## 4 Transponder Calibration Results
### Tandem Mission for Jason-2 and Jason-3  
There was a tandem mission for Jason-2 and Jason-3 in 2016. In that period, transponder calibration was initiated on 28 March 2016, approximately two months after the Jason-3 launch (i.e., 17 January 2016). It ended on 23 September 2016, when Jason-2 shifted to a different orbit. During this tandem period, both satellites were flying as a group of satellites together on the same orbit but separated by just 80 s (560 km, Jason-3 directly behind Jason-2). Both satellites have been calibrated almost simultaneously with the sea-surface facility at Gavdos and the transponder at the CDN1 Cal/Val site along their descending pass No. 18. The first four cycles in Jason-3 have not been measured with the transponder, as previously explained.  
Figure 30 illustrates the differences in range bias of Jason-3 with respect to Jason-2, as measured with the transponder during their tandem period. The mean value for this range bias difference has been estimated to be JA3 \(-\) JA2 = +1.37 cm \(-\) (\(-\)0.40 cm) = + 1.77 cm \(\pm\) 0.7 cm for that specific tandem period and based upon 15 calibration points. It should be pointed out that during Cycles 285-289 (28 March 2016, till 7 May 2016), there have been some functional problems with meteorological sensors at the CDN1 site, and models have been applied to determine atmospheric delays. If we take into account the up to now duration of transponder calibrations for Jason-3 (Cycles 5-80) and compare it against Jason-2 (Cycles 267-303), then this difference between the two satellites comes to a mean value of JA3 \(-\) JA2 = + 0.76 cm \(-\) (\(-\)1.2 cm) = +1.96 cm \(\pm\) 0.8 cm.  
A comparable performance has been observed between Jason-3 against Jason-2, if both satellites are calibrated with sea-surface techniques using the Gavdos Cal/Val for their tandem period. This tandem calibration is shown in Figure 31. Even though the results were based upon a very small sample of 16 data values for the tandem mission, Jason-3 appears to measure shorter sea-surface heights by 2.66 cm with respect to Jason-2. Biases for these sea-surface heights were based on the descending orbit No. 18 and on the GDR-D products.  
Figure 30: Range bias results for Jason-3 with respect to Jason-2 over their tandem mission with the transponder at the CDN1 Cal/Val site in western Crete. The SGDR-D and Level-2 data products have been used for the descending orbit No. 18.  
During their tandem mission, both satellites measured the same sea region south of Gavdos, but on another ascending orbit No. 109 (Figure 32). The mean value of Jason-2 bias for Cycles 281-298 has been computed as JA2 = +2.25 cm (No. 109). Also, the mean value for the Jason-3 bias for its corresponding Cycles 1-19 has been determined as JA3 = \(-1.05\) cm (No. 109). Therefore, based on the other orbit No. 109 and sea-surface calibration, the difference between those satellite observations amounts to JA3 - JA2 = (\(-1.05-2.25\) cm) = \(-3.3\) cm. This analysis reconfirms that Jason-3 measures lower sea-surface heights by 3.3 cm with respect to Jason-2. The produced results seem to agree with other publications on the subject [61].  
Figure 33 shows a profile of sea-surface biases of Jason-3 and Jason-2 with respect to distance from the point of closest approach at the Gavdos Cal/Val. It can be clearly seen that Jason-3 measures lower heights than Jason-2.  
Figure 31: Bias results with sea-surface techniques at the Gavdos Cal/Val site for Jason-3 with respect to Jason-2 over their tandem mission (28 March 2016, to 23 September 2016). Jason-2 seems to observe heights for the sea surface higher than those of Jason-3 by +2.66 cm. Results were based on very few data points (only 16 data values for their difference). The GDR-D products have been used for this for descending orbit No. 18 only.  
Figure 32: Bias results with sea-surface techniques at the Gavdos Cal/Val site for Jason-3 with respect to Jason-2 over the ascending orbit No. 109 (18 February 2016, to 18 August 2016) (16 data values).