## 4 Transponder Calibration Results
### Transponder Versus Sea-Surface Biases: Jason-2 and Jason-3  
Figure 27 presents, at first, the Jason-2 range calibration using the transponder at the CDN1 Cal/Val site along its descending orbit No. 18 and for cycles 267-303 (2 October 2015, to 23 September 2016). Figure 28 depicts the Jason-2 bias derived using sea-surface calibration and based on the Gavdos Cal/Val for the same period (Cycles 265-303). Sea calibration follows a few seconds immediately after the transponder calibration at CDN1 transponder Cal/Val. The gap of range bias results in Figure 26 between Cycle 277 (9 January 2016) and 285 (28 March 2016) exists as the transponder had not been operating during that period.  
The mean value of Jason-2 for its range bias with the transponder along the descending Pass No.18 is computed as range bias (Jason-2, Cycles 267-303) = \(-\)1.2 cm \(\pm\) 0.6 cm (Figure 27), whereas during the same period, the sea surface bias amounts to a SSH bias (Jason-2, No. 267-303) = +1.7 cm \(\pm\) 0.5 cm (Figure 28). Biases derived using transponder and sea-surface techniques exhibit absolute values of about the same magnitude but with opposite signs, as expected. These results support and also corroborate each other as they are determined using diverse calibration techniques, locations, settings, and instrumentation.  
Figure 27: **(Top)** Jason-2 range biases determined by the CDN1 transponder Cal/Val, for Cycles 267–303. The mean value does not include Cycles 285–290. During March–May 2016, there had been a malfunction in the meteorological sensors and no reliable atmospheric delays had been determined. Delays were based solely on models for that period. Results are in cm. **(Bottom)** The boxplot of the Jason-2 range bias. Values are symmetrical around the central location value (mean, red horizontal line). The higher and lower horizontal box lines represent the 75 th and 25 th percentiles, respectively. The outer limits of the boxplot represent maximum and minimum values.  
Similar conclusions were found for the Jason-3 altimeter with transponder and sea-surface calibrations. Jason-3 transponder results for Cycles 5-80 are shown on Figure 29. The first four transponder cycles could not be calibrated and are missing in Figure 29. During that period, fine-tuning had to be put into action with parameters to upload onto Jason-3 such that its altimeter would be able to observe the transponder at that mountain height (i.e., 1050 m altitude).  
The transponder produces a mean value of +0.76 cm \(\pm\) 0.4 cm for the range bias of Jason-3. Results have been produced with Level-2 data products and precise orbits. The distinctive variation pattern on the results of transponder biases in Figure 29 could be generated by yaw steering applied in Jason-3 satellite (as well as in Jason-2). No such variation pattern appears, for example, in Sentinel-3A where no such yaw steering is implemented.  
Figure 28: Jason-2 SSH biases derived using the Gavdos Cal/Val for Cycles 265–303 with GDR-D data products. Results are shown in cm.  
Figure 29: **(Top)** Range bias results with the transponder at the CDN1 Cal/Val site in West Crete for Jason-3 and for the descending orbit No. 18 with SGDR-D products. **(Bottom)** The boxplot of the Jason-3 range bias. It appears that the values are symmetrical around the central location value (mean = +0.76 cm, red horizontal line). The higher and lower horizontal box lines represent 75 th and 25 th percentiles, respectively. The outer limits of the boxplot represent maximum and minimum values.  
Sea surface calibration generates a Jason-3 bias of \(-\)0.74 cm \(\pm\) 0.4 cm (Figure 24f), observed a few seconds immediately after transponder calibration along the same descending orbit No.18. Sea-surface results for Jason-3 were based on cycles 1-80 (18 February 2016, till 11 April 2018) for orbit No. 18 and with GDR-D data products.