# Independent System Calibration of Sentinel-1B
## 3 Internal Calibration  
The internal calibration (ICal) is used for the radar instrument monitoring and compensation of different effects disrupting the imaging signal. Temperature drifts and internal hardware characteristics influence the radar signal, causing gain and phase fluctuations during data acquisition. These changes in the signal are caused by thermal effects, ageing of the SAR electronics, degradation or extreme conditions in space.  
The Sentinel-1B radar instrument is a replica of the Sentinel-1A one. Therefore, the same internal calibration approach is applied, though special attention is required for the On-Ground Characterization (OGC). Different OGC values and correction curves are expected for Sentinel-1B since these depend on the inherent instrument characteristics.  
The Sentinel-1 complex ICal facility is based on the acquisition and evaluation of internal calibration measurements, satellite-dependent OGC corrections, leakage suppression algorithms and the application of the Pulse-Coded Calibration (PCC) method [13]. Different types of internal calibration pulses are generated within the instrument, each one being routed through a different part of the instrument. The proper combination of these calibration pulses allows the characterization of all critical elements seen by the nominal radar signal along the transmit and receive instrument paths.  
Calibration pulses with the same settings as the imaging data are acquired for replica signal retrieval; full bandwidth calibration pulses are acquired for instrument drift analysis over time (see Section 3.1) and for front-end TRM characterization (see Section 3.4). However, Sentinel-1B OGC showed that these internal calibration pulses are disturbed by coherent and phase-modulated leakage signals and can therefore not be used without previous correction [14]. While coherent leakage signals result from the cross-talk of the transmit path to the receive path when acquiring calibration pulses, phase-modulated leakage signals come from the radiating aperture and mainly irradiate into the choke flanges between the antenna panels. The same degradation effects were already observed for Sentinel-1A and therefore, dedicated correction algorithms were developed and implemented to guarantee the instrument stability. Sentinel-1A leakage suppression results are given in [3].  
Additionally, coherent spurious signals could be observed during Sentinel-1B OGC affecting both calibration and imaging pulses. These harmonics of the system clock overlap with the nominal signal in the Analog-to-Digital Converter (ADC). Noise pulses are used for the identification and extraction of the spurious signals, allowing their suppression from the raw data, see [3] for details on the analysis with Sentinel-1A data.  
In order to minimize the impact of all these disturbances on the SAR data, range compression during on-ground processing is realized by an ideal chirp instead of by the replica function derived from the calibration pulses. This requires the derivation and monitoring of the instrument time delay to compensate for the impact of the ADC sampling uncertainty (see Section 3.2).