# Identification of C-Band Radio Frequency Interferences from Sentinel-1 Data
## 2 Sentinel-1 Acquisition Timeline  
We address here Sentinel-1 IW-mode, the default acquisition mode at low and mid latitudes (up to 60\({}^{\circ}\)) over still landmasses. At the beginning of each burst, as shown in Figure 1, the antenna beam is pointed backward of \(-0.6^{\circ}\). During the acquisition it sweeps forward up to +0.6\({}^{\circ}\) at the burst end. The system starts acquiring since the transmission of the very first pulse, although no backscatter is foreseen, at least for the two-way sensor-Earth travel-time, which corresponds to 8-10 pulses, that we define \"rank\" pulses, and are here exploited for RFI monitoring. The S1 acquisition timeline is made of a fixed pattern of three distinct component. The first part is the preamble, including a number of warmup echoes for instrument stabilization and noise and internal calibration measurements. The central part is the \"actual\" SAR acquisition and can be several minutes long. For IW mode the adopted timeline is listed in Table 1, the base scheme (duration about 5.4 s) is repeated an integer number of time according to the mission planning. The final part of the data-take is the postamble and includes again internal calibration and noise pulses. The IW data-takes are split into slices of 25 s to ease processing and data dissemination. The slicing procedure is not data driven and the data \"cuts\" can occur at every time instant within the scheme described in Table 1. At the end of each burst a few calibration pulses are acquired to monitor instrument status during the data-take. Most of the calibration pulses do not foresee any transmission \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Type** &
\begin{tabular}{c} **Pulse** \\ **(s)** \\ \end{tabular} & **Rank** & **PRF (Hz)** & **Notes** \\ \hline IW1 & 1409 & 9 & 1717.1 & \\ TxCal & 12 & & 1717.1 & Same pol. of data \\ TxCal & 8 & & 1717.1 & H pol \\ ISO & 1548 & 8 & 1451.6 & \\ IW2 & 1548 & 8 & 1451.6 & No TX \\ RxCal & 20 & N/A & & No TX \\ IW3 & 1410 & 10 & 1685.8 & \\ EpdnCal & 20 & N/A & & No TX \\ IW1 & 1409 & 9 & 1717.1 & \\ TACal & 20 & N/A & & No TX \\ IW2 & 1548 & 8 & 1451.6 & \\ ApdnCal & 20 & N/A & & No TX \\ IW3 & 1410 & 10 & 1685.8 & \\ ApdnCal & 20 & N/A & & No TX \\ \hline \hline \end{tabular}
\end{table} 
Table 1: IW timeline, cyclically repeated. Figure 1: Sketch of TOPSAR acquisition mode, here represented as the three swaths Sentinel-1 IW. and hence the \"rank\" echoes (belonging to IW1 and IW3) acquired after such pulses should contain only noise and RFI coming from the Earth, if any. On the other hand, for the second sub-swath, IW2, the case is slightly different, since the calibration pulses, named \"TxCal ISO\" in Table 1, are effectively transmitted, but only one cycle over two (as shown in the table) and with antenna beam pointing boresight (approximately 30\({}^{\circ}\) with respect to Nadir) and no electronical steering. This means that there could be some potential ground backscatter echo in the first rank pulses of IW2, but only every odd cycle. The quality of the rank echoes is fundamental for the RFI assessment: we expect that S1 is behaving like a receive-only C-band radiometer. Figure 2 shows the rank echoes power gathered over a very long strip of 730 s, covering from North Africa to arctic. The return power from each of the 20,000 pulses is plotted on the right for both V and H polarizations. It is the superposition of a smooth trend, fast ripples, within 1 dB, and sporadic big spikes, due to RFI. The smooth trend is a good representation of thermal noise, and one can appreciate the sudden changes in the land-sea and see-land transitions, due to the different albedo, like in Crete Island, at 7 s, and the Baltic Sea, at 400 s. A zoom of the ripple pattern has been plotted on the right panel in the same figure: one can observe the saw-tooth behavior with a periodicity of two cycles that matches the timeline listed in Table 1. The shape and the periodicity let then think to transients in the receiver gain after calibration.