# Independent System Calibration of Sentinel-1B
## 3 Internal Calibration
### Instrument Drift within Data Take  
The instrument drift was analyzed during the commissioning phase to confirm the instrument stability over time. It is derived by the compression and evaluation of the full bandwidth calibration pulses (100 MHz) acquired periodically and interleaved with the imaging radar data.  
Figure 5 shows Sentinel-1B instrument drift in amplitude and phase for a long IW Data Take (DT) with a duration of 25 min; the corresponding front-end temperature variation is also represented over time. The temperature information is extracted from the House-Keeping (HK) data available in the Level-0 product headers. As depicted in the top plot, instrument front-end temperatures increase in all panels about 20 \({}^{\circ}\)C during SAR imaging operation. In the same time period of about 25 min, the  
Sentinel-1B instrument drifts about 0.25 dB in amplitude and about 18 \(deg\) in phase (bottom plot). The instrument drift shown in Figure 5 can be considered as linear. In that case, the theoretical drift in amplitude and phase over time is of 0.01 dB and 0.6 \(deg\) per minute, respectively. This demonstrates the high stability of the Sentinel-1B instrument over long data takes.  
The instrument stability was also demonstrated for long data takes acquired in different transmit polarizations and different modes. Table 1 shows the results for three such long DTs.  
\begin{table}
\begin{tabular}{c c c c} \hline  & **IW** & **SM6** & **SM6** \\ \hline Polarization & DV & DH & DV \\ Date & 17 May 2016 & 26 May 2016 & 25 May 2016 \\ Duration & 25 min & 25 min & 25 min \\ Mean Temp. & 14 \({}^{\circ}\)C & 12 \({}^{\circ}\)C & 13 \({}^{\circ}\)C \\ Temp. Drift & 23 \({}^{\circ}\)C & 20 \({}^{\circ}\)C & 23 \({}^{\circ}\)C \\ Amp. Drift & 0.25 dB & 0.2 dB & 0.23 dB \\ Phase Drift & 17.2 \(deg\) & 13.9 \(deg\) & 19.6 \(deg\) \\ \hline \end{tabular}
\end{table}
Table 1: Sentinel-1B instrument amplitude and phase drift for long DTs acquired with different polarizations and acquisition modes.  
Figure 5: Sentinel-1B front-end temperature drift (**top plot**); instrument drift in amplitude and in phase (**middle and bottom plot**) of the co-polar channel (VV) for a 25 min long IW data take acquired during the commissioning phase on 17 May 2016.  
Finally, the instrument drift was monitored on a daily basis during the 3-month commissioning phase (CP) for data sets acquired in different SAR acquisition modes. Figure 6 shows the long-term instrument drift with a mean value of 0.15 dB in amplitude (top plot) and 4 _deg_ in phase (bottom plot). Moreover, the instrument drift remains smaller than 0.25 dB for the amplitude and smaller than 8 _deg_ for the phase for almost all DTs acquired during the CP. Note that StripMap mode data (in yellow) show a slightly lower drift compared to the rest of the data analyzed, both in amplitude and phase. This is mainly due to the shorter acquisition time of the SM data analyzed, with a typical duration of about 1 to 2 min.  
The analysis confirms the high radiometric stability of the Sentinel-1B instrument over time and even for long DTs, independently of the imaging mode, transmit and receive polarization.  
Beyond that, at raw data level, this instrument drift measured by means of calibration pulses is compensated for during SAR data processing, so that the remaining instrument drift in the SAR image is smaller than 0.1 dB in amplitude and only a few degrees in phase. This accurate internal calibration is the basis to ensure the stability of the instrument and thus to achieve the required absolute radiometric calibration accuracy of 1 dB (3 \(\sigma\)).  
Figure 6: Sentinel-1B instrument drift within a DT in amplitude (**top plot**) and phase (**bottom plot**) during the three months of CP from June to August 2016. Different colors indicate different acquisition modes.