Ground-based synthetic aperture radar (GB-SAR) sensors represent a cost-effective solution for change detection and ground displacement assessment of small-scale areas in real time early warning applications.

The main differences between GB-SARs and classical Orbital or Airborne SARs are the way the aperture is synthesized, the transmitted signal waveform and its power. In GB-SARs, the radar sensor is placed on a mechanical platform of linear displacement, transmitting a triangular FMCW signal of few watts of output power. Despite its flexibility in terms of rapid deployment and easy operability, some limitations have to be considered, as the reduced length of the aperture and the long scanning time, which degrade the cross-range resolution. High rate Stepped Linear Frequency Modulated Continuous Wave (SLFM-CW) GB-SAR sensors are capable to perform faster scans, reducing the scanning time in the order of seconds

The instrument employed for the measurements is the RISKSAR-X GB-SAR sensor. It has been used in different applications, such as the monitoring of rock falls in the Montserrat mountain (North-Eastern Spain), the characterization of urban subsidence in Sallent (North-Eastern Spain) or the monitoring of landslides in Canillo (Andorra). The system operates with a triangular SLFM-CW ramp signal generated by a Direct Digital Synthesizer (DDS) that is up-converted to the desired frequency by an active frequency multiplier. The transmitted power is delivered by a solid state power amplifier. The receiver unit consists of two parallel low-noise chains with a direct Zero-IF demodulator, where a sample of the transmitted signal is used as local oscillator. The sensor is fixed on a linear motion unit and operated in StripMap mode.

A view of Montserrat Range from Guilleumes area

The four polarimetric channels response of the area of Guilleumes (Montserrat Range, Catalonia, Spain)

A view of el Forn de Canillo (Andorra). ARS group monitored this scenario during one year, periodically once per month

PhD Ruben Iglesias was in charge of the GB-SAR operation in Canillo (Andorra). The Sensor is placed in a 2 m long Linear Motion Unit, taking a measurement every 10 minutes for a period of about 12h, during the afternoon-evening-night

HH Reflectivity map (SLC) of the scenario of Canillo. Relative low image resolution is due to the short aperture length (2m). Nevertheless it is enough to distinguish different prominent targets together with the distributed ones