## 7 Results and Discussion
The range and azimuth biases (mean ALE values) estimated from Figure 6c may be considered calibration biases: their removal from the product timing annotations would center the scatterplot on (0, 0) and increase the mean geolocation accuracy of the delivered products. This change was in fact implemented in the IPF for the slant range bias on 5 May 2015, effectively shifting the mean range ALE to 0 m. Note that since PD has been compensated here, the delivered products would not be as accurate _without_ PD compensation. However, investigations are underway, with ESA considering how to fully correct for the bistatic effect and even incorporate PD estimates into the product annotations, so that out-of-the-box product geolocation would already provide the best possible accuracy. In Figure 7, the analog of Figure 6 is shown, but for the corresponding GRDF products. While the effects of the PD and bistatic residual corrections are the same, the best-case ALE standard deviation (Figure 7c) is not as accurate as the SLC case. This is unsurprising, as GRDF products are further processed via a slant-to-ground-range polynomial, and their SM sample spacing is fixed at 4.0 m in both dimensions, reducing the available range resolution compared with SLC products. More information is lost during the _detection_ step: the conversion of the complex values to amplitudes. Less information is then available during the oversampling step where the CR peak position is measured in the image raster. On the other hand, the _mean_ biases in Figures 6c and 7c are nearly identical. This is strong evidence that the slant to ground range processing step does not introduce any unexpected biases itself, and is functioning as planned. The SM products may provide high ranging accuracy, but to compare the accuracies of the different primary OSV sources, the ALE was estimated in the same way as for Figure 6c, this time using different OSV sources. A comparison is shown in Figure 8. In all three cases, orbit interpolation was performed using ESA's EO_CFI library. The ALE using predicted OSV files is shown in Figure 8a. Along-track errors of over 200 m were occasionally observed, probably due to orbit propagation error over the large time interval between file generation (_i.e._, when the orbit is propagated) and the actual acquisition, although most values were below \(\sim\)50 m. Note that predicted OSV files (AUX_ORBPRE) are the only case where the file is generated before the acquisition. Restituted (AUX_RESORB) and precise (AUX_POEORB) files are generated _a posteriori_ based on GNSS measurements. Much more encouraging is the ALE scatter in Figure 8b, based on the restituted OSVs. These are available in near-real-time, _i.e._, a latency of less than 3 hours after product acquisition. Comparing the restituted with the precise OSV result, which is repeated in Figure 8c, it is clear that both are nearly equivalent, with only a loss of \(\sim\)1 cm in the azimuth and range standard deviations in the restituted case. For all but the most time-critical applications, the AUX_RESORB orbit files should therefore be sufficient. All SM mode products fulfil the accuracy requirement of 2.5 m specified for this mode using near-real-time restituted OSVs (see Tables 7-1 and 7-2 in [2]). In fact, even the GRDF products without PD or bistatic residual corrections applied (_i.e._, the \"out-of-the-box\" scenario shown in Figure 7a) meet this requirement. Figure 7: StripMap GRDF estimates using precise state vectors; each point represents a single product over _Torny-le-Grand_ (green) or _Dubendorf_ (black); (**a**) no path delay or bistatic residual corrections; (**b**) path delay correction but no bistatic residual correction; (**c**) path delay and bistatic residual corrections applied. (a) no PD, no bistatic; (b) PD, no bistatic; (c) PD, bistatic. ESA and the geometric calibration team are currently discussing how to best incorporate the calibration results into future products.