## 6 Conclusions  
PAZ, the satellite EO SAR sensor of the Spanish National Earth Observation Program, is a powerful observation tool that will be used in the different applications defined in Copernicus, the European Earth observation program: land monitoring, marine monitoring, emergency management and security. Taking into consideration the increasing interest in passive radars and the potential good features of a system such as PAZ, a complete study about PAZ's feasibility as an opportunity illuminator in passive radar applications was carried out. Bistatic SAR systems have been the object of study, and some works dealing with the generation of passive radar images have been published. In this paper, the feasibility of PAZ as an opportunity illuminator in a passive bistatic radar system in surveillance applications was \begin{table}
\begin{tabular}{c c} \hline \hline
**Nominal height** & 20,200 km \\ \hline
**Orbits per day** & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: GPS orbit parameters. Figure 25: System geometry analysis for Navstart 59 (USA 192), on 25 May 2015, at 12:20:00: (**a**) total variation of the bistatic range during the acquisition time; (**b**) virtual displacement associated with the satellite orbit. \begin{table}
\begin{tabular}{c c} \hline \hline
**Central frequency** & 1557 MHz \\ \hline
**Bandwidth** & 2 MHz \\ \hline
**Power** & 27 W \\ \hline
**Modulation** & Spread spectrum \\ \hline
**Transmission rate** & 50 bps \\ \hline \hline \end{tabular}
\end{table} 
Table 5: GPS signal parameters used in the third geometry study. considered. Although a major drawback of that system is its intermittency in acquisition, constrained by the orbit parameters and its on-demand operating principle, PAZ is a powerful detection tool, with a finite lifetime, factors that justify the search for alternative applications. A detailed study was carried out to analyze the influence of waveform, platform movement, variable bistatic geometry, transmitted power and on-demand operation mode. The main conclusions are the following:  
* The transmitted signal is characterized by periodic ambiguity peaks that limit the coverage area and the target dynamics. After a study of instrumented coverage and system sensitivity, the limits imposed by the signal AF are beyond the limits imposed by these other factors, so the ambiguity peaks have no practical effect on system performance. * OI availability: A study of potential bistatic geometries was carried out in order to determine the time availability and the instrumented spatial coverage. The orbital parameters, the acquisition modes and the operation schedule impose critical limitations on the OI availability. Taking into consideration these limitations, the following potential applications are proposed:
* A PR system exploiting the signal emitted by PAZ can be used as a low-cost, easily deployable and configurable calibration tool in the commissioning phase of the SAR sensor or in posterior maintenance processes. The cost associated with the PR receiver is expected to be reduced due to the availability of commercial systems for direct receiving of sensor X-band data. * PR based on PAZ can also provide additional information, especially that related to the speed and trajectory of moving targets, to improve the information extraction from the acquired SAR images in the operational phase. This application can be really important in areas of special interest for monitoring specific phenomena. * As a PR can exploit the signal transmitted by other SAR sensors, GNSS and communication satellites, BSAR images can be generated to fill time gaps during which PAZ is not available. * Instrumented coverage area: This is determined by the OI antenna footprint. The spotlight acquisition mode simplifies the geometry of the system, because it allows the definition of a constant coverage area during SAR sensor operation.