# Independent System Calibration of Sentinel-1B
## 3 Internal Calibration
### TRM Characterization  
Sentinel-1 TRM characterization is based on the PCC method, also known as the PN-Gating method [13], verified for the first time in flight by TerraSAR-X [15]. This method allows all individual paths of an active phased array antenna to be measured simultaneously and under most realistic conditions. Applying this method, each TRM is code-controlled from pulse to pulse with a different code sequence applied to the phase. The number of different code sequences is defined by the number of TRMs or individual signal paths being investigated. A total of 280 TRMs in H polarization and 280 TRMs in V polarization compose the Sentinel-1B front-end antenna. These are grouped into 14 tiles, each also being fed by one TRM. As each polarization signal is received by its own receiving channel, a total of 280 different code sequences are used for the front-end TRMs and 14 for the tile amplifiers. Sentinel-1 features a dedicated calibration mode for TRM characterization, called Radio Frequency Characterization (RFC) mode. This mode only contains calibration pulses acquired with the PCC method. Since only one transmit channel is available per RFC acquisition, RFC data are acquired and independently analyzed for the two polarizations on transmit. The main goal of the RFC mode is to derive the excitation coefficients of each individual front-end sub-array, driven by an individual instrument path on transmission and on reception. The method is also used for TRM health monitoring, by comparing the in-flight measurements against reference values, in order to detect anomalies or drifts from the expected values. The reference values are derived from averaged RFC data acquired in-orbit to make sure that they represent the nominal instrument temperature conditions. Deviations from the reference values are stored in the so-called error matrix tables that can be fed into the antenna model to derive the actual antenna patterns. Figure 10 shows the settings derived for the 280 Sentinel-1B TRMs in the front-end for the transmit path and V polarization. The x-axis represents the 280 TRMs in the front-end (V-channel); the y-axis shows the amplitude and phase deviations of each TRM for 76 RFC measurements acquired during the CP between June and August 2016. In other words, each green point represents one evaluated RFC measurement for one single TRM. The blue line indicates the mean value of these 76 measurements and provides the average offset of each front-end TRM with respect to its reference value. Sentinel-1B front-end monitoring results depicted in Figure 10 show the status of the SAR antenna after launch and during the CP. One failed Electronic Front-End (EFE) unit (composed of four TRMs, two for each polarization) could be detected in tile number 5 by applying the PCC method. However, this has no impact on the SAR data performance (radiometric performance or image quality), since the two failed TRMs per polarization represent less than 1% of the complete SAR antenna. All other  
Figure 9: Sentinel-1B Front-End mean temperature for the DTs analyzed during the CP. front-end TRMs were working well with deviations in amplitude within 0.5 dB and in phase within 10 \(deg\). This demonstrated the health and proper performance of the majority of the SAR antenna after launch. More important though is the stability of the individual TRMs over time and under different temperature conditions. This is important because the settings of the individual TRMs define the shape of the antenna patterns and consequently the ability to accurately compensate the antenna patterns in the SAR data by means of the antenna model. In order to monitor and evaluate the Sentinel-1B front-end status and stability, different analyses were performed during the CP. First of all, a 24-h RFC campaign was carried out on 15 May 2016 to evaluate Sentinel-1B stability under different temperature conditions (hot and cold cases). RFC acquisitions were acquired continuously for both transmit polarizations.