Compact active transponders have been recently proposed as a powerful and cost effective solution to monitor motion of critical geological areas. Continuous monitoring of the ground deformation can be pursued by the recourse to SAR interferometry; precise measurements can be obtained when time coherence persists over long times on the inspected zone at least in a few points (permanent scatterers).
Sometimes the introduction of coherent radar targets can be a good solution to overcome the problem of phase instability, especially in vegetated and/or unurbanized areas.
In the past, passive devices such as corner reflectors were used in such areas but their drawbacks due to their large size and weight and loss of time coherence due to geometric deformations and material degradation has limited their use.
The availability of smaller, lighter, and less complex radar transponders is a recently presented as an alternative to passive devices, but also such compact and low cost devices can suffer some drawbacks, such as misplacement of the focused source on the ground due to the internal HW delay and distortion of many ground points due to the strong impulse response of the transponder that creates the typical cross pattern on the focused image.
It would be desirable to have the phase precise information on the ground facilitated by the presence of compact active transponders with the possibility of switching on and off the visibility of the transponder on the acquired raw image.
In this paper we propose a noise like pure transmitter to be used as a phase reference system, emitting a pseudo noise known signal at the background noise level. The presence of the transmitter can be extracted properly focusing the pseudo-noise transmitted sequence, stored on the sensor on-board memory. The correlation based range processing allows to focus the single transmitter, neglecting the influence of other ground returns, and the azimuthal chirp and range walk are preserved in the range focused transmitter signal, as they result dependent on the Doppler shift due to the SAR platform relative motion with respect to the ground.
Once extracted the phase information of single point scatterer, the raw data image can be obtained by a standard focusing algorithm. In this case, the presence of the transmitter is not evident in the scene as it is seen as a noise source by the focusing algorithm.
There are several problems to be addressed in the definition of the system but some simplification of the required hardware can be pursued.
Preliminary simulation results of the proposed approach are presented in the paper.