## 4 Error Budget of Height Estimation Accuracy
### Height Estimation Accuracy
The best results are obtained when the bistatic off-nadir angle attains lower values (5-10\({}^{\circ}\)). This can be explained considering the variation of the bistatic angle for each of the four selected values of the off-nadir angle (Figure 7). Indeed, when the bistatic antenna off-nadir angle is minimum, a larger bistatic angle is established, producing a better stereo effect and thus improving the height estimation accuracy. The  
Figure 11: Height uncertainty as a function of target latitude due to vector parallax difference uncertainty in the hybrid method for four bistatic off-nadir angles (1/10 of an image pixel assumed as co-registration uncertainty). Figure 12: Performance comparison between the analyzed methods for spaceborne bistatic SAR stereo radargrammetry considering 1/10 of an image pixel as co-registration uncertainty and four bistatic off-nadir angles: (a) projection of bistatic parameters, (b) bistatic rigorous stereo SAR problem, (c) hybrid method. small variations of the height uncertainty within the observed latitude can be used to design an orbit (i.e. to select adequate RAAN and anomaly separations) which allows more favourable bistatic angles to be achieved at latitudes where more accurate DEM generation is required. However, it must be noted that the best geometric conditions (\(\theta\)_\"_ = 5-10\({}^{\circ}\)) could introduce radiometric disparities between monostatic and bistatic images which could influence co-registration algorithms performance [13], hence it is more conservative to consider a larger co-registration error (Figure 13). The method of projection of bistatic parameters and the bistatic rigorous SAR stereo problem are more robust with respect to radiometric disparities leading to a LE90 point-to-point error of 4-5 m over the whole range of covered latitudes, also considering a co-registration error dropped to 1/2 of an image pixel. Finally, it is possible to provide some guidelines to improve performance. It is clear that a reduction of monostatic resolutions would be useful, however if the reference monostatic mission is held, and present trend is obviously towards high resolution missions, the simplest way to reduce height uncertainty is to upgrade the absolute and relative navigation accuracy. This can be carried out without hardware modifications, but by means of a better filtering of GPS observables. Indeed, in recent years robust algorithms have been developed, able to estimate the absolute position with a decimetre accuracy [42] and the relative position with a centimetre one, even for baselines of hundreds of kilometres [43, 44] and highly variable along the orbit [45]. In this context it is useful to investigate the gain in relief reconstruction accuracy achievable with 25 cm errors on each component of the absolute position and  
Figure 13: Performance comparison between the analyzed methods for spaceborne bistatic SAR stereo radargrammetry considering 1/2 of an image pixel as co-registration uncertainty and low bistatic off-nadir angles: (a) projection of bistatic parameters, (b) bistatic rigorous stereo SAR problem, (c) hybrid method. with 10 cm error for the relative baseline, along with 1/10 of an image pixel as co-registration error, representing the best achievable performance on area characterized by favourable conditions of surface coverage and slope characteristics (Figure 14). In this case LE90 point-to-point drops to 1.5-2.5 m.