baseline, smaller incidence and shorter radar wavelength will improve the altimetric precision significantly.  
The antennas are installed outside the capsule of Tiangong II and form a 2.3-m baseline. The baseline length is quite short for the high altimetric precision requirement. In order to compensate for the short baseline deficiency, a near-nadir incidence is chosen. A bonus brought by the near-nadir incidence is that the received signals are mainly from specular reflection, which improves the signal-to-noise ratio (SNR) a lot and further increases the altimetric precision. The relationship between SNR and altimetric precision is explained in Section 2.3.  
Theoretically, the shorter the radar wavelength the higher the altimetric precision. However, short radar wavelength has its drawbacks, and rain attenuation will decrease the SNR of reflected signal and eventually worsen the altimetric precision [19]. Moreover, when an electromagnetic wave intersects with the ocean surface, short radar wavelength will lower the possibility of specular reflection because less surface appears smooth [16]. This will definitely worsen the precision as well since most of  
Figure 1: Schematic diagram of the resolution and swath of an altimeter, interferometric imaging radar altimeter (InIRA) and synthetic aperture radar (SAR) (the sketch does not represent the actual geometrical size).  
the reflected signals come from specular reflection under near-nadir incidence. Hence, the Ku-band antennas are chosen for InIRA above all these considerations.  
In order to achieve a wide swath, the relative variation of the incidence is quite large. The nominal incidence of InIRA is 1\({}^{\circ}\)-8\({}^{\circ}\), as shown in Figure 2a; a 47-km-wide swath can be achieved within the incidence range, while a large relative variation of incidence also makes the ground-range resolution of InIRA change significantly. The ground-range resolution has changed almost eight times from \(\sim\)213 m in the near range to \(\sim\)27 m in the far range, as shown in Figure 2b.The variation of ground-range resolution also causes the multi-look number varies significantly among the swath. Averaging N neighboring pixels, also referred to as multi-looking, is normally used to reduce the phase error while processing the interferometric data [20]. The multi-look number is determined by the ratio between grid resolution and ground-range resolution. Supposing that the grid resolution along the ground-range direction is 1 km after multi-looking, then the phase error suppression by multi-looking in the near range of the swath will be much less than in the far range.